Bulletin No. 239 n 0 ( D, Petrography and Mineralogy, 30 

benes \ E, Chemistry and Physics, 42

DEPARTMENT OF THE INTERIOR

UNITED STATES GEOLOGICAL SURVEY

CHARLES D. WALCOTT, DiKECTOK

EOCK CLEAVAGE

BY

CHARLES KENNETH LEITH

WASHINGTON

GOVERNMENT PRINTING OFFICE

1905
K . A 

\U°"3
CONTENTS.

Page.

LETTER OF TRANSMITTAL .................................................. 9

INTRODUCTION ............................................................ 11

Definition of cleavage.............................................. 11

Importance of metaclasic structure or secondary rock cleavage ...... 12

Literature on secondary rock cleavage ............................. 13

Secondary cleavage in its relation to a parallel arrangement of mineral constituents. ........................................... 14

Manner in which a parallel arrangement of mineral constituents

is brought about............................................ 15

Two structures included under cleavage, one of them not dependent upon a parallel arrangement of mineral constituents..... 17

False cleavage ............................................... 18

Cleavage in its relations to direction of application of the pressure

producing it ............................................... 18

Purpose of present paper ......................................... 19

Material used in investigation..................................... 20

Experimental work .............................................. 21

Acknowledgment ..................."........................J.... 21

PART I. FLOW CLEAVAGE ................................................. 23

Chapter I. Mineral arrangement in rocks with flow cleavage ............ 24

Mica ........................................................ 24

Hornblende.................................................. 28

Quartz ...... 4 ................................................ 31

Feldspar..................................................... 35

Chlorite........................... ........................ .40

Calcite....................................................... 40

Tremolite, actinolite, gri'merite, and other ainphiboles .......... 42

Kaolin and talc. .......'...................................... . 42

Garnet ....................................................... 42

Tourmaline.................................................. 43

Staurolite .................................................... 43

Ottrelite (and chloritoid) ..................................... 44

Andalusite (chiastolite) ...................................... 44

Sillimanite (fibrolite) ......................................... 45

Summary statement concerning facts of arrangement in rocks

with flow cleavage ......................................... 45

Chapter II. The observed relation of secondary rock cleavage to the

parallel arrangement of mineral particles............................. 49

Relative importance of minerals in producing flow cleavage ......... 50

Mica .............................. i......................... 51

Hornblende.................................................. 52

Quartz....................................................... 54

Feldspar...................................................... 55

3
4 CONTENTS.

PART I. FLOW CLEAVAGE Continued. Page. 

Chapter II. The observed relation of secondary rock cleavage to the 

parallel arrangement of mineral particles Continued. 

Eelative importance of minerals in producing flow cleavage Cont'd.

Calcite....................................................... 58

Chlorite ..................................................... 59

Kaolin and talc .............................................. 59

Tremolite, actinolite, gri'merite, and other amphiboles.......... 59

Tourmaline.................................................. 60

Staurolite.................................................... 60

Ottrelite..........................................^.......... 61

Andalusite ........ L ....._..................._................ 61

Sillimanite............. ...................................... 62

Garnet ...................................................... 62

Summary statement of effect of principal cleavage-giving minerals (mica, hornblende, quartz, and feldspar) on flow cleavage.. 63 

Chapter III. The processes through which the secondary parallel arrange- -- 

ment of minerals is brought about................................... 65

Section I. General account of possible processes of rock flowage yielding parallel arrangement..................................:..... 66

Crystallization and recrystallization ........................... 67

Rotation....................i................................ 71

Gliding...................................................... 74

Granulation ................................................. 75

The combined effect of recrystallization and granulation on size 

of grain.................................................... 76

Section II. Relative importance of processes of arrangement in individual minerals ................................................ 78

Mica ......................................................... 78

Hornblende.................................................. 80

Quartz ...................................................... 82

Feldspar..................................................... 86

Calcite ...................................................... 88

Chlorite..................................................... 90

Tremolite, actinolite, grimerite, and other amphiboles.......... 90

Garnet, Staurolite, tourmaline, andalusite, chloritoid, etc........ 91

Section HI. General predominance of process of recrystallization ... 92 

Section IV. Order of recrystallization ............................. 93

Chapter IV. Miscellaneous observations on the internal structure of 

cleavable rocks..................................................... 96

Section I. Why some minerals show crystallographic parallelism and 

others do not .................................................. 96

Section II. Molecular shape and recrystallization .................. 97

Section III. Flow cleavage as a molecular phenomenon............. 99

Chapter V. Observed relation of flow cleavage to the elongation and 

shortening of rock masses............................................ 102

General ......................................................... 106

Chapter VI. Relations of the elongation and shortening of rock masses, 

and hence of flow cleavage, to stress................................. 109

Strain ............................................................ 109

Stress ........................................................... Ill

Relation of stress to strain ........................................ 112

Application of general principles of relations of strain and stress to 

rock flowage and flow cleavage.................................. 112
CONTENTS. 5

Page. 

PART II. FRACTURE CLEAVAGE, COMPARISON WITH FLOW CLEAVAGE, SUMMARY

STATEMENT OF CAUSES AND CONDITIONS OF SECONDARY ROCK CLEAVAGE...... 119

Chapter I. Fracture cleavage.......................................... 119

Relations of fracture cleavage to stress ............................. 121

Chapter II. Comparison of fracture cleavage and flow cleavage .......... 125

Explanations of flow cleavage may not apply to fracture cleavage.... 126

Explanations of fracture cleavage may not apply to flow cleavage.... 126

Superposition of the two structures and gradations between them.... 130

Relative importance of fracture cleavage and flow cleavage.......... 133

Chapter III. Summary statement of causes of secondary rock cleavage... 135

Flow cleavage..................................................... 135

Fracture cleavage ................................................ 139

Relative importance of fracture cleavage and flow cleavage .......... 139

Chapter IV. Comparison of present statement with previous statements

concerning secondary cleavage ...................................... 140

PART III. ORIGINAL CLEAVAGE _...................................... ..... 143

Bedding in clastic sediments .................................. 143

Bedding in nonclastic sediments....................... i....... 146

Flow structure in igneous rocks................................ 146

Miscellaneous................................................ 149

Conclusion .................................................. 150

Superposition of secondary flow cleavage on original cleavage.... 151

Superposition of fracture cleavage on original cleavage.......... 152

INDEX ................................................................... 211

ILLUSTRATIONS.

Page. 

PLATE I. Specimens illustrating plane parallel and linear parallel types of flow

cleavage.....;................................................ 158

II. Photomicrographs of micaceous schists.................. ^......... 160

III. Photomicrographs of micaceous schist, showing biotite flakes lying

across the schistosity ....................................... ..'. 162

IV. Specimens of hornblendic schists illustrating plane-parallel and

linear-parallel types of flow cleavage..............-. .. -. - I64

V. Photomicrographs of hornblendic schists, showing arrangement of

hornblende crystals parallel to columnar axes................... ' 166

VI. Photomicrographs of hornblendic schists, showing the arrangement

of hornblende cleavage in slides cut across the schistosity ........ 168

VII. Photomicrographs of " leaf gneiss " .............................. 170

VIII. Photomicrographs of micaceous and quartzose schist............... 172

I.. Photomicrographs of micaceous and quartzose schist. 1............. 174

X. Photomicrographs of micaceous and quartzose schist............... 176

XI. Photomicrographs illustrating recrystallization of quartz and slicing

of feldspar..................................................^. 178

XII. Specimens illustrating granulation and slicing of feldspar crystals... 180

XIII. Photomicrographs of porphyritic constitutents developed after rock

flowage has ceased ............................................ 182

XIV. Photomicrograph of micaceous and quartzose schist with cleavage

across bedding, and photomicrograph of slate with false cleavage . 184

XV. Photomicrographs of artificially deformed marble. After Adams... 186

XVI. Schistose marble ................................................ 188

XVII. Cleavage bands in Ordovician slates and parallel quartz veins in

Cambrian slate................................................ 190

XVIII. Specimens illustrating fracture cleavage .............:. ............ 192

XIX. Specimens illustrating fracture cleavage........................... 194

XX. Specimens illustrating fracture cleavage........................... 196

XXI. Specimens illustrating fracture cleavage........................... 198

XXII. Specimen showing both flow cleavage and fracture cleavage in the

same block ................................................... 200

XXIII. Specimen showing both flow cleavage and fracture cleavage in the

same block ................................................... 202

XXIV. Specimens illustrating parallel arrangement of feldspars in rock

masses which have not undergone secondary rock flowage ....... 204

XXV. Chemical sediments with cleavage................................ 206

XXVI. Specimen showing elongation of pebbles in the plane of rock cleavage........................................................... 208

XXVII. Specimens showing nndistorted pebble and pebbles elongated in

plane of rock cleavage......................................... 210

7
ILLUSTRATIONS.

FIG. 1. Kough parallel arrangement sometimes shown by the greatest and mean

diameters of mica plates lying in the plane of rock cleavage....... 25

2. Sketch showing the manner in which mica plates feather out, one

against the other, in rocks with flow cleavage .................... 26

3. Biotite with its own cleavage inclined to the rock cleavage, as marked

by the adjacentmuscovite plates................................. 27

4. Biotite and quartz in a band with parallel muscovite plates adjacent.. 27

5. Habit of common hornblende crystal .............................. 29

6. Sketch showing tendency toward parallelism of dimensional axes and

cleavage of hornblende, as viewed in basal section................. ^29

7. Slicing of hornblende.. v ......................................... 30

8. Cross section of hornblende crystal parallel to clinopinacoid, showing

positions of cry stall ographic and elastic axes..................... 30

9. Slicing of quartz grain in schistose conglomerate.................... 32

10. Sketch showing granulation of quartz where feldspar is comparatively

unaffected by deformation........................................ 33

11. Twinned albite crystal with brachypinacoidal development.......... 36

12. Twinned albite crystal with .macropinacoidal development........... 36

' 13. Microscopic view of fresh anorthosite .............................. 37

14. Microscopic view of anorthosite after granulation ................... 37

15. Sliced feldspar in rhyolite-gneiss..............................'..... 38

16. Feldspar showing incipient "slicing" in schist-conglomerate ........ 38

17. Sliced feldspars in micaceous and chloride schist.................... 38

18. Feldspar in parallel lenses with crystallographic parallelism ......... 39

19. Parallel grains of calcite in schistose marble ........................ 41

20-21. Diagrams showing the rotation accompanying a given-amount of shortening .......................................................... 72

22-23. Diagrams showing angle of rotation of large particle under differential

movement...... ................................................ 73

24-25. Diagrams showing angle of rotation of small particle under differential

movement ..................................................... 73

26. Secondary development of parallel sericite by crystallization in cracks

formed by fracture............................................. 79

27. Pebbles of quartzite with constituent quartz grains elongated by recrystallization in the planes of elongation of the pebbles and of the rock 

as a whole ..................................................... 103

28-29. Diagrams showing irrotatidnal strain (pure shortening).............. 110

30. Diagram showing scission ......................................... 110

31-32. Diagrams showing equivalence of a strain ellipsoid in scission to a strain

ellipsoid in pure^shortening combined with a rotation............. Ill

33. Diagrams illustrating rotation of particle during rotational strain and

the resulting change in direction of transmitted stresses ........... 117

34. Fracture cleavage in slate emphasized by ferruginous staining ....... 120

35-36. Diagrams showing development of fissility, which by cementation or 

welding may yield fracture cleavage along the longer and shorter 

diagonals of a deformed portion of the rock stratum............... 122

37. Fracture cleavage after fissility in bedded rocks, showing application

of principles illustrated in figs. 35 and 36......................... 123

38. Diagram showing the theoretical position of strain ellipsoids in quadrant of a ceresin block flattened between two plates, with friction 

between wax and plates........................................ 129

39. Diagram showing fracture cleavage superposed on flow cleavage ..... 131

40. Fracture cleavage crossing flow cleavage............................ 132
LETTER OF TRANSMITTAL.

DEPARTMENT OF THE INTERIOR, 

UNITED STATES GEOLOGICAL SURVEY, 

. Washington, £>. C'., May 17, 1904. 

SIR: I have the honor to transmit herewith the manuscript of a 

paper on rock cleavage, by Charles Kenneth Leith, and to recommend 

its publication as a bulletin.

The paper embodies the results of a very careful and laborious 

investigation of facts concerning rock cleavage and a discussion of 

their theoretical significance. Its publication will place the subject of 

rock cleavage in a much more satisfactory shape and be of material 

assistance to all structural geologists.

Very respectfully, C. W. HAYES,

Geologist in Charge of Geology. 

Hon. CHARLES D. WALCOTT,

Director United States G-eological Survey.

ROCK CLEAVAGE.

By CIIAKLES KENNETH LEITII.

INTRODUCTION.

DEFINITION OF CLEAVAGE.

Rock cleavage, as commonly defined in geological text-books, in 

effect, is a rock structure by virtue of which the rock has the capacity 

to part along certain parallel surfaces more easily than along others. 

It is possessed by a considerable proportion of the rocks of the lithosphere. It is usually distinguished from actual partings of a similar 

nature. The parallel structures may be original or secondary.

(1) Original structures are induced in the rock mainly during its 

solidification from a magma or deposition in water, though perhaps 

modified by subsequent static metamorphism. They comprise sedimentary bedding, flow structure of lavas, certain gneissic structures, 

and pegmatite structures.

(2) Secondary structures are induced by deformation through rnetamorphic processes subsequent to the formation of the rock. They 

have been given various names, such as cleavage, slatiness, schistosity, 

foliation, fissility, etc.

Generally, the secondary structure only has been considered under 

cleavage, but, it is apparent that there is nothing in the definition 

of the term, rock cleavage, as above given, to prevent its application to any of these structures, whether original or secondary. It 

was so applied by Le Conte. a The writer believes that the facts 

presented in this paper will justify the conclusion that there is no 

essential difference between the original and the secondary structures; 

that in some cases they are not to be discriminated with certainty, 

and that both should be included under rock cleavage. Therefore, 

the term cleavage will be used as above defined; it will be confined 

to structure, and will have no significance as to origin. Where the

"Says Le Conte: "This structure is usually treated under metamorphic rocks, as a kind of metamorphism; but it is found in rocks which have not undergone ordinary metamorphic changes, and 

It is produced by an entirely different cause." Le Conte, J., Elements of Geology, 4th ed., 1896, 

p. 189.

11
12 BOCK CLEAVAGE. [BULL. 239.

origin of the structure is clear the term original, or secondary, may 

be prefixed.

The application of terms pertaining to cleavage, as here used, is 

summarized in the following table:

'Original cleavage. (Protoclase or original-cleavage rock.) 

Bedding.

Flow structure in lavas. 

Parallel structure in certain gneisses. 

Pegniatitic structure.

Parallel structure due to the arrangement of feldspars in 

certain gabbros, etc.

'Flow cleavage. Including in whole or 

in part: " Ultimate cleavage" of 

Sorby,' 'Cleavage'' of most writers, 

'' Slaty cleavage," " Cleavage 

proper," etc. 

Schistosity.. (Schists!) 

Slatiness. (Slates.) 

Parallel structure in certain gneisses. 

(Comes partly under schistosity.)

Rock cleavage. (Cleavable rock.)

Secondary cleavage.

(Metaclaseor second-'

ary-cleavagerock.)

Fracture cleavage. Including in whole 

or in part:

Close joints cleavage.

False cleavage.

Strain slip cleavage.

Slip cleavage.

Ausweichungs cleavage.

Fissility in part. (The term is retained for closely spaced parallel 

partings.)

Eift.

In this classification several new terms required for the systematic 

classification of cleavage structures and cleavable rocks have been 

introduced. Protoclase may be defined as a rock possessing a cleavage originall}7 developed during sedimentation under water or cooling 

from magma, such as bedding, flow structure, etc. Metaclase may be 

defined as a rock possessing a cleavage secondarily developed during 

rock deformation. Secondary cleavage is considered under the heads

of fracture cleavage and flow cleavage. Fracture cleavage is conditioned by the existence of incipient, cemented, or welded parallel 

fractures, and is independent of a parallel arrangement of the mineral 

constituents. Flow cleavage is conditioned solely by a parallel 

arrangement of the mineral constituents. Fracture cleavage is a phenomenon of the zone of fracture, and flow cleavage of the zone of rock 

flowage. The structures are correlative, just as jointing and folding 

are correlative. The fitness of these terms will not be argued here, 

but it is hoped that it will appear from the following discussion. 

IMPORTANCE OF METACLASIC STRUCTURE OR SECONDARY ROCK

CLEAVAGE.

Secondary rock cleavage is found wherever rocks have been much 

deformed under conditions of rock flowage, and especially in mountainous areas and regions of ancient crystalline rocks. The correct
(

J. LEITH.] . LITEKATURE. "13

interpretation of the structure throws light on the nature of rock 

deformation in general and its relation to the stresses producing it, 

and hence is of fundamental significance in determining the nature 

of mountain-making movements and stresses. Since so large a proportion of the rocks of the lithosphere develop this peculiar structure 

in adjusting themselves to the earth's stresses, there is warrant for 

the attempt to determine the true nature of the structure and the reason for its occurrence.

LITERATURE ON SECONDARY ROCK CLEAVAGE.

The literature on secondary rock cleavage is voluminous, but the 

conclusions thus far reached are so diverse that the geologist is still in 

doubt as to the true explanation of the phenomenon. An attempt is 

made below to outline the well-known hypotheses and to indicate what

points are still under controversy.

The English geologists seem to have been the first to seriously

1 attempt the explanation of secondary rock cleavage. Among them

I Sedgwick" should receive the first mention. His theory (proposed in

^ 1829), that cleavage is due to the parallel arrangement of individual

particles making up. the rock mass, caused by "crystalline or polar"

forces acting on the whole mass "simultaneously in given directions"

and "with adequate power," was long and widely accepted. In 1838

Fox b published an account of the development of cleavage in clay used

in the separation of copper and zinc in an electric battery. Because of

the work of Sedgwick and Fox, cleavage was usually explained in the

first half of the last century as due to "crystalline," "polar," or

"magnetic" forces, terms evidently carrying somewhat vague mean-'

ings and well illustrating the state of knowledge on the subject at that

time.

About the middle of the last century there appeared notable contributions to the subject by Darwin, c Dana,rf Sharpe,*3 Sorby/Tyndall,-/

aSedgwick, Adam, Remarks on the structure of large mineral masses, and especially on the chemical changes produced in the aggregation of stratified rocks during different periods after their depositions: Trans. Geol. Soc. London, 2d ser., vol. 3, pt. 1,1835, pp. 4G1-48G.

'»Fox, R. W., On the lamination of clay by electricity: Edinburgh New Philos. Jour., old ser., vol. 

25, 1838, pp. 196-198.

c Darwin, Charles, Geological Observations on South America during Years 1832-1836, pp. 161-168. 

Published in 1846.

dDaua, J. D., Silliinan's Am. Jour., 1st ser., vol. 45,1843, pp. 107-108.

<?Sharpe, Daniel, On slaty cleavage: Quart. Jour. Geol. Soc., vol. 3,1847, pp. 74-105, and vol. 5,1849, 

pp. 111-129. On the arrangement of the foliation and cleavage of the rocks of the north of Scotland: Philos. Trans. Royal Soc. for 1852,1852, pp. 445-461. Contains references to early work by DC la 

Ml Bechc, Austin, Hunt, Murchison, and others.

/Sorby, TI. C., Edinburgh New Philos/Jour., old ser., vol. 65,1853, pp. 187-148; ibid., new ser., vol. 4, 

1856, p. 339 et scq.; ibid., vol. 6,1857, p. 316 et seq.; Philos. Mag., 4th ser., vol. 11,1856, pp. 20-36; ibid., 

vol.12, 185G, pp. 127-128; Quart. Jour. Geol. Soc. for Nov., 1863, pp. 401-406.

oTyndall, John, Comparative view of the cleavage of crystals and slate rocks: Philos. Mag., 4th 

ser., vol. 12, 1856, pp. 35-47,129-135.
14 ROCK CLEAVAGE. [BULL. 239.

and Phillips. a All subsequent contributions have contained in whole 

or in part the essential conclusions of these men. Among those who 

have added to or modified the theories of rock cleavage proposed by 

the scientists above named, the following should be particularly mentioned: King, 6 Daubree, c Barker,^ Becker,6 Van Hise/ Hoskins, 9' 

and Adams and Nicolson. 7i Many other names might well be included, 

such as Dale,* Hutchings/ and Reade and Holland/' but the men named 

have suggested, emphasized, or modified some particular feature of 

the subjectj and a discussion of their conclusions will indicate with a 

fair degree of clearness the present state of knowledge of the subject, 

and will obviate a tedious bibliographical discussion.

SECONDARY CLEAVAGE IN ITS RELATION TO A PARALLEL ARRANGEMENT 

OF MINERAL CONSTITUENTS.

All the writers above named, except Tyndall, King, Daubree, and 

Becker. have assumed rock cleavage to be for the most part dependent 

on a parallel arrangement of the greatest, mean, and least dimensions 

of the mineral particles of unequal dimensions making up the rock 

mass. Sorby* was the first to maintain that the best cleavage exists 

in rocks which contain minerals '' whose length and thickness differ 

most." Dana 771 and Van Hise 71 have in addition proposed and amplified 

the idea that the mineral cleavage of the parallel-arranged particles

a Phillips, John, Report on cleavage and foliation in rocks, and on the theoretical explanation of 

these phenomena (pt. 1): Kept. 26th Meeting Brit. Assoc. Adv. Sci., held 1856, pp. 369-396. Published 

in 1857.

bKing, W., On the structure of rocks called jointing: Trans. Royal Irish Acad., vol. 25,1875, pp. 

605-662.

c Daubree, A., Geologic experimentale, vol. 1,1879, pp. 407-418.

dHarker, A., On slaty cleavage and allied rock structures, with special reference to the mechanical 

theories of their origin: Rept. 55th Meeting Brit. Assoc. Adv. Sci., held 1885, published in 1886, pp. 813- 

852. Contains bibliography.

e Becker, G. F., Finite homogeneous strain, flow, and rupture of rocks: Bull. Geol. Soc. America,

vol. 4, 1893, pp. 13-90. Sehistosity and slaty cleavage: Jour. Geol., vol. 4, 1896, pp. 429-448. Experiments on schistosity and slaty cleavage: Bull. U. S. Geol. Survey No. 241,1904, 32pp.

/Van Hise, C. R., Principles of North American pre-Cambrian geology: Sixteenth Ann. Rept. U. S. 

Geol. Survey, pt. 1,1896, pp. 633-668. Deformation of rocks, pt. 3, Cleavage and flssility: Jour. Geol., 

vol. 4,1896, pp. 449-483. Metamorphism. of rocks and rock flowage: Bull. Geol. Soc. America, vol. 9, 

1898, pp. 269-328. A treatise on metamorphism: Mon. U. S. Geol. Survey, vol. 47,1904, pp. 748-763.

0Hoskins, L. M., Flow and fracture of rocks as related to structure: Sixteenth Ann. Rept. U. S. 

Geol. Survey, pt. 1, 1896, pp. 845-874.

ft Adams, F. D., and Nicolson, J. T., An experimental investigation into the flowage of marble: 

Philos. Trans. Royal Soc. London, ser. A, vol. 195,1900, pp. 363-401.

iDale, T. N., The slate belt of eastern New York and western Vermont: Nineteenth Ann. Rept. 

U. S. Geol. Survey, pt. 3, 1899, pp. 163-307. Contains bibliography. Structural details in the Green 

Mountain region and in eastern New York: Sixteenth Ann. Rept. U. S. Geol. Survey, pt. 1,1896, pp. 

549-570.

JHutchings, W. M., Clays, shales, and slates: Geol. Mag., new ser., dec. 4, vol. 3,1896, pp. 309-317, 

343-350.

fc Reade, T. M., and Holland, Philip, The green slates of the Lake district, with a theory of slate 

structure and slaty cleavage: Proc. Liverpool Geol. Soc., 1900-1901, pp. 101-127. The phyllades of the 

Ardennes compared with the slates of North Wales: Proc. Liverpool Geol. Soc., pt. 1, 1897-98, pp. 

274-293, and pt. 2,1899-1900, pp. 463-478.

l Philos. Mag., vol. 12, 1856, p. 128.

»»Loc. cit., p. 107.

?i,jQur. Geol., vol. 4, 1896, p. 453.
LITERATURE. 15

may aid in giving a secondary rock cleavage. Dana's conclusion is 

expressed in the words, "Modern igneous rocks are laminated, and in 

general more or less so according to the quantity and cleavability of 

the cleavable minerals they contain. Mica, the most perfectly foliated 

mineral, produces, when abundant and not overruled by other constituents, the most perfectly laminated rock." Van Hise has concluded 

that'' rock cleavage is due to the arrangement of the mineral particles 

with their longer diameters or readiest cleavage, or both, in a common 

direction."

On the other hand, Tyndall" and Daubre"e 6 performed experiments 

from which they concluded that secondary rock cleavage is independent of any parallel arrangement of the constituent particles. King c 

also maintained that cleavage, while perhaps in part due to a flaky or 

dimensional arrangement of the mineral constituents, " is essentially 

the result of pressure exerted against divisional planes, chiefly belonging to jointing, that existed in any given rock prior to its becoming 

affected by such pressure." Becker/* largely from a mathematical and 

experimental analysis of the relations of cleavage to pressure, later 

reached the conclusion that cleavage is essentially independent of any 

parallel arrangement of the mineral constituents, although such parallel arrangement may be present as an incidental result of the development of cleavage/

MANNER IN WHICH A PARALLEL ARRANGEMENT OF MINERAL CONSTITUENTS IS BROUGHT ABOUT.

Those who maintain that the parallel arrangement of minerals is an 

essential condition for secondary rock cleavage differ among themselves as to the manner in which this arrangement is brought about. 

Sedgwick and Fox, followed by others, referred the parallel arrangeftPhilos. Mag., vol. 12, 1856, p. 37.

' Geologie experimentale, vol. 1, 1879, p. 413.

eTrans. Royal Irish Acad., vol. 25,1875, p. G57.

rtBull. Geol. Soc. America, vol. 4, 1893, pp.-79, 80.

eBecker summarizes his view as follows (Nineteenth Ann. Kept. U. S. Gcol. Survey, pt. 3, p. 206):

"Slaty cleavage is produced when a solid hut plastic mass, firmly supported on one side, experiences a pressure on the opposite side which is not perpendicular to the supporting surface. The 

resulting cleavage has a direction intermediate between that of the applied force and the fixed support. The cleavage itself makes with the deforming force an angle which may vary between a very 

small value and one equaling or even exceeding 45°. The firm support of the deformed rock required 

by the theory may be afforded either by a purely material resistance or by any combination of forces 

which prevents the mass from rotating as a whole Avhile undergoing deformation. Lateral pressures 

not equal in all directions appear to be of minor importance so long as they do not interfere with the 

condition that the angle between the resultant force and the fixed support shall differ sensibly from 

90°. The origin of the cleavage as conceived in this theory is incipient "solid flow,'' which is a different thing from liquid flow. The production of cleavage should usually "be accompanied by the formation of master joints at angles to the cleavage approaching 90°, and the direction of the pressure is 

perpendicular to the intersection of the cleavage with such joints, intersecting (but not exactly 

bisecting) the obtuse angle. The grain of the slate should be parallel to this intersection. In general 

there should be an elongation in the direction of the grain and a contraction in the plane of cleavage 

at right angles to the grain."

See also Becker, G. F., Experiments on schistosity and slaty cleavage: Bull.U. S. Geol. Survey No, 

241,1901.
16 BOCK CLEAVAGE. [BULL. 239.

ment to "crystalline," "polar," or "galvanic" forces. Darwin" 

supposed the arrangement to have been induced by "tension," 

"before .the final consolidation of the mass and the total cessation 

of molecular movement." Sorby 6 assumed the previous existence 

of flat particles and supposed their arrangement to be due to rotation during the secondary deformation of a rock by shortening of 

the rock mass in one direction and elongation in a perpendicular 

direction, but he concluded also that other particles may have been 

flattened in situ, and that some of the parallel flat particles are 

secondary developments, particularly in "schists," which he distinguished from "slates." Sharpe c supposed every particle in a 

deformed rock to have taken part in the elongation and shortening 

which the rock underwent, thus accounting both for the flatness of the 

particles and for their parallel arrangement. Sharpe^ concluded also 

that, while "no connection has been detected between cleavage and 

crystallization beyond the tendency of mica and talc to arrange themselves along the planes of cleavage," still on these planes " there would 

be the least resistance to their intrusion or formation " and the development of such minerals "may have been a subsequent operation." 

Van Hise e believes the causes of the parallel arrangement conditioning rock cleavage to be, first and of most importance, the parallel 

development of new minerals; second, the flattening and parallel rotation of old and new mineral particles; and third and of least importance, the rotation into approximately parallel positions of particles 

which had originally a random arrangement. Van Hise thus differs 

from Sorby in maintaining the predominance of the new development 

of minerals, which was suggested by Sorby only as a minor detail. 

Adams and Mcolson deformed limestone at temperatures of 300° and 

400° C., and obtained a flattening of calcite particles, and thus a parallel structure, through twinning and gliding, a movement whicn 

occurs also in the deformation of metals and ice. At ordinary temperatures the flattening produced in this way was associated with actual 

granulation of the particles. Comparing the results of experiments 

with the deformation observed in marbles in the field, they concluded 

that while " recrystallization undoubtedly plays an important, and in 

many cases probably a chief, part in the great movements which are 

observed to have taken place in the limestones of contorted districts, 

this process is by no means the only one by which such movements are 

brought about. "^ They may be brought about by .the development of 

cataclastic structure and through gliding and twinning of the constita Philos. Mag., vol. 12, 1856, p. 168.

£> Edinburgh New Philos. Jour., vol. 55, 1853, pp. 137-148. Quart. Jour. Geol. Soc., vol. 19, 1863, 

pp. 401-406.

cQuart. Jour. Geol. Soc., vol. 3, 1847, p. 74-105. 

<*Quart. Jour. Geol. Soc., vol. 5,1849, p. 129.

cJour. Geol., vol. 4,1896, p. 453. Sixteenth Ann. Kept. U. S. Geol. Survey, pt. 1,1896, p. 635. 

/Philos. Trans. Koyal Soc. London, ser. A, vol. 195,1900, p. 398.
LITERATURE. 17

uent crystal particles. The latter processes give a parallel arrangement of the particles. They concluded further that the "flowage of 

the granite and other harder, crystalline rocks" (and, presumably, a 

parallel arrangement of the deformed particles) might be induced in 

much the same way."

The processes resulting in the deformation of rock masses have been 

discussed by many other writers,'' who do not emphasize the development of parallel arrangement of mineral particles, resulting from the 

deformation.

TWO STRUCTUKES INCLUDED UNDER CLEAVAGE, ONE OF THEM NOT 

DEPENDENT UPON A PARALLEL ARRANGEMENT OF MINERAL CONSTITUENTS.

Sorby, c in 1857, showed that cleavage properly included two distinct phenomena, a capacity to part along incipient parallel fractures 

independent of any; parallel arrangement of the minerals, and a 

capacity to part parallel to, and conditioned by, the parallel arrangement of the mineral constituents. The first he called "close-joints 

cleavage," and the second "ultimate structure cleavage." A similar 

view was held in 1878 by Professor Heim,d who discriminated an 

"ausweichungs cleavage" which results from a succession of small 

displacements or faults, usually in connection with overfolds, and 

independent of any parallel arrangement of mineral constituents, from 

another cleavage called in part "micro-cleavage," which is dependent 

for its existence on the parallel arrangement of the mineral constituents. Harker, 6 in 1886, used the term cleavage " in a sense sufficiently 

wide to include not only the structure discussed under the name slaty 

cleavage proper, but also other structures, which though effectively 

identical with it have been produced in a different manner;" that is, 

in the manner described by Sorby and Heim for their "close joints" 

and "ausweichungs" cleavage. Van Hise.^ in 1896 distinguished 

cleavage or capacity to part, dependent on a parallel arrangement of 

the mineral constituents, from fissility, '' a structure in some rocks by 

virtue of which they are already separated in parallel laminae in a 

state of nature," and do not necessarily require a parallel arrangement 

of the mineral constituents, although this is usually developed in rocks 

showing this structure. As thus defined, fissility is a phenomenon 

quite separate and distinct from cleavage. But Van Hise in a subsequent discussion used the term fissility also for rocks in which the

« Adams, F. D., and Nicolson, J. T., An experimental investigation into the flowage of marble: 

Philos. Trans. Royal Soc. London, ser. A., vol. 195,1900, p. 399.

''See excellent summary by Adams, op, cit.

eRept. 27th Meeting Brit. Assoc. Adv. Sci., held 1857, p. 92. Published in 1858. .

rfUber den Mechanismus der Gebirgsbildung, vol. 2,1878, pp. 51-58.

ellept. 55th Meeting Brit. Assoc. Adv. Sci., held 1885, p. 836. Published in 1886. Contains bibliography.

/Sixteenth Ann. Kept. U. S. Geol. Survey, pt. 1, 1896, p. 633. Jour. Geol., vol. 4, 1896, pp. 449-450.

Bull. 239 05 2
18 KOCK CLEAVAGE. [BULL.239.

parallel fractures are incipient or have been cemented or welded, giving the rock a capacity to part, or cleavage, rather than an actual 

parting. Other terms which have been used for cleavage developed 

in this manner are rift, fault-slip cleavage, slip cleavage, and false 

cleavage.

FALSE CLEAVAGE.

"Falge cleavage," or "grain," or "bate cleavage," normal to cleavage proper, was noted by several of the early observers, Sedgwick," 

Phillips, b and De la Beche,c and by many later investigators. Sharpe 

and many following him have concluded that the structure is due to 

breaking along the greater or mean axes of the mineral particles 

arranged parallel, but others have held that the structure is due to 

obscure divisional planes developed in the manner of fracture cleavage.

CLEAVAGE IN ITS RELATIONS TO DIRECTION OF APPLICATION OF THE 

PRESSURE PRODUCING IT.

Concerning the relations of secondary cleavage or parallel arrangement of minerals, or both, to pressure, there is again considerable difference in the views held. Sharpe,^ Sorby, 6 and TyndalK held that 

cleavage develops in planes normal to the greatest pressure, the two 

latter having produced it in such planes experimentally. Sharpe supposed also that heat and "galvanism" may have helped pressure, and 

Sorby admitted the existence of another cleavage structure, his " closejoints cleavage," presumably developed with inclination to the pressure 

in the manner of joints. King argued that slat}^ cleavage is the result 

of " pressure exerted against divisional planes, chiefly belonging to 

jointing, that existed in any given rock prior to its becoming affected 

by Such pressure," 6' the preexisting divisional planes having developed 

in planes inclined to the greatest compression in a manner common to 

jointing structures. Daubree h produced cleavage parallel to the direction of flow by forcing plastic clay through a cylinder. . Becker,* in 

1893 and 1904, maintained on mathematical grounds that slaty cleavage is developed along planes inclined to the greatest pressure. In

a Synopsis Classification British Paleozoic Rocks4 London, 1854, p. xxxv.

t> Phillips, John, On a group of slate rocks ranging east-southeast between the rivers Lune and 

Wharfe, from near Kirby Lonsdale to near Malham, and on the attendant phenomena: Trahs. Geol. 

Soc. London, 2d ser., vol. 3, pt. 1,1829, pp. 1-19.

e Geol. Observer, 2d ed., 1853, p. 588, fig. 239.

d Quart.. Jour. Geol. Soc., vt>l. 3, 1847, p. 75.

ePhilos. Mag., vol. 11, 1856, p. 26.

/Philos. Mag., vol. 12,1856, p. 37.

ffTrans. Royal Irish Acad., vol. 25,1875, p. 657.

''Ge'ologie exp6rimentale, vol. 1,1879, p. 413.

iBecker, G. F., Finite homogeneous strain, flow, and rupture of rocks: Bull. Geol. Soc. America, 

vol. 4, 1893, PP. 13-90, Experiments on schistosity and slaty cleavage: Bull. U. S. Geol. Survey No. 241, 

1904.
LEITH.] PURPOSE OF PRESENT PAPER. 19

1896 Van Hise and Hoskins" held that cleavage, during deformation, 

develops at any instant in planes normal to the greatest pressure, but 

that the final direction of cleavage may be inclined to the direction of 

greatest pressure which has produced the deformation. This conclusion was based on field observations by Van-Hise and on mathematical 

analysis by Hoskins. Their " fissility," corresponding essentially with 

Sorby's " close-joints cleavage," develops, they maintain, in the manner 

of fractures in shearing planes inclined to the greatest pressure.

PURPOSE OF PRESENT PAPER.

It will be fundamentally assumed that secondary rock cleavage is of 

two kinds, which differ widely in their essential causes and conditions. 

Abundant evidence of the truth of the assumption appears, it is believed, 

in the facts here recorded. The cleavage developing during rock flowage (or the deformation of rock without conspicuous fracture) will be

called flow cleavage, and that developing through the deformation of 

rock by fracture and subsequent cementation will be called fracture 

cleavage.

In distinguishing the two kinds of cleavage the writer is in essential 

accord with Sorby, Heim, Harker, Van Hise (if his fissility be in part 

called cleavage), and others. The term fissility is retained for closely 

spaced parallel partings as defined by Van Hise, but is not extended 

to include the capacity to part fractures, which is properly a cleavage, 

and is here called fracture cleavage.

One of the principal causes of confusion in the discussion of cleavage 

has been the attempt by some authors to make the explanation of one 

kind of cleavage apply to all cleavage. On the one hand, the parallel 

arrangement of mineral constituents has been held to be essential to 

all cleavage, and, on the other, the parallel fractures independent of 

&ny arrangement have been strongly urged as sufficient cause for all 

cleavage. 6 An attempt'will be made to show that each of these explanations is adequate for one kind of cleavage, but not for all cleavage. 

Especial emphasis will be placed on the proof that incipient or cemented 

parallel fractures, yielding what is here called fracture cleavage, will 

not explain what is here called flow cleavage, or cleavage dependent 

upon the parallel arrangement of the mineral constituents. This will 

require detailed discussion of the internal arrangement of the mineral 

constituents of rocks with each kind of cleavage, the relations of this 

arrangement to the observed cleavage, the 'nature of the processes 

bringing about the arrangement, and the relations of the arrangement 

to pressure. Especial attention will be paid to the causes and conditions of flow cleavage, which is the most characteristic structure in 

rocks ordinarily considered cleavable.

"Sixteenth Ann. Kept. U. S. Geol. Survey, pt. 1,1896, p. 638. Jour. Geol., vol. 4,1896, p. 457. 

fcBecker, G. F., Experiments oh schistosity and slaty cleavage: Bull. U. S.Geol. Survey No. 241,1904.
20 KOCK CLEAVAGE. [nuu..239.

The prevailing differences of opinion concerning the relations of 

cleavage to the arrangement of mineral particles, the processes bringing about the arrangement, and even the relations of cleavage to pressure, may be traced in part to vague and incomplete knowledge of the 

intimate nature of cleavage itself. Students of the subject have made 

numerous careful observations, but with a few exceptions have confined 

their observations to one aspect of the subject. There is thus a need 

of systematic microscopical study of cleavage of rocks of all kinds 

from many localities, with a definite purpose of ascertaining the exact 

arrangement of the mineral particles, the relations of the observed 

arrangement to cleavage, and the relations of cleavage to the deformation of rocks. The present investigation has attempted to supply insome degree this observational deficiency. With the basis of observed 

fact thus obtained it is possible to present with some degree of confidence 

certain general conclusions as to the origin and conditions of cleavage.

The phenomena of flow cleavage will be treated in Part I. Part II 

will be devoted to fracture cleavage, and a comparison of fracture 

cleavage and flow cleavage. Original cleavage of bedding, flow structure, etc., will be treated independently in Part III.

MATERIAL USED IN INVESTIGATION.

The material studied has been drawn from the rock collection of the 

section of pre-Cambrian and metamorphic geology of the United States 

Geological Survey, numbering 40,000 specimens and upward of 15,000 

thin sections, representative of. the principal crystalline schist areas of 

North America; from the metamorphic rocks belonging to ihe Wisconsin Geological Survey; and from the University of Wisconsin collection of metamorphic crystalline rocks from various parts of North 

America and Europe, including specimens of most of the well-known 

metamorphic cleavable rocks of both continents. From the Survey 

and university material a large number of typical specimens and slides 

were selected for special study. Where any feature was especially 

well exhibited in the slide, or where the slide had been cut in planes 

indeterminate with reference to the rock cleavage, new sections 

were cut, usually in three mutually perpendicular directions, with 

known relations to cleavage. Perhaps 1,000 specimens and slides were 

selected for close study, and of these 250 slides were specialty prepared. In addition to the metamorphic crystalline rocks, a considerable number of semicrystalline and partly consolidated sediments in 

the same collections were examined. Finally several specimens and 

slides of typical cleavable rocks from various parts of North America 

were loaned by men who had collected and studied them. It is believed 

that the conclusions as to the causes and conditions of rock cleavage, 

which are based on the study of .the 'great variety of cleavable rocks 

available from widely separated localities, will apply to cleavable rocks
I-EITH.] EXPEEIMENTAL WORK. 21

in general. It is inevitable, however, that certain pnenomena are 

more apparent than others in the particular slides examined, and thus 

that these phenomena will be overemphasized in the following- discussion and others will receive less attention than they should. Further 

work will doubtless show modification of emphasis to be necessary.

Illustrative specimens and slides are cited in the text. Where no 

letters are attached the numbers refer to the collection of the section 

of pre-Cambrian and metamorphic geology of the United States Geological Survey. The letter W. with a number signifies Wisconsin 

Geological Survey, U. W., University of Wisconsin, H., Hobbs, and 

C., Clements.

EXPERIMENTAL WORK.

This report, in substantially the present form, was submitted for 

publication jn the spring of .1903, and was referred to a committee

consisting of Messrs. Whitman Cross and George F. Beckei'. On the 

recommendation of Doctor Becker, publication was postponed until 

he and the writer might jointly perform experiments, which had been 

planned by Doctor Becker, on the artificial development of cleavage. 

In December, 1903, a few days were spent on these experiments in the 

chemical and physical laboratory of the United States Geological Survey at Washington. It was planned to continue the experiments at a 

subsequent date, but opportunity did not recur. The results, which 

were not decisive, are referred to on pages 126-130. So far as they 

were carried, the experiments did not seem to the writer to require 

a modification of the views here stated. Further, so largely is the 

present report a record of observed facts of cleavage produced by 

nature, with only such general conclusions as would obviously follow, 

that it is not thought likely that further experimental work would 

require essential modification, although it would doubtless make possible a more definite and mathematical treatment of the subject and aid 

in' the interpretation of the facts observed."

ACKNOWLEDGMENT.

The investigation nere reported upon was undertaken at the suggestion of Dr. C. R. Van Hise, was carried on under his supervision, and 

the results reviewed by him. The paper, as it stands, is in considerable part an expression amplified, modified, and accompanied by

a While the above paragraph was in proof, the writer received a report by Doctor Becker on Experiments on schistosity and slaty cleavage (Bull. U. S. Geol. Survey No. 241, 1904), which covers the 

experimental work above referred to, as well as work previously and subsequently done by Doctor 

Becker. The conclusions reached arc essentially the same as in Doctor Beckcr'sprevious papers (cited 

p. 14), in which, as it seems to the writer, the attempt has been made to apply one explanation to 

two distinct cleavage phenomena. Doctor Seeker's conclusions are, as he states, "founded on 

experiment and analysis." He does not attempt to describe the facts as shown by the rocks themselves. His method is deductive. The writer's point of view is almost altogether different. He has 

attempted, after the inductive method, to study the facts in the field and laboratory and to point 

out their significance.
22 ' BOCK- CLEAVAGE. [BULL.239.

detailed proof of general ideas which he had obtained from his own 

observations on rock cleavage.' For this material aid, as well as for 

unusual facilities for study, the writer is pleased to acknowledge his 

indebtedness to Doctor Van Hise.

Thanks are due Prof. L. M. Hoskins, of Leland Stanford Junior University, and Prof. E. R. Maurer, of the University of Wisconsin, for 

helpful suggestions and criticisms of the parts of this paper dealing 

with the laws of mechanics.

For the loan of specimens and slides the writer is indebted to 

Profs. William H. Hobbs and J. Morgan Clements, of the University 

of Wisconsin; Dr. Samuel Weidman, of the Wisconsin Geological 

Survey; Prof. Frank D. Adams, of McGill University, Montreal, and 

Messrs. Whitman Cross and Ernest Howe, of the United States Geological Survey, Washington; and for a large number of measurements 

of the dimensions of parallel arranged minerals, to Mr? Sydney H. 

Ball, of the University of Wisconsin.
PART I.

FLOW CLEAVAGE.

By flow cleavage is meant the cleavage dependent on the parallel 

arrangement of the mineral constituents of the rock, an arrangement 

which will be shown to be developed during rock flowage.

In studying flow cleavage the following queries have been kept in 

mind:

Do the particles of the same mineral species in a cleavable rock show 

variation in shape, or are they uniform throughout? If they vary, 

do their dimensions show variations proportional to the amount of 

deformation which the rock shows? Are the greatest and mean axes 

of the mineral particles always parallel to the rock cleavage? Do the 

particles of the same mineral species show a parallelism of their vector 

or directional properties i. e., properties related to direction in 

crystals, such as cohesion, elasticity, optical, thermal, electrical and 

magnetic properties, and crystalline form? Do the minerals ever 

exhibit a parallelism of vector properties and not a dimensional parallelism, or vice versa? Are they arranged dimensionally parallel and 

with no parallelism of their vector properties ? Are both dimensional 

and vector properties parallel; and if so, to what extent? To what 

extent is cleavage independent of any parallel arrangement of the 

mineral constituents ?

23
CHAPTER I.

MINERAL ARRANGEMENT IN ROCKS WITH FLOW CLEAVAGE.

Comparatively few minerals make up the great bulk of the rocks 

possessing secondary flow cleavage. The micas, including muscovite 

and biotite, chlorite, hornblende, quartz, and feldspar, are in greatest 

abundance, making up perhaps nine-tenths of them. Other minerals, 

characteristic though less abundant, are calcite, tremolite, actinolite, 

garnet, tourmaline, staurolite, chloritoid, sillimanite, etc. Olivine 

and the pyroxenes are rare in the typical rocks showing flow cleavage, 

for the most part having been altered to one or more of the 

minerals above named. In the following discussion the first-named 

group of minerals will receive the most attention. Discussion of the 

remaining minerals on the same scale would unduly extend the paper 

and complicate the conclusions. Many of the features of arrangement 

of the constituents^ cleavable rocks below described are well known, 

indeed almost axiomatic, to petrographers, but it is necessary again to 

present them in order to secure a firm "basis for generalizations.

MICA.

Of the minerals in rocks with flow cleavage the micas are by far 

the most characteristic. Muscovite, sericite, and biotite are the common micas of these rocks, and these alone are considered; statements

concerning them will apply nearly as well to the rarer micas. The

common association of the micas is with quartz and feldspar, making 

up rocks which are called, because of the relative abundance of the 

constituent minerals or because of their texture, micaceous schists, 

micaceous gneisses, quartzose schists, slates, phyllites, etc. It is 

preferred in this paper to use the mineral prefix as an adjective, as in 

the term micaceous schist.

The micas are monoclinic but pseudo-hexagonal in s}7mmetry. The 

basal plane has a hexagonal or rectangular outline. The characteristic 

physical feature is the perfect cleavage parallel to the base.

As is well known, mica in cleavable rocks is arranged in plates parallel to its own perfect basal cleavage. The plates are observed to be 

parallel or nearly parallel to an observed plane of cleavage in the 

micaceous cleavable rocks. In sections cut normal to the plane of 

cleavage the mica cleavage plates appear as narrow laths with parallel 

extinction, usually crowded into layers. Sections cut parallel to the

24
ARRANGEMENT OF MICA. 25

plane of rock cleavage show basal planes of the micas, the biotites 

nearly isotropic and the muscovites highly refracting. Study of the 

micas in slides in these two directions yields interesting results.

Mica plates looked at normal to their flat surface do not show any 

great uniform difference in dimensions. They are commonly of rather 

irregular shape, sometimes hexagonal and sometimes rectangular, but 

in any case with slightly unequal dimensions as shown by average 

measurements. So far as there is any difference in the dimensions of 

the plates, the}'' may show a small degree of parallelism of their greatest and mean diameters in the plane in which they lie. In some slides 

a parallelism somewhat similar to that indicated in fig. 1 is to be seen. 

This in most cases would probably be. overlooked if the other minerals 

in the slide did not usually show a better parallel arrangement of the 

same kind, and thus indicate where the parallelism of the micas is to

be looked for, It will be seen that the mica plates are not strictly

parallel to a plane, and hence a section cut parallel to the plane most

nearly common to all plates is likely to show

oblique sections of many plates, and. any faint

tendency to parallelism of the greatest and mean

diameters of the plates in this plane may not

appear in the slide. .

Slides cut across the plane of cleavage of rock FIG. i.-Kough parallel ar-

. . , . . , ... rangement sometimes containing biotite or muscovite snow roughly par- shown by the greatest 

allel aggregates of mica plates, -bounded by their and mean diameters of

,,.,,, , -_ .. mica plates lying in the excellent basal cleavage planes. In cross section pia'ne of rock cleavage. 

they look like laths. The laths are commonly 

concentrated into layers, with other minerals, such as quartz, intervening (PI. X). Each mica crystal shows several cleavage laminae, 

perhaps due to incipient movement, but the crystal is still evidently a 

unit.

While there is a characteristic parallelism of mica plates, the deviations from this arrangement are numerous and marked. These irregularities may be seen to be in the neighborhood of harder minerals 

which have been relatively rigid during deformation, but are present 

also where the harder minerals are evenly arranged in bands or are 

very subordinate in quantity. Irregularities are also occasionally due 

to the development of micas in cracks or along the cleavage of minerals, 

such as feldspar. The irregularities in the parallel arrangement are 

of two kinds: (1) They may be due to actual bending or breaking of 

the mica plates themselves (PL II, J?), in which cases there may 

be considerable gliding or slipping between the mica laminae. These 

bendings are sometimes so great as to make the cleavage more nearly 

parallel to an axis than to a plane, giving it an approach to linear or 

"pencil" cleavage (PI. I). The minute folds so formed may also give 

a capacity to part parallel to one of the limbs of the fold (PI. XIV, j5),
26 BOCK CLEAVAGE. [BULL. 239.

thus yielding a structure to which in part the term false cleavage has 

been applied. (2) Irregularities are formed by unbroken or unbent 

plates abutting against each other at low angles, producing a characteristic feathering out of one set of laminae diagonally against another 

set, as shown in fig. 2 and PI. II, A. A hasty examination of such 

separate groups of plates gives the impression of bending and breaking, particularly where the micas are about a core of some harder 

mineral. The angles which these contiguous plates make with one 

another average less than 15°. Not infrequently this change in direction is accompanied by a change in substance; one mica may be a 

muscovite and the other a biotite. This curious feathering out of the 

laminae of the mica against one another is exceedingly characteristic, 

and is usually present where there has been no contortion of the mica 

plates. For its explanation see page 114. It is to be noted that while 

the irregularities about hard particles give a structure with a mesh 

appearance, where the micas come together at the ends of a hard particle they do not cross each other, but change their direction in either 

of the ways above described until they run nearly parallel. - 

The above description applies to both biotite and muscovite. The

two micas, however, show 

these general features in 

different degrees, and bioFIG. 2.-Sketch showing the manner in which mica plates fcite shows Certain features 

feather out, one against the other, in rocks with flow cleavage. . _ ,

which have not yet been

mentioned. It may sometimes be seen in cross section in stumpy 

porphyritic crystals whose cleavage lies at any angle to the rock 

cleavage (PL III). a The other constituents of the rock may bend 

around them, may stop at their peripheries, or may pass through them 

without change of direction; in other words, in the last-named case the 

biotites have the same characteristics of occurrence as have certain of 

the chloritoids, garnets, etc., described on pages 43-45. Rarely, when 

the cleavage of porphyritic biotite is uniformly normal to the main 

rock cleavage, it is found to be parallel to a later developed "false" 

cleavage (PI. XIV, £). The porphyritic biotites show a characteristic 

stumpy habit as compared with the biotites developed in plates parallel to the plane of rock cleavage. There may be occasionally 

observed a breaking down of the porphyritic biotites along their 

cleavage, the resulting slices having a tendency toward parallelism 

with one another and with the plane of rock cleavage. Where the 

process has gone far the porphyritic micas have given way entirety to 

aggregates of parallel mica plates. The greater diameters of the porphyritic biotites may be parallel or transverse to their cleavage, and 

thus may form any angle with the rock cleavage.

«Sp. 1106 H., Williamstown, Mass., slight arrangement; so. 14930, si. 7672; sp. 14971, si. 7709, Black 

Hills, S. Dak ; sp. 4662 H., southern Connecticut.
LEITH.] AKRANGEMENT OF MICA. 27

FIG. 3. Biotite with its own cleavage inclined to 

rock cleavage, as marked oy the adjacent muscovite plates. (Sp. 14901, si. 7651, Black Hills, 

South Dakota.)

Occasionally the biotites occur in flattened lenses with muscovites 

passing around them, as shown in fig. 3. In this case the cleavage of 

the biotite is seldom parallel to that of the niuscovite and of the rock, 

but usually makes a considerable angle with the plane of rock cleavage. 

. Again, the biotite may be associated with quartz, and the two have 

very much the same relation to the adjacent muscovite. The sketch 

in fig. 4 shows the biotite and quartz in the same band with muscovite passing around them.

In spite of these differences and irregularities in the biotite and 

muscovite, commonly the greater diameters of the cleavage plates are 

parallel, as first described.

As the shapes and dimensions of the micas are commonly those of 

cleavage pieces of mica crystals, 

the shapes and dimensions must 

have uniform relations to other 

physical properties of the cry stals. 

Hence so far as there, is uniformity of arrangement of the dimensional axes of mica plates, there 

is also uniformity of arrangement 

of the crystallographic or vector 

properties. Only one dimensional axis of the mica plates, the shortest, 

has a tendency toward uniform parallelism in the particles of a cleavable rock. Hence only the 

physical properties parallel to 

the shortest diameter of the 

mica particles are parallel. 

These are the properties repreFJG. 4.-Biotite and quartz in a band with parallel mus- Sented by the Crystallographic

covite plates adjacent. (Sp. 14901, si. 7651, Black axis C and the elastic axis ft.

Hills, South Dakota.) rr\i i j.- i t The elastic axes £ and ft in

the micas lie in the plane of cleavage. Do these axes in the different 

mica plates in a secondary cleavable rock lie parallel? In the case of 

the biotites it was hoped that the determination, by the use of conconvergent polarized light, of the direction of the plane of the optic 

axis, indicated by the slightly opening cross, would give some information bearing on this subject. However, the optic angle is so small 

and the basal plane so nearly isotropic that the determinations of the 

direction of the plane of the optic axes were uncertain and unsatisfactory. A number of such determinations were made in various slides, 

and such as they were seemed to indicate no parallelism whatever. It 

is to be noted that the physical differences in the directions of the 

crystallographic axes in this plane are very slight, and if such characters ever affect the orientation of a mineral they would be little likely 

to do so here.
28 ROCK CLEAVAGE. [BULL.239.

In the muscovites, while there are no uniform dimensional differences in the diameters of the plates, the behavior with reference to 

light, represented by axes £ and ft, varies somewhat widely. A quartz 

wedge used with basal sections of muscovite indicates clearly the positions of these axes. About 100 determinations of the position of the 

optic axes in parallel muscovite plates were made from a dozen selected 

slides, and no certain parallelism was found in the arrangement of the 

axes of elasticity. One slide seemed to show a predominance of basal 

sections with the It axis corresponding to the greater dimensional axes 

of the parallel arranged particles, and the £ axis in the direction of the 

mean dimensional axes, but other basal sections in the same slide and 

in many other slides showed no such arrangement.

Dimensions. There is little or no difference between the length and 

breadth of mica plates, as noted in the discussion of basal sections. 

Between thickness and either length or breadth there is, of course, a 

marked difference. The ratio of the least to the greatest diameters of 

the mica plates is generally uniform." Four hundred measurements'of 

both biotite and muscovite from selected slides give an average 

ratio of greatest to least dimensions of 100 :16. From this common 

figure, however, there are many variations. The biotites and muscovites show a uniform difference; the average for biotite is 100 : 20, 

and that for muscovite is 100 :14. The various porphyritic and 

semiporphyritic forms which the biotites sometimes show have an 

average ratio of greatest to least diameter of 100:40, while in 

the same slides" the mica cleavage plates parallel to the plane 

of rock cleavage, mainly muscovite, have ratios running from 100 : 20 

to 100 :10. In one case b the ratio of length or breadth to the thickness of the porphyritic biotites averages 100 : 65, while the muscovites 

parallel to the plane of rock cleavage average 100 :13. The measurements of porphyritic biotite above noted are of the greatest and least 

diameters regardless of the mineral cleavage.

HORNBLENDE.

Common hornblende is a characteristic constituent of a large number of metaclasic rocks. Hornblendic schists are common resultants 

of the metamorphisni under dynamic conditions of intermediate and 

basic rocks, particularly of igneous origin, such as diorite, basalt, or 

diabase. They also result from the metauiorphism of the sedimentary 

rocks, such as feldspathic and ferruginous gi^wackes and slates.

Common hornblende is monoclinic in symmetry, with columnar habit

«Sp. 12480, si. 14846, Wissahickon Creek, Pennsylvania; sp. 14884, si. 9448; sp. 1490], si. 7051, Black 

Hills, South Dakota. 

bSp. 14930, si. 7672, Black Hills, South Dakota.
LtilTH.] AEEANGEMENT OF HOENBLENDE. 29

£T^

FIG. 5. Hubitof 

common hornblende crystal.

and prismatic cleavage (fig. 5). Basal sections show the prismatic 

cleavage in planes intersecting at an angle of 124°.

Examination of the specimens and slides of the hornblendic schists 

shows the hornblende to occur in characteristic columnar crystal 

form with linear parallelism or fascicular arrangement (PI. V). In 

many cases the crystals are aggregated into indefinite layers. In individual instances there is wide variation from the general parallelism. 

This variation is likely to be in a plane, and not at angles to this plane 

that is, the hornblende crystals, while not parallel to one 

another, are parallel to a plane. a Where the hornblende 

crystals, are best arranged parallel to one another, a section cut parallel to their long dimensions shows the prism 

faces of the hornblende with only an occasional basal section (PL V). Sections cut across the plane of rock cleavage show mainly basal sections with prism faces less 

conspicuous (PL VI). In many cases the longer axes of 

th'e cross sections of the hornblende have a tendency. 

(perhaps scarcely noticeable) to lie approximately in the 

plane of rock cleavage, which thus bisects the acute angles 

of the hornblende cleavage (fig. 6, and PL VI). 6 In slides 

showing the best arrangement, however, there are numerous wide 

variations from such parallelism of the axes of the cross sections of

the crystals.

In addition to parallel columnar crystals 

of hornblende, porphyritic hornblende 

crystals may frequently be observed having 

considerable variety of shape and arrangement. They are usually short and broad, 

and their cleavage and greater diameters 

make almost all angles with the plane of 

rock cleavage. All stages of the breaking 

down of such hornblendes may be observed. 

This breaking down occurs usually, though 

not always, along the crystallographic cleavage planes and in some cases results in a 

'fairly good parallelism of the slices (fig. 7).

In other cases hornblende crystals of different color and evidently of 

secondary origin are seen to be developing with parallel arrangement 

out of the porphyritic hornblende.

The hornblende particles in a rock with secondary cleavage being 

crystals with characteristic habit, and such crystals being arranged

FIG. 6. Sketch showing tendency 

toward parallelism of dimensional 

axes and cleavage of hornblende as 

viewed in basal section.

« Sp. 1101 H. Croatia.

bSp. 28501, si. 14854, sp. 28504, si. 14854, sp. 27750, si. 13308, Black Hills, South Dakota.
30 BOOK CLEAVAGE.. [BULL. 239.

FIG. 7. Slicing of hornblende. (Sp. 40685, 

si. 15360, sp. 40690, si. 15363, hornblende 

schist, Mesabi district of Minnesota.)

with their greatest, mean, and least diameters respe lively parallel, or 

more commonly with their greatest dimensions parallel, it follows that 

the vector properties of these diameters must have a proportional 

degree of parallelism in all crystals. These physical properties, represented by the crystallographic axes, a, 5, and c (fig. 8), and the elastic 

axes, a, ft, and £, bear the following relations to the diameters of the 

particles in the common hornblende. The axis of least elasticity, jc, 

lies in the acute angle ft at an inclination of not more than 20° to the 

crystallographic axis, <?, of the hornblende, and hence to the axis of

greatest development. The axis of 

mean elasticity, ft, corresponds to the 

crystallographic axis 5, or direction of 

mean development of the crystal. The 

axis of greatest elasticity, a, lies in the 

obtuse angle B at an inclination of not 

more than 30° to the crystallographic 

axis, $, the direction of least dimensional development. Where-all the dimensional axes are parallel, the 

vector-properties represented by these axes are also parallel. W7here 

the mean and least diameters of the hornblende have no uniform parallelism, the only vector properties that are 

parallel to one another are those parallel to the 

greatest dimension the crystallographic axis 

c and the axis of elasticity £ while the properties represented by a or ~b are parallel to a 

plane and not to each other. Where all the 

hornblende crystals are not parallel to one another,, but are parallel to a plane, the shortest 

diameters of the particles may be uniformly 

at right angles to this plane, and hence the axes

CL and & in different crystals may be also parallel. However, observation shows that when 

the greatest dimensions of the hornblende crystals are not parallel to one another, but only 

to a plane, the mean and least diameters are 

not likely to be parallel to one another.

Dimensions. One hundred and fifty measurements were taken of 

the dimensions of the hornblende crystals in prismatic section from 15 

selected slides. A striking uniformity was found in the ratio of the 

length of the longest diameters i. e., the dimensional axes corresponding to the columnar development to the length of the shorter ones at 

right angles to them. This ratio varies between 100 :25 and-100 :20. 

The ratio between the mean and the least diameters of the crystals, as 

seen in basal sections, averages about 100:60, and corresponds well 

with the ratio calculated for the crystal form.

PIG. 8. Cross section of hornblende crystal parallel to clinopinacoid (parallel to axes a 

and c in fig. 5), showing positions of crystallographic and 

elastic axes.
LEITH.] AEEANGEMENT OF HOENBLENDE AND QUARTZ. 31

There appears to be a difference between the dimensions of hornblende crystals in secondary cleavage rocks and the dimensions of 

hornblende crystals in original igneous rocks which have not been 

deformed since their solidification. Measurements (125) of original 

hornblende crystals in a number of selected hornblendic granites and 

diorites and more basic hornblendic rocks showed that the ratio of 

length to breadth or thickness ranged from 100:30 to 100:75, with 

100:40 common.

QUARTZ.

Quartz is a common and abundant constituent of the more acidic 

cleavable rocks. Its common associates are mica and feldspar in rocks 

which are named quartzose schists, micaceous schists, micaceous 

gneisses, slates, phyllites, etc., according to the relative abundance or 

the cleavage-making importance of the minerals composing them.

The quartz crystal is hexagonal, commonly of short, columnar, 

habit, and lacks good cleavage. The quartz observed in rocks with 

secondary cleavage is described below according to its characteristic 

shape. Later it will be shown that the shape is dependent on the 

manner of development.

(1) Quartz occurs in particles that have more or less modified crystal 

outlines and grade into irregular, rounded to subangular forms; the 

crystal outlines are rare, while the irregular, rounded to subangular 

forms which grade into the forms described under (3) are common. 

The crystal forms when present rarely $how any tendency toward 

parallelism, dimensional or crystallographic. The irregular forms 

sometimes have a dimensional arrangement and sometimes not. In 

no case do they have a uniform crystallographic arrangement. The 

ratio of the greatest to the least diameters of this group varies from 

100:100 to 100:50 or less. A common ratio is not far from 100:75.

(2) Quartz occurs in angular fragments, which apparently owe 

their shape mainly to the fracturing of larger particles, and in forms 

that grade from angular fragments to the grains described under 

(1). Gradations in the fracturing of the larger particles are to be 

observed, beginning with undulatory extinction and ending with 

granulation of every original particle. The fine lines of undulatory 

extinction perhaps associated with lines of inclusions that represent 

the intersection of a plane with the -slide, not infrequently occur 

in sets crossing each other at angles approaching 90° and making 

an angle of 45° to the plane of rock cleavage. Not infrequently, in 

intermediate stages, the residual material left in grains which have 

been subjected to granulation may be seen to have roughly lenticular 

shapes (augen), the long direction being parallel to the plane of rock 

cleavage. This lenticular appearance is emphasized by the occurrence of granulated quartz in tails at the ends of the nongranulated
32 ROCK CLEAVAGE. [BULL.289.

material. Parallel quartz particles developed by the parallel fracturing 

or slicing of a large quartz individual are also not uncommon. The 

fractures may be mainly in one set of planes and are either parallel to 

or inclined at a low angle to the plane of rock cleavage as conditioned 

by the other constituents of the rock (fig. 9). Close to the point 

where the slicing occurred, the length of such slices may be from two 

to six times their breadth, but a short distance away particles of 

apparently the same shape have been broken into small pieces. In 

advanced stages of granulation the quartz particles have been strewn 

out and left in nearly even layers.

As soon as any of the micaceous minerals begin to be in evidence 

the quartz is likely to show undulatory extinction. The advanced 

stage of the granulation of the quartz is usually reached before the 

feldspar begins to show signs of pressure. Early stages of granulation of the quartz, with the mica well developed and the feldspar still

FIG. 9. Slicing of quartz grain in schistose conglomerate (Metropolitan, Mich., large slide No. 1).

unaffected, are well exhibited in some of the Black Hills schists. A 

schist 68 from the Medicine Bow Mountains, Wyoming, shows welladvanced granulation of the quartz, while the feldspars show only 

peripheral granula'tion.

The granulated quartz particles are usually unequidimensional and 

in 100 fractured particles the ratio of the greatest to the least dimensions ranged from 100:50 to 100:100, with an average of 100:75,

The most angular pieces are in many cases the small ones, although it 

can hardly be said that, on the average, the smaller the particles the 

more angular they are. The longer diameters of the particles 6 in the 

earlier stages of the granulation are commonly in random positions. 

In later stages they show a very slight tendency to parallelism to the 

observed cleavage in the rock, as above noted-. This is due to rotation, as will be seen below. c

The dimensional arrangement of the quartz particles, formed by 

fracturing in advanced stages of the granulation, carries with it, as 

shown by the varying" extinction, no crystallographic or vector 

arrangement whatever. In early stages of fracturing the angular

o Sp. 15570, Medicine-Bow Mountains, Wyoming.

b Sp. 14815, si. 7577, sp. 14817, si. 14849, Black Hills, South Dakota; sp. 923 U, W., Dittersdorf, Saxony.

c Sp. 15665, sL 8084, Medicine Bow Mountains, Wyoming.
ARRANGEMENT OF QUARTZ. 33

pieces derived from the breaking down of a single large particle may 

have similar crystallographic arrangement, but the particles derived 

from different crystals show no such parallelism.

(3) Quartz appears in a variet}^ of elongated lens-shaped, ribbonshaped, and spindle-shaped forms, with their longer axes parallel or 

inclined to the plane of cleavage. Lenticular or augen forms composed of single individuals developed by peripheral granulation have 

already been described in connection with the granulated forms discussed under (2), p. 31. Other flattened lenses, with outlines curved 

and even, made up usually of several individuals, give no evidence of 

development by granulation and slicing. They lie with their longer 

axes in the plane of rock cleavage, and are usually a little more elongated in one direction than in another. 

In some cases this elongation is very, 

marked and the cross section is a flattened lens." Whether or not the 

lenses show a difference of dimensions in the plane of rock cleavage,- 

they commonly have considerable 

length or breadth as compared with 

their thickness (Pis. VII, VIII, IX, 

X). It is not uncommon to see a lens 

extending the length of a hand specimen, and to have a thickness of only 

0.25 mm. Each lens may be made PIG. i0._sketch showing granulation of

Up Of a number Of Crystal individu- quartz (A) where feldspar (B) is compara11. .-IT n mi T ^ lively unaffected by deformation. als lying side by side. The individuals themselves usually, though not always, have their greater dimensions in the plane of rock cleavage, the ratio of the greatest dimension 

in the plane of rock cleavage to the thickness of the crystal averaging 

perhaps 100:50, but varying from 100:20 to 100:100. These figures 

were determined by 200 measurements from 20 selected slides.

The lenses above described, consisting of several individuals lying 

side by side, grade into shorter lens-shaped forms, consisting mainly of 

a single individual, although the "tails" maybe formed of smaller 

separate particles. These forms are really phenocrysts (PI. XI, A). 

These single crystals in flattened lenses in turn grade into spindle-shaped 

forms which are elongated in one direction and shortened in all directions normal to this. The ratio of the length to the thickness of the 

individual crystals varies widely. In the quartz-porphyry from the 

Thuringer Wald described by Futterer,6 it varies from 10:1 to 2:1.

«Sp. 18845, si. 10185, Laurentide Mountains, Canada.

fc Futterer, Karl, Ganggranite von Grosssachsen und die Quartzporphyre von Thai im Thuringer 

Wald: A thesis presented to the University of Heidelberg for the degree of Ph. D., 1890, p. 48. Sp. 88.

Bull. 239 05 3
34 BOCK CLEAVAGE. [BULL. 239.

The arrangement of the quartzes in parallel lenses and spindles may 

be observed in slides in which the feldspars have not been at all 

deformed. In the quartz-porphyry from the Thuringer Wald (PL XI), 

all* stages of deformation of the quartzes into these forms may be 

observed, while the feldspars in many cases at least are relatively 

undeformed or deformed only by fracture.

In the forms described under (3) dimensional arrangement usualty 

does not carry with it a crystallographic arrangement of the individuals. The crystallographic arrangement is entirely random, as. 

shown by the extinction of different particles at different times when 

revolved under crossed nicols. In the slides which have been examined 

during this investigation the quartz-porphyry from the Thiiringer 

Wald shows a parallelism of the crystallographic c axes to each 

other, to the greatest development of the crystals, and to the plane of 

rock cleavage. This may be an original orientation retained during 

deformation.

In the above lens- and spindle-shaped forms, both in layers and in 

separate individuals, one feature of the crystallographic arrangement 

is of special interest. Not infrequently, where the particles lie in 

such a position that their c axes are in the plane of rock cleavage, and 

particularly where the crystals show strain effects, such as undulatory 

extinction, sections cut normal to the G axis show the quartz not to be 

optically uniaxial, but to be slightly biaxial, and the plane of the optic 

axes to lie normal to the plane of rock cleavage. This position of the 

plane of the optic axes is what would be expected from the flattening 

of a uniaxial ellipsoid of elasticity in the plane of rock cleavage. Futterer® describes this feature in the Ganggranite von Grosssachsen, 

and in the quartz-porphyry of the Thuringer Wald. Other similar 

crystals lying with .their G axes in the plane of rock cleavage are

optically uniaxial, but in such cases strain effects are usually lacking. The biaxial nature of the quartz appears itself to be a strain 

phenomenon.

The relative abundance of the forms of quartz particles above described of course varies widely in different cleavable rocks. Probably, 

however, in the quartzose cleavable rocks as a group the forms 

described under (1) and (2) occur in greatest abundance. The 

flattened lens-shaped forms described under (3) are less abundant,' but 

are very characteristic of cleavable rocks which have undergone severe 

metamorphism. The spindle forms described under (3) are of rare 

occurrence.

It appears from the above account that the occurrence of quartz in

albid., pp. 20, 34,
LEITH.] ARRANGEMENT OF QUARTZ AND FELDSPAR. 35

cleavable rocks is essential^ different from that of mica or hornblende. 

Mica and-hornblende are usually in crystals with characteristic habit, 

with good dimensional arrangement, and for the most part with a good 

crystallographic arrangement. Quartz, on the contrary, is characteristically without crystal habit; the shape of the particles Varies somewhat with the nature and amount of the deformation which the rock 

has undergone, and, except in rare instances, there is no crystallographic parallelism.

FELDSPAR.

Feldspar is present in considerable quantity in the gneisses, micaceous schists, hornblendic schists, chloritic schists and slates, and, in 

fact, in all of the metaclasic rocks, with the exception of^certain ultrabasic and quartzose schists.

In the following discussion the various feldspars are not discriminated except in descriptions of individual specimens. The differences 

in arrangement between the varieties are numerous, but for the purposes of this paper a general statement covering the feldspars as a 

group is sufficient.

The feldspars occur (1) in forms that have more or less modified 

crystal outlines, (2) in small, nearly equidimensional particles, with 

irregular angular outlines, and (3) in ellipsoidal or lenticular forms 

with curved or angular outlines. Between these all intermediate shapes 

are to be observed.. In each of these classes the feldspars show minor 

differences of occurrence, the description of which would involve the 

discussion of the processes by which they are arranged. These differences will be ignored here as far as possible and will be described in 

connection with the processes of arrangement on pages 87-89.

(1) The feldspars that have more or less modified crystal outlines 

may have their longer diameters arranged (a) at random or (5) parallel 

to the plane of rock cleavage.

(a) The crystals with random arrangements in many cases crowd 

the other parallel constituents of the rock, which bend around them. 

In other cases the parallel constituents are not crowded, but about 

against the feldspars without any bending and even pass through them 

without change of direction. a

(b) Where the crystals lie with their greater diameters parallel to 

the plane of rock cleavage, there is a tendency toward uniformity of 

arrangement of the crystallographic axes and of the feldspar cleavage which requires special description. Several good cases of this 

arrangement have been observed and two of these will be described 

in detail.

aSps. H. 1060,1068, Greylock, Mass.
36 ROCK CLEAVAGE. [BULL. 239.

The first is that of the Hoosac schist" of Massachusetts, representing 

an extreme case of metamorphism. It is a micaceous quartzose schist 

with large albite phenocrysts. The mica and quartz are arranged in 

bands. The porphyritic albites include parallel particles of quartz and 

biotite (PI. XIII, .4). The micas abut against the albites frequentty 

without the slightest change of direction; they are not crowded, and do 

not bend around the albites. The feldspars themselves lack evidence 

of mechanical deformation. Their greater dimensions lie in the plane 

of rock cleavage. The ratio of the greatest to the least dimensional 

axes of the feldspar particles,' determined b}^ 100 measurements from 

10 selected slides, varies from 100 : 50 to 100 :75.

An examination of these feldspars in sections cut across the plane 

of rock cleavage shows that the elongated feldspars have a marked 

tendency toward parallel extinction. The angle of extinction with 

the long side of the crystal and with the rock cleavage varies from 0 

to 19°. Many crystals also show simple twins; the twinning plane 

cuts across the short direction of the ciystal, and 

frequently lies normal to the plane 

of rock cleavage (PL XIII, A). 

Other crystals show a twinning 

plane parallel to the longer dimensions of the crystal that is, parallel to the rock cleavage. Albite 

with a simple twinning has two 

habits, a common tabular development parallel to the brachypinacoid, and a less common tabular 

development parallel to the macropinacpid (figs. 11, 12). The relation of the twinning lines to the 

shape of the feldspar sections in this rock shows that crystals of both 

habits evidently are present. The greatest diameters of the feldspar 

particles lie in the plane of rock cleavage but are not parallel.

Another case of parallel arrangement of feldspar crystals is in a 

mashed feldspar rock from the Wausau district of Wisconsin, described 

by Weidman. 5 This rock has been subjected to severe metamorphism, 

acquiring thereby a parallel arrangement of its mineral constituents/ 

The feldspars are in separate parallel tabular crystals, and also in the 

parallel flattened lenses -described in a subsequent paragraph. Where 

they are in ciystals (albite), the tabular brachypinacoidal faces are parallel to each other and to the rock cleavage. The longest diameters are 

parallel to the plane of rock cleavage, but not necessarily to each other.

«Sp. 18062, sis. 14972,14973,14974, Hoosac tunnel, North Adams, Mass. 

6 Bull, \Viscousin Geol. Nat. Hist. Survey, in preparation.

c A considerable portion of the rock shows an original parallel arrangement of different origin, 

which will not be here discussed, but which is not confused with the particular phenomenon described.

FIG. 11. Twinned albite crystal with 

brachypinacoidal development.

FIG. 12. Twinned albite crystal with 

macropinacoidal

development.
LEITH.J AEEANGEMENT OF FELDSPAE.

FIG. 13. Microscopic view of

fresh anorthosite. (Cf. 

Adams, Geol. Survey, Canada: vol. 7,1894, pt. J.

This is the arrangement shown by part of the albite crystals of the 

Hbosac schist. The average ratio of the greatest to the least diameter 

is 100: 20. 

In albite the axis of elasticity, jc, is approximately normal to the 

brachypinacoid, while the axis of elasticity, a, is nearly normal to the 

macropinacoid. In the parallel arranged albites above described, therefore, these axes have different positions, depending upon which of the two tabular developments 

is present and controls the arrangement. 

Where the tabular development is parallel to the 

brachypinacoid, the common case, the axis £ is 

normal to the cleavage of the rock. The axes a 

and ft are not parallel, but lie in the same 

plane. Where the tabular development is 

Hiacropinacoidal and the feldspar is arranged

parallel to this plane, the axis of greatest

elasticity a alwa'ys stands normal to the plane

of rock cleavage determined by the dimensions

of the crystals. The axes £ and ft in this case are not parallel to each

other, but are parallel to the plane of rock cleavage.

(2) The angular particles clearly owe their shape principally to the 

granulation of larger particles, although they may be partly original and 

partly due to other causes. The first effects noticeable in the mechanical deformation of a feldspar crystal are twinning lamellae and peripheral granulation. With increasing" 

deformation there is gradation from 

these phenomena to complete granulation of the particles. At an intermediate stage of deformation the f eldsparcrystal may have a lenticular form. 

Where the small particles ground off 

from a large particle lie close at hand, 

forming a "tail," the group forms 

a conspicuous lenticule, or "augen." 

At. later stages of granulation the 

minute particles not infrequently lie 

in a layer of uniform thickness.

In the sheared anorthosite from north of Montreal, described by 

Adams,a feldspar particles in parallel, elongated, sometimes ribbonlike forms, with irregular angular borders may be seen lying in a 

matrix of finely granulated material (figs. 13 and 14; also PI. XII, J3). 

Measurements show that the larger feldspars have been granulated 

into nearly 70,000 small particles. The residual feldspars show strain

a Adams, F. D., Report on the geology of a portion of the Laurentian area lying north of the island 

of Montreal: Geol. Survey of Canada, vol. 8, pt. 1,1890, PI. VII.

FIG . 14. Microscopic viewof anorthosite after 

granulation. The residual feldspars may lie 

with their longer axes parallel or inclined 

to the elongation of the rock as a whole.
38 ROCK CLEAVAGE. [BULL. -239.

FIG 15. "Sliced" feldspar in rhyolite-gneiss. 

(Berlin, sp. 3813, Wis. Survey.)

shadows grading into minute fractures. Granulation of feldspars is 

well exhibited in the Berlin rhyolite-gneiss of Wisconsin (PI. XII, A). 

Occasionally the fracturing of feldspar particles has resulted in their 

division into parallel slices, either parallel or inclined to the plane of 

rock cleavage as conditioned by the other constituents of the rock

mass (figs. 15,16,17; PL XI, B}. 

Usually the fractures producing 

such slices are in one set of planes 

at a low angle with the plane of 

elongation of the rock mass as a 

whole, but sometimes also in intersecting planes.

In the early stages of mechanical deformation there is practically no parallel arrangement of the 

greater diameters of the minute granules developed by granulation. 

In subsequent stages the angular particles, through rotation, have a 

very slight tendency to lie with their 

longer diameters parallel to the plane of 

rock cleavage. In many thin sections 

of rocks with fair secondary cleavage, 

the mica |ind hornblende, and even the 

quartz, have their longer diameters parallel, while the angular feldspar particles 

$how no tendency to such arrangement.

Where best developed the dimensional parallelism is hardly noticeable 

oh a cursory examination. A reason for this appears in the fact that 

the angular particles as observed in the cleavable rocks have such slight 

differences in dimension. The ratio of the greatest dimensional diamFiG. 16. Feldspar showing incipient 

"slicing" in schist-conglomerate. 

(Metropolitan, Mich., large slide, No. 

1.)

FIG. 17. "Sliced" feldspars in micaceous and chloritic schist from southern Appalachians.

eters to the least, obtained from 100 measurements from 10 selected 

slides, is in extreme cases 100:40, while the average is more nearly 

100:60.

The slight dimensional arrangement of the minute feldspar granules 

carries with it no tendency toward a parallel crystallographic arrangement, so far as the observations made in this study have gone, except 

in the infrequent cases of the slightly moved slices of individual feldspar particles. Diligent search has failed to reveal even a faint tend-
IJHTH.] ARRANGEMENT OF FELDSPAR. 89

i

ency toward parallelism; if present at all it must be in exceptional 

cases! This comes from the fact that the relations of the dimensions 

of the granulated feldspar particles to the crystallographic properties 

are ordinarily not uniform, and even where they are partially uniform, 

the tendency toward dimensional arrangement due to difference in 

dimensions is so slight that uniformity in crystallographic arrangement is not evident.

It is to be noted that most of the fragments formed from a single 

sliced individual have close approach to crystallographic parallelism, 

although there is no parallelism between the pieces formed from the 

slicing of different feldspar particles; also that there has frequently 

been rotation of these slices toward a common plane inclined to the 

planes of fracture and parallel to the longer diameters of other constituents which have developed in a parallel position by another process shown later

to be recrystallization."

(3) The remaining occurrences of feldspar to be described have lenticular or ribbon forms with theirflatsurfaceintheplane 

of rock cleavage. These, forms show all 

gradations into both the angular .feldspar 

particles and the feldspar crystals above 

described. In the nepheline-syenite of the __ 

Wausau district and in a hornblendic FlG. i8.-Feidspo7~in~paraiiei lenses

Schist from the Mesabi district Of Min- with crystallographic parallelism.

,.,i i /. IT ., . ,1 The longer axes of the lenses are

neSOta" the lenses Of alDlte either OCCUr as 'approximately in the brachypinaseparate individuals or lie end to end and coid- From hombiendic schist of

" . T , i Mesabi district of Minnesota. form Jens-shaped layers of considerable

extent (fig. 18). Many of the feldspars in the lenses or layers show 

no uniform relations of their dimensions to the crystallographic 

properties. 0 The feldspars extinguish at various angles, like the 

quartz lenses of similar shape described in the preceding section. 

The ratio of the greatest to the least axes of the individuals is 100:30. 

In other cases the greater dimensions of the albites of the lenses or 

layers correspond roughly with the tabular developments parallel to 

the brachypinacoid, in which case there is a tendency to extinguish 

parallel to the longer diameters of the lenses, although the extinction varies from 0 to 20° or 30°. <* There is a characteristic lack of 

strain effects, although the rock gives evidence of deformation. The 

axis of elasticity .c is parallel to the shorter diameters of the lenses 

and thus transverse to the cleavage of the rock. 

In general the feldspar particles in rocks with secondary flow cleavaCf. Lehmann, Atlas, PI. XXI, fig. 3. cSp. 6022A, Wausau, Wis. Cf. Weidman. 

<>.Sp. 45492, si. 15759, Mesabi district of Minnesota. dSp. 6024C, Wausau, Wis. Cf. Weidman.
40 BOOK CLEAVAGE. . [BULL.239.

age are present in quantity in the order in which they are described 

above. The common parallel arrangement of feldspars is thus dimensional, although, even where the dimensional arrangement is best, the 

approach to parallelism of the longer diameters of the particles is not 

close. Rarely the parallel arrangement is also crystallographic, in 

which case the good basal cleavage of the feldspar is normal to the 

plane of the greatest and mean dimensional axes, and hence to the 

plane of rock cleavage with which the longer axes are parallel. The 

poorer brachypinacoidal cleavage or clinopinacoidal cleavage is in most 

cases parallel to the plane of tabular development and the rock cleavage, but in a few cases is normal to it.

CHLORITE.

Chlorite is characteristic of chloritic schists or slates. The development of such cleavable rocks may be in part under conditions somewhat different from those under which metaclasic rocks containing 

mica or hornblende are developed. 05

Not infrequently chloritic schists result from the alteration of biotitic schists, and in such cases the chlorite is essentially pseudomorphic.

Chlorite is monoclinic, but pseudo-hexagonal, with prismatic habit, 

and with excellent basal cleavage.

In cleavable rocks the mineral occurs in cleavage plates that have 

their flat surfaces parallel to the rock cleavage in a manner similar to 

that Nof the micas. It appears also in irregular and ill-defined aggregates. Because of its tendency to hexagonal development, the greater 

diameters of the cleavage plates are not markedly unequal in length, 

and thus no parallelism of greatest and mean diameters in the plane 

of rock cleavage can be looked for. The ratio of the thickness of the 

plates to the diameter is about the same as in the micas, in average 

cases perhaps 16:100. As in biotite, the axis of greatest elasticity a 

is normal to the cleavage plane. The axes fr and JC lie in the plane of. 

rock cleavage, but are not parallel in different cleavage plates. Chlorite 

has the same manner of aggregation into layers etc., as the micas.

CALCITE.

Cleavable marbles are not abundant, notwithstanding the great 

number of limestone formations which have been subjected to 

extreme metamorphism. Neither is calcite an abundant secondary 

constituent in cleavable rocks in which other constituents, such as mica 

and hornblende, are predominant. The conditions favorable for the 

development of such constituents apparently do not favor the development of secondary calcite. Indeed, under such conditions there is a 

marked tendency toward the replacement of calcite by silicates. The

a See discussion of development of chlorite in Van Hise's Treatise on Metamorphism: Mon. 

U. S. Geol. Survey, vol. 47,1904.
LEITH.J ARKANGEMENT OF CHLOEITE AND CALOTTE. 41

arrangement of the calcite in cleavable marbles is of principal interest, 

and to this the description below is mainly restricted.

Calcite has hexagonal rhombohedral form, with3 in general, a tendenc}7 to columnar development parallel to c when crystal outlines are 

present. Its cleavage parallel to the rhombic, faces is excellent, and 

polysynthetic twinning parallel to £R is a common feature.

Calcite in marbles with flow cleavage appears in two ways (1) in 

small nearly oval grains, and (2) in small angular pieces. Another 

occurrence is discussed under fracture cleavage on pp. 119-121.

(1) Some of the most cleavable marbles examined, especially those 

from Talledega Mountain, Georgia," were seen to have a distinctly 

granular aspect and to be made up of oval grains of calcite of somewhat uniform size, arranged with their dimensional diameters in a 

common direction parallel to the observed rock cleavage (fig. 19). 

Betvyeen these grains may be a considerable

amount of finely granulated material. Few 

strain effects, such as fractures or close 

polysynthetic twinning, are to be observed. 

The dimensional parallelism is well marked. 

No crystallographic parallelism accompanies 

the dimensional parallelism. The grains 

extinguish at different angles, and their

0 it Fro.. IP. Parallel grains of calcite cleavage lies at all angles with the greatest in schistose marble,

dimensions of the particles, slides ^i, m, Miedega MounIn the cleavable marbles the ratio of length

of these parallel grains to breadth, as determined by 100 measurements 

from 10 selected slides, is from 100:32 to 100:60. This ratio is not far 

different from that obtained in marbles showing no cleavage, in which 

a number of measurements give an average of 100:73.

(2) The calcite particles may sometimes be seen in irregular angular 

pieces. Commonly their greatest and least diameters are not far 

different from those described under (1), and gradational phases into 

(1) are noted. There is a slight tendency to parallel arrangement, 

which is. dimensional and not crystallographic. In those rocks examined the angular particles are not so common as the ellipsoidal grains 

described under (1).

While the accessory calcite occasionally- found in micaceous schists, 

quartzose schists, or hornblendic schists, is perhaps partly original, 

it is in many cases a secondary replacement of other minerals after 

the cleavage of the rock has. been produced. It may till irregular 

elongated areas left by the solution of other minerals, although, in 

general, in rocks of this sort there is little tendency for parallel 

dimensional arrangement of the calcite. In no case is crystallographic 

arrangement observed.

«Sp. 18083, si. 9626, Green Mountain Range, Vermont; sp. 18192; sis. I, II, III, IV, Talledega Mountain, Georgia; near Comstock, N. Y.; Cambro-Silnrian marbles of western Massachusetts described in 

Mon. U. S. Geol. Survey, vol. 23, 1894, Laurentian marbles.
42 BOOK CLEAVAGE. [BULL. 239. 

TREMOLITE, ACTINOLITE, GRUNERITE, AND OTHER AMPHIBOLES.

These minerals are especially characteristic of cleavable rocks formed 

by the metamorphism of calcareous and sideritic rocks and known as 

trernolitic schists, actinolitic schists, and griineritic schists. They are 

characteristically developed by contact metamorphism, but are formed 

also at considerable depths under conditions of high temperature and 

pressure. They are all monoclinic, with a marked prismatic or acicular development. They occur either in single crystals, or, characteristically, in sheaf-like or radial aggregates.

In cleavable rocks the undoubted characteristic occurrence of these 

minerals is in radial and fascicular aggregates with entirely random 

arrangements. Occasionally the individual crystals are arranged with 

their longer dimensions in the plane of rock cleavage, though rarely 

parallel to each other in this plane. The arrangement is not conspicuous and might be overlooked, but on examination in a slide the 

crystals, especially those of actinolite or griinerite, are see^i to cross 

each other at acute angles, perhaps 40°, and lines bisecting these angles 

are seen to have a common direction. The best cases of parallelism 

are observed where the crystals are in single individuals. Where the 

crystals are in the sheaf-like and radial aggregates, the rays lying in 

the plane of rock cleavage are likely to have a greater development 

than those crossing it. Between random arrangement and this tendency to parallelism- there is complete gradation; but the former is 

believed to be by far the more common. Where the parallel arrangement is best developed, no data are at hand to determine whether or 

not the amphiboles of this group have their shorter axes parallel, like 

hornblende. From analogy to the hornblende it is reasonable to suppose that this is sometimes the case. So far as this may occur, the

crystallographic and elastic axes would have the same relations to the

plane of rock cleavage as in common hornblende.

KAOLIN AND TALC.

Talc is largely the result of weathering, but in places it forms cleavable rocks. It is similar to mica and chlorite in habit and arrangement.

GARNET.

Garnet is probably the most common of the porphyritic minerals of 

the "crystalline schists." In extremely metamorphosed cleavable 

rocks, and especially .in the micaceous schists and in phyllites, porphyritic garnets may be seen.

The garnets are isometric, and thus show no dimensional or crystallographic variations along their different axes. In the rocks 

examined their diameter is commonly not less than 1 mm., and is
ARRANGEMENT OF MINERALS. 43

occasionally 4 cm. Being equidiniensional, the individual crystals 

can be said to have no dimensional arrangement with reference to 

the plane of rock cleavage. However, an examination of secondary 

cleavable rocks shows that, in many cases at least, the garnets are 

more numerous in certain planes of cleavage than in others. They 

are likely to occur along planes where mica has been abundantly 

developed, and are not so numerous in the quartz and feldspar layers.

TOURMALINE.

Tourmaline is characteristic of the micaceous schists and certain 

feldspathic schists. Its development is commonly supposed to be 

largely due to the contact action of the magma and the introduction 

of extraneous material into the metamorphosed rock.

The mineral is hexagonal (trigonal), with a marked columnar habit.

In the rocks with secondary cleavage the columnar crystals of tourmaline have a marked tendency to lie parallel to the plane of rock cleavage, but not to one another/' In a few cases they are parallel to one 

another as well as to the plane of rock cleavage. 6 There is no parallelism of the shorter axes, except to a plane, when the longer axes 

are parallel. .' » 

So far as the crystals have dimensional arrangement the crystallographic properties also are parallel. The crystallographic axes, c, 

or the principal optic axes (negative) in the different particles thus lie 

in the plane of cleavage and rarely are also parallel to one another in 

this plane. The crystals vary considerably in the ratio of their greatest to their smallest diameters; 100 :60 and 100:20 are extremes from 

25 measurements in 5 selected slides and specimens.

STAUROLITE.

Staurolite is characteristic of micaceous schists. It is orthorhombic, 

and characteristically in short tabular crystals and cross twins, in 

each case with a well-marked development parallel to the inacropinacoid. The arrangement of the staurolite in the secondary cleavable 

rocks may be observed macroscopically, and especially well on a 

weathered surface. An examination of numerous specimens shows 

that the staurolites have a strong tendency to lie with their flat surface 

parallel to the plane of cleavage, 13 although many exceptions to this 

rule are noted. The greater axes of the crystals in this plane are not 

parallel. .

As the staurolites in crystal form have uniform relations of their

«Sp. H. 1049.1, Hoosac Mountain, Massachusetts; sp. H. 4154B, near Seymour, Conn.; sp. H. 4151, 

Beacon Falls, Conn.; sp. H. 4083, A, near Danbury, Conn.

l> Sp. 14889, Black Hills, South Dakota; sp. 29633, Laramie, Wyo.

cSp. H. 1105, west side Couanicut Island, Narragausett Bay; sp. H. 1084, Conn., sp. 14919, Black 

Hills, South Dakota.
44 ROOK CLEAVAGE. [BULL. 239;

crystallographic axes to the dimensional axes, it follows that the crystallographic properties of the crystals are arranged to the same degree 

as the dimensional axes. The crystallographic axis c and the axis of 

greatest elasticity a thus lie in the plane of cleavage.

OTTRELITE (AND CHLORITOID).

Ottrelite and chloritoid are found principally in the micaceous 

schists or phyllites. They are closely related in their properties and 

occurrence, and are commonly not discriminated, although they seem 

to differ in chemical composition. The minerals are monoclinic and 

the basal cleavage is good, although it is not so perfect as in mica.

They occur in the micaceous schists or phyllites in a variety of ways 

in radial or sheaf-like aggregates, in thin scales or plates, and in simple 

crystals. In the rocks examined the ottrelite has predominantly crystal form. The crystals are more or less hexagonal, tabular plates, with 

a good basal cleavage parallel to their longer dimensions. "Hourglass" twins are not uncommon.

In the typical ottrelite-schist from Ottrez, Belgium,® the ottrelites 

h,ave no parallel arrangement whatever, dimensional or crystallographic. In rocks from other localities, however, there is a tendency 

for the dimensions of the plates to control the arrangement of the 

crystals, causing the greater dimensions to lie more nearly in the 

plane of cleavage than normal to it. Within this plane the greater 

dimensions of the crystals make all angles with one another. The 

mica-schists of the Black Hills, South Dakota,6 show this arrangement. The tendency toward parallelism is slight, even where most 

marked, and in rocks showing such arrangement many ottrelite crystals lie squarely across the rock cleavage and at all angles to it. The 

result is that any rock with secondary cleavage containing ottrelite is 

likely to show ottrelite cleavage faces on any surface, but more conspicuously in the zone normal to the plane of rock cleavage, for in the 

plane of rock cleavage.the mica cleavage (the ottrelite is commonly 

associated with mica) is likely to obscure the cleavage of the ottrelites.

The average ratio of the length of the crystals to the breadth, taken 

from 100 measurements from 10 selected slides, is 100:40.

ANDALUSITE (CHIASTOLITE).

Chiastolite is not important as a cleavage-giving mineral, but it is 

characteristic of certain dense cleavable hornfels in which long, prismatic, chiastolite crystals stand out conspicuously against a finegrained, dark-colored background. The mineral is orthorhombic with 

columnar habit. In most of the rocks with secondary cleavage that 

were examined the chiastolite crystals were either inclined or perpena Sp. H. 1104, Ottrez, Belgium. & Sp. 14874, Black Hills, South Dakota.
WITH.] AEBANGEMENT OF MINEKALS. . 45

dicular to the plane of rock cleavage, 05 but in certain of the rocks there 

is a tendency for the long directions of the crystals to lie roughly in 

the plane of rock cleavage. 6 The greatest and mean axes have no 

parallelism in this plane. The average ratio of length to breadth, 

measured in 30 crystals from 3 slides, Is 100 :10. .

As chiastolite occurs in crystals whose dimensional and crystallographic properties are uniformly related, it follows that the crystallographic properties are parallel so far as the dimensional axes are 

parallel. The properties represented hy the c axis are thus parallel 

to a plane and not to a line.

SILLIMANITE (FIBROLITE).

This is a rare constituent of the cleavable rocks as a whole, and even 

where most abundant makes up but a small portion of the rock. It is 

orthorhombic, with a very Strong columnar habit.n

In the cleavable rocks sillimanite is seen to occur in single, slender, 

rod-like forms, with characteristic. cross fractures, and also in sheaflike aggregates and parallel bunches. The individuals and the crystals 

in aggregates have a strong tendency toward a parallelism of their 

longer axes. c They furthermore have a tendency toward concentration into definite layers/* The individuals are always slender, 

frequently almost hair-like, and their length as compared with their 

thickness is very great. On account of the fineness and cross fracture 

average measurements are not practicable. The crystallographic axis 

c and the axis of least elasticity, £, lie in the plane of rock cleavage.

SUMMARY STATEMENT CONCERNING FACTS OF ARRANGEMENT IN ROCKS 

WITH FLOW CLEAVAGE.

The above facts are believed to afford a basis for the following 

generalizations:

Rocks with flow cleavage are made up principally of flat v or elongated particles with a variety "of shapes and dimensions. Particles of 

a particular mineral have characteristic shapes and dimensions and are 

easily distinguished from particles of other minerals. Occasionally, 

however, they show minor variations of form due to differences in 

origin and behavior during deformation. To illustrate: Mica is in 

thin plates; hornblende is columnar; quartz is seldom in crystal form, 

but is in angular or lenticular grains; feldspar is sometimes in crystal 

or lenticular form, but usually in angular to subangular grains. The 

ratio of the greatest to the least diameter varies in mica from 100:20 

to 100:10; in hornblende, from 100:20 to 100:25; in quartz from

aSp. H. 71.1, Yaqui Gulch, California.

bSp. H. 1112.1, H. 71, Gefrees, Fichtelgebirge, Germany.

cSp. 14 Bayley, sp: 504 Bayley, sp. 90G W., Saxony, sp. 14901, si. 7651 Black Hills, South Dakota.

rf Sp. 504 Bayley. .
46 BOOK CLEAVAGE. [BULL.239.

100:50 in common cases to 100:15 in exceptional cases; in feldspar it 

is about 100:60. The uniformity of this ratio for a given mineral 

species disproves the idea sometimes held that the degree of flatness or 

elongation of particles in a rock with secondary cleavage is proportional to the amount of deformation which the rock has undergone.

The unequiaxial particles of the rocks with flow cleavage have a 

tendency to lie with their greater, mean, and least diameters parallel 

to one another, although this tendency varies with the nature of the 

mineral, with the processes which have been effective in bringing 

about £he arrangement (as will be shown in a subsequent chapter), 

and with the amount of deformation which the rock has undergone. 

There is also a tendency for minerals of the same kind to be aggregated into layers. It is obvious that there must be complete gradation 

from the random arrangement of minerals of undeformed rocks to the 

parallel arrangement of minerals of a very cleavable rock. A parallel 

arrangement is reached at different stages in the development of rock 

cleavage by different minerals. In the same rock it may be reached 

by mica, chlorite, and hornblende and not by feldspar and quartz. In 

a further stage of deformation it may be possessed by mica, chlorite, 

hornblende, and quartz and not by feldspar. In still more advanced 

stages of deformation it is acquired by tourmaline, actinolite, griinerite, 

tremolite, staurolite, sillimanite, andalusite, chloritoid, etc., so far as 

they receive it at all. Calcite has not been compared directly with 

these minerals, but unquestionably takes on a parallel arrangement 

early in the deformation.

The crystallographic or vector properties of the particles of any 

mineral are parallel only when they have uniform relations with 

the parallel dimensional axes. This is commonly the case where the 

particles are either crystals or cleavage plates from crystals that have 

the characteristic shape and habit of the mineral species represented. 

In arranging themselves dimensionally such crystals or cleavage plates 

must necessarily arrange their crystallographic axes. The relations 

of dimensions to vector properties are uniform in most mica, chlorite, 

and quartz particles, and in only a small portion of the feldspar 

particles, while in calcite they show no uniformity whatever. While 

in a particular mineral these relations may be uniform, in different 

minerals they may vary. A vector property may be parallel to the 

greatest dimensional axis in one mineral and to the least dimensional 

axis in another. Thus the degree of parallelism of vector properties 

in the rocks with flow cleavage as a whole may be ascertained by listing 

under each mineral the vector properties corresponding to the parallelarranged greatest, mean, and least dimensional axes, and seeing how 

far these properties correspond in all the minerals in the rocks.

While the arrangement of the vector properties of the particles of 

a cleavable rock is thus subordinate to, and entirely dependent upon,
SUMMAEY OF MINERAL ARRANGEMENT. 47

the dimensional arrangement, and while the arrangement of the 

physical properties with reference to the dimensional axes varies 

with different minerals, in some of the more common cleaA^able rocks 

the net effect of the parallel arrangement of the vector properties 

of the mineral particles, incidental to and resulting from the 

dimensional arrangement, may be. such as to give the cleavable rock 

as a whole a definite polarity with reference to these properties i. e., 

to give the cleavable rock certain properties for certain directions 

and other properties for other directions. Experiments on heat, 

electric properties, cleavage, and hardness of cleavable rocks seem 

to show some such condition, .although it is probably due more to 

the dimensional than to the crystallographic arrangement of the 

individual particles. A discussion of these properties in cleavable 

rocks is outside the scope of this paper. However, attention will be 

called to one property, the behavior of the cleavable rocks with 

reference to light, which has been noted in this investigation. '

The axis of greatest elasticity a the axis affording the easiest vibration of light for rays propagated normal to it has a position normal 

to the plane of rock cleavage in the micas, in chlorite, and in talc; 

within a few degrees of the normal to the plane of rock cleavage in the 

crystals of hornblende showing triaxial parallelism and in the smaller 

number of feldspar crystals showing parallel arrangement of their 

crystallographic properties. In most of the feldspar ciystals the axis 

a lies in the plane of the greatest dimensions, while the axis £ is normal to the plane of rock cleavage. The arrangement of the mean and 

least elastic axes in some parallel minerals is known, and in others is 

not known; in the rocks showing flow cleavage as a whole, sufficient data 

are not yet at hand to warrant any definite statement concerning the 

uniformity or nonuniformity of arrangement of the mean and least 

axes. These minerals include the most abundant minerals appearing 

in the cleavable rocks with uniform crystallographic arrangement. 

 The minerals not mentioned are the porphyritic minerals which are 

commonly supposed to crystallize either after movement has entirely 

ceased or near the close of the deformation, such as tourmaline, garnet, 

staurolite, sillimanite, etc.

In other words the above facts show that so far as the common 

cleavable rocks have any uniform effect on the transmission of light, 

due to the optical orientation of the individual particles, they allow 

the light ray to travel faster normal to the plane of rock cleavage 

(but vibrating parallel to this plane) than the rays traveling parallel to 

the plane of rock cleavage. The possible significance of this in its 

bearing on molecular structure is discussed on pages 97-99.

Repeating the main conclusion, then, the mineral particles in a rock 

with flow cleavage, so far as they are arranged, have their respective 

dimensional axes parallel, but only incidentally and partially are their
48 . BOOK CLEAVAGE. ^1*239.

vector properties parallel. The mineral particles have been arranged 

with their dimensional axes parallel regardless of any influence which 

the vector properties may have had, except in so far as they modified the dimensions. Even in minerals such as quartz and calcite, 

in which the physical properties along the principal and lateral axes 

are strongly differentiated, these vector properties show no tendency to parallelism because they have no uniform relations to the 

parallel-arranged dimensional axes of the particles. Moreover, the 

vector properties have no influence in arranging the crystal, where 

there are no dimensional differences in the principal axes. When two 

of the three principal dimensional axes of a particle are of the same 

length, so that there can be said to be no parallel dimensional arrangement of such axes, the vector properties show no parallelism, although 

in the absence of dimensional control they would have an opportunity 

to show any influence they might have. For instance, the vector 

properties tying in the plane of the mica cleavage are not parallel, 

in spite of the fact that the length and breadth of these plates are not 

so far different, but that the vector properties, had they had any influence, could have succeeded in arranging themselves without violating 

the principles of dimensional control.
CHAPTER II.

THE OBSERVED RELATION OF SECONDARY ROCK CLEAVAGE TO 

THE PARALLEL ARRANGEMENT OF MINERAL PARTICLES.

In the rocks with secondary flow cleavage examined during this 

investigation, which are believed to be fairly representative of such 

rocks in general, a plane, or surface, of cleavage has been observed to 

be uniformty parallel to the greatest and mean, or greatest and least 

dimensions, or both, or even to the greatest alone, of the parallel constituent mineral particles. This has been observed microscopically 

in thousands of specimens, and microscopically in several hundred 

specially prepared slides. Not only has a cleavage been observed to 

coincide with the longer diameters of the parallel particles, but it has 

been apparent that the excellence of such cleavage is proportional to 

the degree of the arrangement and to the inequality of the dimensions 

of the particles. In addition to this essential condition of parallel 

arrangement, another condition, dependent on the first, has been 

observed to . be of great importance the parallel arrangement of 

mineral cleavages. This has usually aided the dimensional arrangement, but has rarely tended to cause a cleavage in a plane different 

from that caused by the dimensional arrangement of the mineral 

constituents. The characteristic segregation of particles of the same 

mineral species into layers may develop also planes of weakness that 

are due to intrinsic strength of the minerals, quite independent of any 

dimensional arrangement. Minute foldings of parallel-arranged constituents, as in micaceous slates, may yield a plane of weakness parallel 

'to the limbs of the folds (PL XIV, j5), causing a cleavage which has 

been called in part "false cleavage."

It is needless to say that the agreement of secondaiy flow cleavage 

with parallel mineral arrangement, above emphasized, is not a newly 

observed relation. It has been described and noted by so many geologists that the coincidence of the two structures has been accepted as 

almost self-evident. Yet, as will be shown in Part II, there is a cleavage, here called fracture cleavage, which is independent of such a 

parallel structure, and largely because of this certain geologists have 

concluded that all cleavage may be independent of a parallel arrangement of minerals. A comparison of cleavage independent of a parallel 

arrangement and cleavage dependent upon such an arrangement in 

 other words a comparison of fracture cleavage and flow cleavage is 

made in Part II.

Bull. 239 05 4 49
50 ' ROCK CLEAVAGE. [BULL.239.

RELATIVE IMPORTANCE OF MINERALS IN PRODUCING FLOW

CLEAVAGE.

If the dimensional arrangement of the particles in the secondary 

cleavage rocks, and the attitude of the mineral cleavage of the particles are known, there are at hand the facts of occurrence necessary 

to an intelligent discussion of the relative cleavage-producing capacities of the minerals occurring in rocks with secondary cleavage.

Before taking up the detailed discussion, however, it may be well to 

explain what is meant when it is said that one rock has a better cleavage 

than another, or that one mineral affords a better rock cleavage than 

another. In the following discussion excellence of secondary rock 

cleavage, parallel either to a plane or a line, will be considered as 

determined by (1) the facility of parting, (2) the evenness of the 

parting, (3) the number of planes or lines of parting in a given thickness of rock. The extent to which a given mineral contributes these 

properties to a rock determines its cleavage-producing capacity.

By observation it is known that secondary flow cleavage in rocks 

occurs parallel to the greater diameters of the constituent particles, 

or parallel to the mineral cleavages or both. Usually, but not always, 

they are in the same plane. It is further a matter of direct observation that the perfection of cleavage depends upon the following factors:

Shape and arrangement. The degree of flatness or elongation of 

the particles, and their degree of parallelism, is here meant. The 

greater the difference between the length or breadth and thickness 

of parallel particles and the better the parallelism the better the cleavage. The shape and arrangement must be considered together, 

because particles having shapes favorable to the development of cleavage can produce no cleavage unless they have parallel arrangement, 

and vice versa, particles well arranged produce poor cleavage unless

they show marked differences in dimensions.

According to the relative differences between the, dimensional axes, 

the parallel particles may tend to produce cleavage parallel to a plane, 

to two intersecting planes, or to a line. Where two of the dimensions are not far different and are both greater than the third, the 

cleavage produced by the parallel arrangement is in a plane; where 

two are much smaller than the third the cleavage may be parallel to a 

line; where the three diameters are of different lengths there may be 

cleavage parallel to the plane of the greatest and mean diameters, 

and a poorer cleavage in the plane of the greatest and least diameters; that is, normal to the first one. This less perfect cleavage conditioned by the greatest and least dimensions of the particles is what 

has been called grain, particularly in descriptions of slates.

Thickness. The thinner the cleavage-making particles the greater 

the number of planes or lines of parting ^between or through the minerals possible in a given thickness of rock.
LEITH.] EFFECT OF MICA ON FLOW CLEAVAGE. 51

 Cohesion ^principally mineral. The cohesion of the particles is the 

bond by which molecules of like character are held together. Mineral 

cleavage is a property of cohesion. The cleavage of the particles 

themselves and its attitude with reference to the plane of the greatest 

and mean dimensional axes help to determine the perfection of rock 

.cleavage. The more nearly parallel to the greater dimensions of the 

particle, and the better the mineral cleavage, the better the rock 

cleavage produced. Usually, but not always, the mineral cleavage is 

parallel to one or both of the greater dimensions.

Adhesion. The adhesion of the particles is the bond by which 

molecules of unlike character are held together. Where adhesion is 

les£ than cohesion, the less the adhesion the better the parting. 

Where adhesion is greater than cohesion, parting occurs within the 

particles by overcoming cohesion, and variation in adhesion has no 

effect on cleavage.

Abundance. The more abundant a good cleavage-producing mineral is and the more area it occupies in the plane of its greater diameters the better the rock cleavage produced. Aggregation of good 

cleavage-giving particles into layers may give a good cleavage in such 

layers, but may leave intermediate layers with a poor cleavage or with 

none at all. This applies especially to the coarser rocks (schists). 

It is well known that fine-grained slates, yielding the easiest and most 

even planes of parting, show no marked concentration of their different cleavage-giving minerals into la}7 ers.

Below an attempt is made to indicate the extent to which these factors are applicable to individual minerals, and to determine the relative 

importance of the minerals as cleavage producers. The cleavageproducing factors are discussed in the same order as above.

MICA.

Shape and arrangem.ent. The particles have a ratio of greatest or 

mean diameter to thickness averaging perhaps 100:16, which is the 

greatest difference found in any of the cleavage-making minerals. 

They have a strong tendency to parallelism. The greatest and mean 

diameters do not show any uniform differences in dimensions, and 

hence the only cleavage produced by the parallel arrangement is 

parallel to the plane of the mean and greatest diameters.

Thickness. The thickness of the particles in the rocks examined 

varies from about 0.01 to 0.40 mm., but averages perhaps 0.15 mm., 

giving a great number of mica plates and of possible planes of parting 

in a given thickness of rock.

Cohesion. Mica has perfect mineral cleavage strictl}7 parallel to its 

greater dimensions. It is the only important cleavage-giving mineral 

in which this relation holds, unless chlorite and talc be here included. 

The cohesion of the cleavage laminae is probably less than in any other
52 EOCK' CLEAVAGE. [BULL.239.

mineral with which the mica is commonly associated in a cleavable 

rock, and usually less also than its adhesion with other minerals.4 As 

a result the parting in a rock containing mica is likely to be caused 

by the separation of the mica plates along- their cleavage.

Adhesion. As stated above, the adhesion of the micas to other 

particles is usually greater than its cohesion along its own cleavage 

planes, so that parting seldom occurs between micas and adjacent 

minerals. When concentrated into the layers the adhesion of the 

separate crystals of mica may be less than the cohesion of their cleavage, but as the shape of the separate crystals is that of cleavage plates 

the adhesion in this case might as well be called cohesion. Whether 

the parting be through the individual micas or between them -the 

parting planes of rocks with secondary rock cleavage containing mica 

show mica as a dominant constituent.

Abundance. Mica is very abundant and widespread in the rocks 

with secondary cleavage. In the rocks with coarser cleavage it has a 

tendency to be concentrated into layers, which are usually continuous 

and large in area. In the slates this tendency is slight.

When considered with reference to all of these factors mica is found 

to be a good producer of rock cleavage, affording great facility of 

parting parallel to a plane, and causing even planes of parting and a 

large number of possible planes of parting in a given thickness of rock. 

Indeed, as will be seen later, mica is probabty more important as a 

cleavage producer than all other associated minerals combined.

HORNBLENDE.

Shape and arrangement. The ratio of the length to the shortest 

diameter varies between 100 :20 and 100 :25. The ratio of the two 

shorter diameters is about 100:60, The long axes are commonly 

parallel to one another, and in these cases it is not uncommon to find 

a tendency for the least and mean axes also to be parallel. In other 

instances the longer axes are not parallel, but lie in the same plane, 

and the mean and least axes may not be parallel. In the first case the 

rock cleavage is almost anywhere in the zone parallel to the long axes, 

although when the least and mean axes are arranged, probably it is a 

little better in the plane of the greatest and mean axes than in the 

plane of the greatest and least axes. If the effect of their concentration 

into "layers is disregarded the parallel hornblendes themselves tend to 

produce a linear parallel type of cleavage rather than a plane parallel 

type. There is an axis of cleavage rather than a plane of cleavage. 

Where the hornblendes have their longer diameters parallel to a plane 

and not to each other the tendency to parting may be only parallel to 

this plane.

Thickness. The smallest diameter in the rocks examined varied 

from about' 0.03 to 0.20 mm,
LBITH.] EFFECT OF HORNBLENDE ON FLOW CLEAVAGE. 53

Cohesion and adhesion. Hornblende has good prismatic cleavage 

in two planes inclined at about 28° to the plane of the greatest and 

mean dimensional development. Where the hornblende particles lie 

with all three dimensional axes respectively parallel, the prismatic 

cleavages are inclined 28° to the plane of cleavage produced by the 

dimensional arrangement. The hornblende cleavage therefore assists 

but little in producing a parting parallel to this plane. It may, however, and probably does, have an effect in producing the linear parallel 

type of cleavage, just as do the normal crystal dimensions. The 

cleavage is parallel to the unit prism faces and produces forms identical with the normal crystal outlines. It is hard to determine whether 

the parting is more easy around the particles than through their cleavage. The resulting forms are practically the same. The glistening 

cleavage surface shown on fresh fracture would seem to indicate that 

the fracture has been through the mineral cleavage planes to some 

extent at least, and when examined with a lens of high power the

hornblende crystals are seen to have a grooved and irregular surface. 

Whether the parting is through or around the hornblendes, the fractured surface has irregularities which probably correspond roughly to 

the cleavage surface or to the prismatic shape of the hornblende.

Abundance. : Where hornblende occurs at all in a secondary cleavable 

rock it is ordinarily in considerable abundance, not infrequently making up much the greater bulk of the rock. There is also a tendenc}7" 

for the hornblendes to lie in layers. However, even where abundant 

and in layers, they do not occupy all of the area of the potential plane 

of parting. An average case would be one in which the hornblendes 

occupy two-thirds of the area, and from this proportion there is gradation to disappearance. While individual hornblendes tend to produce 

a linear-parallel parting by their shape and cleavage, the effect of their 

concentration into layers is to make the parting approach the planeparallel type. In general, therefore, so far as the hornblendes are not 

concentrated into layers they tend to give a linear-parallel parting; so 

far as they are in hiyers they tend to give the plane-parallel type of 

parting. The common case is perhaps intermediate between these two 

extremes. a

In shape and aggregation, the hornblendes are not so well adapted 

to give the plane-parallel type of rock cleavage as the micas. They 

are better adapted by their shape, cleavage, and aggregation to give a 

linear-parallel type of parting. In either case their effect is much 

greater than that of quartz or feldspar.

aSp. 40686, si. 15361, Mesabi district of Minnesota.
54 > BOOK CLEAVAGE. . [BULL. 239.

QUARTZ.

Shape and arrangement. The quartz particles described under (1) 

and (2) on pp. 31-33, that is, the angular and subangular particles due to 

fracturing, showing all gradations in shape into original, angular, subangular, rounded or crystal-shaped grains, to the slight extent that 

they are arranged, exhibit a tridimensional parallelism. They may 

help to produce a rock cleavage in the plane of their greatest and mean 

diameters, or the plane of their greatest and least diameters (grain 

or side), or even in the plane of the least and mean diameters, depending upon the relative differences between the dimensional axes. The 

ratio of the greatest to the least dimensional axes in the rocks measured 

varied from 100 : 50 to 100:100. The lengths of the mean axes varied 

from almost that of the greatest to almost that of the least. When in 

slices inclined to the prevailing plane of elongation of the rock mass, 

the effect of the quartz on cleavage is small. The inclination of the 

slices to the prevailing cleavage is usually slight and causes only local 

variations from the prevailing surface of cleavage.

The quartz individuals in the layers and lenses described under (3), 

on page 33 have a good tridimensional arrangement which may again 

yield cleavage in the plane of the greatest and mean diameters and in 

the plane of the greatest and least diameters. But the layers are so 

thin compared with their length that the cleavage is mainly parallel 

to their flatness.

In the exceptional spindle-shaped forms described under (3), on 

page 33 the ratio of length to thickness varies widely. In the slides 

examined, it averages iOO : 50. The least and mean diameters of these 

forms are practically the same, so that any tendency to parting which 

these minerals might condition would be anj^where in a zone parallel 

to the greatest diameter. However, these grains are phenocrysts and

in themselves probably have little effect.

In general, the dimensional inequality of the quartz individuals is so 

slight, and their arrangement so poor, that their effect as individuals 

in the production of rock cleavage is small. When aggregated into 

layers, as described on page 33, their effect may be considerable.

Thickness. In thickness the quartz particles, of course, vary 

widely, ranging from large conspicuous phenocrysts to submicroscopic size. A common thickness for the lenses or layers of quartzes 

described on page 33 is from 0.06 to 0.40 mm.

Cohesion. The cleavage of quartz is so poor that it probably has 

no effect on the parting. The cohesion is so great that parting seldom 

occurs across or within the grains. It is a fact of observation that 

cleavage surfaces of schists or slates seldom show fracture surfaces of 

quartz.

Adhesion. The adhesion with other minerals is usually great. It
LEITH.] EFFECT OF QUARTZ ON FLOW CLEAVAGE. 55

is again a matter of observation that cleavage surfaces of schists or 

slates containing quartz seldom show quartz on such surfaces. When 

associated with feldspar in the " leaf gneisses" it may sometimes 

appear, but in its common occurrence with mica in the mica-quartzschists, mica alone appears.

Abundance. Quartz is one of the most abundant minerals of the 

rocks with secondary cleavage as a whole, but varies in individual 

cases from nearly 100 per cent to disappearance. In the coarser schists 

. it has a tendency to aggregation into layers. The la}7 ers may be contin'iious, large in area, and relatively thin and even. The effect on the 

parting due to the abundance of the particles and to the aggregation 

of the particles into layers is considerable, notwithstanding the fact 

that the individual particles are so poorly adapted by their dimensions 

and arrangement to induce a good parting.

The total effect of the quartz in the production of cleavage is much

less than that of either the mica or the hornblende. The parting is 

less easy. The plane of cleavage is as a rule less smooth, although 

probably, when the quartz is in the recrystallized layers, it is nearly as 

smooth as that due to hornblende. The planes of parting are much 

less numerous for a given thickness. When compared with the feldspars, as will be seen further on, quartz probably has the greater 

effect on rock cleavage.

FELDSPAK.

Shape and arrangement. The feldspar with crystal and lenticular 

outlines, (1) page 35, has in part its longer diameters in the plane of 

rock cleavage. The diameters in this plane are not respectively parallel. The ratio between the greatest and least diameters varies from 

100 :50 to 100 :75. So far as any rock cleavage is produced by the 

shape and arrangement it is parallel to a plane. The angular feldspar 

particles described under (2), on pages '37-39, have a very slight tendency to parallel arrangement. Such as it is, it is probably usually 

tridimensional. The difference between the greatest and least diameters is not large, 100:40 being an extreme and 100:60 to 100:75 

being common ratios. The parting may be parallel to the plane 

of the greatest and mean axes and also to the plane of the greatest and 

least axes (grain and side). But in either case, owing to the poor 

arrangement and the small inequality in the axes, this tendency 

to parting is very slight. The residual feldspars left after granules 

have been rubbed off may themselves lie in irregular parallel lenticules 

(fig. 14). Where the feldspar has been sliced (see p. 38) the longer 

axes of the slices usually lie at low angles to the plane of rock cleavage 

(figs. 15, 16, 17). The ellipsoidal and lenticular forms described on 

page 39 usually have their longer diameters in the plane of rock 

cleavage. The individuals show a ratio of greatest to least axes of 

100:30. '
56 BOCK CLEAVAGE. [BULL. m

In the cleavable rocks feldspar with random arrangement is probably as abundant if not more abundant than feldspar with parallel 

arrangement, so that even were the effect of the arranged feldspar 

considerable, the total effect of feldspar on cleavage due to its dimensional arrangement would be verjr slight.

Thickness. The thickness, of course, varies widely, from the largest 

phenocrysts to submicroscopic size. In the slides examined the 

arranged feldspars were not thicker than 3 mm.

Cohesion. As the parallel arrangement of feldspar particles is mainly 

dimensional and not crystallographic, it follows that the cleavages of 

the different feldspar particles in general are not parallel and therefore have no uniform effect on the rock cleavage. In exceptional 

instances the feldspar particles exhibit both dimensional and crystallographic parallelism, and the feldspar cleavages have an effect on the 

rock cleavage. The go'od basal cleavage stands at right angles to the 

greater dimensions of the crystal, and hence to the plane of rock 

cleavage, to which the greater dimensions of the crystals are parallel-. 

As the greater diameters of the crystals are parallel only to the plane 

of rock cleavage and not to each other, the basal cleavages of the different crystals, while alwaj^s at right angles to the plane of the greatest and mean diameters, may be at any angle to one another. The 

poor macropinacoidal or braclrypinacoidal cleavage may stand either 

normal or parallel to the rock cleavage determined by the dimensional 

arrangement of the constituent particles, depending upon the nature 

of the tabular development of the feldspar, whether brachypinacoidal 

or macropinacoidal. In the former case it is parallel to it; in the latter case it is at any azimuth normal to the plane of the dimensional 

cleavage, but of course always at right angles to the basal cleavage.

The characteristic arrangement of the basal feldspar cleavage normal

to the plane of dimensional cleavage, to which the greater axes of 

the feldspars lie. parallel, has a tendency to produce another rock cleav-. 

age at right angles to the rock cleavage conditioned by the dimensional 

arrangement of the constituent minerals of the rock. The cross 

cleavage may be well seen in the examination of hand specimens. On 

the ragged edges of parting normal to the best rock cleavage anywhere 

in a zone about the specimen, there may be seen numerous glistening 

cleavage faces "of feldspars, many of them undoubtedly basal and some 

of them pinacoidal; while on the cleavage surfaces of the rock the 

feldspar cleavages usually do not appear. The basal cleavage of the 

feldspar would tend to make a better rock cleavage than a rock cleavage produced by the arrangement of the pinacoidal cleavage of the 

feldspar alone, or by the arrangement of the greater and mean dimensional axes, or both; but the total cleavage-producing effect of the 

feldspar in any plane is very small compared with that of the other
LEITH.] EFFECT OF FELJDSPA& ON FLOW CLEAVAGE. 57

minerals usually present, and thus the cross cleavage produced by the 

feldspar is usually insignificant compared with the cleavage formed by 

the dimensional arrangement of the other constituents.

Adhesion. The adhesion of the feldspar particles to adjacent particles is usually great, probably greater than its own cohesion or the 

cohesion or adhesion of the particles with which it is associated, with 

the exception of quartz. As a result feldspars do not appear uniformly on the cleavage face of the schist except where the feldspar is 

associated with quartz, as in the case of leaf gneisses. In the case of 

uniform crystallographic arrangement, the feldspar cleavage surfaces 

may appear as indicated in the preceding paragraph.

Abundance. Feldspar is one of the most abundant of the minerals 

of the cleavable rocks. It shows a slight tendency to aggregation 

into layers, particularly where granulated and in the coarser schists. 

Where the feldspars have crystal outlines they are usually porphyritic and not concentrated into the planes of rock cleavage; where in 

layers the effect on cleavage is probably greater than where not so 

aggregated.

In general, feldspar exerts its effect on rock cleavage in two ways: 

(1) By its shape, dimensional arrangement, and aggregation it gives a 

little assistance to other minerals in producing rock cleavage parallel 

to the greater dimensions of the particles; (2) in the rare cases where 

feldspar has crystallographic as well as dimensional arrangement the 

mineral cleavages of the feldspar condition a poor rock cleavage, both 

parallel and normal to the greater diameters of the particles.

Feldspar and quartz have less effect on the production of rock cleavage than any of the other principal cleavable rock constituents. With 

respect to the effects of the mineral cleavages on rock cleavage feldspar is more important than quartz. If the effect of an arrangement 

parallel to their flatness is compared, it seems that feldspar is far'.less 

important than quartz. As the arrangement parallel to the flatness 

is more common than parallelism of mineral cleavage in both quartz 

and feldspar, it is seen that the general effect of quartz on cleavage is 

probably more important than that of feldspar. For this reason feldspar has been placed last in the group of principal cleavage-making 

minerals. In its cleavage-producing capacity it is, however, unique 

among the minerals in affording rock cleavage in two directions at 

right angles to each other.

The comparatively slight effect which feldspar has in the production 

of cleavage is well illustrated in certain f eldspathic rocks of the Wausau district of Wisconsin. Here in a number of places the feldspars may 

be observed to have tridimensional parallelism, yet in breaking out a 

hand specimen only a very poor rock cleavage, parallel to the basal 

cleavages of the feldspars, or parallel to the greater dimensions of the
58 BOCK CLEAVAGE. [BULL. 239.

parallel crystals may be observed. a The same lack of cleavage when 

the feldspar is well arranged appears in certain of the Archean feldspathic schists of the Mesabi district of Minnesota. 

CALCITE.

Shape and-arrangement. The shape of the calcite particles varies in 

different slides. Where in angular and ellipsoidal grains (p. 41) the 

ratio of the longest to the shortest axes of the particles varies-widely, 

but 100 : 50 was found to be an average for several slides. The paral-. 

lelism of the longer axes is not marked, but still a distinct tendency 

toward it can be detected. The rock cleavage produced depends on 

the'differences in the three dimensional axes and the degree of arrangement, and is (1) parallel to the plane of the greatest and mean diameters this may be fairly good and (2), parallel to the plane of the 

least and greatest diameters a very poor cleavage. » Thickness. The thickness varies from submicroscopical to, perhaps, 

several centimeters.

Cohesion. The calcite cleavage is rhombohedral. In practically all 

cases this cleavage is at random angles to the plane of the greatest 

development of the particles. In one instance the acute angles of the 

cleavage faces were observed to show a slight and doubtful tendency 

to lie in the plane of rock cleavage, giving the rhombohedral cleavages 

an inclination of 37£° to the plane of the greatest dimensional axes and 

to the rock cleavage.

Adhesion. Adhesion of the particles to one another and to other 

particles is not far'different from the cohesion of the mineral along 

cleavage planes. In the most cleavable marbles the plane-of parting 

is a smooth one and shows but few calcite faces, which indicates that

the rock cleavage follows the greater diameters of the constituent particles. Cleavable marbles with a poor cleavage commonly show the 

calcite cleavage faces on the rock cleavage surface.

Abundance. Calcite, when present at all, is likely to make up a 

considerable proportion of a rock, where in subordinate amount there 

is a strong tendency to be concentrated into layers.

The total effect of calcite in producing rock cleavage measured by 

the importance of the above factors may be considerable in the cleavable marbles (see PI. XVI). So far as observed the effect is due 

entirely to the shape and parallel arrangement. It must be remembered, however, that cleavable marbles are rare when compared with 

the abundance of noncleavable marbles, which have undoubtedly been 

under conditions of rock ffowage.

aSp. 5678,'Wausau, Wis., cf. Weidman.
USITH.] EFFECT OF MINEEALS ON FLOW CLEAVAGE. 59

CHLOKITE.

\i

In general the same statements can be made concerning chlorite that 

were made above for mica. It has much the same shape. The parallel structure is not so common as in mica, but locally it is quite as good. 

Chlorite is not classed with the micas because in the typical rocks with 

secondary cleavage it is certainly far less abundant.

KAOLIN AND TALC.

Talc has essentially the same effect on cleavage as chlorite, but is a 

rare constituent in the rocks with secondary cleavage as a whole.

TREMOLITE, ACTINOLITE, GRtJNERITE, AND'OTHER AMPHIBOLES.

Shape and arrangement. The particles are commonly greatly elongated. The ratio of the length to the breadth is great. The crystals

lie for the most part in random directions, but in some cases are parallel to a plane, and perhaps to a line. , Kosettes or sheaf-like aggregates also are c'omraon, and the individuals of these aggregates may 

have a greater development in the plane .of rock cleavage than normal 

to it.

Thickness. The rosettes are sometimes a foot in diameter, while 

there is every gradation from this to submicroscopic size. Commonly 

the crystals are very thin.

Cohesion. Where the particles are at all definitely arrange'd, the 

inclination of the planes of the prismatic cleavages to the plane of the 

greater diameters is not known; the particles are for the most part too 

fine to determine this. From the general arrangement of the individual 

particles it may be stated that it is probable that in general there is 

no uniformity in this inclination, but that in certain cases, from analogy with the hornblendes, the acute angles of the prismatic cleavage 

lie in the plane of rock cleavage, in which cases the planes of prismatic cleavages lie at angles of 2Y° to the plane of rock cleavage. 

The effect of this factor in the production of the rock cleavage must, 

however, be practicably negligible because of the imperfect dimensional arrangement of the particles.

Adhesion. Concerning the adhesion of the particles, it can be said 

that cleavage surfaces of rocks containing them commonly show the 

particles on the surface. It is presumable that most of them appear 

with the normal outlines of the particles, but some of them undoubtedly present the cleavage surfaces of the minerals.

Abundance. The minerals are not abundant in the rocks with 

secondary cleavage as a whole. They are, however, often found in the 

derivatives of calcareous and sideritic rocks. In such rocks they 

show a tendency toward segregation into layers. This is shown in 

some cases by the greater development of the rays of a rosette or
60 BOCK CLEAVAGE. X [BULL.239.

. sheaf-like aggregate in certain planes. Their concentration into layers 

affords a poor parting.

The total effect of these minerals in the production of cleavage is 

relatively slight. In some comparatively few instances their parallelism and aggregation are such that a poor parting is produced; but their 

common occurrence is in blades which interlock to such an extent 

that parallel planes of parting are not possible, and indeed in places 

the minerals by their interlocking prevent the parting which the other 

constituents of a rock have a tendency to condition.

TOURMALINE.

, Shape and arrangement. The crystals are elongated in the prism 

zone. The ratio between the longer and the shorter diameters varies 

considerably, a number of measurements of different specimens showing a maximum of 100:20 and a minimum of 100:60. The elongated 

particles have a marked tendency to lie parallel to the plane of rock 

cleavage. In this plane there is also to be observed at times a tendency for the crystals to lie with their longer dimensions parallel.

Thickness. The absolute thickness, of course, varies greatly. The 

crystals measured vary from 0.1 to 1 cm. in thickness.

Cohesion. The particles themselves have practically no cleavage.

Adhesion. In breaking a cleavable rock containing tourmaline, the 

parting commonly occurs around the tourmaline individuals. The 

adhesion with the other particles with which it is associated is thus 

slight.

Abundance. Tourmaline, even where most abundant, does not form 

any considerable proportion of a rock with secondary cleavage, and 

occupies no large area in the plane of possible parting. It also shows 

little or no tendency toward concentration into layers,

The total effect of the tourmaline on the production of cleavage is 

v slight. Such as it is, it is due to its shape and arrangement and weak 

adhesion to the adjacent particles. It can produce but few planes of 

parting, and these planes are very uneven. It may aid other minerals 

in producing cleavage by preventing fracturing across its longer 

dimensions.

STAUROLITE.

Shape and arrangement. Staurolite occurs as tabular individuals, frequently with cross twins. The crystals have a strong tendency to lie with their tabular development approximately parallel to 

the plane of rock cleavage, although there are many exceptions to 

this.

 Thickness. The absolute thickness varies widely, from submicroscopic to 2 cm. or more.
EFFECT OF MINERALS ON FLOW CLEAVAGE. 61

Cohesion. The staurolite cleavage is so poor that parting is seldom 

or never observed to occur parallel to it.

Adhesion. The adhesion of the particles to the adjacent minerals is 

slight, although probably greater than that of garnet or tourmaline. 

Oh cleavage surfaces of rocks the staurolites are likely to appear with 

their natural crystal surfaces.

Abundance. Staurolite cr3*stals, even where most abundant, occupy 

but a small proportion of the area of possible parting:. They are 

seldom or never concentrated into layers.

Staurolite has practically no effect in the production of cleavage. 

Such as it has is due to its tabular development and low adhesion and 

its tendency to lie with its tabular development in the plane of rock 

cleavage.

- OTTRELITE.

Shape and arrangement. As shown on pp. 44-45, ottrelite occurs in

a variety of forms, but only one shows any parallel arrangement in 

rocks with secondary cleavage. This appears in flat crystals with a 

ratio of the greatest to the least diameters varying from 100:50 to 

100:35.

Thickness. The absolute thickness of the crystals measured varies 

from 0.24 to 0.90 mm. averaging perhaps 0.45 mm.

Cohesion. The cleavage of the particles is basal, and corresponds 

with the longer diameters of the particles. Hence, so far as the ciystals have any arrangement, the mineral cleavage is parallel to the 

plane of rock cleavage.

Adhesion. The adhesion of the particles to other particles is greater 

than its own interior cohesion or the cohesion of any of the other 

minerals with which it is associated except, perhaps, mica. The brilliant basal cleavage is all that usually appears on a surface of rock 

cleavage.

Abundance. Ottrelite crystals are not abundant, and show no concentration into layers.

The total effect of the ottrelite in the production of cleavage is practically nothing.

ANDALUSTTE.

Shape and arrangement. These crystals are greatly elongated in 

the prism zone. The ratio of length to thickness was found fr.om a 

number of measurements to average about 100:10. The crystals run 

in everjr direction through the rock, but there is a greater tendency 

for them to lie parallel to the plane of rock cleavage than transverse 

to it.

Thickness. The absolute thickness in the specimens measured is 

small, 0.15 mm. being, perhaps, an average.
62 BOOK CLEAVAGE. ' [BULL. 239.

Cohesion. The cleavage of the particles themselves is poor, and the 

rock cleavage is seldom seen to follow the mineral cleavage.

Adhesion. The adhesion of the particles to the other minerals with 

which the}T are associated is usually slight, as shown by the fact that 

the crystals commonly appear on the cleavage surfaces of the rock containing them.

Abundance. When most abundant andalusite makes up but a small 

proportion of the rock, and shows but a slight tendency toward concentration into layers.

Compared with the other subordinate cleavage-producing minerals 

andalusite may have considerable effect in the production of cleavage 

on account of its shape, parallelism to a plane, and low adhesion. It 

has also a negative effect in preventing cross fracture. Its importance is indicated by the number'of crystals usually appearing on a 

cleavage surface of the rock containing this mineral.

SILLIMANITE.

Shape and arrangement. This mineral occurs as slender acicular 

crystals, which are sometimes almost hairlike. Average measurements 

are difficult to obtain as the crystals are characteristically broken 

across, and it is not possible to determine the limits of the individuals. 

There is a strong tendency to parallelism of their longer dimensions, 

either in individuals or in sheaf-like bunches or aggregates.

Thickness. The absolute thickness is very small, as is indicated 

above.

Cohesion. The mineral cleavage parallel to the brachypinacoid is 

perfect; but the crystals are not flat, and this cleavage probably does 

not have any uniform relation to the plane of rock cleavage. The 

crystals are so fine that no direct observations of this relation have 

been made.

Adhesion. The adhesion of the particles to other particles is small, 

as is shown by the fact that the sillimanite commonly appears on the 

fracture surface of the rock with secondary cleavage in which it occurs.

Abundance. Sillimanite is not abundant, and forms no considerable 

proportion of the rocks in which it is present. It is, however, concentrated into layers to a, considerable extent.

When the mineral is present in a rock with secondary cleavage, its 

effect on the parting may be considerable. The effect comes from 

its shape, parallelism, poor adhesion, and its concentration into layers.

GAKNET.

Garnet is isotropic, has no cleavage, and has but a slight tendency 

to concentration into layers; hence the effect on the production of 

cleavage is practically nothing.
LKITH.] EFFECT OF MINEEALS ON FLOW CLEAVAGE. 63

SUMMARY STATEMENT OF EFFECT OF PRINCIPAL CLEAVAGE-GIVING MINERALS (MICA, HORNBLENDE, QUARTZ, FELDSPAR) ON FLOW CLEAVAGE.

Shape and arrangement. With reference to the inequality of the 

dimensions of the fragments the micas stand easily first, hornblende 

second, quartz third, and the feldspars last. Between the micas and 

hornblendes the difference is wide; between hornblende and quartz it 

is still greater; between quartz and feldspar"it is little.

The degree of parallel arrangement varies with the inequalit}^ of 

the axes of the fragments; the micas have the best arrangement, while 

the feldspars have the poorest.

The parallel unequiaxial particles of cleavable rocks tend to cause 

the best cleavage parallel to the plane of the greatest and mean 

diameters. In mica there.is no uniform difference between the mean 

and greatest diameters, and hence the mineral affords no other cleavage. In hornblende -there is but little disparity between the least 

and mean diameters, xand hence the greatest and least diameters give 

a cleavage almost as good as that afforded by the greatest and mean 

diameters. There is an axis rather than a plane of cleavage. In 

quartz and feldspar the difference between the greatest anql mean 

diameters is usually slight, but when arranged there may be a tendency 

to produce other cleavages parallel to the plane of the greatest and 

least diameters (side)" and rarely and doubtfully even parallel to the 

plane of the least and mean diameters (end).

It is to be remembered that all of these minerals may exceptionally 

occur as porphyritic constituents in rocks with secondary cleavage, in 

which case there may be no dimensional arrangement.

Thickness. With reference to the absolute thickness, and hence the 

number of possible planes of parting in a given thickness of rock, the 

order on the average is mica, hornblende, quartz, and feldspar, although 

local conditions may change this order.

Cohesion. In the effect of its own mineral cleavage one group of 

minerals the micas (and chlorite, kaolin, and talc) stands preeminent. 

Not only is the cleavage of the micas exceedingly good along one plane, 

but this plane corresponds exactly with the plane of the greatest 

dimensional development of the crystals. The mineral cleavage and 

the dimensional arrangement work together. Next in importance is 

the hornblende cleavage. The hornblende cleavage aids the dimensional arrangement of this mineral in producing the linear-parallel 

type of parting. The influence exerted by the feldspar cleavage is 

relatively slight. Of the two feldspar cleavages one is commonly ' 

parallel to the plane of the greatest dimensions, and the other, the best, 

normal to it. This latter is a conspicuous feature. Feldspar is thereoSuch cleavage has been called "false cleavage" by Sharpe.
64 BOOK CLEAVAGE [BULL. 239.

fore unique among cleavage-producing minerals in affording two 

cleavages at right angles to each other.

Adhesion. In general there is great adhesion between the mineral 

particles, as evidenced by the fact that the minerals appearing on the 

rock cleavage surface are likely to exhibit their mineral cleavage 

planes, indicating that parting has occurred within the micas, the 

hornblende, or the feldspars. Less commonly it occurs between the 

hornblende and adjacent minerals, or between the quartz and feldspar 

and adjacent minerals.

Abundance. As to abundance, no comparative statement can be 

made. The minerals vary widely in proportion in different rocks. 

Quartz and the feldspars, however, in general probably make up a 

greater bulk of the rocks with secondary cleavage than do the hornblendes or the micas. As to aggregation into layers, particularly in 

the coarser cleavable rocks, the micas are again far in the lead, with 

hornblende and quartz next, and the feldspars probably last. This 

gives a better parting but fewer planes of parting in a given thickness of rock. The tendency of all minerals toward concentration into 

layers rather than toward even distribution is the characteristic feature for the coarser cleavable rocks. For the slates the grain is so 

fine that this concentration can not be observed.

Mica, then, stands easily first as a cleavage producer. It is more 

important probably than all the other minerals together, because of 

the parallelism of its perfect mineral cleavage with the rock cleavage 

afforded by its greater dimensional arrangement, a condition found in 

no other mineral. Its cleavage is parallel to a plane. Hornblende 

stands next in importance, and locally may produce a ccleavage nearly 

as good as that made by the micas. Its effect is to produce either a 

plane-parallel or a linear-parallel type of cleavage, or some combination of these. Quartz and the feldspars are relatively unimportant 

compared with the first two minerals. Quartz probably has a greater 

effect than feldspar, due to its better dimensional arrangement. Feldspar, however, when arranged by recrystallization, exerts through its 

own cleavage a unique influence in producing cross rock cleavage.
CHAPTER III.

THE PROCESSES THROUGH WHICH THE SECONDARY PARALLEL 

ARRANGEMENT OF MINERALS IS BROUGHT ABOUT.

The production of secondary parallel structures has been observed 

to be associated with the deformation of rocks. A consideration of 

the factors of rock deformation in general will enable us to see more 

clearly the nature of the particular kind of deformation, which is 

accompanied by the production of parallel structures.

As has been shown by observation and experiment the principal 

factor in the deformation of rocks is differential pressure. The statement is almost axiomatic. For the present purpose, it is immaterial 

whether the pressure* be produced vertically by the superincumbent 

weight of strata, horizontally by the lateral thrust due to the crustal 

shortening of the earth, by the intrusion of igneous rocks, by increase 

of temperature, or by any combination of these or other causes. Other 

factors affecting the manner of rock deformation are the physical and 

chemical properties of the constituent minerals, the original arrangement of the minerals, and the conditions of temperature, moisture, 

and time under which rocks are deformed. According to the relative 

importance of these factors, rocks yield to pressure in different ways. 

The adjustment may be accomplished by coarse fracturing and differrential movement; by minute slicing; by fine fracturing extending 

only through the individual mineral particles of the rock, called granulation; by chemical change or redistribution, called crystallization 

or recrystallization; by the molecular-mechanical change of gliding 

along crystal planes; by parallel rotation of its constituent parts, without either molecular change or fracturing; or by any combination of 

these methods. Where the rock has permanently changed its form 

without losing its integrity, the molecular attraction being strong 

enough to hold the particles together, it is said to have flowed. All 

of the processes through which the rock may change its form may be 

effective in rock flowage, the essential limitation being that fracturing 

must not be so extensive or so preponderate over the other processes 

as to disintegrate the rock.

One of the common results of "rock flowage" is a parallel arrangement of the mineral constituents of the rock, giving it a "flow" cleavBull. 239 05 5 " 65
66 BOOK CLEAVAGE. [BULL.239.

age. So common is such an arrangement in rocks which have been 

deformed by "flowage" that the existence of a parallel arrangement 

is commonly accepted as proof that a rock has flowed. "For those not 

accepting this as proof, however, there is other evidence as is noted on 

pp. 102-108. '

Where rock deformation occurs by slicing a "fracture cleavage" 

may be developed independent of any parallelism of mineral constituents. This occurs through the development of incipient fractures 

or the welding or cementation of parallel fractures, yielding parallel 

planes of weakness.

The conditions under which fractures are prominent may be observed 

near the outer part of the lithosphere. The conditions under which 

fractures are less extensive or subordinate to other processes of 

deformation are believed to exist at greater depth. Hence for convenience in discussion, the outer part of the lithosphere- has been 

designated by Van Hise a the zone of fracture, while the deeper zone 

where rocks are deformed without fracture more extensive than granulation, has been designated the zone of rock flowage. It is readily 

understood that there is no sharp and regular plane of demarcation 

between these zones, but everywhere gradation and irregularity. For 

some rocks the zone of fracture extends to great depth; for others 

the zone of flowage extends to the surface. Local physical conditions may also cause these variations for the same rock. An attempt 

has been made by Van Hise to determine, on a basis of observed fact 

and mathematical deduction, the approximate depth to which the zone 

of fracture may extend, the result obtained being about 12,000 meters 

for the hardest rocks. Below this depth fractures probably do not 

exist; the rock flows when deformed. Thus flow cleavage, as a result 

of rock flowage, is essentially a phenomenon of the deep-seated zone 

of flowage. It appears at the surface through the removal of the

overlying rock by erosion.

In this chapter flow cleavage will be discussed. The consideration 

of fracture cleavage is deferred to Part II.

SECTION I. GENERAL ACCOUNT OF POSSIBLE PROCESSES OF 

ROCK FLOWAGE YIELDING PARALLEL ARRANGEMENT.

It follows from the foregoing that the processes resulting in the 

change of form in the rock during flowage must also be the ones which 

in some way result in bringing about the parallel arrangement of the 

mineral constituents; These processes in their relation to parallel 

arrangement are:

(1) Crystallization and recrystallization, resulting in flattening of

a Van Hise, C. R., Principles of North American pre-Cambrian geology: Sixteenth Ann. Kept. U. S. 

Geol. Survey, pt. 1,1896, pp. 589-603.
LEITH.] PEOCES'SES OF MINEEAL ABEANGEMENT. 67

old mineral particles and growth of new ones in planes of easiest relief. 

These processes might conceivably also result in segregation of minerals of the same kind into bands, with or without parallel arrangement of . the constituent particles, and might thus afford a parallel 

parting due to differing tenacity in or between different bands.

(2) Gliding along definite planes in minerals, resulting in flattening 

of the mineral particles. Gliding, ciystallization, and recrystallization' 

are the only processes which have been suggested to explain the flattening of mineral particles without fracture.

(3) Rotation of particles, of whatever origin, toward a parallel dimensional arrangement.

(4) Granulation without rotation may produce a parallel arrangemen.t of the mineral constituents by leaving residual parallel grains 

and by breaking the particles into slices which have a rude parallelism. Through the crystallization of other constituents,'or through

the cementing of such fractures by infiltration of mineral material, or 

through the uniting of the parts due to molecular attraction under 

pressure, the rock majr retain its integrity.

The nature of each of these possible processes will be considered, 

after which will be taken up their relative importance in the arrangement of individual minerals and in the arrangement of the particles of 

the inetaclasic rocks as a group.

CRYSTALLIZATION AND KECRYSTALLIZATION.

Crystallization and recrystallizaiion include the growth of old crystals, fortunately orientated, in the plane of easiest relief, and the production of entirely new crystals with their greater dimensions in the 

same plane.

Attention has been paid to this subject by many investigators, but 

the latest and fullest discussion, especially in application to the development of secondaiy rock cleavage, has been made by Van Hise,ft whose 

work is briefly summarized below.

Crystallization and recrystallization are brought about by solution 

and deposition of minerals principally through the agency of water.

Water has long been known to have the power of taking minute 

quantities of mineral matter, including silicates, into solution. At 

ordinary temperature this process is exceedingly slow, yet it occurs 

to an appreciable extent; at higher temperatures it takes place much 

more rapidly. Barus has shown 180° C. to be the critical temperature 

for glass. At higher temperatures glass is dissolved in water very 

rapidly from 180° to 220° C. it remains in a colloidal state, and 

beyond 220° it is in clear solution. In-lowering the temperature the 

glass goes through a reverse series of changes. Temperatures as high

« Van Hise, C. E., Bull. Geol. Soc. America, vol. 9, 1898, pp. 269-328; Mon. U. S. Geol. Survey, 

vol. 47,1904.
68 BOOK CLEAVAGE. [BULL. 239.

as 180° C..exist in the crust of the earth at a depth of about 5,000 

meters, with the normal temperature gradient, though in regions of 

volcanic or dynamic action this temperature may exist near or at the 

surface. Barus has shown the solution of glass to be accompanied by 

a diminution of the volume of the glass-water system, and probably also 

by the development of heat. Vice versa, the deposition of glass from 

the glass-water system is accompanied by increase in volume of the 

glass-water system and by absorption of heat. Pressure and temperature therefore have an important influence on the solution and deposition of the glass. If refractory glass behaves in this way, minerals 

in rocks are likely to yield to similar changes even more readily.

All rocks contain water, in supercapillary, capillary, and subcapillary openings. Under varying conditions of high temperature and 

pressure of the lower zone of flowage now under discussion this water 

must be active in taking mineral material into solution and depositing 

it. The water takes the material from the mineral particles and deposits 

it in an unstrained crystal form, either on the parts of the same particle 

or elsewhere. There is probably no extensive migration of material, 

for the openings are minute. If an entirely new mineral or ciystal is 

developed the process is called crystallization; if an old particle is 

regenerated or changed in form the process is called recrystallization. 

However, there is no sharp line between the two. The material for 

crystallization may come from minerals close at hand, and thus, so far 

as the rock as a whole is concerned, the process is really recrystallization. In the following the two terms are separated when practicable, 

but in many cases it seems advisable to use the term recrystallization to 

cover the general process of molecular change for the rock as a whole.

Pressure is the dominant factor. If the pressure on a solution near 

the solubility temperature of the solute increases, solution takes place; 

if it decreases, deposition occurs. In a rock undergoing deformation, 

pressure on capillary and subcapillary filaments of water probably 

varies from time to time and from place to place. It must vary in 

different-sized openings. This ma}^ result in crowding the water out 

of the places of great pressure and concentrating it in places of less 

pressure. If near the critical temperature, deposition may be supposed to take place wherever the lessening pressure, causing supersaturation, allows it. ' .

On the other hand, with the pressure near the critical point, changes 

of temperature will cause solution and deposition. A rise of temperature may enable the water to take more material into solution,-while 

a fall of temperature may cause supersaturation Of the solution a'nd 

consequent deposition. Different parts of the same rock mass may be 

at different temperatures, and solutions moving from place to place 

almost certainly find conditions of temperature favorable for deposit-
r.EiTir.] PEOOESSES. OF MINEEAL ARRANGEMENT. v_ 89

ing their load; also, the temperature at the same point may vary 

from time to time.

A third factor in influencing deposition is the selective influence of 

the solid mineral particles with which a solution conies in contact. 

There is a tendency for minerals to select from solution material like 

that of which they are composed and add it to themselves in ciystallographic continuity. Of course,.some minerals, under given conditions, have much greater power in this way than others; guch'are those 

favored by possessing a mineral habit, cleavage, or density best 

adapted to the conditions of existing pressure. Certain particles 

fortunately oriented may grow or survive, while adjacent particles, 

not.so .oriented, may be destro}7 ed.

Another factor affecting the solution and deposition of mineral 

material is the physical condition of the mineral particles. If the 

minerals with which water is in contact are in a state of strain, 

and thus have energy potentialize.d in them, solution is known to 

be greatly accelerated. As the mineral material after deposition 

is not in a state of strain1 the process of recrystallization obliterates 

evidences of strain. If a strained condition of the minerals accelerates solution a condition of no strain in the mineral in contact with 

the solution does not accelerate solution, and thus may be of negative 

assistance to deposition. The strain. effects present in a mineral are 

probably dependent largely on the position of the mineral crystal 

with reference to pressure, minerals unfavorably situated being most 

strained and hence likely to be the first to go into solution.

With the proper combination of the above factors old mineral particles grow and new particles develop with their greater diameters in 

the directions of relief from pressure. The relations of these directions of relief to pressure are discussed on pages 109-118.

The process of molecular change by crystallization or recrystallization is thus one of solution and deposition of mineral material by the 

water contained in rocks. In its accomplishment much or little water 

may be used, but an extremely small amount only is necessary. Also 

the amount of material in solution may be much or little, but it is 

usually so small compared with the adjacent solid minerals that the 

rock throughout the process of its change of form by recrystallization 

is essentially a solid. Indeed, there is evidence that changes between 

solid particles may occur, tb a yet unknown extent, without the intervention of water, but the important process is believed to be as above 

described.

Crystallization or recrystallization under rock deformation tends to 

result in the condensation of chemical systems that is, the minerals so 

developed during rock flowage usually have a higher specific gravity 

than the average of the minerals from which they are derived.
70 EOCK CLEAVAGE. . [BULL.239.

Thus far Van Rise's discussion has been followed in the main. 

Below are discussed certain additional features of the subject which 

have appeared during the investigation here reported.

Criteria for determining crystallization or recrystallization. Criteria for determining the probability of crystallization or recrystallization of a given mineral during rock flowage may in part be deduced 

from what has been said on the principles of recrystallization, but in 

large part they have been brought to mind by a study of individual 

minerals during this and other investigations. They are as follows:

(1) The conditions of depth, temperature, moisture, or time under 

which the rock is deformed. Often important evidence can be obtained 

which will show this, and thus determine the presumptive importance 

of recrystallization. The presence of water is' usually an essential 

condition.

(2) The nature of the rock itself as showing the probability of its 

yielding by recrystallization, rotation, or fracture.

(3) The mineral associations and the character of the mineral itself. 

Not infrequently the mineral is known to be of a secondarj^ nature from 

its characteristics of color, zonal growth, etc.

(4) Presence of minerals in a metaclasic rock which were not present in the rock from which the metaclasic rock can be shown to have 

been derived.

(5) The occurrence of minerals with their greater diameters at 

right angles to-an original structure such as bedding which does 

not show sufficient deformation to warrant the assumption that the 

minerals at right angles to the bedding reached their position through 

rotation. Such minerals could not have been deposited in this position; 

they could not have been rotated to this position; development in situ 

is the only alternative.

(6) The shape of mineral particles in the rocks with secondary cleavage, as compared with the shape of those in original igneous rocks.

(7) The manner of contact of the minerals. In the micas, for instance, 

the laminae of one crystal feather out diagonally against those of 

another, giving close diagonal contact, which could be produced only 

by molecular adjustment.

(8) The size of the particles. Crystallization or recrystallization 

has a tendency to increase the size of the grain (see pp. 76-78). If the 

grain is coarser than seems necessary for the amount of deformation 

the rock has undergone, or coarser than in the original rock, crystallization or recrystallization, the only constructive processes known, 

must be the cause. This evidence is decisive in many cases, as for 

instance, in a coarse schist derived from a mud stone.

(9) Better than the size of grain is the evenness of grain. It is 

shown (pp. 71-78) that recrystallization combined with granulation 

has a marked tendency to produce an even grain.
PEOCES8ES OF MINERAL ARRANGEMENT. 71

(10) The lack of strain effects commensurate, with the deformation 

shown by the rock as a whole or by adjacent minerals. When strain 

effects are present in most minerals they are conspicuous; when absent 

or slight this is easily determined.

(11) The complete parallelism of minerals, or, what amounts to the 

same thing, the lack of bending or breaking due to interference of 

particles, which would necessarily be in evidence if the particles had 

been much rotated.

(12) Recrystallization has a tendency to segregate minerals of the 

same kind into bands, in this showing great similarity to original flow 

structure.

Crystallization or recrystallization accompanied l)y rotation. Crystallization or recrystallizatipn is usually accompanied by rotation of 

the particles, as will be shown in the discussion of the relations of parallel structure to pressure (pp. 109-118).

So-called flattening of particles in situ largely a process of recrystallization. It is believed that the so-called "flattening of particles in 

situ," so frequently cited, occurs in most cases through the process of 

recrystallization above described. But gliding along mineral cleavage 

planes may be partially responsible in some cases.

BOTATION.

The part which rotation plays in producing a parallel arrangement of 

mineral constituents during deformation is most difficult to determine.

Effectiveness of rotation in bringing about an approach to parallelism of mineral constituents in rocks with secondary cleavage may 

depend upon the'conditions*of temperature, moisture, pressure, and 

speed under which the rock is deformed and upon the characteristics 

of the particles themselves their size, shape, and manner of aggregation. The interrelations of these two factors, determining the rotation 

of a mineral particle, are exceedingly complex.

When differential pressure is applied to the complex mass of particles forming a rock, and the rock flows, there is a tendency to a rotation of all particles which are not already arranged with the.best 

possible attitude toward the pressure there acting i. e., on all particles 

on which couples are effective under the given stress conditions. This 

applies to all particles, whether original, recrystallized, or recrystallizing.

If there is freedom of movement of the particles, sufficient rotation 

is likely to cause approach to parallelism. In a soft, plastic clay which 

is deformed at the surface under conditions where recrystallization is 

not possible the particles have such freedom of movement that approximate parallelism of the particles may be reached, but this requires 

a very considerable amount of shortening of the rock mass. Also in 

the deep-seated zone where rock deformation is brought about mainly
72 BOCK CLEAVAGE. [BULL. 239.

by recrystallization, considerable freedom of movement may be allowed 

certain particles by the process of recrystallization affecting the particles of the rock unevenly. In an ideal case where the particles have 

no interference the degree of parallelism reached by a certain amount 

of rotation may be indicated in figs. 20 and 21, taken from Harker. If 

a circle with a definite number of radii representing the greater diameters of original particles be compressed into, an ellipsoid with its 

shorter diameter one-fourth the length of the diameter of the circle, 

the radii making an angle of less .than, say, 10°, with the plane of 

greatest elongation will be twelve times the number within the same 

angle in the undistorted form.

The shape of the minerals also is likely to have some effect on the 

freedom of movement. One would suppose that in a complex of long 

narrow crystals there would be more interference in rotation than in

a complex of short, stumpy 

crystals. If this is the case 

the crystal best adapted to a 

parallel arrangement under 

differential pressure that is, 

one with great differences in 

dimensions is not adapted 

for rotation into such parallel arrangement. Suppose, 

for instance, that the long, 

slender hornblende crystals in 

a rock had random arrange- 

°ment, and the attempt were 

made to rotate them toward 

Jn T% . u . Flo' a1' a common plane or line. It FIGS. 20 and 21. Diagrams showing the rotation accom- L 

panying a given amount of shortening. After Barker, may D6 dearly seen that long 

Kept. 55th Meeting Brit. ASSOC. Adv. sci., 1885, P. 822. before approximate parallelism is reached the crystals will mutually interfere, and then 

because of unequal transverse support they are likely to be broken in 

their further movement. This fact of necessary interference is dwelt 

upon here because later it will be shown that many long, slender 

crystals which now lie parallel in schists could not have been rotated 

to this position because they do not show such interference effects.: 

Where the slender crystals are few, or are so arranged that the}7 will 

not interfere, rotation may be effective in bringing about their parallelism, as has been shown during this investigation.

The size of the particles may also have an effect on the degree of 

parallelism reached by rotation. With a given amount of deformation under certain conditions the smaller the particles affected the more 

will they be rotated.. Figs. 22, 23, 24, and 25 illustrate this.

If a particle with the diameter X Y is rolled between two plates P P',
LBITH.] PROCESSES OF MINERAL ARRANGEMENT. 73

until the one plate is past the-other to a distance S S', the diameter 

X Y, which was originally normal to the plates, as shown in fig. 22, will 

have the position shown in fig. 23. Now roll a particle of smaller size 

with the diameter X' Y' (fig. 24) between two plates until the upper plate 

is past the lower plate the same amount as in fig. 23. In this case the 

diameter X' Y' of the smaller particle will rotate to a much greater 

angle with its original position than did the diameter of the larger 

particle. If the circumference of the first particle is 2 inches and 

that of the second 1 inch, a movement of oneTfourth of an inch

 p'

Y

Fig. 22. Fig. 23.

FiGS. 22 and 23. Diagrams showing angle of rotation of large particle under differential movement

amounting to SS'.

will revolve the axis of the large particle 45°. A similar movement 

will revolve the axis of the smaller particle 90°. Therefore the amount 

of revolution which a particle undergoes in a given amount of movement is inversely as its circumference. This has been shown experimentally in clay mixed with particles of mica. With a given amount 

of movement it has been shown that the finer the particles of mica so 

mixed the more nearly parallel they are. Granulation produces, 

smaller particles and hence may aid rotation in bringing particles into 

parallelism.

It is thus seen that rotation is likely to be effective so far as there 

is freedom of movement, so far as the particles are eq nidi men sional,

Y' s'

 p'

Y' pFig. 24. . Fig. 25.

FIGS. 24 and 25. Diagrams showing angle of rotation of small particle under differential movement

amounting to SS',

 and so far as they are small. It will be shown (pp. 109-118) that 

rotatory stresses are always present in the strains commonly occurring 

in rocks, and that all particles, whatever be their origin, so far as they 

are not already in the best possible attitude toward pressure, or normal 

to this position, are likely to feel the effects of these strains.

For rotation the obvious criteria are as follows:

(1) The presence of strain effects, which are certainly more characteristic of rotation than of recrystallization. One of the most characteristic of these is angular outlines, indicating that the shape is due to
74 KOCK CLEAVAGE. [BULL. 239.

granulation. Where the particles are angular and clearly due to the 

granulation of larger particles it is certain that any parallelism of 

their longer diameters that is observed in the rocks with secondary 

cleavage is due to rotation, as no other process is possible in such a 

case. In such rocks the granulation of larger particles can be observed 

in all its stages, and accompanying the granulation may be seen all 

stages of rotation of the fractured particles. These granulated particles are not markedly unequidimensional, and hence there is usually 

little interference. The occurrence of angular particles apparently 

due to granulation is one of the best criteria observed for determining 

that rotation has occurred. The lack of .angular character, however, 

is not evidence that rotation has not occurred.

(2) The absence of positive evidence of recrystallization. The 

reverse condition, the presence of evidence of recrystallization, does 

not show that rotation has not occurred.

GLIDING.

Certain crystals may change their form under pressure by differential movements along "glide" planes without open fractures." The 

glide planes frequently follow the mineral cleavage. After the movement the glide planes may be planes of secondary twinning. Movements 

with minute fracturing along twinning or cleavage planes of minerals 

have also been called gliding by some writers. They are considered in 

this paper under'' slicing." Gliding is accompanied by a minute change 

of form. For only a few minerals has this change of form been 

shown to have any considerable value. The best gliding observed is 

found in calcite, and the process will be described in connection with 

that mineral (pp. 88-90). - "

The principal and almost the only criterion for the detection of gliding 

is the presence of repeated twinning and slip planes, sometimes giving a 

reedy or fibrous structure. Both granulation, and recrystallization 

destroy these gliding planes, and if abundant evidence of either of 

these processes is present, they may hide the evidence of gliding. a 

For this reason the part gliding has played-in flattening and elongating 

mineral particles can not be determined certainly from a study of 

the metaclasic rocks themselves. Its relative importance may be 

argued only from the presence of evidence of crystallization adequate 

alone to explain the observed facts. Experiments by Professor 

Adams still in progress are likely to show to what extent gliding may 

be effectiye.

a Adams, F. D., Experimental investigation into the flow of marble: Philos. Trans. Royal Soc. 

London, ser. A, vol.'195,1901, pi. 24, fig. 4. Mugge, O., Ueber die Plasticitat der Eiskrystalle: Neues 

Jahrbuch fur Mineral., etc., vol. 2,1895, pp. 212-228.
PKOCESSES OF MTNEEAL AKK ANGEMENT. 75 

GRANULATION.

This process probably is most effective in its assistance to rotation 

and recrystalli.zat.ion (p. 76), but even where re-crystallization or rotation is practically absent, it may still produce a very imperfect parallel 

structure.

Granulation may leave residual flattened particles. When a rock is 

put .under pressure and fracturing- occurs, the fractures are usually 

highly inclined to the greatest pressure, as shown on p. 112. Probably where the material is not confined on the sides, the fractures for 

the most part have positions 'inclined less than 45° to the^ greatest 

pressure. However, where the material is not ideally brittle or is confined on all sides it has sometimes been observed that the fractures tend 

to take positions at angles greater than 45° to the greatest pressure, 

carving the grains into flattened cones or lenticular forms, with 

diamond-shaped cross sections. It may be that the flatness of the'

residual grains is in part really due to the minor recr}7 stallization or 

gliding of the particles and not to the direction of fracture. Such 

flattened particles, when formed by fracture without the aid of the 

processes above outlined, will have their greater dimensions parallel, 

and so this process must be mentioned as one of the possible ones in 

the production of parallel structures. It is simply one of the results 

of granulation.

A better parallel arrangement is developed where the granulation 

occurs mainly along one set of shearing planes rather than along two 

intersecting sets, taking off slices or minute granules from the side 

of the original particles and leaving the remnant of the particles as 

thin, ribbon-like fragments with parallel arrangement."

While granulation usually fractures particles in such a way that the 

resulting smaller particles have no great differences in dimensions, it 

has been observed that the material broken from the particles is sometimes in slices whose long dimensions follow the directions of fracture 

(figs. 15, 16, and. 17). The length of such slices is not infrequently 5 

or 6 times the thickness, although a°ratio of 3:1 is more common. At 

the time and place of breaking these particles may be arranged with 

their longer diameters at angles 45° or less to the general plane of 

rock cleavage. The special kind of granulation that results in parallel 

slices is for convenience of discussion called " slicing" in this report.

The phenomenon of slicing may be particularly well seen in a 

mineral which itself has a good cleavage to control the direction of 

slicing. In hornblende, for instance, slicing with little or no rotation 

has been observed to divide a large particle into a number of thin 

ones lying with their greater diameters at a low angle to the plane of

«See discussion of Adams's anorthosite, p. 37.
76 BOCK CLEAVAGE. [BULL.239.

schistosity (fig. 7, p. 30). Where slicing follows the mineral cleavage, 

stages of its gradation into gliding are to be observed.

Assistance of granulation to recrystallisation. Granulation forms 

small particles and brings about a state of strain in particles, both of 

which are favorable to recrystallization. .With a given mass the finer 

the subdivision the greater the surface. The finer the subdivision, 

therefore, the greater the reaction, ceteris paribus, between mineral 

and solvent. Also, the deformation leading up to granulation induces 

a state of strain in a mineral in which energy is potentialized. Minerals 

thus strained are attacked much more readily by solvents than minerals 

not so strained. A further and important assistance is rendered b}r 

the development of heat due to the mechanical crushing.

Assistance of granulation to rotation. Granulation aids rotation in 

two ways. It produces smaller particles and allows greater freedom 

of movement. So far as slicing occurs, the greater dimensions of the 

slices just after granulation may lie at angles not larger than 45° to 

the plane of rock cleavage, from which position a comparatively 

slight amount of rotation will carry their longer diameters into the 

plane of rock cleavage. Evidence of such rotation frequently accompanies slicing. In many observed instances the fractures are at even 

smaller angles to the plane of cleavage, and a small amount of rotation 

has induced a close approach to parallelism.

'THE COMBINED EFFECT OF KECRYSTALLIZATION AND GRANULATION ON

SIZE OF GRAIN.

As a matter of observation recrystallization tends to increase the 

size of the particles in a rock mass undergoing rock flowage; but to 

varying extents in particles of different mineral species. The increase 

in size comes from the fact that small particles present greater surface

per Unit volume for the action of solvents than larger particles, and 

the material of the smaller particles goes into solution and is ultimately added to the larger particles because of the "surface tension 

which exists on the boundary surfaces between solids and liquids, as 

on those between liquids and gases the so-called free surfaces of 

liquids. This tension acts so that the surfaces in question are reduced 

in size, with the consequent enlargement of individual crystals (the 

total amount of precipitate remaining practically unaltered), i. e., with 

the coarsening of the grains. " a Granulation tends to subdivide the 

particles of a rock mass undergoing rock flowage, but to varying 

extents with different minerals. Other things being equal, the larger 

particles of the rock are likely to feel more of the differential pressure, and to be granulated to a greater extent than the smaller ones. 

Whether acting separately or together, recrystallization and granulaaOstwald, W., The Scientific Foundations of Analytical' Chemistry, Macrnillan & Co., London, 

1895, p. 22.
LEITH.] SIZE OF GRAIN. 77

tion tend to cause a uniformity in the size of particles of the same 

mineral species in a rock, a fact of direct observation in cleavable 

rocks. But in a given rock the uniform size of the particles of one 

mineral species thus produced is not the same as that of another mineral species.

This difference in size of grain in different mineral species results 

not only from difference in mineral habit, but from the fact that 

recrystallization and granulation affect different minerals to different 

degrees. In some minerals, under given conditions, recrystallization 

.or granulation occurs, one almost to the exclusion of the other, and 

in other mineral particles both processes occur. The size of grain 

depends upon the balance obtained between the two; in Qther words, 

it is the net result of a contest between the constructive process of 

recrystallization, tending to produce larger grains at the expense of 

smaller ones, and the'destructive process of granulation tending to 

break down the larger particles. The recrystallization of micas and 

hornblende, without granulation, produces a characteristic uniformity 

of grain for each of these minerals. Granulation of quartz and feldspar particles, without recrystallization, produces a characteristic uniformity of size of grain for each. Granulation and recrystallization, 

acting together on quartz and feldspar, tend to cause uniformity of 

size of grain in each. As would be expected, the mineral particles 

showing evidence of recrystallization alone, such as mica and hornblende, are larger than the ones in which the process of granulation 

or of granulation and recrystallization combined have been effective, 

such as quartz and feldspar.

It should be remembered that rocks with secondary cleavage are 

 composed very largely of mica, hornblende, quartz, and feldspar, and 

for the most part of two or three of these minerals, and hence there 

are usually, as shown by observation and measurement, but two or 

three characteristic sizes of grains, giving, in connection with parallel 

arrangement, a striking impression of uniformity of grain, in marked 

contrast to the grain observed in many of the rocks before rock flowage has developed a cleavage in them.

After rock flowage has ceased it not infrequently happens that 

recrystallizatiou continues or again begins, and then the tendency of 

large grains to develop at the expense of small ones is not opposed by 

the tendency of granulation to break down the larger particles as they 

form. To this condition may be ascribed the porphyritic crystals 

frequently seen in cleavable rocks which have developed apparently 

without regard to the prevailing cleavage in the rock, and show 

included in them other constituents of the rock with a dimensional' 

arrangement parallel to the plane of rock cleavage (Pis. Ill and XIII). 

The principal cleavage-giving minerals mica, hornblende, quartz, and 

feldspar m&y be sometimes observed to develop under these con-
78 EOCK CLEAVAGE. .[BULL.239.

ditions. The minerals characteristic of cleavable rocks, but not having 

any great effect on rock cleavage such as garnet, tourmaline, chloritoid, andalusite, chiastolite, staurolite, etc. are known to develop 

commonly under such conditions and only rarely during rock flowage.

SECTION II. RELATIVE IMPORTANCE OF PROCESSES O,F 

ARRANGEMENT IN INDIVIDUAL MINERALS.

MICA.

N The origin and parallel arrangement of mica plates in the micaceous 

schists, slates, and phyllites are believed for the most part to be the 

result of crystallization or recrystallization, for the following reasons:

Micaceous schists are rocks which are known in many cases to have 

been in the zone of rock flowage, under conditions of high pressure and 

heat, and with a considerable water content; all of these are conditions 

favorable to the development of mica by crystallization or recrystallization. Mica certainly is not a characteristic mineral of the belt of 

weathering.

The micaceous schists and phyllites are very common products of the 

alteration of water-bearing feldspathic sandstones or graywackes and 

shales or muds, originally containing little mica, and the}^ are also the 

result of the metamorphism of various igneous rocks with or without 

mica as an original constituent.

The occurrence of mica in large flakes in schists derived from rocks 

originally containing small flakes or none at all is the most decisive 

evidence that can be offered of its development by recrystallization.

In certain cleavable rocks derived from sediments the mica plates 

can be seen to stand at right angles to the original bedding, and this 

original bedding may not show deformation sufficient to warrant the 

assumption that the micas now found at right angles to the bedding 

were originally parallel to the bedding and owe their present positions 

to rotation. They could not have been originally deposited in this 

position; they could not have been rotated; the alternative explanation 

is crystallization in situ/' (PI. XIV, A.)

The common association of the micas in cleavable rocks with secondary minerals, such as .garnet, staurolite, etc., well known to be 

secondary developments by new crystallization, is presumptive evidence that the micas themselves may have resulted from secondary 

crystallization.

When the original nature of the rock is in doubt, certain features 

to be observed in the micas of metaclasic rocks with secondary cleavage 

are evidences of recrystallization. Petrographers sometimes discriminate secondary mica from original mica by criteria of color, shape, or 

distribution. The micas commonly lack strain effects such as would be

a Sp. 14745, si. 9380, Little Falls, Minn.
PEOCESSES ARRANGING MICA. 79

expected in minerals of their shape if original and rotated to their 

present position. They have more uniformity in size and in general 

a larger size than mica particles in noncleavable rocks. Their manner 

of feathering out against each other precludes the idea of their rotation as original minerals to this position.

The micas may be found in layers, these layers alternating with 

quartz layers which have certainty been recrystallized. The association of the quartz and biotite andx the nature of the contact of the two 

minerals are good evidences of their new crystallization (PI. IX, JB).

Sericite is a common development in the peripheral granulation of 

quartz and feldspar, assuming a characteristic parallel position (fig. 

26). That the parallel position of the sericite is due to crystallization 

in situ is scarcely open to doubt.

Rotation of original grains and the retention of the arrangement 

of favorably orientated original micas probably enter also to a very 

limited extent into an explanation of the present arrangement of the 

micas. In a rock with secondary cleavage developed from a rock 

originally containing mica it is probable that 

some of the flakes now present were originally 

present, and have either, retained their original positions or have been rotated. The micas 

lying with their cleavage plates at a low angle to FIG. 26. The secondary dethe plane of rock cleavage would be the most veiopment of parallel serfr -° . . cite by crystallization m

likely to be rotated to this plane. Occasionally cracks formed by fracture. 

there may be observed the actual bending of < H- 10f Sericite-schist-

 / o . Taraus.)

'mica plates in a common direction, showing the 

direction of rotation. In the false cleavage described on pp. 25-26 the 

parallel mica plates may be seen in various stages of subsequent rotation along certain zones.

Two other processes may have a slight effect in arranging the micas, 

gliding and slicing that is, the differential movement between cleavage laminse without actual fracture and'similar differential movement 

with fracture. It is doubtful whether mica shows to any extent true 

gliding in the cleavable rocks, but it is certain that many mica plates 

owe their arrangement to the slicing of larger mica plates along cleavage planes and the strewing out of the slices along parallel planes. 

This differential movement or slipping of the mica plates has been 

frequently observed and described. It is to be noted, however, that 

this process is not likely to be effective unless the cleavage of the 

original mica particles is at low angles to the plane of rock cleavage.

The micas Avhich sometimes appear with their dimensional axes 

parallel to the prevalent cleavage of the rock, but with their mineral 

cleavage inclined to this plane (figs. 3, 4), may owe their unusual position to granulation or slicing which has not been controlled by the 

mica cleavage, much as the pyroxenes in the mashed anorthosite,
80 ' BOOK CLEAVAGE. [BULL.239.

described by Adams, a are strewn out by granulation in bands with the 

pyroxene cleavage at any angle to the rock cleavage. But there is 

good evidence that associated quartz in bands is itself recrystallized, 

and it seems probable therefore that recrystallization has affected the 

mica also.

As we are concerned mainly with parallel structures, it. is scarcely 

necessary to discuss the origin of the random mica particles developed 

with their own cleavage and longer diameters inclined to the prevalent 

cleavage of the rock mass (PI. III). It is easy to prove that such 

minerals are for the most part porphy ritic developments after the rock 

flowage developing the prevalent cleavage had ceased; the proof is on 

the same kind of evidence used to prove the later development of garnets, tourmaline, chloritoid, and other minerals of that class.

HORNBLENDE.

It is clear that newly crystallized or recrystallized hornblende particles, which are observed in schists, very greatly preponderate over 

original particles, and hence that the arrangement is due largely to 

recrystallization, for the following reasons:

Hornblendic schists are characteristic developments from rocks 

which have been in the zone of rock flowage under such conditions of 

great pressure and temperature as to afford favorable conditions for 

recrystallization.^

In many cases the hornblendic schists have resulted from the metamorphisrn of greenstones or'basic igneous and sedimentary rocks in 

which the hornblende as an original constituent was either wanting or

. ,<).' ;'' '. s , . ! to

in subordinate quantity. Numberless gradations from such original 

rocks to typical hornblendic schists have been noted in many regions

by many observers.

Where original and recrystallized hornblende occur together in a 

slide, the new hornblende may sometimes be discriminated by its 

lighter color or its occurrence as enlargement borders of the original 

hornblende.

Uniformity of size is characteristic of hornblende particles in cleavable rocks, and this has been shown to be an evidence of recrystallization. , r,,

Hornblende particles observed.in.rocks with secondary cleavage have 

a greater length relative to breadifti.pr thickness than the hornblendes 

observed in'original igneous roc&s/as shown by the measurements 

given on page 31. In the few'specimens measured the average length 

also was found, to be somewhat greater than that of original hornblendes; 

but many more measurements would be necessary to warrant a positive generalization on this point. The additional relative or absolute
LEITH.] PEOCESSES AEEANGING HOENBLENDE. 81

length is reached without fracturing, and recrystallization is the only 

process to which it can be attributed.

Hornblende may frequently be seen to have developed, through alteration of feldspar and other material. In the mashed rhyolite-gneiss 

from Berlin, Wis., it is developed in the tails of the granulated feldspars and does not itself show granulation. In these tails it is parallel 

to similar hornblende occurring in the groundmass.-

White hornblende crystals in cleavable rocks are frequently considably broken and have many irregularities in arrangement, in general 

they show a degree of parallelism in their different parts and to the 

plane of rock cleavage and freedom from strain effects which can not 

be expected in hornblende crystals owing their parallel arrangement 

to rotation alone. The natural inference is that both the arrangement and origin of the particles must be due to recrystallization. In 

a hypothetical case, if a rock in which are numerous slender hornblende crystals with random arrangement be deformed by rock flowage, 

the hornblende crystals interfere, and, because of unequal transverse 

support, break or bend at numerous places in order to accommodate 

themselves to the new conditions. They may become only imperfectly 

arranged parallel to a plane or line. This interference of the particles in a mass undergoing strain can be shown to be a mathematical 

necessity. In the rocks with secondary cleavage bending and breaking, due to interference, are subordinate phenomena, and hence it is 

concluded that0 crystallization has been important.

From the facts above cited it is concluded that recrystallization of 

hornblende under pressure has produced the greater part of the hornblende making up the hornblendic schists; hence that recrystallization 

is mainly responsible for the parallel arrangement, although doubtless it has been assisted somewhat by rotation.

So far as parallel hornblendes now present in rocks with secondary 

cleavage are original constituents (for the above reasons they are 

believed to be subordinate in quantity) it must be supposed that they 

were either in their present position before deformation commenced, 

or were in a favorable position for rotation to their present position, 

or are the result of slicing oi larger original hornblendes.

Slicing has certainly been instrumental in bringing about the arrangement of many of the hornblendes in hornblendic schists developed from 

original hornblendic rocks. a In a number of rocks have been observed 

all stages in the process of slicing which has followed the direction of 

the cleavage of the hornblende (fig. 7). Slices when first formed are 

likely to be at almost any angle except 90° to the plane of rock cleavage, 

but it is clear that the majority of them were already at angles less 

than 450 to the plane of rock cleavage before any rotation occurred.

<i Sp. 40(>85, si. 15360; sp. 40600, si. 15363, Mesabi district, Miim.

Bull. 239 05 0
82 BOCK CLEAVAGE. [BULL.239.

In slides of hornblende-schists there are freqently to be observed 

random hornblende crystals which lie with their columnar development at high angles to the plane of rock cleavage. These may be 

original or secondary developments with exceptional arrangement due 

to local variation in stresses; they may be original'crystals which have 

not been rotated during rock flowage, or they may be secondary 

developments after deformation has ceased.

QUAKTZ.

The quartz particles described under (1) page 31, as having modified 

crystal outlines and rounded subangular shapes show no direct evidence 

of granulation or recrystallization or gliding, and in the absence of 

such evidence, no opinion is offered as to the cause of their parallel 

arrangement, so far as they have any.

The quartz particles described under (2) pages 31-33, are clearly, both 

in shape and in arrangement, the result of granulation and slicing, with 

or without subsequent rotation. Evidence of this has already been 

presented and will not be repeated.

The quartzes in elongated lenses and spindle-shaped grains described 

under (3) pages 33-35, may in part develop by granulation or slicing 

in the manner described under (2) pages 31-33, but most of them, 

and especially those with more or less even and rounded outlines lack-, 

ing strain effects, are believed to owe their origin and .arrangement, 

or at least their final configuration, mainly to recrystallization, for the 

following reasons:

(1) Many rocks with secondary cleavage containing quartz in this 

form are -known to have contained considerable moisture and to have 

been in. the zone of rock flowage under conditions of severe rnetamorphism favorable to recrystallization. The alteration of muds and 

sands under such conditions has frequently }delded such forms.

(2) Where the quartz in this form is associated with secondary hornblende or mica in cleavable rocks, as it commonly is, and arranged with 

its longer diameters parallel to the longer diameters of these minerals, 

which are characteristic secondary minerals resulting from recrystallization, the presumption is that recrystallizing forces have also 

affected the quartz. Other common associates of quartz in this form 

are garnet, staurolite, tourmaline, ottrelite, etc., which are known to 

be new developments. The manner of association of quartz and mica 

is also in some cases evidence of recrystallization. The parallel mica 

plates may be observed to occur strictly parallel to the periphery of 

flattened quartz individuals, and the ends of the mica plates not infrequently project into clear, limpid quartz which has evidently grown 

around the mica. (PL IX, J., J5.)

(3) In the rnetaclasic rocks, which have evidently undergone much 

deformation, the elongated and spindle-shaped quartz forms described
LKITH.]_ PROCESSES ARRANGING QUARTZ. 83

under this head are commonly larger than the quartz grains in undeforuied rocks in the same rock mass. Rocks become deformed and 

cleavable in parts, and such parts may be coarser grained than the 

remainder of the formation. If an increasing size of grain in a formation is found to correspond with the greater metamorphism occurring at any place, it may be inferred that some constructive process 

has occurred to increase the size of the grains. Recrystallization is 

the only constructive process known.

In the Black Hills of South Dakota is a great series of rocks with 

secondary cleavage which have resulted from the metamorphism of 

fine-grained banded sediments. The size of the grain in these rocks 

varies \vith the amount of metamorphism, and in general not with the 

original character of the graywacke and slate, although in earlier 

stages the size of the recrystallized grains corresponds roughly with 

that of the original grains; it is possible to select a graded series of 

rocks resulting from the metamorphism of a graywacke slate in which 

the grain increases as the cleavage and metamorphism are more 

advanced. This is but one of many illustrations that might be cited.

(4) As important as the size of the grain is the evenness of grain in 

determining recrystallization. While different degrees of metamorphism are likely to be associated with different sizes of grain, rocks 

showing the extremes of metamorphism, the best cleavage, and abundant evidence of recrystallization have a curious uniformity of grain 

in particles of the same mineral species. There are no phenocrysts 

nor matrices. Quartz in the elongated lenses is seen to share in this 

uniformity of grain, affording confirmatory evidence that its development in such cases is by recrystallization.

(5) In a rock with secondary cleavage quartz particles are frequently 

seen to lie in narrow layers of rather uniform thickness composed of 

numerous narrow individuals lying side by side, as shown in fig. 4 

and Pis. VI1-X. The individuals are frequently joined by narrow 

zones of diffuse extinction, due to the inclination of the contact plane. 

The layers for a considerable distance, while extinguishing in various 

parts, are still continuous crystalline masses of quartz without mechanical breaks and without other strain effects commensurate with the 

deformation they must have undergone. It is believed that these 

features can be explained best by recrystallization. It is perfectly clear" 

that the long band as it now stands could not have been rotated to its 

present position as a whole. It is possible, and indeed probable, that 

some of the individual parts represent small quartzes or fracture 

pieces of quartz which have been rotated into approximate parallelism 

or carved in situ by granulation or slicing. However, the continuity 

and evenness of banding can not be explained in any such way. Granulation or slicing could scarcely yield such close fitting parts, and 

these processes would furthermore result in the development of
84 ROCK CLEAVAGE. [BULL.239.

minute granules strewn out in the plane of rock cleavage. Even 

if granulation and slicing could yield elongated individuals so peculiarly fitted together, they would be likely to taper off at the ends 

and give way to finety granulated material, as in the case of the granulated feldspar figured on p. 37, which shows the best parallel arrangement due entirely to granulation which has been observed in this 

investigation. The absence of associated granulated material with 

the quartz bands is evidence that either granulation has never occurred 

or if it has occurred that recrystallization has coalesced' the separate 

granules; and if this has happened, it is reasonable to assume that 

recrystallization has also affected the original residual particles.

The narrow quartz bands frequently resemble veins at first glance. 

If they were veins they would still be the result of crystallization. 

But close examination shows points in which they differ from veins. 

The individual quartzes in a vein do not have great elongation in the 

direction of rock cleavage; they form a mosaic. If a vein, there ought 

to be found discoloration of some sort due to weathering along the 

walls of the vein; this is uniformly absent, and even if present, it 

might still be explained as material pushed aside during the recrystallization of the quartz. No common conditions are known in which 

numerous veins would develop in this way parallel to the plane of 

rock'cleavage for long distances without breaking 'across it at places, 

although this might happen' exceptional^ in planes of cleavage along 

which minute jointing had occurred. Neither do veins show the 

evenness and parallelism shown by these bands. Lastly, the material 

of these bands in many cases is the only other material present in the 

rock besides the micas or feldspars. If the quartzes represent veins, 

we must suppose the rock before the vein action commenced to have 

been entirely composed of parallel mica plates or layers of feldspar,

which is not probable.

(6) In the case of the quartz-porphyry of the Thuringer Wald, so 

frequentty referred to, the shape of the quartzes is again indicative of 

recrystallization. The long curving tails on these quartzes, which 

show undulatory extinction and minor fracturing, but still retain 

their essential unity, could scarcely have resulted from any other 

process.

(7) Where the quartzes are seen to have cores of older quartz surrounded by rims of new quartz, usually divided by a ring of ferrite 

or other inclusions, therejs conclusive proof of the partial recrystallization or growth of the quartz. The old and the new quartz are in 

crystallographic continuity. The new quartz, while in optical continuity with the original quartz, is usually added only along directions 

of easiest relief.

Quartz is sometimes seen completely inclosing other minerals which 

tire original. AVhile this might supposedly happen before the final
l*f*H.J PROCESSES ARRANGING QtJARTZ. ° $5

solidification of the rock from a magma, in rocks with secondary cleavage it probably commonly indicates recrystallization of the quart/ 

subsequent to .the original solidification of the rock.

(8) If in a rock which had been clearly subjected to great pressure, 

as evidenced by the breaking of the feldspars and of some of the 

quartzes, some of the quartzes are comparatively free from strain 

effects or fractures, it can be supposed that the quartzes have 

readjusted themselves to the prevailing conditions by crystallization 

or recrystallization, thus obliterating their strain effects. Of course, 

strain effects may still be present and recrystallization have occurred. 

Indeed, in most cases recrystallization is believed to lag behind 

deformation to such an extent that some strain effects are visible. 

The statement should perhaps be that if the quartz shows any less 

fracturing or strain shadows than other minerals of the rock, this is 

evidence of the recrystallization of the quartz.

A characteristic strain effect in quartz is the inclusions SO frequently 

observed. These are common!}7 in planes which in slides appear like 

lines. They consist largely of cavities containing gas and water and 

minute mineral particles. Observation shows these planes of inclusions to be closely associated with other pressure effects, such as 

undulatory extinction and fracturing. There is frequently complete 

gradation from tenuous and ill-defined planes of inclusion to wellmarked planes and fractures."

F.urther, these inclusions may be in planes intersecting one another 

at high angles, frequently at an inclination of 45° to the plane of greatest 

diameters of the quartz or to the plane of rock cleavage. According 

to principles given, pp. 121-124, they may be phenomena of fracture 

in shearing planes. In a quartz in which there has been no strain it 

is believed that these planes of inclusions are less numerous and have 

less regularity of arrangement; but this is hard to prove, as it is so 

extremely difficult to find a rock containing quart/ which can be 

proved not to have undergone strain.

When tested by the above criteria the shape and arrangement of the 

elongated quartz bands or elongated spindle-shaped quartz masses (3), 

seem probably to have resulted mainly from recrystallization. Yet 

there may have been also subordinate gliding, granulation, arid rotation, for recrystallization obliterates evidences of these processes.

When the quartz grains are nearly equidimensional, or the grain 

is liner, the above criteria are less effective. Many quartz crystals 

Avhich are nearly round and show no evidence of recrystallization 

by any of the above criteria, have undoubtedly been recrystallized. 

Where the grain is exceeding!}7 fine it is impossible to apply these 

criteria, and one is unable to tell by direct observation what processes

<< Sec figs. 3 and 4 of Van Hise's paper on the pre-Cambrian Rocks of the Black Hills: Bull.Geol. Soc. 

America, vol. 1,1890, pp. 216, 217.
86 BOOK CLEAVAGE. (BULL.289.

or what combination of processes have been effective in producing- the 

fine-grained o-roundinass of many cleavable rocks.

Quartz crystals have been made to glide under pressure experimentally, but it is certain that little.or no evidence of such gliding is now 

to be observed in the quartz of the cleavable rocks. If gliding has 

occurred, evidence of it has been obliterated by recrystallization and 

granulation. The only strains in quartzes now to be observed that are 

of the same order as those of gliding are those of undulatory extinction, and the comparative absence of this phenomenon, as seen above, 

is regarded-as evidence that reciystallization has occurred.

In general the quartz particles showing evidence of recrystallization 

iii cleavable rocks are not so numerous as those lacking such evidence, 

but if this statement is .confined to quartz particles showing parallel 

arrangement, the evidence of recrystallization preponderates over 

that of any other processes causing such a parallel arrangement. 

Recrystallized quartz particles are certainly less abundant than recrystallized mica or hornblende, and probabty more abundant than recrystallized feldspar. Evidence of granulation and rotation is more abundant in quartz particles than in mica or in hornblende particles, and 

probably less abundant than in feldspar particles, but these processes 

have yielded but a poor arrangement. So far as recrystallization 

works upon granulated and rotated .particles it tends to obliterate 

evidence of granulation and rotation, and hence the evidence now 

observable may not in all cases measure the extent to which granulation 

and rotation has actually occurred.

FELDSPAR.

In the presentation of the facts of arrangement of the feldspars the 

feldspar particles were classified according to shape.

The feldspars occurring with more or less modified crystal outlines 

and random arrangement described under (1) on page 35 may be either 

original or recrystallized. From their shape they could not have been 

granulated. The crystals described under (la), which have random 

arrangement and around which the other constituents of the rock bend,I

are original as compared with other constituents, for they have evidently acted as rigid units during the deformation of the rock, the other 

constituents being crowded about them and thus frequently showing 

strain effects. If rec^stallized, they would be likely to have a parallel 

arrangement or to include the other constituents, as in the case described 

below. ° 

The crystals with random arrangement described under (la) as not 

crowding the other constituents of the rock but containing them in 

lines parallel to the plane of rock cleavage must be recrystallized, for the 

minerals contained are quartz and mica in parallel bands which them-
LEITH.] PROCESSES ARRANGING QUARTZ AND FELDSPAR.'. 87

selves can be proved to be recrystallized. The development of the 

feldspars must have been subsequent to the recrystallization of the 

quartz and .mica. If original in their present form and arrangement 

the crowding- must inevitably have occurred during the development 

of the parallel arrangement in the other constituents of the rock. 

Furthermore, the}7 lack strain effects which they must have shown if 

they were there during the deformation of the rock. Their large size 

as compared with that of other constituents results from the fact that 

their development by recrystallization occurred after the rock deformation ceased, when granulation no longer breaks down the grains as fast 

as recrystallization builds them up.

Crystals with more or less modified crystal form occurring in dimensional parallelism to the plane of rock cleavage (1 b, pp. 35-37) owe 

their shape and probably their parallel arrangement to recrystallization. 

That recrystallization is responsible for their development is proved 

by the evidence cited above, that is, the presence of reciystallized 

quartz and mica through the feldspars without change of direction and 

further by the lack of strain effects commensurate with the deformation which the rock has- evidently undergone. In some cases, also" 

such crystals have been observed to be the alteration products, with 

mica and garnet, of a larger feldspar crystal. The association with 

other recrystallized minerals such as hornblende, is further presumptive evidence of recrystallization. Rarety the feldspar crystals under 

this head have received their arrangement through slicing along feldspar cleavage planes and rotation. 6

The angular particles described under (2) on pages 37-39 might be 

either in original form, granulated, or recrystallized, but their shape is 

probably in most cases, if not in all, either original(i. e., the same as in 

the undeformed rock from which the cleavable rock is derived) or due 

to granulation. If recrystallization has affected them it has left them 

with a shape quite different from that characteristic of recrystallized 

particles described above and below. All stages of granulation from 

the original mineral to the finely granulated particles may actually be 

seen in many slides. Where such gradations are absent the angular 

form is itself presumptive evidence that the form is due either to 

granulation or original character. Evidence of the rotation of .such 

granulated particles toward the common plane of cleavage is almost 

uniformly present. In certain cases, as in the anorthosite described 

by Adams (pp. 37-38 and figs. 13 and 14) the elongated forms are clearly 

the result of granulation or slicing along the peripheries, leaving 

residual parallel grains. In many other cases the evidence for recrystallization or granulation is not decisive.

The lenticular and ribbon forms of feldspar described under (3) on 

page 39 are in part, both in shape and arrangement, the result of

«Sp. 6021 W., Wausau, Wis. ?>Sp. 0021W. in part, Wausiiu, Wis.
88 BOOK CLEAVAGE. [BULL.239.

recrystallization. The criteria for determining this are identical with 

those used in determining the origin and processes of arrangement of 

similar bands of quartz (pp. 82-86).

In general the feldspars, which clearly owe their shape and parallel 

arrangement to recrystallization, are relatively rare as compared with 

particles lacking such evidence and probably arranged largely by rotation and granulation or as compared with still more abundant particles lacking the parallel arrangement. Feldspars without parallel 

arrangement, but showing evidence of recrystallization, are often 

observed. " Cores and enlargements and new growths along cracks are 

not uncommon phenomena, but such reciystallization has not yielded 

a parallel structure and thus does not properly come within the limits 

of this discussion. Many feldspars show evidence that some combination of these processes of arrangement has been effective, but usually 

the one or the other is preponderant.

Gliding is not observed to be important. - While secondary twinning 

as an initial stage in granulation is a common phenomenon, in no case 

has this twinning been observed to have caused &ny appreciable flattening of the mineral. Where any such possible flattening is present with 

the twinning, evidence of granulation, rotation, or recrystallization is 

commonly more conspicuous.

In an exceedingly fine-grained groundmass it is practically impossible 

to determine, from any phenomena shown by the particles themselves, 

what processes have been effective, though it may be possible to determine their probable origin by ascertaining the dominant processes 

which have affected the coarser constituents of the rock. a

CALCITE,

In explaining the parallel arrangement of calcite grains several 

processes may be considered, the common ones recrystallization, 

granulation, and rotation and gliding. Any or all of these processes 

may be effective in given cases, and it is extremely difficult to deterx-mine criteria by which to separate them. Granulation and accompanying rotation are mainly responsible for the shape and arrangement 

of the angular fragments described on page 41. This is evidenced 

mainly by the shape of the fragments and their gradation to unbroken 

crystals. The process determining the shape and arrangement of 

the small ellipsoidal particles (described on p. 41) is not so clear, 

but it is thought to be mainly recrystallization, because of the 

general evenness of texture of the grains, their ellipsoidal outlines, 

and their comparative freedom from strain effects. Yet.it is apparent 

that granulation may have aided recrystallization and that its characteristic evidences may have become obliterated by recrystallization.

aSp. 14929, si. 9460, Black Hills, South Dakota.
PROCESSES ARRANGING FELDSPAK AKD CALOTTE. 89

Gliding may have been partly effective in any of the above cases. In 

experiments on the deformation of marble tinder pressures, Adams 

has shown that deformation of the calcite crystals may occur by gliding along twinning planes of the crystal i K, This results in dimensional parallelism of the ciystals and not in crystallographic parallelism, for gliding may occur, no matter what the original arrangement 

of the calcite is, and no amount of the gliding will make parallel the 

relative crystallographic directions of different crystals. The main 

criterion by which gliding is determined is the increased amount of 

twinning to be observed. Where the movement occurred by gliding, ' 

the crystals were found to be finely striated by polysynthetic twinning 

grading into minute fractures, giving a reedy or fibrous appearance 

(PI. XV). In the calcite particles occurring in cleavable rocks more 

or less of such twinning is to be observed, and the presumption is 

that at least some of it is a secondary phenomenon due to pressure and is accompanied by a change in form. But the difficult}^ lies 

in ascertaining how much of the twinning is original and how much 

is secondary, and thus in determining the amount of the deformation 

brought about in this way. In the particular slides examined!, in but 

few cases did the twinning lines seem to be much more numerous than 

that common for an unmashed marble. If gliding had occurred extensively, subsequent recry stall ization ma}^ have obliterated it. In connection with his description of his experiments on the flowage of 

marble, Adams publishes descriptions of 42 cleavable marbles for 

comparison. He concludes that while recrystallization undoubtedly 

plays an important and in many cases probably a chief part in the 

deformation of marble, the processes of gliding and granulation are 

also effective."

The facts observed in this study seem to indicate that recrystallization and granulation are by far the most common of the processes 

producing the arrangement of calcite grains in cleavable rocks. The 

ease of recrystallization of calcite has been commonly recognized. It 

is beautifully illustrated under conditions of low pressure by the 

cementation of marl and by the common change of limestone to 

marble.

The process of gliding has probably assisted both of the above 

processes, though in the particular rocks examined in this study the 

evidence of such assistance has been slight. In the observations of 

cleavable marbles made by Adams more evidence of gliding was 

found, and in his experimental work the process has been demonstrated to occur.

If the parallel arrangement of the calcite crystals of schistose mar 

bles is often clearly the result of recrystallization, arid calcite is

<i Adams, F. D., and Nicolson, J. T., An experimental investigation into the flow of marble: I'hilos. 

Trans. Royal Soc. London, vol. 196,1901.
90 " BOCK CLEAVAGE. [BOLL. 239.

known for other reasons to recrystallize very easily, the question 

naturally arises why parallel structures in marbles are not more general, for it is exceedingly rare to find marbles with a parallel structure. 

Crystalline granular marbles without parallel structure frequently 

appear between very schistose gneisses. In the exceptional cases 

where parallel arrangement is attained the conditions must be somewhat unusual. Following Van PIise, a it may be suggested that the 

ready recrystallization of calcite and its consequent tendency to develop 

large individuals by the merging of smaller ones may soon obliterate 

evidence of parallel arrangement after rock fiowage has ceased, and 

that a parallel arrangement appears only when the rock flowage and 

development of a parallel structure has occurred so recently that subsequent recrystallization has not had time or the proper conditions to 

obliterate the parallel structure. In other words, the parallel structure developed by granulation or recrystallization under dynamic 

conditions may be obliterated by subsequent recrystallization under 

static conditions.

CHLORITE.

No presentation of evidence is necessary to show that chlorite in 

parallel flakes in cleavable rocks is a development by crystallization or 

recrystallization. The development of chlorite from other minerals 

has been so frequently observed and described that its secondary nature 

is accepted as a matter of course.

In many schists chlorite is partially- or wholly pseudomorphous 

after the mica's, in which case its parallel arrangement is that of the 

mica. In other cases chlorite is a development from minerals of appropriate composition which originally had no parallel arrangement. 

In either case the shape and arrangement are due to recrystallization.

TREMOLITE, ACTINOLITE, GRUNERITE, AND OTHER AMPHIBOLES.

- These minerals are in all cases certainly the result of crystallization 

in metaclasic rocks, as may be proved by field occurrence, where their 

development may be traced from rocks originally having none of these 

minerals. Their arrangement in certain cases in the plane of rock 

cleavage may be explained (1) by recrystallization during rock flowage; (2) by recrystallization after movement ceased, the plane of rock 

cleavage affording directions of easiest development of the crystals; 

(3) by rotation of recrystallizihg and recrystallized particles. What 

combination of these factors in a given case has been effective in producing the very partial arrangement observed in these minerals is 

difficult to determine. If (1) were effective we should expect to find

aVnn Hise, C. R., A treatise.on metamorphism: Mon. U. S. Geol. Survey, vol. 47, 1904, pp. 754-755.
PROCESSES ARRANGING MINERALS.

si better arrangement than we do find. If (3) were effective we should 

expect to find considerable breaking and interference of the fibers. 

Explanation (2) seems to afl'ord. the best explanation of the facts observed and this is accordingly given emphasis, although it is not meant 

to imply, that the two others may not have been effective to a subordinate degree.

GARNET, STATJROLITE, TOURMALINE, ANDALUSITE, CHLORITOID, ETC.

Garnet, staurolite, tourmaline, andalusite, chloritoid, and the other 

crystals of this class in cleavable rocks are uniformly porphyritic. 

Their development in such rocks is largely by recrystallization, which 

occurs after rock flowage has ceased and is favored by high pressure 

and temperature, as is evidenced by their high specific gravity and 

characteristic association with great masses of intrusive igneous rocks. 

Evidence of their development by the recrystallization of the rock mass 

and recrystallization later than the rock flowage that has produced cloavage is shown by the following facts: (1) They appear in rocks clearly 

derived b}^ rock flowage from rocks originally lacking such minerals. 

(2) They frequently lie at high angles to the prevailing cleavage in the 

rock. (3) They do not show the degree of mechanical deformation that 

they would necessarily show had they developed in their present positions before rock flowage had ceased. Many of the crystals are long and 

acicular, and would surely have been broken if any considerable movement had occurred subsequent to the development. (4) They include, 

within their outlines, minerals in part similar to those in the remainder of the rock with an, arrangement of their greater diameters in 

the plane of rock cleavage (PI. XIII),. an arrangement in part at 

least developed during rock flowage. (5) The mica and the other constituents of cleavable rocks, which are certainly developed by recrystallization during the process of deformation, are frequently seen to 

end abruptly at the periphery of the minerals of this group and to 

pass them by without any change of direction or crowding. If rock 

flowage under which the micas developed had occurred subsequent 

to the formation of the porphyritic crystals, crowding and bending 

of the micas'must inevitably have occurred and would doubtless not 

be obscured by their subseqent recrystallization. (6) The usual large 

size of minerals of this group, as compared with their associated 

mineral particles, is due to their development subsequent to rock 

flowage, when granulation is no longer acting to break down the 

crystals.

While the development of this group of crystals is believed to liave 

been mainly subsequent to the development of the rock cleavage, it is 

true also that in some cases further flowage occurred subsequent to 

their development, as shown by fracturing of the crystals and the
92 ROCK CLEAVAGE. [BULL.M

crowding- of the other constituents. The very fact that the effects of 

further movement are so conspicuous confirms the conclusion that the 

crystals not showing these effects developed after the movement 

ceased.

SECTION III. GENERAL PREDOMINANCE OF PROCESS OF RECRYSTALLIZATION.

By comparing the discussion of the cleavage-producing capacit}^ of 

mica, hornblende,, quartz, and feldspar which make up the great bulk 

of the rocks with secondary cleavage, with the discussion of the processes which have determined their shape and arrangement, it appears 

that the mineral grains yielding the best flow cleavage are the ones 

which are described as showing conspicuous evidence that their shape 

is due to recrystallization, and if our idea of the factors and conditions 

controlling recrystallization is correct, the arrangement of such reciy s- - 

tallized particles is also due largely to the same process. It is an 

observed fact that in general, so far as evidence of recrystallization is 

absent, the parallel arrangement also is lacking. (The reverse proposition does not hold.) Yet there is evidence, also, that granulation 

and slicing occasionally develop an excellent parallelism and may have 

been effective also in producing the parallel arrangement of particles 

which clearly bear the stamp of recrystallization as the last and dominant process. Evidence of rotation of original and granulated particles 

unaccompanied by recrystallization is conspicuous only in certain 

original and granulated mineral particles which are not so well adapted 

to give a good rock cleavage as the particles in which evidence of 

recrystallization is apparent. It is certain, however, that rotation has 

accompanied recrystallization in many grains which now show evidence 

only of recrystallization, for during the rock flowage which developed 

the cleavage rotatory stresses must have been generally active, as will

be shown on a subsequent page (pp. 109-118). There is practical!}' no 

evidence of gliding in these principal cleavage-giving minerals.

The relative importance of the various processes giving the parallel 

arrangement to the remaining minerals of metaclasic rocks is a matter 

of little consequence for the reason that such minerals have for metaclasic rocks as a whole so little effect on rock cleavage. In calcite all 

processes, recrystallization, granulation, gliding, and rotation are 

effective; the order given is probably also the order of their importance. Calcite is the mineral best adapted to change its form by .gliding, and is the only one in which this process has yet been shown to 

be effective, but even here this process is probably subordinate to 

recrystallization. From evidence to be observed in the rocks themselves, gliding is therefore practically negligible. In the remainder 

of this group of minerals the origin is almost entirely recrystalliza-
LEITH.] PREDOMINANCE OF RECRYSTALLIZATION. 93

"tion, and the arrangement, so far as there is arrangement, is doubtless 

due mainly to the same cause., although rotation may have had some 

effect. 

Investigations have shown in certain instances that the cleavability 

of a rock varies with its composition." This expresses also the fact 

that cleavability varies with mineralogical composition. In the development of flow cleavage in a noncleavable rock there is both chemical 

and mineralogical change and redistribution of substances. This 

occurs through recrystallization. Hence the close relation not only 

between cleavability and composition, but between these two and 

recrystallization.

SECTION IV. ORDER OF RECRYSTALLIZATION.

It has been seen that certain minerals develop secondarily in cleavable rocks by recrystallization more readily than others. It is not 

possible to make definite statements concerning the order of facility of 

crystallization or recrystallization of different minerals, because of the 

fact that certain of the cleavage-making minerals, such as hornblende, 

usually develop to the exclusion of others, such as micas; the cleavage 

rocks in which hornblende and biotite or hornblende and muscovite 

occur together are very few. We know definitely that quartz generally recrystallizes before feldspar, and mica and hornblende before 

quartz and feldspar. In a few cases b we know also that the biotites 

have developed before the hornblendes, but how generally this statement can be made to apply is a matter of doubt. As to the micas, it 

is certain that the muscovite and biotite, when they occur together, 

usually develop simultaneously; they are interleaved in a most intricate 

fashion, and the development of one or the other is apparently a matter 

of the substances available. Exceptionally the muscovite evidently 

crystallizes before the biotite. The recrystallization of calcite has 

not been compared directly in the same" slide with the development of 

the minerals above named, but it is known that this mineral recrystallizes very easily and probably relatively early in rock flowage. 

Chlorite perhaps in some cases develops simultaneously with the 

micas, but it is certain that in other cases it develops later as an alteration of micas or other minerals. Where it is developed to the exclusion of the micas it is of course difficult to compare its ease of recrystallization with that of the micas. Actinolite, griiuerite, and tremolite 

are certainly in many if not in most cases crystallized out later than the 

principal cleavage-making minerals, as is evidenced by their unbroken 

form and random arrangement in rocks which show parallel arrangeaSee articles by Reade and Holland in Proc. Liverpool Geol. Soc. for 1899-1900, pp. 463-478, and Tor 

1900-1901, pp. 101-127. See also Marker, Kept. 55th. Meeting Brit, Assoc, Adv. Sci., 1885, p. 828. 

(>Sp. 18037, .si. 9589, near Hudson River, New York,
94 ROCK CLEAVAGE. [BULL. 239.

ment by crystallization or recrystallization of other constituents. The 

same statement may be made with reference to the group of heavy 

minerals, such as garnet, staurolite, andalusite, chloritoid, tourmaline, 

etc. They are all developments by crystallization very late in the 

process of deformation or after deformation has entirely ceased. 

Their development is also later than, the recrystallization of calcite 

or chlorite and, in most cases at least, of actinolite, griinerite, and 

tremolite."

It is interesting to ascertain whether or not the order of recrystallization or crystallization of the various minerals of cleavable rocks 

bears any uniform relation to any of their mineral properties, and to 

attempt to ascertain what properties have caused the minerals to 

recrystallize in this order during rock flowage. So far as one can judge 

from the above facts, there is no obvious arrangement of the mineral 

properties corresponding with the order, of recrystallization. The 

specific gravity in the four principal schist-making minerals mica, 

hornblende, quartz, and feldspar ranges roughly from higher in the 

easiest and first recrystallized to lower in the last recrystallized. In 

chemical composition there is apparently no order. The symmetry 

shows no regular variation. The mineral habit varies from greater inequality of the dimensional axes to less inequality. In the less important 

minerals which undoubtedly crystallize later in rock flowage, there is 

the same irregularity and uncertainty in the order of properties. 

Instead of having a.lower specific gravity than the principal cleavagegiving minerals above named, these minerals have a higher one. In 

symmetry and shape they show greater variety; in some instances the 

crystals have less inequality of dimensional axes than the principal 

cleavage-making minerals and in other cases more.

The conditions determining the order of development of the various 

cleavage-making minerals by crystallization or recrystallization during rock flowage are evidently somewhat complex, and not evidenced 

by a simple graded series of mineral properties. The form and kind 

of mineral developing at any stage in the process may be conditioned 

b}7 factors of tempei^ature and pressure, substances available, crystal 

shape and symmetry of the particles which may develop at that point, 

their specific gravity, etc. For different minerals and conditions 

different factors are dominant, and a comprehensive discussion of 

the causes of the order of development during rock flowage would 

involve a detailed description of many factors affecting each mineral, 

which would make a paper in itself. Van Hise 6 has made an 

exhaustive study of these factors, and the reader is referred to his 

work on metamorphisin. One principle, however, may be empha-

«See slides and specimens from Republic Mountain and Humboldt Mountain, Marquette district,. 

Michigan. 

& Van Hise, C. K., Mon. U. S. Geol. Survey, vol. 47,1904.
OEDEIt OF RECRYSTALLIZATION. 95

sized. While the minerals developing at any place or time during 

rock flowage are limited in their variety by the substances available 

in the capillary and subcapillar}^ solutions, it may be supposed that if 

at any point such substances are present that any one of several minerals might develop, the mineral best adapted by its high specific 

gravity and especially shape, as shown by the above facts, to the conditions of pressure there existing will" be the first one to form, although 

other factors may have a modifying effect. It is an observed fact that 

minerals developed by recrystallization during rock flowage have as a 

rule higher specific gravities than the minerals originally present in the 

rock or minerals developed under conditions of weathering, and thus 

the development of a rock with a good cleavage through recrystallization means a diminution of volume. It is further a matter of observation, as shown in the foregoing pages, that the minerals showing

the best evidence of recrystallization are the ones which by their

shape and dimensions are best adapted to the conditions of unequal 

pressure which have existed during the deformation of. the rock.
CHAPTER IV.

MISCELLANEOUS OBSERVATIONS ON THE INTERNAL STRUCTURE OF CLEAVABLE ROCKS.

SECTION I. WHY SOME MINERALS SHOW CRYSTALLOGRAPHIC 

PARALLELISM AND OTHERS DO NOT.

It has been shown that minerals of different kinds in metaclasic rocks 

do not exhibit a parallelism of their ci^stallographic or vector properties, but that particles of the same mineral species do show such 

parallelism in so far as these properties have uniform relations to the 

dimensions of the particles. In other words, the minerals are always 

controlled in their arrangement by their dimensions, and in so far as 

the vector properties happen to be in uniform positions with reference 

to such dimensions these properties are themselves parallel. The 

parallel crystallographic arrangement of'particles of the same mineral 

is thus a phenomenon subordinate to, and dependent upon, the dimensional arrangement of the particles. This uniform relation of the 

crystallographic properties to dimensions and consequently the parallel 

arrangement of the crystallographic properties are found in cleavable 

rocks only in the particles of minerals which have a strong tendency 

to occur with strongly marked crystal habit, such as mica and hornblende. In developing by recrystallizatiou during rock cleavage, as 

these minerals do for the most part in cleavable rocks, their tendency 

to take on definite crystal forms, working under the general law of 

dimensional control, requires that the crystallographic axes of the 

particle shall also be arranged in parallel positions. Minerals such as 

quartz, feldspar, and calcite, which lack a strongly marked crystal habit 

or cleavage form in metaclasic rocks, commonly do not show a parallelism of the crystallographic properties of the different particles. It 

is known that the tendency to the development of quartz and calcite in 

columnar forms is slight. Any small accident of crystallization may 

cause the development of short, stumpy crystals whose length is not 

greatly different from their thickness. This is shown in quartz by a 

tendency to develop terminal fa'ces, manifesting itself in striations on the 

pyramidal faces, in this contrasting strongly with hornblende, which has 

a strong tendency to columnar habit without the development of terminal faces. For this reason in meeting the requirements of dimensional 

96
Msmi.] CAUSE OF CEYSTALLOGEAPHIC PAEALLELISM. 97

arrangement under rock flowage, quartz, and calcite in reciystallizing 

do not necessarily arrange their crystallographic axes. The tendency 

toward developing in characteristic crystal shape is not strong enough 

to orient the crystallographic properties when the mineral is arranging 

itself dimensionally under pressure. The same statements apply in a 

less degree to feldspar.

Under exceptionally favorable circumstances it would be natural 

to expect that the tendency, slight though it is, for quartz, feldspar, 

and calcite to take on characteristic crystal shape with characteristic 

dimensions, would cause the crystallographic properties to be parallel, 

and this, except for calcite, seems to be the case in exceptional 

instances, as shown in Chapter II.

The reverse proposition, that all minerals showing strongly marked 

crystal habit in cleavable rocks have also crystallographic parallelism, 

does not hold, for the fact has repeatedly been emphasized that many

minerals, such as tourmaline, chloritoid, etc., and exceptionally even 

feldspar and biotite, develop late in the process of rock flowage or 

after rock flowage has entirely ceased, and usually lack not only crystallographic but even dimensional parallelism.

SECTION II. MOLECULAR SHAPE AND RECRYSTALLIZATION.

There is a significance in the entirely new development by recrystallization of minerals, such as mica and hornblende, with strongly marked 

crystal habits and with parallel dimensional and crystallographic 

arrangements. The outward form of a crystal is commonly taken to 

be a manifestation of its internal structure. The characteristic dimensions of a crystal may be supposed to be conditioned by the molecules 

themselves, because of their shape or manner of aggregation or both. 

It may be supposed further, then, that the dimensional control of the 

arrangement observed in the crystals developed entirely by recrystallization in metaclasic rocks will apply equally well to the very first 

molecule or group of molecules of these minerals which began to 

develop; in other words, that the very first molecule or group of 

molecules which were brought together by crystallization or recrystallization to make up the crystals of these minerals were controlled in 

their initial position by the conditions of unequal pressure existing 

during the rock flowage. They were actually squeezed or. held in 

parallelism by the differential pressure. Whatever molecular property 

gives the characteristic shape to crystals causes the arrangement of 

the molecules under pressure. So far as fortunately oriented crystals 

survive or continue to grow, it may be that the molecular properties 

determining their shape meet the requirements of dimensional control. In discussions of the molecular structure of a mineral, it has 

ordinarily been assumed that the shape of crystals is due to moleBull. 239 05 7
98 BOCK CLEAVAGE. [BULL.239.

cular grouping, i. e., the molecules have been considered as essentially equidimensional bodies, and the differences in symmetry and 

shape of crystals have been considered to depend upon the spacing 

of the molecules. One would expect, therefore, that, as the first 

molecules of the newly developing crystal aggregated themselves 

in a rock" undergoing flowage, their tendency to unequal spacing in 

different crystallographic directions would give the crystal at once 

unequal dimensions. This would cause its orientation with its shorter 

diameters parallel to the shortening oiL the rock mass, and in mineral 

particles so arranged the greater molecular spacing would be looked 

for in the direction of the greatest dimensional axes of the crystals 

that is, normal to the shortening of the rock mass. But it has further 

been held by ciystallographers that mineral cleavage is dependent upon 

the spacing of molecules, the molecules being spaced more closely in 

the planes of mineral cleavage than transverse to them. On such a 

hypothesis it must be assumed that in developing by recrystallization 

during rock flowage, the micas, for instance, take a position with the 

greater spacing of the molecules parallel to the shortening of the rock 

mass, a direction in which one would expect the molecules to be most 

closely spaced. If the molecules were most widely spaced in the direction of the least dimensional diameter of the mica, as required by the 

theory of mineral cleavage, it is difficult to understand how the minute 

beginnings of mica crystals, or the first aggregates of molecules which 

can be called ciystals, could arrange themselves, as they unquestionably 

have, under the law of dimensional arrangement. Such considerations, together with the complete dimensional control of the arrangement of newly developing mineral particles in a rock undergoing rock 

flowage and the complete dependence of rock cleavage upon the parallel arrangement of unequidimensional mineral particles, suggest that 

possibly the more minute constituents or structural elements of these 

crystallographically parallel mineral particles may be arranged under 

similar laws; the molecules themselves may have unequal dimensions 

and be arranged under the law of dimensional control shown by the 

crystal particles. It would follow that minerals dimensionally and 

not crystallographically arranged, such as most quartz, calcite, and 

feldspar particles, have molecular shapes such that in the beginning 

of their development they may meet the requirements of dimensional 

control without necessarily arranging their crystallographic axes.

. As another possibility the molecules might be both dimensionally 

arranged and unevenly spaced, giving several possible combinations 

of arrangements. It might well be that with the parallel dimensional 

arrangement of the molecules there is a wider spacing in directions 

normal to the greater diameters of the molecules. This would allow 

newly developing mica particles to meet the requirements of dimensional control and at the same time to arrange their mineral cleavage,
LEITJI.] MOLEOULAE SHAPE AND BECEYSTALLIZATION. 99

assuming this to be conditioned by the wider spacing of the molecules in planes normal to the shortest dimensional diameter.

An interesting and perhaps significant fact in this connection is the 

common position of the optic axis a normal to the greater dimensions 

in such minerals as the micas. This means that the rays propagated 

.normal to the plane of cleavage and vibrating parallel to it travel 

more rapidty than others. The behavior of light in such crystals may 

be influenced by the possible dimensional arrangement of the mineral 

molecules here suggested. However, a discussion of the possible 

nature of such influence must include so many factors, many of them 

of yet doubtful nature, that it must be left to some other time and 

place.

SECTION III. FLOW CLEAVAGE AS A MOLECULAR PHENOMENON

 It is clear that as flow cleavage is a capacity to part, not an actual 

parting, whether along mineral cleavages or between mineral particles 

due to their weak adhesion, it is a molecular phenomenon. Parting 

can occur only by breaking certain bonds of molecular attraction. It 

is necessary only that the bonds of molecular attraction should be less 

strong along certain parallel lines or planes than along others. A definition of flow cleavage, in terms of molecular attraction, would thus 

read as follows: Flow cleavage is a capacity to part along parallel 

planes or lines or surfaces, due.to weaker molecular attraction along 

such parallel planes or lines than in other directions in the rock. 

This allows of parting parallel to a number of sets of lines or planes, 

but where one set gives an easier parting than another only the easier 

one is likely to appear.

When a rock with flow cleavage is cleaved, it is observed that the parting occurs either between separate mineral particles, which act as units 

during parting, or along cleavage planes in the mineral particles themselves or commonly both. Where the mineral particles act as units, the 

parting has been observed to occur most readily along planes or lines in 

which the fewest mineral particles are to be met with. In the common 

cases this is obviously in the plane of the greatest and mean diameters, 

or in any planes or surfaces parallel to the greatest diameter of the mineral particles. The parting occurs less readily in the plane of the greatest and least axes, and still less readily, or not at all, in the plane of the 

least and mean axes of the particles. Where the particles act as units 

in the parting, the adhesion of the particles to one anotfeer is weaker 

than the internal cohesion of the individual particles. The planes of 

parting are separate entities; they are finite in number, and have definite positions in the rock. For convenience in discussing flow cleavage, 

the parting so .occurring between the mineral particles rather than 

through them may be called intermineral or adhesive cleavage. Such 

cleavage is likely to yield few possible planes of parting visible in the
100 BOOK CLEAVAGE. [BOLL.239.

hand specimen. Where during the parting of a metaclasic rock the 

particles do not act as units, but break along their own parallel 

mineral cleavages, the number of possible planes of parting through 

the minerals so arranged may be almost infinite in number. The cohesion of the substances of the particles is less than the adhesion of 

the particles to one another. Cleavage so produced may be called cohesion cleavage, to distinguish it from the intermineral or adhesion cleavage. Because of its fineness it might perhaps be distinguished as 

microcleavage. As already intimated adhesion and cohesion cleavage 

commonly occur together- in rocks.

All flow cleavage belongs under one or both of these heads. The 

terms are superfluous for ordinary descriptive purposes, but are convenient in a discussion of this phase of the subject.

The preceding discussion of the effect of the individual minerals on 

the production of cleavage has given data for a general statement of 

the relative importance of intermineral and cohesion cleavage in metaclasic rocks as a whole.

Cohesion cleavage seems to be important so far as mica and hornblende are present in a metaclasic rock, and adhesion cleavage so far as 

quartz and feldspar are present. The presence of mineral cleavage 

faces in the rock cleavage partings is evidence of the cohesion cleavage, 

while their absence is taken as evidence of adhesion cleavage. But, 

while the presence of mineral cleavage faces on the rock cleavage parting surface is necessary when the rock is parted along cohesion cleavage planes, it does not follow that the presence of such mineral cleavage planes always indicates that the rock cleavage was of the cohesion variety alone, for the shapes of hornblende and mica particles 

in cleavable rocks are determined by mineral cleavage, and it is 

very difficult to tell on the rock cleavage surface whether the mica 

and hornblende there appearing have been parted along their own 

cleavages or are in their original forms. While adhesion cleavage 

can occur alone, cohesion cleavage is rarely, if ever, present to the 

exclusion of the adhesion cleavage. It is to be remembered that the 

micas and hornblendes never make up the entire mass of a rock, but 

are usually separated by layers of quartz or feldspar, or both, and 

even if these minerals were not present tKe cleavage would not be 

cohesion cleavage alone, but partly adhesion cleavage between different 

mica or hornblende individuals.

It can be said, then, that so far as quartz and feldspar appear on the 

cleaved surface of a metaclasic rock, as it does in many quartzfeldspar gneisses, the rock cleavage has probably been mainly of the 

adhesion variety. So far as mica and hornblende appear, the rock 

cleavage probably has been both of the adhesion and cohesion varieties. 

While adhesion cleavage is nearly always present, the fact that cohesion cleavage is certainly important for the micas and hornblendes indi-
I.BITH.] .FLOW CLEAVAGE AS A MOLECTJLAK PHENOMENON. 101

cates that cohesion cleavage in cleavable rocks may be as important if 

not more important on the whole than adhesion cleavage.

So far as chlorite is present in the rock the cohesion cleavage is 

dominant. So far as actinolite, griinerite, tremolite, staurolite, tour-' 

maline, and chiastolite are present, and have parallel arrangement, 

probably the adhesion cleavage is important.

Calcite in finely fissile limestones or marbles yields an adhesion 

cleavage alone, as is evidenced by the characteristic absence of calcite 

cleavage on cleavage surfaces of such rocks "and by the absence of 

parallelism of the calcite cleavage as observed microscopically.
CHAPTER V.

OBSERVED RELATION OF FLOW CLEAVAGE TO THE ELONGATION AND SHORTENING OF ROCK MASSES.

If the relation of flow cleavage to the elongation and shortening of 

rock masses is certainly known, its relations to pressure may be worked 

out. The relation of flow cleavage to elongation and shortening of a 

deformed rock mass may be shown by (1) the distortion of pebbles of 

a conglomerate, (2) the distortion of mineral crystals, (3) the distortion 

of volcanic textures, (4) the distortion of fossils, "(5) the distortion of 

beds and attitude of folds, (6) relations of cleavage to intrusives, (7) 

 the position of fractures. It is scarcely necessary to attempt an 

exhaustive discussion of each of these lines of evidence, they are so 

common and well known. But of each a few illustrations to show the 

nature of the evidence may be given.

(1) Distortion of pebbles of a conglomerate. Schistose conglomerates 

show by the distortion of their pebbles, either with or without fracture, the directions of elongation and shortening, although it may 

sometimes be difficult to distinguish the shapes of undeformed pebbles 

from those of deformed pebbles. As illustrative examples may be 

cited: Conglomerates from the iron districts of the Lake Superior 

region a (See PI, XXVII), from Crystal Lake in California, from the 

Black Hills of South Dakota, 6 from Madoc, Ontario, 6 from the Green 

Mountains of Vermont, and from the Front Range of Colorado. Sederholm^ describes squeezed conglomerates from Finland, Marker 6 cites 

a number of instances in his discussion of rock cleavage, and Lehmann-^ figures several in his crystalline schist report. Instances could 

be cited from almost every knowii area of pre-Cambrian.sedimentary 

rocks.

In all of these conglomerates the matrix is schistose or cleavable, 

and the schistosity or cleavage is approximately parallel to the greater

aSp. 25718, north of Felch Mountains, Michigan;, sp. 42094, fracture in pebble, north shore of Lake 

Superior; sp. 45810, Pine River, Wisconsin. 

&Sp. 14818, Black Hills, South Dakota. 

cSp. 18391 and 18392, Madoc, Ontario.

dSederholm, J. J., Archean sedimentary formations: Bull. Geol. Survey of Finland, No. VI. 

e Barker, Alfred, Kept. 55th Meeting Brit. Assoc. Adv. Sci., held 1885, published in 1886, p. 837. 

/ Lehmann, Origin of the Crystalline Schists; Atlas, PI. VII, figs. 5 and 6; PI, XVII, fig. 4.

102
RELATION OF CLEAVAGE TO DEFORMATION. 103

diameters of the flattened pebbles, although bending at the ends of the 

pebbles in following their peripheries. Where the flattening of the 

pebbles has been extreme, as in a specimen illustrated in PL XXVI, 

A, JB, the longer diameters of the pebbles not only coincide with the 

cleavage of the rock, but at the squeezed end of the specimen the outlines of the pebbles may not be distinguished.

In some instances also, and probably in many, the flattening of the 

pebbles- has resulted in developing a cleavage within them, and this 

cleavage has been observed to be parallel to the flatness of the pebbles. In a schistose quartzite conglomerate a from the Metropolitan 

district, Michigan (fig. 27), and in a schistose breccia-conglomerate from 

Madoc, Ontario, sections through the pebbles show that the individual 

quartzes making up the complex pebbles have been uniformly elongated by recrystallization in the same plane

in which the pebble as a whole 

has been elongated. In the 

same rocks, also, there is evidence of slicing and granulation of certain particles in 

the pebbles, resulting in a 

parallel arrangement slightly 

inclined to the plane of elongation of the pebbles. Par- 

'allelism of constituent minerals is excellently developed 

in much elongated slate, 

schist, and diorite pebbles in 

a slate conglomerate from 

 Pine River, Wisconsin (PI. 

XXVII, A} J5, C}. While in a few cases the parallel arrangement of 

the mineral constituents may have existed in the pebble before it 

became a part of the conglomerate, there is no question that the prevailing parallel structure has been developed by the secondary deformation which has affected the rock as a whole. In these cases, and in 

others, the cleavage of the pebbles may be good or poor, depending 

on the nature of the constituents, and on the extent to which the 

matrix has taken up the deformation, but, such as it is, the cleavage 

is parallel to the longer diameters of the minute particles making up 

the pebbles, to the plane of elongation of the pebbles as a whole, and 

finally to the schistosity or cleavage of the rock mass as a whole.

a Van Hise, C. R., Principles of pre-Cambriaa geology: Sixteenth Ann. Kept. U. S. Geol. Survey, pt. 1, 

1896, PI. CXV. N

FIG. 27. Pebbles of quartzite with constituent quartz 

grains elongated by recrystallization in the plane of 

elongation of the pebbles and of the rock as a whole. 

Schistose conglomerate from Metropolitan, Mieh. 

Large slide.
104 BOCK CLEAVAGE. [BOLL.239.

Illustrations might be multiplied, but the cases here cited will serve 

to show the bearing of this line of evidence on the subject in question.

(2) The distortion of mineral crystals. The distortion of crystals,- 

either by recrystallization or by granulation, is a commonly observed 

phenomenon. In the description of the features of occurrence in the 

rocks with flow cleavage in Chapter II instances of this are cited. 

Numerous slides have been observed in which the plane of cleavage is 

marked by mica plates or hornblende crystals, while the associated 

quartz and feldspars show fractures at angles with the plane of cleavage (figs. 9, 16, 17). Where, accompanying these fractures, there has 

been displacement of the parts, as is frequently the case, this displacement is observed to work toward extending the fractured particle in 

' the plane of rock cleavage and shortening it normal to the plane of 

cleavage. Lehmann figures a number of instances of this. a Van 

Hise 6 quotes Keith as making the following statement:

Near Blowing Rock, N. C., is a mashed porphyritic granite in which porphyritic 

crystals of feldspar are flattened in various degrees, and their greater diameters are 

upon the average parallel with the secondary structure. In many cases the feldspar 

crystals are fractured in a direction diagonal to the cleavage, and in some cases in a 

single feldspar crystal there are two sets of diagonal fractures approximately at right 

angles to each other and each inclined about 45° to the cleavage.

(3) Distortion of volcanic textures. Ancient volcanics, particular^ 

of pre-Cambrian age, rarely occur over considerable areas without 

showing cleavage structure in part. ( In the Lake Superior country 

a greenstone with original ellipsoidal parting frequently shows a flattening of the ellipsoids in one direction, with or without fracture, and 

in such cases the ellipsoids and matrix have a flow cleavage parallel to 

the longer diameters/ In the pre-Cambrian rocks of Lake Superior 

are many schistose volcanic rocks containing amygdules and spherulites. The elongation of the amygdules^ and spherulites in planes 

parallel to the cleavage in the rock is of common occurrence. The. 

elongation of spherulites and perlitic textures may be particularly 

 well observed in certain areas of isolated pre-Cambrian volcanics in 

the Fox River Valley of Wisconsin. Here there is also Un agreement 

in direction between cleavage and the greater diameters of the distorted fragments of a volcanic breccia. Harker 6 describes the agreement in direction of cleavage and flattening of the fragments in a 

schistose volcanic ash in the boulder clay at Nantlle. In specimens 

from the Black Hills, and from the Menominee district of Michigan, 

the flattening of fragments in breccias is to be observed.f

« See figs. 1, 4, and 5, PI. I, Lehmann's Atlas, cit. ' 

b Van Hise, C. R., Deformation of rocks: Jour. Geol., vol. 4, 1896, p. 460. 

cClements, Mon! U. S. Geol. Survey, vol. 36, PI. XI. 

rtSp. 27750, Crystal Falls district, Michigan.

eSp., 14823, Black Hills, South Dakota; sp., 25669, Menominee district of Michigan. 

/Harker, Slaty cleavage: Kept. 55th meeting Brit. Assoc. Adv. Sci., held in 1885, published in 1886, 

p. 837.
. RELATION OF CLEAVAGE TO BEFOEMATION. 105

(4) Distortion of fossils. The elongation of fossils in the plane 

of cleavage has been observed in so many cleavable rocks that 

no reference will be made to the individual cases. Harker, in his 

article on slaty cleavage, cited, refers to a number of such cases. In 

these distorted fossils it has been observed that the mean axis of- strain 

has been one of absolute elongation in many cases.

(5) Distortion of beds and attitude of folds. Frequently there may 

be observed the thickening or thinning of a bed or layer of rock 

through the processes of rock flowage. Any cleavage which is present in such a distorted bed is likely to be normal to the shortening.

Folds often show the direction of shortening of the deformed rock 

mass. The well known relation of cleavage to simple folds, i. e., a 

position roughly parallel to their axial planes, shows plainly the 

development of cleavage normal to the greatest compression which 

the rock mass has undergone. Deviation from this attitude becomes 

apparent as shearing develops parallel to the limbs of folds, as discussed in a subsequent chapter (p. 152). Folds may be indicated by 

the distortion of bedding. Certain specimens from the. Black Hills 

show that the bedding has been shortened in its own plane by minor 

crenulations, and the associated cleavage may be observed to be in 

all cases normal to the greatest shortening indicated by such folds. a 

Similar instances may be cited from the Lake Superior country and 

from almost any other district where cleavable bedded rocks, particular^ slates, occur. *

Sorby has figured and described undulations in a coarse sandy shale, 

which lies between beds of fine shaly slate showing cleavage but not 

^folding. The axial planes of the undulation in the coarse rock coincide with the cleavage planes of the finer rocks. It is clear that the 

direction of compression as shown by the folds is at fight angles to 

the cleavage planes in the slate,above and below. 6 Harker says that 

similar phenomena may be observed in almost any of the slate quarries 

of northern Wales. *

Folds may be indicated by the distortion of gneissic or original 

flowage structure. The distortion of gneissic banding in the preCambrian gneisses is too common and widespread to need detailed 

reference. 6 Wherever, with this folding, a subsequent cleavage has 

been developed, this appears in planes normal to the shortening of the 

rock mass shown by the folds.

(6) Relations to intruswes. Intrusions of great masses of igneous 

rocks, and particularly deep-seated batholiths, are known to compress 

the adjacent rocks in directions normal to the periphery of the intruaSp. 14974, si. 7712, Black Hills, South Dakota.

& Harker, Slaty cleavage: Kept. 55th meeting Brit. Assoc. Adv. Sci., held in 1885, published in 1886, 

p. 824.

o Van Hise, C. R., Principles of pre-Cambrian geology, Pis. CX and CXVII and fig. 162. Lehmanu, 

Atlas, PI. XIV, figs. 2 and 4; PI. XVI, fig. 1.
106 BOCK CLEAVAGE. ' [BULL.239.

/>

sive masses. Cleavage is commonly developed in the surrounding 

rocks parallel to the periphery of the intrusive masses. Its development in planes normal to the greatest compression is thus clear. An 

illustration, cited by Van Hise, is the Black Hills batholith of granite 

which has intruded a sedimentary series, and developed a cleavage 

parallel to the periphery of the granite.

(7) Relations to fractures. Where fractures and cleavage have been 

developed simultaneously, as is possible in heterogeneous rocks, the 

displacement along the fractures may indicate the direction of shortening and elongation of the rock mass as a whole. This is partially covered in the above discussion of the distortion of pebbles, crystals, etc. 

Even if the displacement can not be observed, the position of the 

fractures may indicate the probable direction of the compression of 

the rock mass, for fractures are ordinarily developed in planes inclined 

to such compression, and hence when the cleavage plane is inclined to 

the planes of fracture (figs. 9,16, IT) the presumption is that it is not in 

the shearing planes, but more nearly normal to the compression of the 

rock mass. This criterion affords only a suggestion of the relations 

of cleavage to shortening of the rock mass, for the precise relations 

vary with the nature of the deformation, as shown in subsequent 

chapters.

GENERAL.

From the lines of evidence above cited it appears that wherever the 

directions of shortening and elongation of a rock mass can be determined with certainty any flow cleavage which may be present is.normal to the total greatest shortening which the rock has undergone. 

The greatest, mean, and least diameters of the particles may be observed. 

to have a tendenc}r toward parallelism with the greatest mean and 

least axes of strain in the rock mass.

But there are minor deviations from parallelism, some of which are 

due to the heterogeneity of the rock mass and some of which are due 

to .the manner of deformation.

Says Van Hise: "It is a very common phenomenon in slates and 

schists, both macroscopically and microscopically, for the direction of 

the secondary structure to wrap around the harder particles. As a 

hard grain or pebble is approached the cleavage structure in the 

matrix opens out on each side of the grain, envelops it, and closes in 

again beyond it. The structures nowhere intersect, although upon 

opposite sides of a particle near the ends they converge, and in passing toward either end the}7 turn and become parallel. " a

In a few and rather exceptional cases, where the parallelism of the 

longer diameters of the mineral particles is clearly the result of granulation or slicing, as in certain sheared conglomerates and sheared

a Van Hise, C. E., Deformation of rocks: Jour. Geol., vol. 4,1896, p. 460.
i.K'TH.] RELATION OF CLEAVAGE TO DEFORMATION. .107

anortliosites described in foregoing chapters, the longer diameters of 

the particles may show uniform, though slight, tendency to deviate 

from parallelism with the elongation of the rock mass, as shown by 

the nature of the strain or by the position of the longer axes of other 

particles present Avhich have received their position parallel to the 

elongation of the rock mass by recrystallization.

In Chapter VI it will be shown that the longer diameters of the 

mineral particles sometimes deviate from parallelism with the elongation of the rock mass as a whole, because of the changing relations of 

the strain to the stress producing it during a rotational strain.

The deviations from parallelism under these conditions might be 

considered as evidence that the parallel structure is developed in 

shearing planes or planes of maximum tangential strain inclined to 

the greatest elongation of the rock mass, for where the rock has been 

much shortened even structures developed in shearing planes may

vary only a few degrees from parallelism to the elongation of the 

rock mass. But the deviation may be adequately explained otherwise. (See following.chapter.) It is certain that in the absence of 

the modifying conditions referred to the parallelism of the longer 

diameters of the mineral particles and the longer axes of strain in the 

rock mass is close, and that no uniform deviation can be detected 

either by microscopical or macroscopical observation.

It is clear from phenomena of the kind above described that an 

excellent cleavage may develop with but a comparatively small 

shortening of the rock mass. But the excellence of the cleavage 

seems to be more a matter of the nature, of the mineral constituents 

(each of them having its own uniform influence on- cleavage) than a 

matter of the amount of deformation which a rock has undergone. 

Where the conditions have been favorable to the development of minerals such as mica and chlorite, for instance, a comparatively small 

amount of shortening of the rock mass has developed a good cleavage, 

while, on the other hand, a very considerable amount of shortening has 

sometimes failed to produce anything but a poor cleavage in a rock 

consisting mainly of quartz particles.

It is of interest to ascertain whether the mean axis of strain represents actual elongation or shortening of the rock mass. Many 

instances of elongation of the niean diameters may be cited in the distortion of original crystals, in the distortion of pebbles of conglomerates, in the distortion of original volcanic textures, and in a few 

cases in the distortion of folds. The shapes of newly developed minerals, and especially those of mica and chlorite, are themselves suggestive of the nature of the strain which the rock has undergone. 

The greatest and mean diameters of the mica and chlorite plates have, 

practically the same dimensions, and if the greatest diameter represents 

elongation, as it does beyond reasonable doubt, the mean diameter also
108 BOCK CLEAVAGE. ' [BULL.239.

represents elongation. Instances of shortening of the mean diameter 

also may be observed in the distortion of all of the forms above named, 

and the shapes of certain minerals newly developed are again suggestive 

of shortening of the mean axes of strain in the rock. In hornblende 

the greatest diameters are far greater than the mean and least diameters, which do not differ widely from each other. If the least 

diameter represents a shortening of the rock mass it seems likely that 

the mean diameter does so also, leaving only one direction of elongation parallel to the longest diameter of the hornblende crystals. The 

shapes of the mica and hornblende crystals so characteristic of cleavable rocks is taken as one of the best criteria for determining the 

nature of the strain, and one is tempted to go even further and say 

that when the chemical and physical conditions are favorable for the 

development of either mica or hornblende, mica may develop when the 

conditions are such that there is elongation along the mean axes of 

strain in the rock and hornblende when there is shortening along the 

mean axes of strain in the rock. A review of the facts seems to indicate that in the development of cleavage there has been, for the most 

part, an actual elongation of the mean axes of strain in the rock rather 

than a shortening, and that the relative importance of elongation and 

shortening of the mean axes is about the same as the relative importance 

of mica and hornblende in the production of rock cleavage.
CHAPTER VI.

RELATIONS OF THE ELONGATION AND SHORTENING OF ROCK 

MASSES, AND HENCE OF FLOW CLEAVAGE, TO STRESS.

Thus far in the discussion there has been a basis of observed geological fact. It has been possible to observe the causal relation between 

parallel arrangement of mineral particles and flow cleavage, and 

between the parallel arrangement of the mineral particles and the 

direction of elongation and shortening of rock masses. It is not 

possible to observe directly the relations of cleavage to the stresses 

which have deformed the rock, but as the relations of cleavage to the

elongation and shortening of rock masses are known and as the relations of elongation and shortening of solid bodies to deforming stresses 

may be worked out in their simpler aspects, both experimentally and 

mathematically, and are accepted as proved in physical and engineering 

treatises, the general relations of cleavage to the stresses producing 

it may be stated with some confidence. The first step in the discussion,, then, is a summary of the simpler and most obvious relations 

of deformation of solid bodies to stress. a

STRAIN.

Strain means any change in the relative position of the particles 

of a body. The change may be either of form or volume, or both. 

When the form changes the strain is called distortion. When the 

volume changes, the strain is called dilatation.

Any small sphere in an unstrained mass becomes an ellipsoid after 

strain i. e., a strain ellipsoid, the greatest, mean, and least axes of 

which are called the principal axes of strain. In a special case (simple 

dilatation) all diameters of this sphere are changed equally and the 

resulting ellipsoid is a sphere. If these axes remain constant in 

direction during strain, the strain is called "irrotational" strain; if 

not, it is called "rotational" strain.

"Hoskins, L. M., taken mainly from " Flow and rupture of rocka": Sixteenth Ann. llept. U. S. Geol. 

Survey, pt. 1,1896, pp. 845-872.

Van Hise, C. K., Principles of North American pre-Cambrian geology: Sixteenth Ann. Kept. U.S. 

Geol. Survey, pt. 1,1896, p. 636.

Thompson and Tait, Natural philosophy.

Becker, G. F., Finite homogeneous strain, flow, and rupture of rocks: Bull. Geol. Soc. America, 

vol. 4,1893, p. 22; Jour. Geol. vol. 4,1896, p. 430.

Young, Thomas, A' Course of Lectures on Natural Philosophy and the Mechanical Arts, London, 

2 vols., vol. 1,1807, p. 135.

Peirce, C. S., Manuscript report to the Director of the U. S. Geol. Survey, 1897.

109
110 ROCK CLEAVAGE. [BULL. 239.

Any irrotatiomil strain in which all three principal axes are changed 

in length in such a ratio that the volume remains constant, has been 

called "pure shortening" by Van Hise,"' and this term will be used 

below. In a special case of irrotational strain without change of volume, one of the axes may remain unchanged in length; then the 

strain is known as a "simple shear" (Thompson and Tait &), or a "pure 

shear" (Becker c), or a "simple detrusion" (Young^ and Peirce*). An 

irrotational strain is illustrated in two dimensions in figs. 28 and 29. 

Every particle of the body takes part in the deformation.

FIG. 28. > FIG. 29. 

PIGS. 28 and 29. Diagrams showing irrotational strain (pure shortening).

A rotational strain occurring without change of volume is illustrated 

by the deformation of a rectangle A B C D (fig. 30) into a parallelogram A B C' D' whose base and altitude are equivalent to 

those of the rectangle. All lines parallel to A-B move parallel to it 

through distances proportional to their distances from A-B. It is a 

strain analogous to that assumed by a deck of cards in which each card 

has been slipped a small amount over the card next below. While 

there is differential movement between the 

cr planes, there is no distortion in the planes them-

' selves. The strain is equivalent to an elongation 

and a shortening at directions at right angles to 

each other combined with a rotation. This strain 

has been called "simple shear" (Hoskins, Van 

Hise) and "scission" (Becker). Peirce, in the 

report referred to, prefers the term scission, 

and in the following discussion the term scission will be used. The 

plane of scission referred to on subsequent pages corresponds to the 

plane of slipping between the cards in the above illustration.

It has been proved that scission is equivalent to a pure shortening 

combined with a rotation of the body as a whole. In fig. 32 the flata Van Hise, C. R., Principles of North American pre-Cambrian geology: Sixteenth Ann. Kept. U. S. 

Geol. Survey, pt. 1,1896, p. 636.

b Thomson and Tait, A Treatise on Natural Philosophy, 2d ed.

cBecker, G. F., Jour. Geol., vol. 4,1896, p. 430.

dYoung, Thomas, A Course of Lectures on Natural Philosophy and the Mechanical Arts, London, 

2vols.,vol. 1,1807, p. 135.

e Peirce, C. S., Manuscript report to the Director of the U..S. Geol. Survey, 1897.

FIG. SO. Diagram showing 

scission.
LKITH.] KELATIONS OF CLEAVAGE TO STRESS. Ill

tened ellipsoids have the position they would have if flattened by 

shortening along the direction P Q and- then rotated until the line 

P Q takes the direction P' Q'.

Given any strain ellipsoid, it is impossible to determine from the 

ellipsoid itself whether it was produced in its present position Ijy a

Q "sf cf\

FlO. 31. ' -FlO. 32.

FIGS. 31 and 32. Diagrams showing equivalence of a strain ellipsoid in scission to a strain ellipsoid in

pure shortening combined with a rotation.

pure shortening, by pure shortening combined with rotation by 

scission or by any combination of these strains. For proof of this 

relation of scission to pure shortening see Hoskins."

Rotational and irrotational strain may be combined with each other 

in any proportion, and either or both may be combined with a dilatational strain. All strains may be referred to these types of strain or 

some combination of them.

STRESS.

Stress is the action and reaction between two adjacent parts' of a 

body. When a condition of stress exists at any point in a body there 

are always three rectangular planes, upon each of which the resultant 

stress is normal. Three intersecting lines normal to these three 

planes, respectively, are called the principal axes of stress.

The difference in the intensity of the greatest and least of the 

stresses acting on these three mutually perpendicular planes is called 

the stress difference.

A shearing stress is a stress acting tangentially to the -plane sepa-1 

rating two adjacent portions of a body between which there is a stress. J 

This is always present on all planes except the three principal planes, 

unless the three principal stresses are of equal intensity and of the . 

same kind. Since in any possible stress condition three rectangular 

planes through any given point are free from tangential stress, any 

possible stress may be regarded as equivalent to three normal stresses 

whose directions are mutually perpendicular.

a Sixteenth Ann. Rept. U. S. Geol. Survey, pt. 1,1896, pp. 862-8G5.
112 EOCE CLEAVAGE. . [BULL. 239.

RELATION OF STRESS TO STRAIN.

In isotropic material, the axes of the strain that is occurring at any 

instant are coincident with the stress axes. If the stress axes remain 

constant in direction, the total strain will have its axes coincident with 

the stress axes; but not otherwise. Rotational strain in isotropic material will not occur unless the directions of the principal stresses vary. 

In irrotational strain the greatest, mean, and least axes of strain 

correspond, respectively, with the least, mean, and greatest axes of 

stress throughout the deformation. In a rotational strain the strain 

ellipsoid at any instant is the net result of all the strains developed at 

successive stages of the deformation; it is a compromise between the 

strain ellipsoid which is tending to be formed by the stresses then 

effective and the strain ellipsoids previously formed and rotated. 

Thus the directions along which elongation and shortening in the mass 

are occurring at any instant in rotational strain are not parallel to 

the axes representing the net result of all the elongation and shortening which the mass has undergone since the deformation started. At 

any instant the tendency toward shortening is in the direction of the 

greatest of the stresses, but the accompanying rotation prevents the 

continuation of the same relation of the stress and strain axes, and 

the axis of the greatest total shortening may be at that instant at any 

angle to the greatest axis of stress or the axis of shortening then 

occurring. When deformation occurs by fracture the fractures develop] 

in planes inclined to the greatest principal stress as an expression of-j 

shearing stresses.

APPLICATION OF GENERAL PRINCIPLES OF RELATIONS OF STRAIN 

AND STRESS TO ROCK FLOWAGE AND FLOW CLEAVAGE.

Knowing the relations of flow cleavage to the axes of. strain in a 

rock mass, the simpler relations of flow cleavage to stress may be 

deduced. These may be considered under two heads (1) the cleavage 

strictly parallel to the axes of elongation of the rock mass, (2) the 

cleavage exceptionally inclined to such axes.

(1) Where cleavage is strictly parallel to the axes of elongation of the 

rock mass, as it is for the most part where the structure is due to 

recrystallization, and thus in the great majority of rocks with flow 

cleavage, the term flow cleavage may be substituted for the strain it 

represents in'the above statement of the relations of strain and stress. 

In other words "plane of cleavage" ma}7 be substituted for the plane 

of the greatest and mean axes of the strain ellipsoid or " axis of cleavage" (when linear parallel) for the greatest axis of strain. The relations of flow cleavage to stress then may be stated as follows:

During irrotational strain, flow cleavage tends to develop uniformly 

in the plane normal to the greatest principal stress, or parallel to the
J^ITH.] RELATIONS OF CLEAVAGE TO STEESS. 113

least and mean stresses, or if linear parallel cleavage, it develops along 

the axis of least stress.

During rotational strain flow cleavage tends to develop at any instant") 

in planes or lines normal to the greatest stress or in planes or lines in 

which elongation is occurring at that instant. But the cleavage tending thus to Jbe formed is almost immediately rotated from its position 

normal to the greatest principal stress by variation in relative direction 

of the stress axes and strain axes, and becomes inclined to the plane or 

line in which new cleayage subsequently tends to develop. In other 

words, cleavage is at all times tending to develop normal to the greatest pressure in a rotational strain, but rotation constantly carries it 

from this position. Just as the total elongation of the rock mass in 

rotational strain is the net result of all the .strains developed at successive stages of deformation, so cleavage, which by observation is approximately parallel to the final elongation of the rock mass, is the net result 

of all the strains, and its average position may be finally inclined, 

somewhat to the greatest principal stress/'

It is probable that either dilatational strains alone or irrotational 

strains alone are of rare occurrence in a rock mass. The first of these 

requires three and the second two pairs of forces acting at right 

angles to one another with identical intensity. The common strain in 

nature is a combination of the forms of strain here called pure shortening, scission, and negative dilatation. These strains may occur simultaneously or at different times, or the same strains may recur several 

times. But the final result in any case is a strain ellipsoid, whose axes 

have relations to stress somewhat intermediate between those above 

described for pure shortening and for scission; in other words, for 

a rotational and irrotational strain. Hence the final position of cleavage is usually inclined to the axis of greatest stress of the force whiqh 

has produced it, although tending throughout to develop normal to 

this axis.

Given a strain ellipsoid, there is no way of determining what stages' 

or manner of deformation the ellipsoid has undergone to reach its 

present configuration. Likewise it may not be certainly determined 

what combination of stress and strain conditions have been present 

throughout the development of a given cleavage, although the relation 

of cleavage to the final total strain may be known. But it is believed 

that cleavage sometimes gives evidence of variations in stress and 

strain conditions during its development. Flow cleavage is dependent 

upon the parallel arrangement of its mineral constituents. If it be 

granted that these constituents have developed during successive

aln Daubrde's experiment cleavage was produced parallel to the elongation of the clay mass which 

was forced through a cylinder. While the relations of stress and strain are complex in such a case, 

there is nothing in Professor DaubrCe's discussion of the subject which would indicate a development of the parallel arrangement under any laws different from those here summarized. (Loc. cit.)

Bull. 239 05 8
114 ROOK CLEAVAGE. [BULL. 239.

stages of deformation of the rock mass, as they undoubtedly have, 

then if the strain has been a rotational one the particles developed at 

successive stages ought to lie at slight angles to one another. The 

recrystallized particles forming at any instant with their greater 

diameters exactly normal to the pressure during a rotational strain 

arc constantly being rotated from this position. The result is that 

the longer diameters or direction of growth of particles developing 

by recrystallization at any instant normal to the greatest pressure do 

not quite correspond with the longer diameters or direction of growth 

of particles developed by crystallization in preceding instants, for 

these latter have been rotated from the most favorable positions. . This 

is believed to be the main explanation of the feathering out of mica 

laminae diagonally against one another where there are no other rigid 

particles present to cause local variation in stress (PL II, SA) and figs. 

2,3,4). The fact that the diagonal lapping of mica plates against one 

another occurs around rigid particles shows it to be a phenomenon 

caused by difference in stress directions, and hence its occurrence 

where such particles are absent offers evidence of its formation under 

changing stresses.

That cleavage by observation shows so little variation because of 

rotational strains may be due to the fact that recrystallization or granulation in later stages maj^ have obliterated evidences of cleavage 

formed during the earlier stages of a rotational strain, or, more probably, to the fact noted on a subsequent page that the very development of parallel particles by recrystallization during rotational strain 

so modifies the stress conditions within the mass that the plane of 

easiest development for newly developing particles keeps up with the 

rotation to a greater extent than it otherwise would.

Relative positions of original and newly developing particles during 

deformation. From analysis during an irrotational strain the greatest

elongation of the rock mass is uniformly normal to the greatest pressure, and hence this is the constant plane or line of growth of newly 

developing or recrystallizing particles. In the early stages any rotating original particles may, however, vary in position from nearly 

parallel to the greatest pressure to normal to it. It is only after the 

deformation has progressed to a considerable extent, even supposing 

the rotating particles to have considerable freedom of movement, that 

the rotating.original particles formerly at angles to the plane or line 

of elongation, or what becomes the surface of rock cleavage, bring 

their greater diameters to approximate parallelism to that of the newly 

developing or recrystallizing particles in the plane of cleavage. Where 

there is considerable interference with rotation a considerable amount 

of deformation may not bring about parallelism of the rotated particles. 

If the deformation of a rock mass is a rotational one, as it commonly is, in the early stages there is likely to be the same difference
MOTH.] KELATIONS OF CLEAVAGE TO 8TEESS. 115

in .position between particles with original random arrangement and 

the plane or line of growth of newly developing or recrystallizing 

particles. But in deformation of this kind, all particles, both the particles already present with random arrangements and the newly developing and recrystallizing ones, are being rotated toward the plane of 

scission. Particles with original random arrangement during a rotational strain may be rotated toward the plane or line of easiest relief or 

easiest crystal growth, or away from it, depending upon the original 

position of the particles with reference to the plane of easiest growth. 

Starting with an unstrained rock, it is of interest to note that a rotational strain sufficient to produce elongation of the mass, say, at 35° 

to the plane of scission, will not rotate a particle originally normal to 

the plane of scission to such an extent as to make its greater diameters 

correspond with those of newly developing particles.'* 

Whether the deformation is by rotational or irrotational strain a

sufficient amount of it may bring about substantial parallelism of all 

particles, new or old.

While analysis would seem to indicate that the relative positions of 

original and recrystallizing particles during deformation may be somewhat as above, it is not at all ^certain that this is actually the case in 

the metaclasic rocks; indeed, evidence has already been adduced to show 

that evidence of the rotation of original particles is insignificant as) 

compared with the evidence of recrystallization in developing a paral-7 

lelism. So far as recrystallization does arrange the minerals there may 

be no intermediate stages of arrangement. There is seldom to be 

observed in cleavable rocks stages intermediate between random 

arrangement and parallelism. The parallel arrangement usually 

appears first fully developed along certain zones while the intervening zones are comparatively unmodified, and where it thus appears, 

there is abundant evidence of'its development entirely by recrystallization. If it be true that the parallel arrangement is due largely to 

recrystallization, and that intermediate grades of arrangement, which 

the stress and strain conditions in the rock mass would necessarily 

develop from the rotation of original particles, are absent, then mere 

parallelism of the mineral constituents becomes presumptive evidence 

that the parallelism has been produced by recrystallization with relations to stress and strain above indicated. This evidence could not 

stand by itself were it not supported by detailed evidence discussed 

in Chapter III.

Effect of heterogeneity. The above statements are based on the 

assumption that the rock mass undergoing rock flowage and developing rock cleavage acts essentially like a homogeneous body in its stress 

and strain relations; but a rock is. commonly a heterogeneous body,

«Hoskins, L. M., Sixteenth Ann. Rept. U. S. Geol. Survey; Van Hise, C. B., Deformation of rocks: 

Jour. Geol., vol. 5,1897, p. 186.
116 ROCK CLEAVAGE. . " [BULL. 239.

and hence there are minor variations in the stress and strain relations, 

under the law stated on a preceding page that " in an isotropic material the axes of the strain that is occurring at any instant will, in 

general, differ in direction from the stress axes, the strain being in part 

governed by the structure of the material." The general effect of 

rigid particles is to transmit stresses locally in directions normal to 

themselves. The mesh structure which is frequently seen around the 

peripheries of large particles owes its origin to the local distribution 

of stresses caused by the influence of a rigid particle on the transmission of forces. Here is additional and most positive evidence of the 

development of cleavage in planes normal to the greatest stress, for it 

is evident that stresses are generally transmitted by rigid particles 

in directions normal to their peripheries. The feathering out of 

mica laminae diagonally against one another about rigid particles, as 

described on pages 25-26, is probably to be explained in this manner. 

Frequent reference has been made to the slicing or fracturing of 

rigid particles in planes, sometimes intersecting, that are inclined 45° 

or thereabouts to the plane of rock cleavage conditioned by the 

parallel arrangement of the micas and other constituents (see figs. 16, 

17 and PI. XI, !>}. As fractures develop in shearing planes inclined 

to the principal stresses, the presence of fractures inclined to the 

rock cleavage is itself evidence of the development of the prevailing 

cleavage in planes normal to the greatest stresses.

' The influence of rigid particles in the transmission of forces during 

rotational strain is of interest: There is supposed^ a rotation of the 

greater diameters of all particles, original and recrystallized, toward 

a common plane. These particles all act as transmitters of forces. 

The rotation being all in the same angular direction, the change 

in direction of transmission of pressure due to the rigid particles, 

is also all in the same direction. Hence, so far as rotation tends 

to bring about parallelism/of the longer diameters of the particles themselves, it also tends to bring about parallelism of the 

transmitted pressures in a direction more nearly normal to the plane 

toward which the particles themselves are being rotated. Particles 

newly growing under these local transmitted stresses thus tend to 

bring their longer diameters more nearly into the plane which the 

rotated particles are approaching than they otherwise would. Particles newly developing under rotational strain are constantly beingrotated from a direction normal to the greatest pressure of the rock 

mass, and hence from a position favorable to easiest growth, and 

because of rotation they are not parallel to the direction of growth of 

succeeding stages. But, so far as the particles themselves affect 

the transmission of forces, they tend to transmit forces such that 

newly developing particles in succeeding instants will develop in planes 

more nearly parallel to their own rotated planes than they otherwise 

would. For instance, suppose the longer diameters of a newly devel-
LEITH.] RELATIONS OF CLEAVAGE TO STRESS. 117

oping particle in fig. 33 to have the position AB in a rotational strain. 

At a later period the longer diameter may have the direction A'B'. 

If the stresses remain constant in direction, the plane of easiest growth 

still remains more nearly parallel to AB than to A'B'; in other words, 

the particle has been deflected from a position most favorable to its 

growth under the given stress conditions. But the rotating particle 

itself influences the direction of the transmission of .stresses, with the 

result that a particle CD, newly developing under the influence of such 

local stresses, would tend to take a position not parallel to AB, but 

more nearly parallel to A'B'. This is made clear by simple resolution 

of forces. Rn , the transmitted normal component of the principal 

stress is proportional to the cosine of the angle a. When the angle a. 

is 90°, this component becomes zero. When the angle a is zero, the 

component Rn has the full intensity of the unresolved force.

R

FIG. 33. Diagrams illustrating rotation of particle during rotational strain and the resulting in" 

change in direction of transmitted stresses..

W here the rock mass is made up of many rigid particles their influence 

may be of considerable importance in keeping newly developing particles in the plane of rotated particles previously present or developed.

A special case of the influence of rigid particles exists where the 

particles have a previous parallel arrangement. The deflection in 

direction of transmission of pressure will be uniform, and any newly 

developing cleavage may be uniformly in planes somewhat inclined to 

those of ordinary development of cleavage in a homogeneous rock 

without a previous parallel arrangement. Discussion of the relations 

of cleavage to previous parallel structures is made on pp. 151-152.

(2) Where the flow cleavage "is due largely to granulation or slicing 

and the longer diameters of the mineral particles are uniformly, 

though slightly, inclined to the axes of elongation of the rock mass, 

the relations to pressure ma}^ exceptionally be somewhat different from 

those above cited. The fractures are the result of shearing stresses 

inclined to the greatest normal stress (pp. 111-112), and the carving of 

parallel grains in situ by the kind of fracturing known as granulation
118 BOOK CLEAVAGE. [BULL.289.

or slicing, causes a cleavage which can be said to have developed in 

shearing planes inclined to the greatest pressure. The cleavage of 

the deformed anorthosite described by Adams (p. 37) may be partly 

formed in this way, and many rocks, as noted in Chapter II, show 

evidence of such development in certain minerals. Fig. 13 represents 

an unmashed anorthosite, and fig. 14 its mashed equivalent. The elongated feldspar particles in fig. 14 are clearly the result of carving in 

situ by the process of granulation or slicing along .the sides. All 

stages of the process may be observed in the rock mass. That granulation or slicing does not occur in planes normal to the greatest 

pressure, but is uniformly in planes inclined to this pressure, is a 

well-established principle of mechanics, and hence we- may conclude 

that the elongated particles of feldspar seen in fig. 14 may be really 

carved in situ in shearing planes and lie essentially in shearing planes 

of the rock mass. On this explanation the longer diameters of the 

particles can not be parallel to the elongation of the rock mass represented by CC in fig. 14. To bring about the deformation shown in fig. 

14, it is not necessary to assume that the movement was entirely parallel to A A, for the same result may have been brought about by a 

movement parallel to BB, and in any given case it is^practically impossible to tell what combinations of strain have occurred.

It has been shown (pp. 31 and 37) that as a result of granulation and 

slicing in intersecting planes at low angles to the greater strain axes 

of the rock mass residual grains may lie with their longer diameters 

essentially parallel to these axes in other words, parallel to the prevailing cleavage. Such cases are covered by the general statements 

under (1) above.

Summary. Commonly flow cleavage has been developed both by 

recrysfcallization and by granulation or slicing; recrystallization of certain minerals and granulation of others close by, in which case the parallel arrangement developed by recrystallization is mainly in planes 

normal to the greatest pressure as in (1) pages 112-117, and the parallel 

arrangement developed by granulation or slicing is developed in shearing planes as in (2) pages 117-118. In the early stages the inclination 

of the parallel structure fonned by fracturing to the parallel structure 

formed by recrystallization is apparent (p. 38), and this deviation is 

excellent evidence of the truth of the conclusion that flow cleavage 

may be developed both in normal and shearing planes.

The fact may again be emphasized that the parallel arrangement 

caused in indirect planes by granulation and slicing is a subordinate 

and exceptional phenomenon as compared with that developed by 

recrystallization, and even where present frequently causes only a 

local variation of cleavage from planes conditioned by arrangement of 

crystallized particles; hence flow cleavage is for the most part developed 

in. planes normal to the greatest pressure, and in but small part in 

shearing planes inclined to the greatest pressure.
PART II.

FRACTURE CLEAVAGE, COMPARISON WITH 

FLOW CLEAVAGE, SUMMARY STATEMENT 

OF CAUSES AND CONDITIONS OF SECONDARY ROCK CLEAVAGE.

CHAPTER I.

FRACTURE CLEAVAGE.

Fracture cleavage may be defined as a cleavage dependent for its 

existence on the development of incipient parallel fractures'or actual 

fractures which by subsequent welding or cementation remain planes 

of weakness. It is obvious that the development of such a structure 

is confined to the zone of "rock fracture." Rocks may be fractured 

along parallel planes quite independently of any arrangement which 

the mineral constituents may have, arid the fractures may be cemented 

by infiltration of foreign material, by crystallization of new minerals, 

and by recrystallization of adjacent minerals, or may be welded by 

bringing adjacent minerals <by compression under bonds of molecular 

attraction. After welding the rock may still have a capacity to part 

along such planes more easily than along others, which capacity by 

our definition is truly a cleavage. Such cleavage has been variously 

called in its different aspects false cleavage, close-joints cleavage, 

strain-slip cleavage, fault-slip cleavage, ausweichungs cleavage, rift, 

and fissility. All of these terms may not have quite this significance, 

as will be shown below, but they have been used to designate structures essentially developed as above stated. The reader will doubtless recall many instances of quarry rocks, apparently massive, which 

on the stroke of a hammer break along definite planes nearly or 

quite independent of any parallel arrangement of the mineral constituents that may be present. In the ancient crystalline rocks it is not 

uncommon to find apparently solid graywackes and slates which break 

into polygonal or rhomboidal blocks along weakly cemented or incipient planes of fracture intersecting one another at uniform angles.

119
120 BOOK CLEAVAGE. [BULL. 239.

This may not be apparent in the solid ledge, but becomes apparent 

on weathering or when artificially broken. Cleavage of this kind is 

often excellent; the planes of parting are smooth, even, and continuous for some distance. A distinctive feature is its intermitted 

character, by which is meant its confinement to certain definite 

planes separated by considerable thicknesses of rock which show no 

tendency to cleaved Another distinctive feature is its presence in 

two or more intersecting, planes rather than one. plane. It is apparent that such cleavage may be present in rocks both with and without parallel arrangement of the mineral constituents, and in the 

former case it may occur either parallel to the longer diameters of 

the mineral constituents or at any angle to them. Where parallel to 

the mineral arrangement it is practically indistinguishable from flow 

cleavage (pp. 130-133). In other cases there is little difficulty in distinguishing the two. (See Pis. XVn-XXUI.)

Many illustrations might be cited 

of closety spaced parallel slips along 

one or two sets of planes which have 

been cemented .or welded, forming a 

fracture cleavage. The structure 

described by Sorby (p. 17) as 

"close-joints cleavage" is for the 

most part so developed. The cementation of any of the widespread 

structures which Van Hise (p. 

17) has called fissility, yields a capacity to party which comes under 

this head. PI. XVIII, B illustrates 

a marble which has been sliced into

thin parallel layers, the rubbing 

along the fractures being followed 

by the development by recrystallization of new mica and chlorite, cementing the rock and giving it a 

fracture cleavage. Dale" figures an excellent example of this structure in closely spaced planes. The so-called "false cleavage" is usually the result of closely spaced, parallel, overthrust folds grading 

into minute faults crossing a previously developed flow cleavage. 6 

(See PI. XIV, B.} The "ausweichungs cleavage" is a name applied 

to a similar structure, caused by the development of minute overthrust 

folds passing into faulting. (See Pis. XX, XXI.) Slip cleavage and 

strain-slip cleavage are for the most part other names for phenomena 

of the sorts above described.

2 MILLIMETERS

FIG; 34. Fracture cleavage in slate emphasized

by ferruginous staining. There are 360 cleavage planes to the inch. After Dale.

aDale, T. Nelson, Sixteenth Ann. Kept. U. S. Geol. Survey, pt. 1, 1896, fig. 91. 

bSp. 14974, Black Hills, South Dakota; sp. 42089, north shore of Lake Superior.
i'K«H.] FKACTUEE CLEAVAGE. 121

The rubbing along fracture planes incidental to the development of 

fracture cleavage frequently and usually results in slickensides or the 

development of parallel arranged minerals, such as chlorite and mica, 

and these may cement the partings. These .minerals are clearly the 

result of recrystallization, and may be supposed to develop in the 

same manner as flow cleavage during a rotational strain or scission; 

that is, to develop in normal planes, but to be rotated .almost at once 

toward the plane of scission,.which in this case is the plane of the 

fractures. It may be said, then, that one of the incidental results of 

the development of fracture cleavage is a parallel dimensional arrangement of the mineral particles. This parallel arrangement, however, 

affects only a minute film of the rock along the parting planes, and 

may have no connection with the texture of the minerals in zones 

intermediate between the fractures. If the fractures be closely enough 

spaced it might happen that nearly all of the constituents of the rock 

might be arranged with their longer diameters parallel as the result of 

the rubbing along fracture planes, but it is argued on a subsequent 

page that very closely spaced planes of fracture are very exceptional 

and local, except where secondary to a previously developed flow 

cleavage, and that it is doubtful whether such fractures are ever 

closely enough spaced, independent of any previously developed 

parallel structure, to develop a parallel arrangement of the mineral 

particles comparable with that shown in fissile schists or slates. In 

practice there is cer.tainly little difficulty, with few exceptions, in 

distinguishing the parallel arrangement developed as a result of rubbing along shearing planes from the parallel arrangement developed 

in normal planes and effecting all the constituents of the rock.

RELATIONS OF FRACTURE CLEAVAGE TO STRESS.

The relations of fracture cleavage to stress and rock deformation 

are different from those of flow cleavage. In the latter the parallel 

arrangement of the mineral particles, and hence the cleavage, is 

developed in planes or lines essentially parallel to the elongation of 

the rock mass. In the former the cleavage is developed in planes 

inclined to the elongation of the rock mass.

In any distortioual strain there are necessarily shearing stresses in 

planes inclined to the greatest pressure. These do not receive expression unless there is fracturing, but whether or not there is fracturing 

the elongation of the mass at any instant is normal to the greatest 

pressure. If fractures occur in irrotational strains, these follow intersecting planes approximately 45° to the greatest pressure planes of 

greatest tangential stress. This angle varies somewhat with the nature 

of the substance and the stress conditions. With a substance not ideally 

brittle it is probable that this angle is somewhat more than 45° to the
122 BOOK CLEAVAGE. [BULL. 239.

d

Fia. 35.

greatest pressure. a With brittle substances the angle is probably less 

than 45°. b Where the two lesser stresses are equal, conchoidal fractures are produced such as occur in ordinary building stone tests. 

The displacement of the fractured parts results in elongation or shortening of the mass in such manner that the axes of the strain for the 

body as a whole, if it be still considered as a unit, have the same relations to pressure as those above stated for flow cleavage. 

If fracturing occurs in rotational strains the fractures are in an

intersecting set in such a 

position that the line of 

greatest pressure intersects the obtuse angles 

made by the fissures, and 

makes a smaller angle 

with the short side of the 

parallelogram of cracks 

bounding the column of 

rock than with: the long 

side. c However, as in 

the case of irrotational 

strain, the displacements 

following the fracturing 

are such as to elongate 

and shorten the mass in 

the manner just indicated. 

While in rotational 

strain the fractures may 

develop in two intersecting planes, they are likely 

to show differences in these two planes. If the rectangle a-l)-c-d in 

fig. 35 be deformed to the parallelogram e-f-g-h in fig. 36, two sets of 

fractures will be formed, but in the direction e-h there is compression, 

and the rubbing along the fractures parallel to e-h produces slickensided surfaces, while along the diagonal f-g there is actual stretching 

of the material with the formation of cracks, and further deformation 

occurs more easily by the widening of these cracks than by the 

formation of new ones. The result is a fewer number of cracks 

normal to the longer diagonal of the rock mass than parallel to it. 

The curving of the fractures along the longer diagonal, shown in 

the figure, is due to the control and adjustment between bedding

aBecker, G. F., Finite homogeneous strain, flow, and rupture of rocks: Bull. Geol. Soc. America, 

vol. 4,1893, p. 57.

bBecker, G. F., op. cit.; Hoskins, L. M., Flow and fracture of rocks: Sixteenth Ann. Kept. U. S. 

Geol. Survey, pt. 1,1896.

cBecker, op. cit., p. 55. .

FIG. 36.

FIGS. 35 and 36. Diagrams showing development of fissility, 

which, by cementation or welding, may yield fracture 

cleavage along the longer and shorter diagonals of the 

deformed portion of the rock stratum. After Van Hise. 

In the center of the stratum the fractures are in the planes 

of the greatest shearing, but on the outside of the layer the

 fractures are in lesser shearing planes, the direction of the 

fracture being controlled to some extent by bedding.
I.EITH.] 1EACTURE CLEAVAGE AND STEESS. 123

planes, which in this case are assumed to be more numerous on thcN 

outside of the layer.

The intermitted character of the fracture cleavage is a characteristic 

phenomenon of fractures in homogeneous rocks. Hoskins, a from an 

analysis of the mechanics of the problem, has concluded that closely 

spaced parallel fractures probably do not form through considerable 

volumes of a homogeneous rock with no previous parallel structure, 

no matter what the nature of strain; and that the extensive development 

of closely spaced parallel fractures must have been conditioned by 

some previous parallel mineral arrangement which has controlled the 

directions of fracture. In this Hoskins is followed by Van Hise, & 

who concludes from field observation that fissility developing in shearing planes is usually secondary to cleavage which develops in normal 

planes, although locally a fine iissility may develop independently of

cleavage. If the stress conditions are such that the rock 

may yield by fracture, it does 

so along afewseparated planes 

inclined to the greatest pressure, under the laws above 

noted. These planes once 

formed, further adjustment to 

pressure is easier by displacement along such planes than 

by the formation of new fracture planes. After there has 

been as much displacement as 

possible in the readjustment, 

the stresses may again accumulate so that the fractured parts are again fractured, but, owing to the 

displacement which they have undergone, the later fractures may not 

be quite parallel to the earlier ones. This is illustrated on a small scale 

by the fractures to be observed in granulation. While in the early 

stages of the fracturing of a given mineral particle belonging to one 

set in an intersecting system the fractures are approximately parallel, 

in later stages they are in &ny possible direction.

On the other hand, Becker" has maintained that parallel and closely 

spaced planes of weakness or fractures may and do develop independent of a previously existing arrangement of the mineral particles, the 

closeness of the spacing depending upon the wave length of the impulse 

producing the fracture. 0

aHoskins, L. M., Flow and fracture of rocks: Sixteenth Ann. Kept. U. S. Geol. Survey, pt. 1,1896, 

p. 873.

6 Van Hise, C.R., Principles of pre-Cambrian geology: Sixteenth Ann. llept. U. S. Geol. Survey, pt. 

1,1896, pp. 655-6.

oBecker, G. F., Finite hoinogentous strain, flow, and rupture of rocks: Bull. Geol. Soc. America, 

vol. 4,1895, p. 16.

FIG. 37. Fracture cleavage after lissilityin bedded 

rocks, showing application of principles illustrated 

in figs. 35 and 36.
124 ' ROCK CLEAVAGE. [BULL.239.

There is practical agreement that closety spaced parallel fractures 

may and do develop, but there is not agreement as to causes or relative importance of the structure. Van Hise, on the one hand, maintains that such parallel fractures are for the most part easily discriminated from flow cleavage; that the structure has not been developed 

independent of a preexisting cleavage over any great area comparable 

in size to that in which original flow cleavage may be observed; andthatthe common development of this kind of structure is in planes somewhat separated by zones of noncleavable material. Becker, on the 

other hand, holds that closely spaced parallel fractures independent of 

any preexisting parallel arrangement do actually develop over wide 

areas; but when we remember that he regards flow cleavage, as well as 

fracture cleavage, as a structure developed in this way, the statement 

can not be set against that of Van Hise, for, as the present paper 

attempts to show, Van Hise and Becker are discussing different phenomena. An examination of the literature on the subject makes it clear 

that the cleavage variously called false cleavage, fault-slip cleavage, 

slip cleavage, ausweichungs cleavage, strain-slip cleavage, and rift, 

here grouped together under the term fracture cleavage, in a great 

majority of cases is a structure affecting well-separated planes with 

intervening zones of noncleavable material, and that only in exceptional instances and for limited areas does it occur in planes as closely 

spaced as the planes of flow cleavage and show external similarity to 

flow cleavage. From the observations which the writer has been able 

to make both in the field and laboratory, it seems doubtful whether 

fracture cleavage ever develops in planes as closely spaced as the 

planes of flow cleavage, as this is typically developed in slates and 

schists. Certainly by far the greater number of structures classed 

under the head of fracture cleavage show an intermitted character 

easily distinguishable from the closely spaced planes of. parting of 

flow cleavage, and the statement is a safe one that fracture cleavage 

usually affects more widely separated planes than flow cleavage, and 

perhaps never affects planes so closely spaced as those of the most 

fissile slates or schists. *
CHAPTER II.

COMPARISON OF FRACTURE CLEAVAGE AND FLOW CLEAVAGE.

Certain features of similarity and difference between fracture cleavage and flow cleavage have been already discussed, but they may be 

repeated as introductory to further considerations.

Fracture cleavage is conditioned by incipient'parallel fractures 

or by the cementation or welding of parallel fractures, and the

parallel arrangement of mineral constituents is in no way essential,- 

although it may have been previously present or may be developed 

along thin films as a result of the rubbing along the parallel fractures. 

The planes of parting may be smooth and even, and may be equally 

good in two or more intersecting sets. They are commonly well 

separated by zones of noncleavable rock, but locally may be very 

closely spaced. The structure is developed in shearing planes inclined 

to the greatest pressure, under the normal conditions and laws of 

fracturing, in the portion of the lithosphere which Van Hise has called 

the zone of fracture.

Flow cleavage is always conditioned by the parallel arrangement of 

the mineral constituents, and is good in proportion to the excellence 

of the arrangement, the degree of the inequality of the axes, the 

arrangement of the mineral cleavages, and other features, as shown in 

Part I. The planes of parting are many and closely spaced, being 

separated only by the mineral particles or even by the cleavage plates 

of the mineral particles. The parting is usually parallel to one plane, 

but rarely in two or more intersecting planes, and in such cases one 

is far better than the other. The structure is developed in planes 

or lines parallel to the greatest elongation of the rock mass, usually 

with the same relations to pressure that the elongation of the rock 

bears. Flow cleavage is developed under conditions and laws of rock 

iiowage in the deep-seated portion of the lithosphere which Van Hise 

has called the zone of rock flowage. The processes of recrystallization, granulation, and rotation are effective in flow cleavage, and not 

in fracture cleavage. The development of flow cleavage is accompanied by a diminution of volume through the development b}r recrystallization of minerals with higher specific gravit}7 , a feature in which 

it differs from fracture cleavage, the development of which causes 

little change in the density of the rock.

: 125
126 ROCK CLEAVAGE. [uuLL.239.

EXPLANATIONS OF FLOW CLEAVAGE MAY NOT APPLY TO 

FRACTURE CLEAVAGE.

This is a self-evident proposition which scarcely needs discussion. 

Flow cleavage is l>y definition a cleavage conditioned by the parallel 

arrangement of the mineral constituents, and the causes of this 

arrangement are the causes of flow cleavage. Fracture cleavage is 

quite independent of a parallel mineral arrangement, except where 

superposed upon a preexisting flow cleavage, and hence the conditions 

and causes bringing about the parallel arrangement of mineral constituents are not processes and causes of fracture cleavage. Where 

fracture cleavage is superposed upon a previously existing flow cleavage the parallel arrangement of the constituents may have a strong 

modifying influence on the newly developing fracture cleavage, but 

the causes of the parallel arrangement can scarcely be said to be a 

cause of fracture cleavage; were the previous cleavage not present 

the fracture cleavage would develop, though perhaps not quite so 

readily or in the same planes or in planes so closely spaced. Again, 

the development of fracture cleavage may be accompanied by the rubbing of the sides against one another, developing slickensides or a 

parallel arrangement of the minerals in films adjacent to the fractures. 

The parallel arrangement in the films is probably developed large!}7 

through recrystallization as in rock flowage; indeed, the films may be 

said to have undergone rock flowage. Such a parallel arrangement is 

in no way essential to the existence of cleavage along the fracture 

planes; the welding of the fractures would yield a fracture cleavage 

as certainly if the parallel arrangement were not there.

EXPLANATIONS OF FRACTURE CLEAVAGE MAY NOT APPLY TO

FLOW CLEAVAGE.

The reverse proposition that the explanation of fracture cleavage 

may not apply to flow cleavage is one of the essential points of this 

paper. King,ft in 1875, emphasized the excellence of rock cleavage 

produced by the regelation of parallel joints, and showed that it possesses features in common with "slaty cleavage," supposedly due to 

parallel arrangement of the mineral constituents. He went still 

further and maintained that slaty cleavage itself is a close joint 

phenomenon, and that any parallel arrangement of the mineral constituents is merely an incidental phenomenon. Becker, a on a basis 

of experiment and mathematical analysis of the relations of stress 

to strain, has been positive in the statement that cleavage is not 

necessarily dependent upon the parallel arrangement of the mineral 

constituents, but can be induced as well in a homogeneous rock (a rock

a See discussion and references, pp. 14-15.
LEITH.] EELATIONS OF FLOW AN.D FEAGTUEE CLEAVAGE. 127

without discrete particles) as in a heterogeneous rock, thus implying 

that secondary cleavage in general may not be conditioned or caused 

by the parallel arrangement of the minerals. Doctor Becker recognizes the existence of a parallel arrangement of the mineral constituents in most cleavable rocks, but insists that this arrangement is 

incidental to rock cleavage, and is one of the results, not a cause or an 

essential condition. He maintains that cleavage is a capacity to part 

along certain planes along which the rock has been strained almost, if 

not quite, to the breaking point, giving it a weak cohesion along such 

planes, and that the development of a parallel arrangement of the 

longer diameters of the mineral constituents is possible only along and 

because of planes so formed, these furnishing directions of easiest 

relief.

It is here held that Doctor Becker's theory of the development of 

cleavage in shearing planes, which he applies to cleavage in general,

applies only to what is here called fracture cleavage, and to this only 

with the modification that the parallel development of mineral constituents along shearing planes occurs as a result of actual rubbing after 

fracture has occurred rather than in planes of weakness along which 

no fracture has occurred. It. is held also that Doctor Becker's theory 

will not apply to the structure here called flow cleavage, which is 

dependent on and conditioned by a parallel arrangement of mineral 

constituents, developed for the most part quite independently of shear-- 

ing planes.

If Doctor Becker's view is the correct one, and the parallel arrange- 

/ment of mineral constituents is a mere resulting incident and not a 

'cause, then there would be no good reason why there should be any 

relation between the nature or excellence of the cleavage and the 

arrangement and shape of the particles themselves. The fact that 

there is this close dependence is shown by the following facts:

Flow cleavage always follows either the mineral cleavage or the 

peripheries of the mineral particles, or both.

 The excellence of the flow cleavage varies directly with the degree 

to which the dimensional axes of the mineral particles approach parallelism.

The excellence of the flow cleavage varies directly with the degree 

of arrangement of the mineral cleavages.

 The shape and dimensions of the particles in a rock with flow cleavage are always uniform and characteristic for minerals of the same 

kind, and thus not determined by the form of any preexisting plane or 

line of weakness.

Where a parallel arrangement of the mineral constituents is absent" 

throughout the body of the rock any cleavage which may be present 

has the definite and distinctive characteristics above described for
128 BOOK CLEAVAGE. [BULL.239.

fracture cleavage, which are quite different from characteristics of 

flow cleavage. This is in itself sufficient evidence that the parallel 

arrangement of mineral constituents in some way affects the nature of 

rock cleavage.

It is known that the parallel arrangement of mineral constituents is 

developed under conditions of rock flowage, a process which, by observation and definition, occurs without fracture, and there is positive 

evidence, stated in Chapter III of Part I, that the parallel arrangement of mineral particles in flow cleavage has developed by processes 

entirely adequate to develop this parallel arrangement without the aid 

of previously existing parallel fractures.

It will be shown (Part III) that original cleavage presents analogies 

to flow cleavage. There can be no question that the parallel arrangement of minerals is the cause of original cleavage, and there can, further, be no question that this parallel arrangement is not determined 

by preexisting fractures in planes of weakness.

Hence, it is concluded that the parallel arrangement of mineral particles, far from being an incidental result of the development of cleavage is itself the fundamental condition for flow cleavage, by which is 

meant the structure, such as slatiness or schistosity, which is ordinarily 

referred to as cleavage.

That flow cleavage does not develop mainly in shearing planes in the 

manner of fracture cleavage is held to be shown by the parallelism of 

the longer diameters of the mineral particles and hence of the cleavage 

to the principal axes of elongation of the rock mass as a whole, evidence 

of which is presented in Chapter V, Part I. In this the writer only 

follows most other investigators of the subject. Doctor Becker would 

contend, because of the amount of shortening which the rock mass 

has undergone, that the longer diameters of the particles, developed 

according to his theory, would frequently incline so little to the principal axes of elongation of the rock mass that this inclination would 

be overlooked, and that the minute inclinations actually observed 

between different particles in a cleavable rock are explained by their 

development in inclined planes. It is believed that these slight variations, which are unquestionably present, have a subordinate effect upon 

cleavage (p. 118), and may be partly explained hy local variations in 

stress because of the heterogeneity of the rock (pp. 106-108) and that 

the variations are too irregular to be explained by Doctor Becker's 

hypothesis.

Further, according to mechanical analysis by Hoskins," it rna}^ be 

doubted whether closely spaced fractures or planes of weakness, 

required by Becker's theory, form in a homogeneous rock independent of a previouly existing parallel arrangement, and the observed

alloskins, L. M., Flow and fracture of rocks as related to structure: Sixteenth Ann. Kept, U. S, 

Geol. Survey, pt. 1,1896, pp. 845-874.
. RELATIONS OF FLOW AND FRACTURE CLEAVAGE. 129

facts intensify this doubt. Any cleavage and resulting parallel 

arrangement thus developed is likely to be in well-separated planes. 

The parallel arrangement of flow cleavage affects all of the constituents in all planes, is not confined to separated planes of parting, and 

shows no evidence of actual rubbing (except when followed by subsequent fracture cleavage). o

Doctor Becker" has described the artificial development of cleavage 

in the flattening of ceresin cylinders and has held this cleavage to be in 

shearing planes. Unfinished experiments of a similar nature, begun 

by Doctor Becker and the writer jointly, do not seem to the writer to 

show the parallelism of cleavage to shearing planes. Indeed, the 

flattened cylinders of ceresin cleave directly parallel to the flat sides, 

with only such minor variation near the edges as would be explained 

by the inclined stresses developed there through the compounding of

FIG. 38. Diagram showing the theoretical position of strain ellipsoids in quadrant of ceresin block 

flattened between two plates with friction between quadrant of ceresin block and plates. After 

Becker, Bull. U. S. Geol. Survey, No. 241, PI. Ill, fig. 11.

the normal stress flattening the mass and the inclined stress developed 

by friction of the mass with the plates confining it. The theoretical 

positions of the flattened strain ellipsoids in such a deformed disk are 

shown in fig. 38. The cleavage developed by splitting the plates 

;seems to be absolutely parallel to the longer diameters of the strain 

'ellipsoids as drawn in the figure. Furthermore, when shavings of 

the ceresin are examined it is found that it is not homogeneous, but is 

flecked with minute bubbles or cavities, which in the flattened disk 

have their axes parallel to the flat sides of the disk, to the strain ellipsoids as drawn in the figure, 6 and to the plane of cleavage developed 

by splitting the disk. These minute flattened air bubbles are apparently planes of weakness which control the cleavage of the mass in 

the same manner as minute parallel mineral flakes might control it. 

Because of these minute planes of weakness the experiments with 

ceresin apparently have no value in showing that a cleavage may be 

developed in a strictly homogeneous substance independent of any 

parallel arrangement of the mineral constituents.

"Bull. Geol. Soc. America, vol. 4,1893, pp. 81-83; Bull. U. S. Geol. Survey No. 241,1904.

&The photographs of cleaved ceresin disks on PI. IV of Doctor Becker's bulletin seem to the 

writer fully to confirm the conclusions above reached. The crescentic breaking at the edge of the 

cake described as following the shearing planes according to his diagram, fig. 14 of PI. IV, correspond 

equally well to the longer diameters of the strain ellipsoids shown in fig. 11 of PI. III. On either 

theory crescentic parting must appear whenever the cleavage occurs on one side or the other of the 

medial plane of the disk parallel to the flattened sides, the only difference being that in one case the 

crescent-shaped edge-of the disk bends away from the medial plane and in the other toward it.

Bull. 239 05 9
130 BOOK CLEAVAGE. [BULL.239.

On Doctor Becker's theory the cleavage ought in the centre of the 

disk to be in two intersecting planes considerably inclined to the flat 

.surface of the disk. The observed cleavage actually runs through 

this portion of the disk readily in one set of parallel planes without 

change of direction other than a gentle curving parallel to the longer 

axes of the strain ellipsoids, as in fig. 38.

When a perfectly homogeneous substance is deformed by irrotational stress, under Doctor Becker's theory cleavage would tend to 

develop equally well in closely spaced intersecting planes. Cleavage 

equally good in two sets of very closely spaced intersecting planes 

has not been observed. If'such planes are prerequisite to parallel 

arrangement of the mineral constituents, how shall a newly developing particle act at the intersection of these planes?

The same argument would apply in a rotational strain, although the 

planes of weakness developed along shearing planes would be mainly 

concentrated in one set and those in the intersecting set would be few 

and far between.

ft is therefore concluded that secondary cleavage includes two distinct phenomena, here called fracture cleavage and flow cleavage, 

developed under different stress conditions, the one developing essentially in shearing planes and entirely independent of a parallel arrangement of the mineral constituents, the other developing essentially in 

normal planes and dependent for its existence on the parallel arrangement of the mineral constituents, and that any explanation of secondary 

rock cleavage, which does not take account of these distinctions, but 

treats cleavage structure as a unit, is sure to lead to confusion.

SUPERPOSITION OF THE TWO STRUCTURES AND GRADATIONS

BETWEEN THEM.

As already noted, a rock in which fracture cleavage has developed

(with, perhaps, incidentally and subordinately a slight tendency to 

. parallel arrangement along slip fractures) may be brought under conditions favorable for the development of rock flowage, and a parallel 

arrangement of all the mineral constituents may develop, either parallel 

or at any angle to the previously existing fracture cleavage. The process of rock flowage being essentially through recrystallization, evidence 

of preexisting cleavage is likely to be obliterated, but in intermediate 

stages the fracture cleavage may remain. Instances may be cited of 

rocks which have been broken up into parallelepiped blocks by fractures in two intersecting sets of planes, these cemented, yielding a 

fracture cleavage, and the whole then strongly compressed, the1 intersecting planes of fracture cleavage being brought nearty into parallelism. On weathered surface erosion m&y work down along the fracture 

cleavage planes, leaving the intermediate areas protruding like pebbles
LEITH.] EELATIONS OF FLOW AND FEACTUEE CLEAVAGE. 131

in a mashed conglomerate. Indeed, a rock of this sort shows very 

close similarity to a mashed conglomerate, and in many cases, as in 

the acidic porphyries of the Vermilion Lake district of Minnesota, can 

be distinguished from true conglomerates only by the most careful 

observation. They differ from the true conglomerates in that pebbles 

and matrix show no variet}^; in that the pebble-like forms are nearly 

all the same size; and, finally, in that the planes of fracture cleavage 

intersect at acute angles around the ends of the pebbles, whereas in 

the true conglomerate deformed by rock flowage the cleavage, indicated by the longer diameters of the parallel-arranged mineral particles, does not intersect at. the ends of the pebbles. King a supposed 

all cleavage to develop in this way by the compression of divisional 

fracture planes into substantial parallelism.

It is difficult to judge of the effect of fracture cleavage in determining the plane of newly developing flow cleavage,, but it is likely that 

it has little effect, for during rock flowage the conditions of pressure

FIG. 39. Diagram showing fracture cleavage parallel to B and C superposed on flow cleavage

parallel to A.

are such that there is little opportunity for movement along shearing 

or fracture planes; the rock is elongated without regard to the existence of a previous fracture cleavage, and hence the parallel arrangement of the mineral constituents is developed without regard to the 

previous arrangement.

When flow cleavage- comes under conditions of fracture we may 

have a fracture cleavage superimposed upon the flow cleavage, either 

parallel or inclined to it (fig. 39 and Pis. XXII, XXIII), and in this case 

the previously existing flow cleavage has a veiy marked effect on the 

position of the newly developing fracture cleavage. 'If the rocks were 

homogeneous the fractures would develop along intersecting planes of 

maximum shear, but the rock being already possessed of flow cleavage, 

the tendency is for the fractures to follow the cleavage planes already 

present, even if they are considerably inclined from planes of maximum 

shearing stress. Fracture cleavage is also likely to develop mainly in one 

plane parallel to the laniinge of a previously existing flow cleavage rather 

than along two or more intersecting planes, although the latter also 

occurs. As ahead}' noted, it is believed that a previously existing

aTrans. Royal Irish Acad., vol. 25,1875, pp. 605-662.
132 EOCK CLEAVAGE. [BULL. 239.

parallel arrangement furnishes the conditions under which closely 

spaced parallel fractures ordinarily develop.

Evidence of slipping along the greater diameters of parallelarranged particles in rocks with flow cleavage is often observed, and 

it is perfectly evident that such slipping is controlled largely by the 

parallel arrangement, and that the parallel arrangement is in no wise 

dependent upon the slipping, although it may be somewhat modified 

or emphasized by it. The cleavage in such a case is conditioned primarily by the parallel dimensional arrangement of the mineral constituents, and the additional episode of slipping parallel to such flow

FIG. 40. Fracture cleavage crossing flow cleavage. After Dale.

cleavage is a modifying condition rather than a primary cause of fracture cleavage, which, if the flow cleavage had not been present, would 

develop, perhaps not so readily and in different planes. As a matter 

of observation, it is believed that it is practically impossible to tell in 

many cases whether or not flow cleavage has been followed by the 

development of parallel fracture cleavage.

False cleavage, fault-slip cleavage, slip cleavage, strain-slip cleavage, rift, and ausweichungs cleavage may all in part represent fracture cleavage superposed upon flow cleavage (Pis. XIV, B and XX), 

although the previous existence of a flow cleavage is not necessary. 

These structures characteristically develop along well - separated 

planes, and are frequently followed by the rubbing of the parts and 

the development of new minerals parallel to the fractures.
RELATIONS OF FLOW AND FRACTURE CLEAVAGE. 133

The so-called "grain" of slates may be in some cases the result of 

the superposition of fracture cleavage on flow cleavage; at least a 

structure so developed 'has sometimes been called grain, but it is 

believed that for the most part it is due to weakness along the plane 

of the greatest and least diameters of parallel-arranged particles, and 

is a structure present in all rocks in which there is a good tri-dimensional parallelism of thesmineral particles.

As folding and jointing or faulting may occur side by side, due 

to many varying conditions, though ordinarily developing in different zones, so we may have fracture cleavage and flow cleavage developing side by side, often with the most intricate relations. Folding commonly passes into faulting or jointing, and flow cleavage grades 

into fracture cleavage. In a given case it is frequently difficult to tell 

where one kind of cleavage begins and the other ends. PI. XX, A 

illustrates a "fault-slip" cleavage. The layers of the rock have been 

minutely crenulated, and the folds have passed into minute fault-slips, 

the cementation of which by recrystallization of new minerals parallel 

to the slips has given a capacity to part along separated fault planes 

which is here defined as fracture cleayage. There has apparently also 

been developed a flow cleavage parallel to the axes of the minute folds, 

and it is exceedingly difficult to tell where the fracture cleavage ends 

and the flow cleavage begins.

RELATIVE IMPORTANCE OF FRACTURE CLEAVAGE AND FLOW

CLEAVAGE.

It is very apparent from the above discussion of fracture cleavage 

and flow cleavage that they are correlative phenomena developed 

under different condition's. Flow cleavage is the better cleavage of 

the two in that it allows of closely spaced planes of parting, whereas 

fracture cleavage does not, and flow cleavage also is the kind which 

appears in rocks which we ordinarity associate with the term cleavage. 

The cleavage parallel to the longer diameters of particles in slates, 

pl^llites, and schists is flow cleavage, and it is not necessary to argue 

that this structure is far more conspicuous, widespread, and characteristic of what are ordinarily thought of as cleavable rocks than 

the fracture cleavage developed in planes inclined to the longer 

diameters of the parallel minerals. Indeed, many writers use the 

term parallel arrangement of mineral particles and rock cleavage 

as essentially synonymous. Because of its manner of development 

flow cleavage obliterates evidence of previously existing fracture 

cleavage, while fracture cleavage is likely to emphasize, rather than
134 BOOK: CLEAVAGE. [BULL.239.

to destroy, flow cleavage. It is concluded, therefore, that the most 

characteristic and widespread structure which we ordinarily think of 

as cleavage comes under the head of flow cleavage, but that there 

is also present abundantly in rocks of the lithosphere another kind 

of cleavage, here called fracture cleavage, developed under conditions quite different from those favorable to the development of flow 

cleavage, and not in any way dependent upon the parallel arrangement 

of the mineral particles.
CHAPTER III

SUMMARY STATEMENT OF CAUSES OF SECONDARY ROCK

CLEAVAGE.

Secondary rock cleavage is a property by virtue of which, some 

rocks may be split along parallel surfaces and which is due to the 

weakness of molecular attraction along such surfaces as compared 

with the attraction in other directions. The property of cleavage is a 

capacity to part; actual partings are not included under this head. It is 

a phenomenon associated with the parallel arrangement of mineral particles, which in turn is induced during " rock flowage " under differential 

pressure. The relations of cleavage to the parallel arrangement of 

mineral particles; the relations of cleavage and parallel arrangement 

to differential pressure, and the processes through which the parallel 

arrangement of mineral particles has been brought about, have been 

matters of dispute among men who have attacked the subject from 

mathematical, experimental, and observational standpoints. The foregoing report is a presentation of evidence on this subject afforded by 

field and laboratory study of the structural and mineralogical characteristics of metaclasic rocks, with only incidental reference to the mathematical and experimental aspects of the question. It is believed tTiat 

a sufficiently great variety and abundance of rocks have been examined, and that the facts observed are so uniform and significant as to 

warrant the following statement of the causes and conditions of 

secondary rock cleavage:

Secondary rock flowage is of two kinds, widely differing in essential causes and conditions. .One has been called flow cleavage, because 

of its characteristic development during rock flowage, under conditions 

and laws of rock flowage and in the zone of rock flowage, and the 

other has been called fracture cleavage because of its characteristic 

development through fracture under laws and conditions governing 

fracture in the zone of rock fracture.

FLOW CLEAVAGE.

Flow cleavage conditioned primarily by the parallel arrangement 

of the principal diameters of unequiaxial mineral particles. It follows these principal diameters and thus intersects the fewest possible 

mineral particles. It is observed to vary in excellence with degree 

of arrangement, and with the inequality of the greater, mean, and

135
136 BOOK CLEAVAGE. [BULL.289.

least dimensional axes of the parallel arranged particles. It is linear 

parallel or plane parallel (parallel to a line or to a plane) and is in one 

plane or two intersecting planes (one of them representing grain), 

according to the degree of inequality of the greater, mean, and least 

dimensional axes of the parallel arranged particles. The ratio of the 

inequality of the axes has been found by measurement to be fairly 

uniform in particles of the same mineral species, and hence the particles of a mineral species wherever found with the same degree of 

arrangement have about the same effect in1 producing rock cleavage.

Flow cleavage is conditioned secondarity by the parallelism of the 

mineral cleavages of the parallel mineral particles. This is a condition clearty dependent upon the arrangement of the dimensional 

axes of the mineral particles; the dimensional axes of the particles 

must be parallel, if there is flow cleavage, while the mineral cleavages may or may not be parallel depending on the uniformity or lack 

of uniformity of their relations to the dimensional axes. Uniform 

relations are observed in cleavable rocks only where the particles have 

dimensions determined by uniform crystal habit, as in the micas, 

hornblende, chlorite, talc, rarely in feldspar, and in the group of 

characteristic but less abundant schist-making minerals which have 

been seen to have a poor dimensional arrangement, tremolite, staurolite, garnet, actinolite, chloritoid, andalusite, tourmaline, sillimanite, 

etc. So far as any of these minerals have dimensional parallelism 

there is a tendency toward parallelism of their mineral cleavages, and 

to this extent the mineral cleavage may cause a rock cleavage. The 

uniform relations of mineral cleavage to dimensions of the particles are 

not observed when the dimensions of the particles are independent of 

the crystal habit, as in calcite and the greater proportion of quartz and 

feldspar particles. In such particles the mineral cleavages are not 

parallel among different particles.

Parallel mineral cleavages may produce a flow cleavage in the same 

plane as that conditioned by the dimensional arrangement, as in mica 

and chlorite; parallel to the greatest axis of dimensional development, 

as in hornblende; or where the mineral has two cleavages one may aid 

the cleavage conditioned by the dimensional arrangement, while the 

other may give the rock a cross cleavage, as in feldspar.

Minerals of the same kind have a tendency to be concentrated into 

bands. As the minerals vary in their cleavage-producing capacrty 

(that is, in shape, dimensional arrangement, and arrangement of their 

cleavages), the rock cleavage is likely to vary greatly in different planes, 

in some cases so widely that practically the flow cleavage is confined 

to a comparatively few and separated planes. When segregated in 

bands, also, difference in strength or cohesion of the different minerals 

may condition planes of weakness quite independent of the parallel 

dimensional arrangement. - ^
LEITH.] SUMMAKY OF SECONDARY BOCK CLEAVAGE. 137

According to the arrangement shown by the minerals, the parting 

following rock cleavage may be between mineral particles or through 

their mineral cleavages. If between the minerals, the flow cleavage 

may, for convenience in discussion of origin, be called intermineral or 

adhesion cleavage; if through the minerals because of their own cleav- . 

age, the flow cleavage may be called cohesion cleavage. In cleavable 

rocks as a whole probably intermineral cleavage is predominant; in 

the most fissile schists and slates both the intermineral and cohesion 

cleavages are present and the cohesion cleavage may be more effective 

than intermineral cleavage. In terms of individual minerals, intermineral cleavage is predominant in so far as quartz, feldspar, calcite, 

trernolite, actinolite, garnet, tourmaline, staurolite, chloritoid, andalusite, and sillimanite are present. Intermineral and cohesion cleavage 

together are effective in proportion to the amount of mica, chlorite, 

hornblende, and rarely feldspar, present.

The processes through which the parallel 'arrangement of minerals 

is brought about, are:

(1) Crystallization or recrystallization of minerals with their respective dimensions in common planes or lines, with or without contemporary or subsequent rotation. Evidence of this is conspicuous in the 

abundant parallel minerals which have been observed to afford the best 

flow cleavage, mica, hornblende, and chlorite, as well as in certain 

minerals which are less abundant in cleavable rocks and which have 

little effect on rock cleavage, such as calcite, tremolite, actinolite, garnet, tourmaline, staurolite, chloritoid, andalusite, and sillimanite. 

Evidence of recrystallization is present but less abundantly in certain 

parallel minerals, which have less effect on rock cleavage, such as 

quartz and feldspar.

(2) Rotation of all the particles in a rock toward dimensional parallelism. Rotation must always occur during the change in form 

induced by rock flowage. The evidence of rotation consists largely 

in the absence of evidence of other processes of parallel arrangement. 

On such evidence, rotation, while probably alwa)rs present, is subordinate to recrystallization as a process of arrangement in the best 

cleavage-giving minerals, and is important only in minerals such as 

quartz and feldspar which have little effect in the production of cleavage when compared with mica, hornblende, and chlorite.

(3) Slicing and granulation, besides affording considerable assistance 

to recrystallization and rotation, are observed to yield a dimensional 

parallelism b}r themselves, by carving the original grains in situ into 

rough dimensional parallelism without any rotation, and by breaking 

slices from the mineral particles which, before any considerable rotation has occurred, may lie parallel. The longer diameters of particles 

arranged in a parallel position by these processes may be parallel or 

inclined to the longer diameters of particles developed in a parallel
138 BOCK CLEAVAGE. [BULL.239.

position by recrystallization. These processes are observed to be of 

some little importance in arranging quartz and feldspar, and to a 

limited extent also are effective in arranging hornblende and mica. 

In any case, however, they are much less effective than either recrystallization or rotation and are important principally in the aid they 

give to recrystallization and rotation.

(4) Gliding, i. e., change of form by differential movements along 

definite planes in crystals without fracture. Evidence of this has 

been observed only in calcite, a mineral better adapted than any other 

to take a parallel arrangement through this process; yet even in this 

mineral gliding is clearly subordinate to recrystallization.

Relations of flow cleavage to differential pressure. The simpler 

relations of stress to the directions of shortening and elongation in a 

rock mass during rock flowage are known. Observed relations of flow 

cleavage to such directions of shortening and elongation give the relations of the structure to pressure. Wherever the plane or line of 

elongation of the rock mass can be observed, the flow cleavage is 

observed to be approximately parallel to the greater elongation of the 

rock mass (or normal to the shortening), but is exactly parallel (with 

such minor deviations as are caused by the heterogeneity of -the rock 

mass) only so far as the cleavage is conditioned by a parallel arrangement caused by recrystallization. Where the cleavage is locally conditioned by a parallel arrangement induced by granulation or slicing, 

it may at times be inclined to the greatest elongation of the rock mass.

Where the cleavage is exactly parallel to the axes of elongation of 

the rock mass, the relations of the cleavage to pressure can be stated 

by substituting the term cleavage for the axes of strain which it 

represents. If the strain is an irrotational one (pure shortening and 

elongation with or without dilatation), the greatest elongation, and 

hence .the flow cleavage, is always developed normal to the greatest 

pressure. If the strain is a rotational one (scission or scission combined 

with pure shortening and elongation, with or without dilatation), at 

any instant the rock mass is being elongated, and hence rock cleavage 

is being developed normal to the greatest pressure, but, due to rotation, the total elongation of the rock mass (which represents at any 

instant the net effect of the stresses producing it) and the cleavage immediately become slightly inclined from its normal position to the greatest 

pressure, and inclined from the position of new cleavage subsequently 

tending to develop in planes where elongation is then occurring normal to the greatest principal stress. Hence cleavage is always 

tending to develop normal to the greatest principal stress, but its final 

position may or may not be inclined to the greater stress depending 

upon the nature of the strain.

Where the cleavage is conditioned in whole or in part by the parallel arrangement caused b}^ granulation or slicing and is not parallel
LEITH.] SUMMARY OF SECONDARY ROCK CLEAVAGE. 139

to the axes of elongation of the rock mass, its development has been in 

shearing planes inclined'to the maximum elongation of the rock mass. 

To these simple relations of cleavage to pressure there are a number of modifications, due to the variation in local stresses caused by 

the influence of rigid particles on the transmission of stresses, and due 

to the limitations of the processes of rotation and recrystallization in 

bringing or keeping the greater diameters of the rock mass in parallelism with the greatest lengthening of the rock.

FRACTURE CLEAVAGE.

Fracture cleavage is a capacity to part along parallel planes, usually 

in intersecting sets, along which there has been either incipient fracturing or actual fracturing followed by cementation or welding. It is 

a structure developed in shearing planes inclined, to the greatest pressure under the laws of fracture and in the zone of fracture in the 

lithosphere. Fracture cleavage may or ma}^ not be accompanied by 

a parallel arrangement of the mineral constituents, but a parallel 

arrangement is not essential .to its existence, and when present is only 

a modifying condition or result and not a cause.

RELATIVE IMPORTANCE OF FRACTURE CLEAVAGE AND FLOW

CLEAVAGE.

Flow cleavage and fracture cleavage are correlative phenomena. 

 Flow cleavage is a structure characteristic of rocks which we ordinarily associate with the term cleavage, such as slates, schists, 

phyllites, etc., and flow cleavage affords planes of parting more closely 

spaced than fracture cleavage and .usually in one plane rather than in 

two or more intersecting planes. Fracture cleavage is obliterated by 

the subsequent development of flow cleavage, while flow cleavage 

m&y be destroyed or emphasized by the subsequent development of 

fracture cleavage in the same rock. The structures known as faultslip cleavage, false cleavage, slip cleavage, and ausweichungs cleavage 

are in most cases varieties of fracture cleavage which have been 

superposed on flow cleavage.
CHAPTER IV.

COMPARISON OF PRESENT STATEMENT WITH PREVIOUS 

STATEMENTS CONCERNING SECONDARY CLEAVAGE.

Sorby," Heim, a Harker,ft and others have considered cleavage to 

include two classes of phenomena similar to those described in this 

paper under "fracture cleavage" and "flow cleavage." Van Rise's" 

discussion of "cleavage" would apply almost in toto to what is here 

called flow cleavage, and his discussion of "fissility" would apply not 

only to the parallel closely spaced fracturing which he termed fissility, 

but to the cleavage, or capacity to part, developed by the cementation 

or welding of such fractures, or to what is here called fracture cleavage. The writer is thus essential^ in accord with Sorby,fl Heim, a 

and Harker a in distinguishing two classes of phenomena under cleavage; and by correlating fracture cleavage with fissility in part, he is' 

in accord also with Van Hise. He differs from those who have 

attempted to discuss cleavage as a single kind of phenomenon resulting from a single group of causes and conditions. King a and Becker a 

in particular have applied to cleavage in general an explanation which 

is here held to apply mainly, with certain modifications, to what is 

here called "fracture cleavage," and Becker's strong presentation of 

fact and argument has had much weight. In this report especial 

emphasis has been laid on the proof that the two phenomena are separate, resulting from different conditions and causes, and that the 

explanation of fracture cleavage will not apply to flow cleavage.

That a parallel arrangement of mineral particles is an essential and 

adequate cause of flow cleavage (or "cleavage" as this term is used 

by many writers) has been assumed by all writers except King* and 

Becker. a The points here added or emphasized are the nature of the 

parallel arrangement of the mineral particles and the dependence of 

cleavage on the parallel arrangement where such arrangement is present, as follows: (1) The control of the arrangement of the mineral 

cleavages by the dimensions of the mineral particles; (2) a statement 

of the relative effects of dimensional and mineral cleavage arrangement in the particles of different mineral species; (3) proof of the 

uniformity in shape, dimensions, and cleavage-producing effect of

a See discussion of literature on pp. 13-15. 

140 
LEITH.] EXPLANATIONS OF CLEAVAGE. 141

particles of the same mineral species; (4) a determination of the relative importance of the dimensional arrangement of the mineral particles and the arrangement of the mineral cleavages in producing flow 

cleavage.

Concerning the processes' through which the parallel arrangement of the mineral constituents in flow cleavage has been brought 

about, no new conclusions are offered, unless certain modifications 

of old ideas may be called new, but evidence is presented to 

show to what extent the several possible processes are effective. 

Recrystallization was hinted at by Sedgwick when he referred to the 

parallel arrangement as due to " crystalline forces." Sorby stated that 

the development of minerals such as the micas in cleavage planes "may 

have been a subsequent operation." Sharpe explained the parallel 

arrangement by the flattening of the individual particles in situ. a Such 

a result is likely to be produced by recrystallization, but Sharpe makes 

no statement as to the process through which it is obtained. While 

a number of other writers have discussed the solution and deposition 

of minerals (recrystallization) as a process .active in the deformation 

of rocks, Van Hise was the first to show the predominance of the 

process of recrystallization in the development of a parallel arrangement of the particles. Moreover, his conception of the process and the 

conditions favorable to it is much more definite than that of his predecessors. The facts observed in the investigation above described are 

in accord with Van Hise's statement. The new features are the presentation of a number of criteria for discriminating the effects of 

recrystallization from those of other processes, and through this means 

the presentation of an additional proof of the predominating importance 

of recrystallization when compared with other processes, and .finally 

a proof of dimensional arrangement and control of mineral particles 

through this process.

The development of a parallel arrangement by the rotation of random particles was first held by Sorby, who has been followed by most 

other writers. The present statement contains nothing new on this 

subject except evidence of its subordinate importance as a process in 

arranging the mineral constituents.

Gliding has been shown by Adams and others to be of importance 

in the production of parallel arrangement of calcite in metaclasic 

rocks. It is believed that the facts stated in this report demonstrate 

that evidence is lacking to show the importance of this process in 

metaclasic rocks in general as compared with the other processes 

described.

The processes of granulation and slicing which produce a parallel 

arrangement of mineral constituents, have been described by Van Hise,

a See references 011 pp. 13-16.
142 - EOCK CLEAVAGE. " [BULL.239.

Adams, and others.* In this paper the writer has attempted to determine the amount and nature of the ettdence of this process in the 

metaclasic rocks.

The statement of the relations of flow cleavage to the shortening 

of the rock mass and to pressure is essentially the same as that of 

practically all others who have assumed the parallel arrangement of 

mineral constituents to be a necessary condition for the existence of 

cleavage, and is especially similar to the statements of Van Hise and 

Hoskins, but it varies from the conclusions of King and Becker, 6 that 

all cleavage is developed in shearing planes inclined to the greatest 

elongation of the rock mass. An advance is made perhaps in showing 

the reasons for slight variations from parallelism to be seen in cleavable rocks and the effect of rigid particles on 'the local distribution of 

stresses within the rock mass.

The explanation of the development of fracture cleavage in planes 

of maximum- shear or jointing here given is essentially the same as 

that applied hy King and Becker, especially Becker, to all cleavage, 

both fracture and flow cleavage (pp. 126-130). Van Hise's explanation of the relations of pressure to fissility (pp. 126-133) apply almost 

without change to the present statement of the relations of pressure 

to fracture cleavage.

It must now be apparent to the reader that the conclusions of the 

present report have many features in common with those long'ago 

reached by Sorby, but that they correspond more nearly in emphasis 

and form with those of Van Hise. Sorb}^ suggested most of the 

factors in the problem; Van Hise showed their relative importance 

and the predominance of recrystallization. Aside from new features 

above referred to, the writer differs from Van Hise only in confining tissility strictly to actual partings as defined by Van Hise and

in applying the term fracture cleavage to the capacity to part formed

by the welding or cementation of fissility partings. To this restriction of the application of the term fissility, and to the use of the term 

fracture cleavage, Van Hise assents. 6

a See discussion and references, pp. 14-17. 

b See discussion and references, pp. 18-19.

c See discussion and references, pp. 19,126-130. See also Treatise on metamorphism: Mon. U. S. Geol. 

Survey, vol. 47, 1904.
PART III.

ORIGINAL CLEAVAGE.

The term original cleavage may be used conveniently to designate the, 

cleavage sometimes possessed by a rock on its first solidification from 

a magma, or deposition from water, as distinguished from secondary 

cleavage, which is produced by secondary processes accompanying 

rock flowage after the solidification or deposition of the rock. A brief

discussion of rocks with original cleavage, or protoclases, is here 

given to show the main points of similarity and- difference between 

original and secondary cleavage. Original cleavage is found in clastic 

sediments and in certain igneous rocks with a banded or flow structure.

BEDDING IN CLASTIC SEDIMENTS.

The bedding of sediments may be a capacity to part along parallel 

surfaces which, rather than an actual parting, is by our definition rock 

cleavage. Such cleavage is probably for the most part due to the differences which are found in the strength of the beds, and which allow 

rupture to occur more easity along softer strata, usually of a shaly 

nature, than along hard layers. But it is certain that the cleavage in 

mechanical sediments is due also to a dimensional arrangement of the 

mineral constituents. Water worn particles or pebbles are commonly 

not round, but ellipsoidal or subangular. While the shape, of course, 

varies considerably Avith the nature and structure of the mineral or rock, 

averages of the greatest and least diameters of a large number of pebbles and of mineral particles from microscopic slides of finer sedimentary rocks show a rough uniformity, not far from 2 to 1. Unexquiaxial 

particles moved by water are deposited in a position determined by 

the configuration of the immediatel}7 underlying floor and by stresses 

there obtaining. . These stresses are those of gravity alone or of 

gravity combined with moving water. The tendency is for gravity to 

bring the greater or mean axes of the particles toward a horizontal 

plane, assuming the floor to be horizontal, while that of moving water 

is to lay the greater diameters of the particles at some angle to this 

plane, usually low, under the. law that the ellipsoid tends to take such 

a position that it presents^the greatest surface to stresses acting upon

143
144 KOCK CLEAVAGE. [BULL.239.

it. Their combined forces deposit the particles in such a manner that 

they sometimes overlap, in the manner of shingles. Gravity is greatly 

predominant in original deposition and has a tendency to produce a 

horizontal arrangement. This tendency is likely to be somewhat 

emphasized later by the weight of overlying rocks. The arrangement may be such that all three dimensional axes of the various particles are respectively parallel, or such that only the least axes are 

parallel, and the other two are axes parallel to a plane and not to one 

another, or such that the particles may lack all but a faint tendency 

toward parallel arrangement, depending upon the configuration of the 

floor and upon whether gravity alone, or gravity combined with moving water, produces the result. A tendency to dimensional arrangement of the particles is thus one of the characteristics of sedimentary 

bedding. The alternation of bauds, of varying coarseness and varying mineralogical composition is also a characteristic feature. When 

a sediment has been cemented to a coherent solid, the bedding 

is not an actual parting, but may remain a plane of weakness 

with a capacity to part, and hence truly a cleavage a cleavage 

primarily due to the dimensional arrangement of unequiaxial par-* 

tides. It is scarcely necessary to add that cementation may be so 

thorough that the rock does not part along bedding planes any 

more readily than elsewhere. In coarse rocks the parallel arranged 

particles themselves may be composite, and the individual minerals 

making up the composite particles may have neither dimensional nor 

crystallographic arrangement. In finer rocks the dimensionally arranged particles may be mineral individuals. In such particles parallelism of c^stallographic properties may be found if there are uniform, 

relations of the crystallographic properties to the dimensional diameters of the particles. The degree of this uniformity varies with the 

different minerals and with the length of time during which the particle

undergoes the wearing of water. Quartz grains in a sedimentary rock 

show little if any crystallographic parallelism. Feldspar may do so rarely 

in arkoses, cla}^s, and muds, where the shape of the feldspar particles is 

conditioned by the normal habit or cleavage of the feldspar. Mica, 

when original in sedimentary rocks derived from the disintegration of 

granite or other mica-bearing rocks, shows characteristic crystallographic parallelism. It may frequently be seen in cleavage plates 

tying parallel to the bedding of quartzites and shales. Hornblende 

has little arrangement, probabty due to its breaking down by water 

action. Other minerals less abundant in sediments may or may not 

show crystallographic parallelism, depending largely on their crystal 

habit and the length of time they have been subjected to the working 

of water, but they are normally in such small quantities in sediments 

that their arrangement and influence on bedding cleavage need not be 

discussed,
ORIGINAL CLEAVAGE. 145

In general, then, the parallel arrangement of particles in sedimentary 

bedding is dimensional, and either complete or partial, usually the 

latter, depending upon the nature of the stresses exerted at the time 

of deposition and the evenness or irregularity of the floor. Where 

the constituent particles of the sedimentary rock are separate mineral 

individuals rather than pebbles made up of many mineral individuals 

there is present in some cases a crystallographic parallelism, depending 

entirely upon the uniformity of the relation between the crystallographic properties of the crystals and their dimensions. The parting 

occurs parallel to the longer diameters of the particles, usually between 

particles and not along the mineral cleavage, although in the case of 

the micas, the rock cleavage ma}^ follow the mineral cleavage, as evidenced by the corresponding mica spangles to be seen on both faces of 

a parted quartz-slate in which the parallel structure is entirely that of 

bedding. Parting along bedding is aided also by the difference in 

nature and texture of different beds.

In all of these phenomena the parallel arrangement of sedimentaiy 

bedding is closely similar in kind to the secondary flow cleavage 

incidental to rock flowage. There are, however, differences in the 

phenomena shown by sediments and rocks with flow cleavage, some in 

kind but mostly in degree. In both rocks the segregation into bands 

of minerals or particles of the same kind or size are characteristic 

features, resulting in planes of weakness perhaps entirely independent 

of a parallel arrangement of the mineral constituents. The particles 

in the flow cleavage rocks due to granulation probably have sharper 

angularities than the angular particles in the sediments. The most 

characteristic particles in flow cleavage rocks, i. e., those developed by 

recrystallization, have more unequal dimensions than particles of the 

same minerals in a sedimentary rock, the water having a tendency to 

minimize the differences in dimension. Characteristic arrangement 

due to dimensions is, accordingly, on an average, poorer in sediments 

than in rocks with secondaiy cleavage, although the differing processes 

bringing about the arrangement in the two cases modify the results. 

The crystallographic parallelism in sediments is not so good as in rocks 

with flow cleavage, for the reason that the rounding effect of the water 

is likely to modify the uniformity of relations between dimensions and 

crystallographic properties. These latter differences, however are not 

great.

These are the differences shown lay the minerals appearing in both 

the sedimentary and secondary cleavage rocks, but there is an additional and very important difference. The relative abundance of the 

different minerals varies. Quartz and feldspar are characteristic of 

sedimentary rocks, while mica and hornblende, etc., are subordinate. 

In the rocks with secondary cleavage mica and hornblende are the charBull. 239 05 10
146 BOCK CLEAVAGE. [BULL.-239.

acteristic minerals, as well as a long list of minerals only sparsely present 

in sediments, such as chlorite, actinolite, staurolite, garnet, sillimanite, 

andalusite, etc. These differences in minerals of course make wide 

differences in average dimensions of the particles present in the original and sedimentary rock (a difference in addition to the variations in 

dimensions shown by minerals appearing both in sediments and flowcleavage rocks). These mineralogical differences have a further effect 

in determining the nature of the parting. In both sediments and rocks 

with secondary cleavage it is parallel to the longer dimensions of the 

particles, but in the former, where quartz and feldspar are dominant, 

the parting is mainly intermineral, while in the latter mica and hornblende are dominant and the T)est parting may follow the mineral 

cleavages, although both kinds of parting may be present in both 

classes of rocks.

In general it appears that so far as the structure itself is concerned 

there is no essential difference between potential parting in the bedding of an indurated sediment and that in a metaclase or rock with 

secondary cleavage. Both are due to differential pressure. Both are 

molecular phenomena conditioned by the same kinds of factors. However, in origin" and in the range of the different factors favoring the 

parallel parting, the two structures are widety different and should 

of course be described under two names.

BEDDING IN NONCLASTIC SEDIMENTS.

Excellent bedding may sometimes be observed in nonclastic sediments which may or may not yield a rock cleavage. The most common 

of the nonclastic sediments limestone sometimes" shows a good bedding cleavage and sometimes not. When present, it is clear that the 

bedditfg cleavage is not due to the parallel arrangement of mineral constituents, but to the intrinsic weakness of certain layers. The subsequent alteration of a nonclastic rock through chemical changes may 

emphasize the bedding, as by the segregation of limestone and chert 

in a limestone formation,-or'the segregation of chert and iron oxide 

in an iron formation resulting from the alteration of iron carbonate. 

Where the original bedding has been so emphasized the capacity to 

part is clearly due to interbanding of materials differing widely in 

nature, certain layers being weaker than others, or the layers of different character having weak adhesion (PI. XXV).

FLOW STRUCTURE IN IGNEOUS ROCKS.

An original cleavage is sometimes found to have been induced in 

igneous rocks prior to or contemporaneous with their, solidification 

from a magrna. In the original flow structure of lavas there is a tendency for any unequidimensional crystals to be arranged with their 

greater diameters parallel to the flowage lines, and there is a marked
WITH.] ORIGINAL CLEAVAGE. 147

tendency for minerals of the same kind to be concentrated into layers, 

both of which phenomena may yield a rock cleavage. The cause of 

the parallel arrangement of flowage is one which need not here be discussed beyond the general statement that the arrangement is probably 

due to rotation of random particles during the flowage of the lava, and 

perhaps also to development in situ under unequal stresses set up during later stages and caused by cooling of the mass. The arrangement 

of particles is dimensional. Commonly this arrangement is partial 

rather than complete, but dimensional axes of one kind in the different 

particles may be parallel, while the others are parallel to a plane and 

notjto one another. A large proportion of minerals in an original rock 

have crystal shape or habit, and thus present uniform relations of 

their crystallographic axes to their dimensions, and hence their crystallographic axes are arranged to the same degree as their dimensions. 

This crystallographic arrangement is found in mica and hornblende, 

and partially also in feldspar. Pressure effects and breaking and separation of particles in the lines of flow are common-.

Certain original gneisses should probably also be described in this 

connection,, although it is believed that the majority of gneisses are 

the result of secondary metamorphic action and therefore should properly come under the discussion of secondary rock cleavage. The 

gneisses with original parallel structure are not surface rocks, but in 

many cases are found in dikes or in other forms whose structural relations show them to have undergone practically no deformation since 

solidification. a Such gneisses can be proved to be igneous in some 

cases at least, and are probably so in the .majority of cases, and if their 

parallel structure is induced prior to solidification it may be in somewhat the same way that the flowage structure of Javas is induced, 

although the parallel development of minerals in situ due to differential stresses set up in the rock during the later stages of its cooling 

may be more important than the rotation of random original particles. 

There is a dimensional parallel arrangement of the particles of the 

gneisses, but the minerals possess characteristic crystal shape to a 

small degree and hence there is little crystallographic parallelism of 

the mineral particles. Mica and hornblende are the only ones which 

show crystallographic arrangement. Quartz and feldspar are exceedingly irregular in outline, and their dimensional parallelism would be 

likely to be overlooked in a hasty examination.

As in the comparison of the original cleavage of sediments and flow 

cleavage, all of the features above described for the original cleavage

a A clear case of the development of an original parallel structure in gneisses is found in the Animas 

Canyon of southwestern Colorado, where, according to Mr. Whitman Cross, dikes of gneiss have intrusive relations to ancient hornblende-schists. The dikes give no evidence of secondary deformation and presumably the parallel structure of the gneiss is original. The writer is indebted to Mr. 

Cross for an opportunity to examine specimens from this locality. (Specimens 10,13,152,153, 2937, 

2943, 2947, 2948.)
148 ROCK CLEAVAGE. [BULL.239.

of igneous rocks are duplicated in secondary cleavage, but in varying 

degree. Indeed, the general aspects of the two are frequently surprisingly similar. This is well shown in the behavior of finer constituents 

in passing around phenocrysts in both original and secondary rocks. 

In kinds of minerals represented in the igneous rocks with original 

cleavage and in rocks with secondary cleavage there are differences. 

Probably mica and hornblende are relatively more important in the 

latter than in the former, while quartz and feldspar are relatively more 

important in the former, although both are present in both kinds of 

rocks. In shape of particles there are minor differences. Angular 

fragments due to granulation in the rocks with secondary cleavage 

probably do not have their counterparts <in so great abundance in the 

original igneous rocks with cleavage. The crystals in the original rock 

are probably not so unequidimensional on an average as those in rocks 

with secondary cleavage and probably show differences in .size, either 

larger or smaller, depending on the dominance of the processes of 

recrystallization or granulation in rock flowage. Differences in the 

relative dimensions of the greatest and least diameters of the particles 

in the original and secondary rocks seem to be indicated by measurements. For instance, secondary green hornblende in schists shows an 

average relation of length to thickness of about 100 :20. Measurements of the same dimensions of similar hornblendes in unaltered rocks 

. give a ratio of about 100:40. The parallel arrangement, even if brought 

about under the same conditions in the two classes of rocks, would not 

be expected to be equally uniform in both, and as a matter of observation, it is not, but the arrangement is brought about under different 

conditions and the cause may not be connected with the dimensions of 

the particles. The crystallographic properties,, therefore, show less 

tendency to be parallel in the original igneous rocks than in the secondary rocks. The concentration of the hornblendes and micas in layers in 

the secondary rocks with cleavage affords an excellent parting not 

attainable in the original igneous rocks where these minerals, if present, are not concentrated to such an extent. The breaking of crystals 

and their separation in lines of flow are strikingly similar in original 

and secondary rocks.

It thus appears in the comparison of rocks with secondary cleavage 

and igneous rocks showing flow structure and cleavage that there is 

a similarity of phenomena throughout. In both the parting is a 

molecular phenomenon and thus a true cleavage. But still sufficient 

and characteristic differences are present to make discrimination possible in some cases. The original cleavage in an igneous rock showing 

flowage structure is in general far less ready than in either the indurated sediments or in the typical crystalline schists or slates.
ORIGINAL CLEAVAGE. 149 

MISCELLANEOUS.

There remains to be discussed a variety of original parallel structures, such as pegmatite, perthite, and parallel arrangements, particularly of feldspar crystals, in rocks both basic and acidic, perhaps due 

to original flowage and perhaps partly pegmatitic in origin. The 

parallel arrangement of pegmatites and perthites has little similarity 

with the secondary parallel arrangement 'incidental to rock flovvage. 

In either case there is no dimensional arrangement of the minerals as 

a whole with reference to the pressure, but rather, an intergrowth of 

two minerals and uniformity in arrangement only with reference to 

each other, due to causes not well understood. It is the peculiar control which the physical properties of a mineral sometimes exert on 

the arrangement of another mineral close at hand. The development 

of minute crystals of one mineral with different arrangement on the 

crystal surfaces of another is an illustration of such control. In

pegmatites the cleavage is that of the constituent minerals, and may 

vary in direction from mineral to mineral;, in rocks with secondary 

cleavage it is of the complex nature already described.

The characteristic arrangement of feldspar parallel to its tabular 

development found in many original basic and acidic rocks presents 

an interesting analogy with certain schists. This parallel arrangement is found in the banded gabbros of the Adirondacks" and northeastern Minnesota, the nepheline-syenites of central Wisconsin, b the 

porphyritic gneiss from the main shaft of the Hoosac tunnel, Massachusetts, and certain other labradorite-porphyrites from America and 

Europe. An examination of the feldspars of these rocks shows them 

to be simply twinned parallel to the clinopinacoid or brachypinacoid. 

In all cases also their tabular development is parallel to this plane, 

and they lie in the rock with these planes parallel. In some cases, .as 

in the banded gabbro and in the nepheline-syenite, the parallelism 

goes still farther and the crystallographic axes in this plane are parallel, as shown by the parallelism of the basal cleavages. This arrangement of the feldspars gives the rock in which it occurs a cleavage 

parallel to the tabular'development of the feldspar, and where the 

basal planes are parallel, a cleavage parallel to them. The rock cleavage so formed is mainly cohesion cleavage, as it follows the brachypinacoidal or basal cleavage of the feldspar. In certain schists a 

similar secondary arrangement of this kind has been found (pp. 35-3T), 

but in this case the secondary, feldspars have not nearly so great differences in dimensions as the original ones, and, furthermore, the rock 

cleavage is not conditioned by the feldspar mineral cleavage, but the 

feldspar is associated with mica and the rock cleavage is largely parallel

«Sp. 14764, si. 9399, near Thompson, Minn.; sp. 18242, sis. 9754, 14972,14973,14974, Westport, N. Y. 

6Sps. 6261, 5821, 5823, 5824, 5825, near Wausau,Wis. Cf. Weidman.
150 BOOK CLEAVAGE. [BULL.239.

to this mineral. The similarity of the development and arrangement 

in the original and secondary rocks brings to mind the possibilit}^ that 

the original parallel structure may be due to differential pressure. The 

rocks showing this arrangement have in many cases certainly not 

undergone mechanical deformation. The parallel arrangement is found 

in fresh massive gabbro in which the feldspars almost certainly 

are original. If this arrangement were attained before the complete 

solidification of the magma, as seems most likely, it could come about 

either through rotation, of unequidimensional particles or through 

development in situ. If such large feldspars were present in the 

magma before solidification, the viscous movement prior to cooling 

would undoubtedly have had a rotating effect on the crystals, and the 

tendency might be to bring them into dimensional parallelism. The 

same result might perhaps be attained by the rotation of such crystals 

under the differential stresses caused by the cooling of the magma. 

Brogger is strongly of the opinion that the forces in a cooling viscous 

magma are unequal in different directions; that at any point they may 

be resolved into three mutually perpendicular differential stresses. In 

neither case is it clear how a good tridimensional parallelism can result 

from the rotation of crystals of which two of the three dimensions 

are not far different. As an alternative the feldspars may be supposed 

to develop entirely after such differential stresses incidental to cooling 

have been set up. In this case it is analogous to the development 

of minerals by recrystallization under conditions of differential pressure in the development of cleavage during rock flowage. This seems 

to offer a basis for a more reasonable explanation of the excellent 

parallel, almost pegmatitic, structures observed in many of the original igneous rocks, than rotation of crystals previously formed. 

Still aiiother possible arrangement yielding parallel parting should

be mentioned, It is conceivable that if in a rock, say a'gneiss, the

materials of differing strength are concentrated in alternate bands, 

there might be a tendency to part in parallel planes, even if the individual particles were not arranged. No case of this has appeared 

during this investigation.

CONCLUSION.

To the cleavage of original rocks, then, such as sedimentary bedding, flow structures in lavas, etc., statements similar to those made 

concerning secondary cleavage may apply. The cleavage is conditioned by the same factors of dimensional and crystallographic 

arrangement, although these factors have different ranges. The relations to pressure are probably similar, although observational evidence 

is partially lacking. The only essential difference is in the processes 

through which the arrangement is brought about. Pegmatites and 

perthites present no dimensional arrangement of the minerals, and
ORIGINAL CLEAVAGE. 151

their cleavage is essentially a local mineral cleavage differing in 

character from the rock cleavage under discussion in this paper. The 

cleavage in such rocks is coterminous with the individual minerals and 

does not extend in the same planes through any great mass of the 

rock; it is then properly not rock cleavage.

SUPERPOSITION OF SECONDARY FLOW CLEAVAGE ON ORIGINAL CLEAVAGE.

It .is evident that secondary flow cleavage can develop, in a rock 

which is either with or without an}7 previous parallel structure, and 

may have any position relative to such previous parallel structure. If 

the flow cleavage is developed parallel to the earlier parallel structure, 

the latter is simply emphasized. If the flow cleavage is developed in 

planes inclined to the earlier structure, both the original and secondary 

structures may be present in the earlier stages of the development of 

the latter. In later stages the earlier parallel structure is necessarily

destined by the development of the secondary cleavage. The secondary cleavage is conditioned by the dimensional arrangement of the 

particles, and the longer diameters of the particles in a rock can not 

be parallel to both original and secondary structures.

Secondary flow cleavage is very commonly developed in sedimentary bedded rocks, and here the original and secondary structure 

may be frequently recognized. The bedding mny be indicated by 

actual fractures due to readjustment between the beds, by variation in 

texture of the different beds, or by variation in the mineral content of' 

the different beds, even though all trace of the original parallel arrangement is entirely destroyed. PI. XIV, A, shows a secondary cleavage 

crossing the bedding of a banded graywacke and slate."

The long dimensions of the individual particles correspond in directions with the secondary cleavage. The original bedding is shown 

only in the alteration in coarseness and mineralogical character of the 

bands. In the hand specimen from which this slide is taken the parting is parallel to the secondary cleavage, and, while the original 

bedding shown by the banding is conspicuous, the parting parallel to 

it is practically nil.

In the instance cited the secondary parallel minerals developed by 

recrystallization across the bedding show greatly varying coarseness, 

and this coarseness corresponds roughly to the texture of the bands 

which they cross. Where an original bedding layer is coarse, the 

micas crossing it are in coarse conspicuous flakes; where the texture 

of the bedding layers is fine the mica crossing it is fine (PI. XIV, A). 

This serves really to emphasize the original alternation in texture of the

« Sps. 14974, Black Hills, South Dakota; 7712, Menominee district of Michigan; 29217, Ocoee area, 

Tennessee; see also sps. 14858,14981,14861,14862, Black Hills, South Dakota; 14745, Little Falls, Minn.; 

25575 and 25584, between Thompson and Livingston, Ga.

For excellent illustrations of cleavage crossing bedding, see Dale's report in Nineteenth Ann. Kept. 

U. S. Geol. Survey, 1899, pt. 3.
152 BOOK CLEAVAGE. [BULL.239.

bedding. Thus while original parallel arrangement is completely 

destroyed by the secondary arrangement, the bedding layers, as shown 

by alteration of textures, are even more conspicuous than before. It is 

believed that this correspondence in texture of original and secondary 

minerals in the early stages of recrystallization is a general phenomenon 

wherever recrystallization occurs that is, the finer the original particles 

the finer the secondary ones developing among them. The micas 

developing in a slate, for instance, tend to be individually smaller than 

those developing in a coarser schist.

The attitude of secondary flow cleavage with reference to the major 

bedding structures in deformed sedimentary rocks has been observed 

for many years and the general relations of the two are known. 

As the sedimentary layers of the earth's crust have been originally 

almost parallel to the surface of the earth, and as the deformation of 

such rocks has been mainly through circumferential shortening of the 

outer portion of the lithosphere, it is in accordance with the law that 

cleavage develops normal to the shortening of the rock mass that cleavage is frequently highly inclined to bedding. ' However, in the deformation of bedded rocks of heterogeneous character the nature of the 

deformation must vary from bed to bed and from time to time, with 

corresponding variations in cleavage. A full discussion of the attitude of cleavage with reference to bedding is not essential to the purposes of this paper and will be omitted: However, there should be 

mentioned the well-known tendency of cleavage to cut across the 

harder layers at high angles and to curve toward parallelism with the 

bedding in the softer layers. This is because of a frequent difference 

in the nature of the strains in the hard and soft layers, if the strata 

have been shortened parallel to their own plane the cleavage in 

general has a tendency to develop normal to the beds; the strain is 

essentially pure shortening. 'But in the folding which accompanies

shortening there must be readjustment between heterogeneous rocks 

or within the softer layers of the series. In the softer layers the 

strain is large!}7 of the kind known as scission. Cleavage developing 

under scission is rapidly rotated toward the plane of scission, which 

in the case of the soft layers would be the plane of bedding; hence the 

frequent tendency of cleavage toward parallelism with the bedding 

in the softer layers, and the peculiar S curve so characteristic of cleavage in rocks of varying hardness (fig. 37).

SUPERPOSITION OF FRACTURE CLEAVAGE ON ORIGINAL CLEAVAGE.

It is apparent that fracture cleavage may be developed without in 

any way obliterating original cleavage, and indeed may emphasize it. 

As in the case of the superposition of a fracture cleavage on a $bw 

cleavage the preexisting cleavage may determine the plane of parting
LEITH.] ORIGINAL CLEAVAGE. 153

even where this is inclined somewhat to what would be the plane of 

parting were the rock homogeneous. Thus it is we have common slips 

along bedding planes and more widely spaced fractures at angles to it. 

The statements made on pages 130-133 concerning the superposition 

of a fracture cleavage on flow cleavage will also apply in the main to 

superposition of fracture cleavage on original cleavage.

One of the results of the superposition of fracture cleavage on original cleavage is the development of "cleavage bands" resulting from 

the softening of material by shearing along the fracture cleavage, 

which allows such bands to weather out readily and gives the rock as 

a whole a conspicuous banding (PI. XVII, A).

PLATES.

155

PLATE I.

157.
PLATE I.

SPECIMENS ILLUSTRATING PLANE-PARALLEL AMD LINEAR-PARALLEL 

TYPES OF FLOW CLEAVAGE.

A. Micaceous schist with plane-parallel type of cleavage. From Black Hills. 

Specimen No. 14858. The black bands represent bedding, and the cleavage 

is inclined to the" bedding. Under the microscope all the constituents of the 

rock, mainly mica and quartz, are seen to .lie with their greater and mean 

diameters in the plane of rock cleavage. It is indeed the tridimensional parallelism of these particles which gives the rock its plane-parallel type of 

cleavage.

jB. Micaceous schist with linear-parallel type of cleavage. From Black Hills. 

Specimen No. 14943. Under the microscope the constituents of -the rock, 

mainly quartz and mica, are seen lying with their longer diameters parallel to 

a line and not to a plane. The micas are for the most part bent and contorted 

in all directions but one, the line of cleavage. The garnets have developed 

subsequent to the development of the cleavage.

C. Micaceous schist with cleavage intermediate'between plane-parallel and linearparallel type. From Black Hills. Specimen No. 14959. The mica plates are 

bent and contorted and have the same arrangement on a minute scale as is 

shown by the rock surface on a larger scale. (See PL II, B.}

158
U. S. GEOLOGICAL SURVEY IULLETIN NO. 239 PL. 1

PLANE-PARALLEL AND LINEAR-PARALLEL TYPES OF FLOW CLEAVAGE.
PLATE II.

159
PLATE II,

*

PHOTOMICROGRAPHS OF MICACEOUS SCHISTS.

A. Photomicrograph of micaceous schist from Hoosac tunnel. Specimen No. 

18062. With analyzer, x 96. The micas, which are entirely new developments by recrystallization, lie in flat plates with their greater diameters roughly 

parallel. Each individual exhibits several twinning lamellae. It will be noted 

that, while there is apparently a bending and irregularity in the mica plates, 

the individuals are for the most part not deformed, and the impression of irregularity is caused by the individuals feathering out against one another at low 

angles. This sort of arrangement is frequently seen about rigid particles which 

have acted as units during deformation, indicating that the arrangement is 

due to differing stress conditions at different places. Where the harder 

minerals are not present it is believed that the feathering out of the mica 

plates results from changing conditions of stress at different times during the 

rock deformation, as well as changing conditions of stress at different places 

in the rock mass (pp. 113-114).

B. Photomicrograph of micaceous schist. From Hoosac, Mass. Specimen H, 

1061.2. With analyzer, x 40. In this rock the mica plates are new developments, but here they have undergone subsequent Reformation and bending.

The difference between this manner of deviation from parallelism and that 

shown in A is apparent.

160
U, S. GEOLOGICAL SURVEY .ULLETIN NO. 239 PL. II

MICACEOUS SCHISTS
PLATE III.

Bull. 239 05 11 161
PLATE III.

PHOTOMICROGRAPHS OF MICACEOUS SCHIST, SHOWING BIOTITE FLAKES 

LYING- ACROSS THE SCHISTOSITY.

A. Photomicrograph of a micaceous schist. From Gogebic district of Michigan. 

Specimen No. 14930. Without analyzer, x 40. Biotite crystals have developed 

in porphyritic fashion with their greater diameters or cleavage or both lying 

at various angles to the plane of rock cleavage. The biotites have the same 

occurrence as mineral^ such as garnet, chloritoid, tourmaline, etc., which have 

developed later than the deformation producing the flow cleavage,

B. The same with analyzer,

162
U. S. GEOLOGICAL SURVEY BULLETIN NO. 239 PL. IN

MICACEOUS SCHIST.

Showing biotite flakes lying across the schistosity. A, Without analyzer; B, with analyzer-
PLATE IV.

163
PLATE IV.

SPECIMENS OF HORNBLENDIC SCHISTS, ILLUSTRATING PLANE-PARALLEL 

AND LINEAR-PARALLEL TYPES OF FLOW CLEAVAGE.

A. Hornblendic schist with plane-parallel type of cleavage. From Mesabi district 

of Minnesota. Specimen No. 45379. The hornblende particles in this rock 

have a tridimensional parallelism, and the rock breaks easily parallel to the 

greatest and mean diameters, and less easily parallel to the greatest and least 

diameters.

B. Hornblendic schist with linear-parallel type of cleavage. From Mesabi district 

of Minnesota. Specimen No. 40686. The longest diameters of the hornblende 

crystals lie parallel to one another, but their mean and least diameters are 

not mutually parallel. The rock accordingly cleaves in any plane parallel 

to the greatest diameters of the hornblende crystals,

164
U. 8. GEOLOGICAL SURVEY BULLETIN NO. 239 PL. IV

HORNBLENDIC SCHISTS, 

Illustrating plane-parallel and linear-parallel types of flow cleavage.
PLATE V.

165
PLATE V.

PHOTOMICROGRAPHS OF HORNBLENDIC SCHISTS, SHOWING ARRANGEMENT 

OF HORNBLENDE CRYSTALS PARALLEL TO COLUMNAR AXES.

A. Photomicrograph of hornblendic schist from the Vermilion district of Minnesota. Specimen No. 28502. 'Without analyzer, x 75. While there are numerous minor irregularities, the general parallelism of the longer axes of the 

hornblende crystals is apparent.

B. Photomicrograph of hornblendic schist from Menominee district, Michigan. 

Specimen No. 26148. Without analyzer, x 75. The parallelism of the green 

hornblende crystals is more marked than in fig. 1.

166
U. S. GEOLOGICAL SURVEf ULLETIN NO. 239 PL. V

. .-- : HORNBLENDIC SCHISTS. 

Showing arrangement of hornblende crystals parallel to columnar axes.
PLATE VI.

167
PLATE VI.

PHOTOMICROGRAPHS OF HORNBLENDIC SCHISTS, SHOWING THE ARRANGEMENT OF HORNBLENDE CLEAVAGE IN SLIDES CUT- ACROSS THE 

SCHISTOSITY. , _

A. Photomicrograph of hornblendic schist from Mesabi district of Minnesota. 

Specimen No. 45416. Without analyzer, x 75. The basal hornblende sections 

at first glance seem to have no orderly arrangement, but on examination it is 

seen that the acute angles of the prismatic cleavage have a slight tendency 

to parallelism:

S. Photomicrograph of hornblendic schist from Vermilion district of Minnesota. 

Specimen No. 28501. Without analyzer, x 90. The acute angles of the prismatic cleavage of the hornblende have a faint tendency to lie in the same 

plane. This is scarcely apparent from the photomicrograph, but an examination of the slide itself leaves no doubt of its presence.

168
 U. S. GEOLOGICAL SURVEY BULLETIN NO. 239 PL. VI

HORNBLENDIC SCHISTS.

Showing arrangement of hornblende cleavage in slides cut across the schistosity.
PLATE VII

169
PLATE VII.

PHOTOMICROGRAPHS OF LEAF GNEISS.

A. Photomicrograph of leaf gneiss from the Laurentian-area north of Montreal. 

Slides furnished by Frank D. Adams. With analyzer, x 60. Doctor Adams 

has described the leaf gneiss as resulting from granulation of a hornblende 

granite, all stages of the process having been noted. (See Part J of Vol. 

VIII of the Geological Survey of Canada, 1895.) The striated feldspars have 

irregular angular shapes such as characteristically result from granulation. 

The two bands of quartz crossing the slide evidently owe their form and 

arrangement finally to recrystallization, although granulation may have been 

an important initial process. The evidence for recrystallization is stated on 

pages 82-87. It will be noted that the quartz individuals have dimensional, 

but not crystallographic parallelism.

B. The same; another view.

170
U. S. GEOLOGICAL SURVEY BULLETIN NO. 239 PL. VII

LEAF GNEISS. 

Two views of the same specimen, After Adams.
PLATE VIII.

171
PLATE VIII.

PHOTOMICROGRAPHS OF MICACEOUS AND QUARTZOSE SCHIST.

A. Photomicrograph of micaceous and quartzose schist from near Westport, N. Y. 

Specimen No. 18259. With analyzer, x 25. The background is composed of 

quartz'and feldspar, with a considerable amount of chlorite, magnetite, and 

other accessory minerals. The quartz and feldspar have irregular angular 

forms such as ordinarily result from granulation. They are similar in their 

form to the granulated feldspar particles shown in PI. VII. The bands crossing the slide are quartz and have the same .characters as the quartz bands 

shown in PI. VII. Granulation probably has aided in the development of 

the quartz bands, but their final configuration is probably due to recrystalli/ation. It will benoted that the quartz has dimensional but not crystallographic 

parallelism.

E. The same without analyzer.

172
U. S. GEOLOGICAL SURVEY 1ULLET1N NO. 239 PL. VIII

MICACEOUS AND QUARTZOSE SCHIST. 

A, With analyzer; B, without analyzer,
PLATE IX.

173
PLATE IX.

PHOTOMICROGRAPHS OF MICACEOUS AND QUARTZOSE SCHIST.

A. Photomicrograph of micaceous and quartzose schist from Hoosac, Mass. Specimen H. 1061. 3. With analyzer, x 60. The mica is clearly secondary and a 

result of recrystallization. The quartz is in clear limpid grains with their 

longer diameters nearly in a common direction. The grains are closely fitting, 

in bands, and strain effects are lacking, and these facts, together with the association with recrystallized mica, go to show that the quartz has itself been 

recrystallized. It will be noted that the quartz has no crystallographic parallelism.

B. The same,'with higher power, x 110. The view illustrates in detail the relation 

of recrystallized quartz grains to recrystallized mica flakes. The mica flakes 

for the most part separate different quartz individuals, but they may be seen 

to bound two or more individuals and to project well into them. It is not 

probable that such a relation could be brought about by granulation, slicing, 

or gliding, and it seems best explained by recrystallization.,

174
JJ. S. GEOLOGICAL SURVEY BULLETIN NO. 239 PL. IX

MICACEOUS AND QUARTZOSE SCHIST. 

Same specimen with different cowers of analyzer.
PLATE X.

175
PLATE X.

PHOTOMICROGRAPHS OF. MICACEOUS AND QUARTZOSE SCHIST.

A. Photomicrograph of micaceous and quartzose schist from Hoosac Tunnel. 

Specimen No. 18062. With analyzer, x 40. The mica is clearly a secondary 

development by recrystallization, and the quartz in the clear bands has been 

as certainly recrystallized, as shown by criteria discussed on pages 82-87.

B. The same, without analyzer; another view.

in
U. S. GEOLOGICAL SURVEY BULLETIN NO. 239 PL. X

MICACEOUS AND QUARTZOSE SCHIST. 

A, With analyzer: B, without analyzer.
PLATE XI.

Bull. 239 05- 12 177
PLATE XI.

PHOTOMICROGRAPHS ILLUSTRATING RECRYSTALLIZATION OP QUARTZ 

AND SLICING OF FELDSPAR.

A. Photomicrograph of recrystallized quartz from Thiiringer Wald. After Futterer 

(fig. 1, PI. II, of Ganggranite von Grosssachsen und die Quartzporphyre von 

Thai in Thiiringer Wald: Mitt. Grossh. Badischen geol. Landesanstalt, vol. 2, 

Heidelberg, 1890). The white area represents recrystallized quartz drawn out 

about the periphery of a feldspar phenocryst, which has remained unaffected 

by recrystallization.

B. Photomicrograph of schistose quartz-porphyry showing sliced feldspar phenocryst in planes inclined to the prevailing cleavage. After Futterer (ibid., 

fig. 2, PL III.). 

\

178
U. S. GEOLOGICAL SURVEY BULLETIN NO. 239 PL. XI

A. RECRYSTALLIZED QUARTZ. 

After Futterer.

S. SLICED FELDSPAR. 

After Futterer.
PLATE XII.

179
PLATE XII.

SPECIMENS ILLUSTRATING GRANULATION AND SLICING OF FELDSPAR

CRYSTALS.

A. Khyolite-gneiss from Berlin, Wis. After Weidrnau (PI. V of Bull. Ill, Wis. 

Geol. and Nat. Hist. Survey, 1898). The feldspar crystals have been crushed 

and strewn out by granulation and slicing. Minute granules may be seen in 

various stages of separation from the feldspars, and the larger feldspars may 

be seen in stages of breaking into slices with their longer diameters inclined 

to the prevalent cleavage of the rock mass.

B. Schistose anorthosite from north of Montreal. After Adams (described in Part 

J of Vol. VIII, Geol. Survey of Canada, 1895). The feldspars and pyroxenes 

have been granulated and sliced and strewn out in the plane of rock cleavage. 

The granulation along the sides has left the residual feldspar particles in irregular lens-shaped individuals with their longer diameters parallel to the rock 

cleavage. Occasionally they are sliced by parallel fractures inclined to the 

plane of rock cleavage.

180
U. 8. GEOLOGICAL SURVEY BULLETIN NO. 239 PL. XII

GRANULATION AND SLICING OF FELDSPAR CRYSTALS. 

A, After Weidman; IS, after Adams.
PLATE XIII.

181
PLATE XIII.

PHOTOMICKOGRAPHS OF PORPHYRIT1C CONSTITUENTS DEVELOPED AFTER 

ROCK FLOWAGE HAS CEASED.

A. Photomicrograph of albite crystal in micaceous and quartzose schist, from 

Hoosac Tunnel. Specimen 18062. With analyzer, x 67. The field is occupied mainly by one large twinned albite. The numerous inclusions of quartz, 

mica, and other minerals to be observed in the feldspar have their longer diameters roughly parallel one to the other and to the prevailing cleavage in the 

rock mass. The albite crystal is supposed to have developed by recrystallization after rock flovvage has ceased, using all the constituents necessary for its 

own growth and leaving the superfluous constituents as inclusions with their 

orderly arrangement unaffected.

B. Photomicrograph of chloritoid crystal in micaceous and quartzose schist, from 

Black Hills. Specimen 14928. With analyzer, x 110. The chloritoid crystal 

here shown has developed later than the rock flowage producing the 'prevailing cleavage of the rock. The chloritoid has grown at the expense of the 

other constituents of the rock, using.all the material necessary for its growth 

arid leaving the excess of material in the form of inclusions, which retain their 

dimensional parallelism with the prevailing rock cleavage.

182
U. S. GEOLOGICAL SURVEY .ULLETIN NO. 239 PL. Xll/

PORPHYRITIC CONSTITUENTS DEVELOPED AFTER ROCK FLOWAGE HAS CEASED. 

^4, Albitci crystal; JJ, chloritoid crystal.
PLATE XIV.

183
PLATE XIV.

PHOTOMICROGEAPH OF MICACEOUS AND QUARTZOSE SCHIST WITH 

CLEAVAGE ACROSS BEDDING, AND PHOTOMICROGRAPH OP SLATE 

WITH FALSE CLEAVAGE.

A. Photomicrograph of micaceous and quartzose schist with cleavage developed 

across original bedding, from Little Falls, Minn. Specimen 14745. With 

analyzer, x 60. A graywacke-slate, in which the banding has been marked by 

difference in texture as well as in composition, has been subjected to deformation, with the result that a cleavage has been superposed upon the original 

bedding at right angles to it. Originally the longer diameters of the particles 

of the bedded rock were parallel to the bedding. Accompanying the development of flow .cleavage most of the constituents of the rock have been 

recrystallized. The quartz particles shown in the light band have been drawn 

out with their longer diameters nearly at right angles to "the former plane of 

their longer diameters, and abundant new mica has developed with its greater 

diameters and mineral cleavage normal to the plane of bedding.

B. Photomicrograph of slate with "false" cleavage, from Black Hills of South 

Dakota. Specimen 14974. Without analyzer, x 60. The longer diameters of 

the particles, mainly mica, quartz, and feldspar, lie, for the most part, in a 

plane intersecting the plane of the page and parallel to its longer sides, but 

in well-separated planes at right angles to this plane the longer diameters 

.of the particles have been deflected into minute monoclinal folds represented by the darker cross lines. In these cross planes also porphyritic 

biotites have developed with their longer diameters parallel. The rock has

two cleavages, one conditioned by the prevailing dimensional arrangement 

of the minute particles and the other conditioned by the planes of weakness along the axes of the minute monoclinal folds crossing the prevailing cleavage. The first cleavage is flow cleavage developed in normal fashion 

during rock flowage, and the second is of the nature of fracture cleavage 

developed later along separated shearing planes in the /one of fracture or 

in the zone of combined fracture and flowage. The rock cleaves into parallelepiped blocks.

184
U. S. GEOLOGICAL SURVEY 3ULLETIN NO. 239 PL, XIV

A. MICACEOUS AND QUARTZOSE SCHIST WITH CLEAVAGE ACROSS BEDDING.

£. SLATE WITH FALSE CLEAVAGE.
PLATE XT.

185
PLATE XV.

PHOTOMICROGRAPHS OF ARTIFICIALLY DEFORMED MARBLE (AFTER

ADAMS).

A. Photomicrograph of Laurentian marble which has undergone granulation. 

After Adams and Nicolson (fig. 3, PI. XXV, of vol. 195, Philos. Trans. Royal 

Soc. of London). With analyzer, x 47. Large crystals of calcite have been 

minutely granulated along their peripheries and the crystals themselves 

have been twisted and twinned as shown by their minute reedy striations.

B. Photomicrograph of Garrara marble deformed artificially. After F. D. Adams 

and J. T. Nicolson (ibid., tig. 4 of PI. XXIV). With analyzer, x 150. The 

marble was confined on all sides, was subjected to high temperature, and 

deformed by a piston. The rock flowed without losing its integrity. The 

flow took place by the processes of granulation and gliding. The calcite 

individuals changed their shape and were distinctly elongated without conspicuous fracture, but developed a reedy or fibrous structure, indicating their 

elongation by gliding along crystallographic planes.

186
U. 5. GEOLOGICAL SURVEY BULLETIN NO. 239 PL. XV

ARTIFICIALLY DEFORMED MARBLE. 

After Adams and Nicolson.
PLATE XVI.

187
PLATE XVI.

SCHISTOSE MARBLE.

Photograph of schistose marble from Talledega Mountain, Georgia. Specimen 25874. 

The rock is made up of minute granules of calcite showing dimensional parallelism, as illustrated by fig. 19, p. 41. . There is no parallelism of the crystallographic 

properties of the calcite particles. The parting is determined by the plane of the 

greatest and mean axes of the parallel arranged granules.

188
U. S. GEOLOGICAL SURVEY

BULLETIN NO, 239 PL. XVI

SCHISTOSE MARBLE.
PLATE XVII.

189
PLATE XVII.

CLEAVAGE BANDS IN ORDOVICIAN SLATES AND PARALLEL QUARTZ 

VEINS IN CAMBRIAN SLATE.

A. Cleavage bands in Ordovician slate, from Rupert, Vt. After Dale (fig. B of PI. 

. XXX, of pt. 3 of the Nineteenth Ann. Kept. U. S. Geol. Suryey). The 

material of this rock was originally nearly homogeneous, and the original bedding crosses the cleavage bands. The softer bands which have been eroded 

out are described by Dale as zones along which slip cleavage (here called fracture cleavage) has developed. The characteristic occurrence of the slip or 

fracture cleavage in well separated planes is to be noted.

B. Parallel quartz veins in Cambrian slate, from Hampton, N. Y. After Dale 

(fig. A, of PI. XXXII, of pt. 3 of the Nineteenth Ann. Kept. U. S. Geol. 

Survey). The veins represent planes of parting along the flow cleavage 

which by their cementation have yielded what is here called fracture cleavage.

190
U. 8. GEOLOGICAL SURVEY

A. CLEAVAGE BANDS IN ORDOVICIAN SLATES. 

After Dale.

_. . . J T

B. PARALLEL QUARTZ VEINS IN CAMBRIAN SLATE. 

After Dale.
PLATE XVIII.

191
PLATE XVIII.

SPECIMENS ILLUSTRATING .FRACTURE CLEAVAGE.

A. Fracture cleavage developed by the cementation of h'ssility openings. From 

Great Basin, furnished by G. K. Gilbert. Specimen No. 42633. The rock 

has been separated into a series of parallel slices and has been subsequently 

cemented by the infiltration of calcite. The rock now has a capacity to part, 

or fracture'cleavage, parallel to the calcite veins.

B. Fracture cleavage in schistose marble. Southern Appalachians. Specimen No. 

S. 2. Each of the bends is made up of one or few calcite individuals with 

dimensional but not crystallographic parallelism. It will be noted that the 

individual bends commonly narrow and feather out. The intervening dark 

material is chlorite and mica, developed along planes of slipping. The closeness of the planes of parting is rather exceptional. It is to be noted, however, 

that they are definite in number and separated by zones of distinctly noncleavable material.

192
U. 8. GEOLOGICAL SURVEY BULLETIN NO. 239 PL. XVIII

ILLUSTRATIONS OF FRACTURE CLEAVAGE.
PLATE XIX.

Bull. 239 05 13 '193
PLATE XIX.

SPECIMENS ILLUSTBATING FRACTURE CLEAVAGE.

A. Fracture cleavage developed by the cementation of fissility openings. From the 

Vermilion district of Minnesota. Specimen No. S. 14. This is from an Archean 

jasper formation which has been much broken by fracturing. The closelyspaced parallel fractures in three sets have been cemented by the infiltration 

of quartz and now remain as potential planes of parting.

B. Fracture cleavage developed by the cementation of fissility openings. From the 

Vermilion district of Minnesota. Specimen No. S. 9. Fracturing has occurred 

along one set of planes, followed by the infiltration of quartz. The rock has 

then been fractured and faulted again in planes crossing the first formed 

fractures and these fractures in turn cemented.

194
U. S. GEOLOGICAL SURVEY BULLETIN NO. 239 PL. XIX

ILLUSTRATIONS OF FRACTURE CLEAVAGE.
PLATE XX.

1.

195
PLATE XX.

SPECIMENS ILLUSTRATING FRACTURE CLEAVAGE.

A. Fault-slip cleavage in gneiss from southern Appalachians. Specimen No. 25602. 

The gneiss has been closely crenulated and the minute folds may be observed 

to pass into minute faults which now represent planes of fracture cleavage. 

The faults may have been cemented or may have been welded by actual pre.s- 

. sure: Parallel to the faults there has also been developed a parallel arrangement of the mineral particles, perhaps due in part to the slipping along the 

fault planes, and it is exceedingly difficult to distinguish between the fracture 

cleavage and the flow cleavage.

B. Schist from Taconic Range, showing minor folding grading into fault-slip cleavage. After Dale (PI. XIV, B, Nineteenth Ann. Eept. U. S. Geol. Survey, pt. 3).

196 '
U. S. GEOLOGICAL SURVEY BULLETIN NO. 239 PL. XX

ILLUSTRATIONS OF FRACTURE CLEAVAGE. 

After Dale.
PLATE XXI.

197
PLATE XXI.

SPECIMENS ILLUSTRATING FRACTUEE CLEAVAGE.

A. Fault-slip cleavage developed in schist. From Marquette district of Michigan. 

Specimen No. 42617. The schist has been closely crenulated and minute folds 

may be seen to pass into faults. The rock cleaves parallel to the prevailing 

schistosity, marked by the longer diameters of the particles; it also cleaves 

parallel to the limbs of the minute crenulations and parallel to the faults. So 

far as the cleavage is parallel to the faults, it may be said to be fracture cleavage; so far as it is parallel to the longer diameters of the particles.on the longer 

limbs of the minute folds parallel to the faults, it may be said to be flow cleavage, conditioned by minor bends in the prevalent parallel arrangement of the 

mineral .particles. The rock then shows a fracture cleavage grading into a 

"false" cleavage along minute folds, both superposed upon an earlier cleavage marked by the prevalent schistosity of the rock mass (pp. 132-133).

B. "Micaceous hematite." From the Lake Superior region. Specimen No. 25730. 

Parallel hematite flakes give the prevailing cleavage to the rock. Minute 

crenulations in this parallel arrangement afford planes of weakness which favor 

cross breaking. The term false cleavage has been applied to such a cross 

structure (pp. 25-26,132-133)..

198
U. 6. GEOLOGICAL SURVEY BULLETIN NO. 239 PL. XXI

A, FAULT-SLIP CLEAVAGE.

B. FALSE CLEAVAGE,
PLATE XXII.

199
PLATE XXII.

SPECIMEN SHOWING BOTH FLOW CLEAVAGE AND FRACTURE CLEAVAGE

IN THE SAME BLOCK.

Slate from Somerville, Mass. The flat upper surface represents a plane of parting 

along flow cleavage parallel to the longer diameters of the constituent particles, 

and the ends and sides of the block represent parting along fracture cleavage 

independent of any parallel arrangement in the block. The block will cleave 

into smaller blocks of similar shapes.

200
U. S. GEOLOGICAL SURVEY BULLETIN NO. 239 PL. XXII

FLOW CLEAVAGE AND FRACTURE CLEAVAGE IN THE SAME BLOCK.
PLATE XXIII.

201
PLATE XXIII.

SPECIMEN SHOWING BOTH FLOW CLEAVAGE AND FRACTURE CLEAVAGE

IN THE SAME BLOCK.

Block of green schist showing both flow cleavage and fracture 'cleavage. FroiMenominee district of Michigan. Specimen No. 25664. The flat upper surface 

(the plane of the page) represents a plane of parting along flow cleavage parallel 

to. the longer diameters of the constituent particles, and the ends and sides of the 

block represent parting along fracture cleavage independent of any paralle 

arrangement in the block. The block will cleave into smaller blocks of similar 

shapes.

202
U. S. GEOLOGICAL SURVEY BULLETIN NO. 239 PL, XXIII

FLOW CLEAVAGE AND FRACTURE CLEAVAGE IN THE SAME BLOCK.
PLATE XXIY.

203
PLATE XXIV.

SPECIMENS ILLUSTRATING PARALLEL ARRANGEMENT OF FELDSPARS IN 

KOCK MASSES WHICH HAVE NOT UNDERGONE SECONDARY ROCK 

FLOWAGE.

A. Gabbro from Westport, N. Y. Specimen No. 18242. The feldspars have 

tabular development parallel to the brachypiiiacoid, and lie in the rock not. 

only with their tabular brachypinacoidal'faces parallel, but with their basal 

cleavage faces parallel, as shown by the reflections of the basal cleavage faces. 

The feldspars are for the most part twinned parallel to the brachypiiiacoid, 

and the twinning plane thus bisects the basal cleavage plane parallel to its 

longest axis.

B. Nepheline-syenite from central Wisconsin. After Weidman. (Bull. Wis. Geol. 

and Nat. Hist. Survey, in preparation.) The light colored protruding ridges 

represent feldspar, crystals with an arrangement almost identical with that 

described for A.

204
U. S. GEOLOGICAL SURVEY BULLETIN NO. 239 PL. XXIV

PARALLEL ARRANGEMENT OF FELDSPARS IN ROCK MASSES WHICH HAVE NOT 

UNDERGONE SECONDARY ROCK FLOWAGE.

-.'.- !" '.:-i' £-'"-.. -." ' .'. : After Weidman. '' -, '. " '.. '. - ' ..-'. ':''. ..',
PLATE XXV.

205
PLATE XXV.

CHEMICAL SEDIMENTS WITH CLEAVAGE. '

A. Cherty iron carbonate showing bedding. From Marquette district, Michigan. 

After Van Hise. (Fig. 2, Mon. U. S. Geol. Survey, vol. 28.) The iron carbonates show a very tine bedding, parallel to which there is a capacity to 

cleave.

B. Ferruginous chert resulting from the alteration of iron carbonate with bedding 

much emphasized by the alteration. From Marquette district, Michigan. 

After Van Hise. (Fig. 1 of PI. XX, Mon. U. S. Geol. Survey, vol. 28.) The 

alteration of the iron carbonates of the Lake Superior region characteristically 

produces ferruginous cherts and jaspilites in which the constituents are segregated into bands, strongly marking and emphasizing the original bedding. 

The banding gives a capacity to part along parallel planes, which is truly a 

cleavage.

C. Cherty limestone with bedding marked by minute bands of chert. From north 

shore of Lake Huron. The segregation of the chert in bands is believed to be 

largely secondary. The rock possesses a capacity to part parallel to the bands, 

although the tendency is but a slight one. The fold passing into a fault

illustrated in the photograph is a result of later deformation. 

206
U. S. GEOLOGICAL SURVEY BULLETIN NO. 239 PL. XXV

"' >' C

CHEMICAL SEDIMENTS WITH CLEAVAGE. 

A and B, After Van Hise.
PLATE XXVI.

207
PLATE XXVI.

SPECIMEN SHOWING ELONGATION OF PEBBLES IN THE PLANE OF 

KOCK CLEAVAGE.

A. Schist-conglomerate. From Black Hills, South Dakota. Specimen No. 14818. 

The longer diameters of the quartxite pebbles may be seen to lie parallel. 

This is due to the compression which the rock has undergone.

B. Same, on opposite side of specimen. This side of the specimen has been compressed so closely and the pebbles so extremely elongated that they are 

almost indistinguishable. The parallelism of the cleavage with the longer 

diameters is marked. The rock cleavage follows not only the longer diameters 

of the elongated pebbles, but occurs within the pebbles themselves parallel to 

the elongated diameters.

208
U. S. GEOLOGICAL SURVEY BULLETIN NO. 239 PL. XXVI

ELONGATION OF PEBBLES IN PLANE OF ROCK CLEAVAGE.
PLATE XXYII.

Bull. 239 05 14 209
PLATE XXVII.

SPECIMENS SHOWING UNDISTO11TED PEBBLE AND PEBBLES ELONGATED 

IN THE PLANE OF ROCK CLEAVAGE.

'A. Greenstone pebble, undistorted, from-.pre-Cambrian conglomerate. Pine River, 

Wisconsin. Specimen No. 45810.

B, C. Distorted pebbles, from pre-Cambrian conglomerate. Pine River, Wisconsin. 

Specimen No. 45810. The flattening of the pebbles was accompanied by 

development of an excellent cleavage in the matrix parallel to the longer 

diameters of the pebbles. The weathering out of this matrix has made it 

possible to recover the elongated pebble in this condition. The pebbles 

themselves have their mineral constituents elongated in the same directions, 

and the pebbles, as well as the matrix, cleave readily parallel to their longer 

dimensions. There is no possibility that the cleavage of the pebbles was 

present before the conglomerate was deformed, for in this case the cleavage 

of all the pebbles would not be so strictly parallel to each other and to the 

cleavage of the matrix. In B the flatness and cleavage of the specimen are 

normal to the plane of the page. In C the flatness and cleavage are parallel

to the plane of the page.

210
U. S. GEOLOGICAL SURVEY BULLETIN NO. 239 PL. XXVII

A. UNDISTORTED PEBBLE,

PEBBLE ELONGATED IN PLANE OF ROCK CLEAVAGE.

C. PEBBLE ELONGATED IN PLANE OF ROCK CLEA-,
INDEX.

A. Page.

Acti.nolite, arrangement of and character

of............................ 42,59-60

crystallization in....................... 93-94

effect of, on form of cleavage....... 59-60,101

 processes arranging .................... 90-91

Adii ms, Frank D., acknowletlgmen ts to.... 22

illustrations from work of........ 170,180,186

on auorthosite.......................... 37

on cleavage ......................... 14,16-17

 on gliding.............................. 141

on granulation ....................... 141-142

on leaf gneiss........................... 170

on pyroxenes in anorthosite ........... 80

on rock flow............................ 74

on slicing............................. 141-142

Adfl.ms, F. D., and Nicolson, ,T. T., on flow

of marble....................... 89

Adhesion cleavage, definition and occurrence of ....................... 99-101

Albite in rocks with flow cleavage, arrangementof...................... 36-37,89

figures showing......................... 36

photomicrograph of.................... 182

Andalusite, arrangement and character of. 44-45,

61-62 

crystallization in....................... 94

effect of, on form o.f cleavage....... 61-62,101

processes arranging ............... \... 91-92

Anorthosite, feldspars in, arrangement of.. 37 

microscopic views of................... 37

Anorthosite, schistose, plate showing...... 180

Arrangement, parallel, causes producing.. 141 

figure showing.......................... 25

flow cleavage due to...........'....... 140-141

in igneous rocks, character and cause

of............................. 146-147

in individual minerals, processes of.... 78

in sediments, character of............ 143-1-15

similarity of flow cleavage and .... 145

literature on............................ 14-17

occurrence of ................ 24-48, 96-97,140

plate showing.......................... 204

processes producing............ 65-95,137-138

relation of flow cleavage and........... 49-64

summarry of facts concerning........... 45-48

See also Minerals, arrangement of.

Ausweichungs cleavage, definition of....... 120

See also Cleavage, fracture. ~

Axes of stress, definition of................ Ill

B. 

Ball, S. H., acknowledgments to ........... 22

Barus, Carl, on critical temperatures....... 67

Bat2 cleavage, definition of................ 19

Pago. 

Becker, G. F., on cleavage................. 14-15,

18,19,21,124,126-130,140,142 

on strain ..................... 109,110,122,123

Bedding, distortion of, relation of flow

cleavage to................... 102,105

original cleavage due to................ 143

 plate showing........................... 206

relations of flow cleavage and.......... 352

See also Cleavage, original.

Bibliography. See Literature.

Biotite, cleavage of, figure showing........ 27

in micas, arrangement and dimensions

of.................:............. 26-2S

Biotite, mnscovite, and quartz, juxtaposition of, figure showing ......... 27

C.

Calcite, arrangemen t of.................... 41

character and occurrence of......... 40-4.1,58

crystallization in ....................... . 93

effect of, on form of cleavage.......... 58,101

effect of rock flowage on ............... 92

parallelism of, figure showing.......... 41

processes arranging.................... 88-90

Carbonate of iron, with bedding, plate

showing........................ 206

Chert, ferruginous, with bedding, plate

showing........................ 206

Chiastolite. See Andalusite.

Chlorite, character and occurrence of...... 40,59

crystallization in ...'.................... 93

effect of, on form of cleavage.......... 59,101

processes arranging..................... 90

Chloritoid, arrangement and character of. 44 

crystallization in....................... 94

photomicrograph of.................... 182

processesarranging..................... 91-92

Cleavable rocks, internal structure of...... 1)6-101

Cleavage, causes of, opinions of former

writers on ...................... 13-1!)

definition of............................ 11

experiments on......................... 21

in igneous rocks, comparison of secondary cleavage and............. 147-148

discussion of...................... 3.46-148

in sediments.......................... 143-146

observation of.......................... 20

photomicrographs showi ng ............ 184

relations of pressure and,-literature on. 18-19 

terms pertaining to...................... :i2

Cleavage, adhesion, character of........... \ 37

definition and occurrence of.......... 9!)-l01

Cleavage, answeichungs, definition of.... 17,120 

Sec also Cleavage, fracture.
212 INDEX.

Page.

Cleavage, bate, definition of................ 18

Cleavage, close-joint, defini tion of........ 17,120

See also Cleavage, fracture.

Cleavage, cohesion, character of........... 137

definition and occurrence of.......... 100-101

Cleavage, false, cause of.................... 49

character of, literature on.............. 18

definition of.......'.................... 18,120

See also Cleavage, fracture.

Cleavage, fault-slip, plate showing....... 196-198

See Cleavage, fracture.

Cleavage, flow, arrangement in, summary

of facts concerning............. 45-48

biotite in rock with, figure showing.... 27

character of ............................ 12,19

conditions of ..................... 125,135-136

definition of............................ 23,99

degree of excellence of................. 50-51

dependence of, on parallel arrangement ................. 127-128,140-141

discussion of........................... 23-118

effect of cohesion and adhesion in ..... 51

effect of minerals on.................... 51-64

summary of ........................ 63-64

effect of stress on..................... 112-114

elongated pebbles in plane of, plate

showing........................ 208

mineral arrangements in rocks with... 24-48 

production of........................... 65-66

relations of deformation to........... 102-108

relations of differential pressures and. 138-139 

relations of elongation and shortening

of rock masses and....... 102-108,142

relations of fracture cleavage and ... 125-134 

relations of molecular attraction and . 99-101 

relations of parallel arrangement and.. 49-64 

relations of sedimentary bedding and.. 152 

relations of strain and stress to rock

flowage and.................. 112-118,

relations of stress and ................ 109-118

relative importance of minerals in producing ......................... 50-64

shape and arrangement of particles in

rocks with...................... 50

similarity of parallel arrangement in

sediments and.................. 145

.specimens illustrating, plates showing........................... 158,164

terms applied to........................ 12

thickness of particles in rocks with .... 50

See also Cleavage, fracture and flow;

Cleavage, secondary. 

Cleavage, fracture, character of.... 12,19,120,123

conditions of........................... 125

definition of...................... 119-121,139

explanations of................... 119-121,142

genesis of, diagrams showing ........ 122,123

in slate, figure showing ................ 120

occurrence of........................... - 49

plates showing ................... 192,194,196

relations of flow cleavage and ....... 125-134

relations of stress and................ 121-124

superposition of, on original cleavage. 152,153 

terms applied to........................ 12

See also Cleavage, fracture and flow; 

Cleavage, secondary.

Page. 

Cleavage, fracture and flow, comparison

of.......................... 19,125-134

explanation of ....................... 126-130

plates showing....................... 200,202

relations oL.......................... 125,134

relations of, to stress, comparison of.... 121

relative importance of............ 133-134,139

superposition of...................... 130-133

figure showing.................... 131,132

Cleavage, intermineral. See Cleavage, adhesion. 

Cleavage, original, definition of............ 143

discussion of.......................... 143-153

superposition of fracture cleavage on. 152,153 

terms applied to........................ 12

See also Cleavage, original and secondary.

Cleavage, original and secondary, comparison of..................... 143,150-153

superposition of...................... 151-153

Cleavage, secondary, causes of........... 135-139

character of............................ 12-13

comparison of igneous-rock cleavage

and........................... 147-148

kinds of ............................ 12,19,135

literature of ............................ 13-19

terms applied to........................ 12

See also Cleavage, original and secondary. 

Cleavage, slaty, cause of ................... 15

plate showing.......................... 190

Cleavage, strain-slip, definition of.......... 120

See also Cleavage, fracture. 

Cleavage, ultimate-structure, definition of. 17 

Cleavage and fissility, comparison of....... 17-18

Cleavage bands, development of........... 153

Cleavage-producing minerals, abundance

of, effectof..................... 51

Clements, J. Morgan, acknowledgments to. 22 

Close-joint cleavage, definition of......... 17,120

See also Cleavage, fracture. 

Cohesion, definition of..................... 51

Cohesion cleavage. See . Cleavage, cohesion.

Conglomerate, pebbles in, distortion of.. 102-103

Cross, Whitman, acknowledgments to..... 22

mention of............................... 21

on parallel structure........ .......< ..... 147

Crystallization, character and effects of... 66-71,

76-78

criteria for determining existence of... 70-71 

order of, factors in ..................... 94-95

parallel arrangement produced by.... 137,141

relations-of molecular shape and....... 97r99

See also Recrystallization-.

Crystallographic parallelism. See Minerals, 

arrangement 'of.

Crystals, mineral, distortion of, relation of

.flow cleavage to.............. 102,104

D.

Dale, T. N., on cleavage................... 14,120

photographs from .................... 190,196

Dana, J. D., on cleavage................ 13,14-15

Darwin, Charles, on cleavage.............. 13,16

Daubre"e, A., on cleavage............ 14,15,18,113
INDEX. 213

Page. 

Deformation of rocks, cause of............. G5-GG

relation of cleavage to................ 102-108

See also Stress; Strain. 

De la Beclie, H. T., on cleavage............ 18

Dctrusion, simple, definition of............ 110

See also Strain, irrotational. 

Difference, stress, definition of............. Ill

Distortion, relation of flow cleavage to... 102,100

E. 

Ellipsoids, strain ........................... Ill

See Strain ellipsoids. 

Elongation and shortening of rock masses,.

relation of flow cleavage to... 102-108 

relation of stress to................... 109-118

Elongation of quartzite pebbles, figure

showing........................ 103

Experiments on cleavage, making of...... 21

F.

False cleavage. See Cleavage, false.

Fault-slip cleavage, plate showing....... 196,198

See also Cleavage, fracture. 

Feldspars, arrangement in........... 144,149-150

arrangement of......................... 55-58

plate showing...................... 204

character of............................ 35-40

comparison of mica, hornblende, quartz, 

and...................7...........'.... 45-46

crystallization in....................... 93-96

effect of, on form of cleavage....... 55-58,63,

300-101 

granulation of, plate showing.......... 180

in parallel lenses, figure showing...... 39

processes arranging .................... 86-88

slicing in, figures showing ............. 38

photomicrograph showing ......... l~&

plate showing...................... 180

Fibrolite. See Sillimanite.

Fissility, comparison of cleavage and...... 17-18

definition of........................ 17,19,120

development of, diagram showing..... 122

plate showing ........................ 192,194

See also Cleavage, fracture. 

Flow, zone of, definition of................. 66

Flowage, rock, cause^-effects'and processes

of............................... 65-67

Flow cleavage. See, Cleavage, flow.

Flow structure in igneous rocks, occurrence

Of.................'............ 146-148

Folds, attitude of, relation of flow cleavage

to............................. 102,105

Fossils, distortion of, relation of flow cleay-

' age to ........................ 102,105

Fox, E. W., on cleavage ................ 13,15-16

Fracture, relation of cleavage and....... 102,106

zone of, definition of.................... 66

Fracture cleavage. See Cleavage, fracture. 

Fractures, parallel, development of...... 123-124

Fntterer, Karl, on quartz in quartz-porphyry........................... 33

photomicrographs from ................ 178

G.

Gabbro with parallel feldspar, plate showing .............................. 204

Garnet, arrangement and character of.. 42^13,02

Page. 

Garnet, crystallization in................... 94

effect of, on form of cleavage .......... 62

 processes arranging .................... 9.1-92

Gilbert, G. K., photograph from............ 192

Glass, solution of, temperature necessary for 67-68 

Gliding, character and effects of........ 66-67,74

criterion for determining .............. 74

parallel arrangement produced by..... 138,

141-142 

Gneiss, cleavage in, plate showing......... 196

parallel arrangement in................ 147

weakness of, along certain layers ...... .150

Gneiss, leaf, photomicrographs of.......... 170

Grain, definition of......................... 18

Grain of slate, cause of..................... 133

Grains, size of, effects of recrystallizatiou

and granulation on............. 76-78

Granulation, character and effects of...... 66-67,

75-78, 92 

figure showing ......................... 33

parallel arrangement produced by..... 137-

138,141-142 

plate showing .......................... ISO

Griinerite, arrangement and character of.. 42,

59-60 

crystallization in....................... 93-94

effect of, on form of cleavage ...... 59-00,101

processes arranging .................... 90-91

H.

Harker, Alfred, on cleavage.. 14,17,93,104,105,140 

on distortion of pebbles ................ 102

Hayes, C. W., letter of transinittal by ...... 9

Heim, Albert, on cleavage ................ .17,140

Hematite, micaceous, cleavage in, plate

showing........................ 198

Heterogeneity.in rocks, effects of, on defor-.

rnation ....................... 115-118

Hobbs, W. H., acknowledgments to........ 22

Holland, Philip, on cleavage............... 14

Holland and Reed, on cleavage ............ 93

Hoosac schist, feldspar crystalsin, arrangement of......................... 36

Hornblende, arrangement of ....'.......... 29-30

axes of, parallelism of, sketch showing. 29 

cleavage of, sketch showing ........... 29

comparison of mica, quartz, and ....... 34-35

comparison of quartz, feldspar, mica,

and............................. 45-46

cross section of......................... 30

crystallization in........................ 93,94

dimensions of.......................... 30-31

effect of, on form of cleavage... 52,53, ] 00-101 

habit of, figure showing................ . 29

in sediments, parallelism in............ 144

processes arranging .................... 80-82

slicing of, figure showing ............... 30

Hoskins, L. M., acknowledgments to ...... 22

on cleavage ........................ 14,19,115

011 rock fracture........................ 128

on strain ................. 109,110,111,122,123

Howe, Ernest, acknowledginents to ....... 22

Hutching, W. M., on cleavage.............. 14

I.

Igneous rocks, flow structure and original'

cleavage in................... 146-148
214 INDEX.

Intermineral cleavage. See Cleavage, adhesion.

Intrusivea, relations o[ cleavage and. 102,105-100

Iron carbonate, with bedding, plate showing .......................<..... 206

Irrotational strain, definition of........... . 109

See also Shortening, pure.

K. 

Keith, W., on rock cleavage................ 14,

18,126,131-133; 140,142 

on rock deformation ................... 104

Lava, parallel arrangement in........... 146-147

Le Conte, J., on rock cleavage ............. 11

Lehmann, Johann, on distortion of crystals ........................... 102-104

on distortion of pebbles................ 102

Light, transmission of, effects of cleavable

rocks on........................ 47

Limestone, cleavage of..................... 146

with bedding, photograph showing.... 206

Linear-parallel flow cleavage, plates showing ........................... 158,164

Literature of secondary cleavage .......... 13-19

M. 

Marble, cleavable, characteristics of....... 41

Marble, deformed, photomicrographs of... 186

Marble, schistose, calcite in. figure showing. 41

fracture cleavage in, plate showing.... 192

plate showing.......................... 188

Material used, character and sources of.... 20-21

Maurer, E. R., acknowledgments to........ 22

Metaclase, definition of..................... 12

Metaclasic structure. See Cleavage, secondary. 

Mica, arrangement of ...................... 24-28

comparison of hornblende, quartz, and. 34-35 

comparison of hornblende, quartz, feldspar, and....................... 45^6

crystallization in....................... 93,94

dimensions of........................... 28

effectof, on form of cleavage. 51-52, G3,-100-101 

feathering of, figure showing...........' 26

in sediments, parallelism in............ 144

processes arranging .................... 78-80

Mica plates, arrangement of, figure showing ............................. 25

Mica-schist of Black Hills, ottrelite in, arrangement of................... 44

Micro-cleavage, definition of............... 17

Mineral crystals. See Crystals, mineral. 

Minerals, arrangement of............ 24-48,96-97

arrangement of, original cleavage due

to............................... 143

production of....................... 65-95

relation of flow cleavage and...... 49-C4

arrangement in, processes of........... 78-92

effect of, on flow cleavage.............. 51-64

summary of......................... 63-64

recrystallization of, order of ........... 93-95

relations of, during deformation....... 114

relative abundance of................ 145-146

relative importance of, in producing

- flow cleavage................... 50-64

Page.

Molecular attraction, relations of flow

cleavage and .................. 99-101

Molecular shape, relations of recrystallization and........................ 97-99

Muscovite, biotite.and quartz, juxtaposition

of, figure showing.............. 27

Muscovite in mica, arrangement and dimensions of .................... 28

' N.

Nepheline-syenite, feldspars in, arrangement of......................... 39

with parallel feldspar, plate showing... 204 

Nicolson, J. T., on cleavage............'. 14,16-17

pjiotomicrogra ph. from ................. 186

Nicolson, ,T. T., and Adams, F. D., on flow

of marble....................... 89

0.

Original cleavage." See Cleavage, original. 

Original structure, synonymous names for. 11 

Ostwald, W., on granulation ............... 76

Ottrelite, arrangement and character of... 44,61 

effect of.................................. 61

Ottrelite-schist, ottrelitein, arrangement of. 44

P.

Parallel arrangement. See Arrangement,

parallel.

Parallelism. See Arrangement, parallel. ' 

Pebbles, distortion of..................... 102-104

elongation of, figure showing .......... 103

plate showing .................... 208,210

undistorted, plate showing............. 210

Pegmatite, parallelism and cleavage in.... 149,

150-151

Perthite, parallelism and cleavage in. 149,150-151 

Phillips, John, on cleavage ................ 14,18

Pierce, C. S., on strain...................... 109,110

Plane-parallel flow cleavage, plates showing ........................... 158,164

Porphyritic constituents, development of,

photomicrograph showing ..... 182

Pressure, effect of, on rotation.............. 71-72

effect of, on solution and crystallization............................. 68

relation of cleavage to direction of, literature on......................'.. 18-19

Pressure, differential, relation of flow cleavage and....................... 138-139

Protoclase, definition of.................... 12

Pure shear, definition of .................... 110

Seecdso Strain, irrotational. 

Pure shortening. See Shortening, pure.

Q.

Quartz, arrangement and occurrenceof.. 31-35,54 

comparison of feldspar, mica, hornblende, and ..................'.. 45-46

comparison of mica, hornblende, and.. 34-35 

crystallization in ....................... 93,94

effect of, on form of cleavage. 54-55,63,100-101 

granulation of, figure showing......... 33

in sediments, parallelism in............ 144

photomicrograph of.................... 178

processes arranging .................... 82-86
INDEX. 215

Page.

Quartz, recrystallization of, .photomicrograph showing ................. 178

slicing of, figure showing .............. 32

veins of, in Cambrian slate, plate showing ............................. 190

Quart/,, biotite, and muscovite, juxtaposition of, figure showing......... 27

Quartzitic pebbles, elongation of, figure

showing........................ 103

Quartz-porphyry, photomicrograph of ..... 178

R.

iileade, T. M., on cleavage.................. 14

.'Reade and Holland, on cleavage........... 93

Recrystallization, effect of granulation on.. 7C

order of..............................:... 93-95

photomicrograph showing ............. .178

predominance of....................... 92-93

Sec also Crystallization.

Rhyolite-gneiss, plate showing............. 180

sliced feldspar in, figure showing....... 8$

Rift. Kr.e Cleavage, fracture.

Rock cleavage. See Cleavage.

Rock flowage, relations of stress and strain

to flow cleavageand.......... 112-118

Rock masses, elongation and shortening of, . 

 relations of flow cleavage 

and....................... 102-108,142

elongation and shortening of, relations of stress and ............ 109-118

Rock specimens, sources of................ 20-21

Rocks, deformation of, cause of............ 05-60

susceptibility of, to flow cleavage...... 50-51

Rocks, cleavable, arrangement in, summary

of facts concerning............. 45-48

internal structure of .................. 96-101

Rotation, character and effects of.... 60-07,71-74

criteria for determining existence of... 73-74 

diagrams illustrating............... 72,73,117

effect of granulation on................ 76

importance of.......................... 92

parallel arrangement due to ........... 137

Rotational strain, definition of............. 109

Sec also Strain, rotational.

Schist, cleavage in, plate showing........ 196,198

feldspar crystals in, arrangement of ... 36 

fracture and flow cleavage in, plate

showing........................ 202

Schists, chloritic, sliced feldspar in, figure

showing......................... 38

Schists, hornblendic, photomicrographs

of.............................. 160,1C8

plate showing.......................... 164

Schists, micaceous, photomicrographs of. 160,162 

plate showing.......................... 188

sliced feldspar in, figure showing...... 38

Schists, micaceous and quartzo.se, photomicrographs of ....... 172,174,176,184

Schist-conglomerate, elongated pebbles in,

plate showing.................. 208

feldspar in, figure showing............. 38

Scission, definition and discussion of..... 110-111

See also Strain, rotational.

Page.

Secondary cleavage. See Cleavage, secondary .

Secondary structures, cause and names of.. 11 

S.ederholm, J. J., on distortion of pebbles.. 102 

Sedgwick, Adam, on cleavage ....... 13,15-16,18

on crystallization...................... 141

Sediments, cleavage in................... 143-146

cleavage in, plate showing............. 206

weakness of, along certain layers. 143,144,146 

Sericite, development of, figure showing... ' 79 

Sharpe, Daniel, on cleavage............. 13,16,18

on crystallization...................... 141

Shear, pure, definition of................... 11.0

See also Strain, irrotational. 

Shear, simple, definition of................. 110

See also Strain, irrotational; Strain, rotational. 

Shearing stress, definition of............... Ill

Shortening, rotation accompanying, figure

showing........................ 72

Shortening, pure, definition of............. 110

diagram showing....................... HO

See also Strain, irrotational. 

Shortening and elongation of rock masses,

relation of flow cleavage to.. 102-108 

relation of stress to................... 109-118

Sillimanitc, arrangcinenland characterof.. 45,62 

effects of................................ G2

Simpledetrusion, definition of............. 110

See also Strain, irrotational. 

Si mple shear, definition of.................. 110

See also Strain, irrotational; Strain, ro- '

tational. 

Slate, false cleavage in, photomicrograph

showing........................ 1.84

fracture and flow cleavage in, plate

showing........................ 200

fracture cleavage in, figure showing... 120 

grain of, cause of....................... 133

Slate, Cambrian, quartz veins in, plate

showing........................ 190

Slate, Ordovician, cleavage in, plate showing ......:.........:............ 190

Slaty cleavage. See Cleavage, slaty.

Slicing, character and effects of............ 75-76

effect of, on arrangement.............. 92

figure showing ................... 30,32,38,39

parallel arrangementproduccdby... 137-138,

141-142 

photomicrograph showing ............. 178

plate showing.......................... 180

Slip cleavage. See Cleavage, slip; Cleavage, fracture. 

Solution, effect of temperature on........... 67

Sorby, H. C., on cleavage................... 13,

14,16,17,18,120,140,142 

on crystallization....................... 141

Staurolite, arrangement and character of. 43-44,

60-61 

crystallization in....................... 94

effects of, on form of cleavage...... GO-61,101

processes arrangi ng..................... 91-92

Strain, discussion of...................... 109-111

effect of, on crystallization ............. 69

relation of rock flowage and flow cleavage to stress and.............. 112-118
216 INDEX.

Page. 

Strain, relation of stress to......:........... 112

See also Shortening, pure; Scission. 

Strain, irrotational, definition of.....'...... 109

diagram showing........i.............. 110

flow cleavage during, development of. 112-113

fractures resulting from, character of. 121-122

Strain, rotational, definition of............. 109

diagram showing rotation ot particle

during.......................... 117

flow cleavage during, development of.. 113

fractures resulting from, character of.. 122

Strain ellipsoids, causes of...........'..... 110-111

definition of............................ 109

equivalence of different forms of, diagram showing.................. Ill

theoretical position of.................. 129

Strain-slip cleavage, definition of .......... 120

See also Cleavage fracture.

Stress, axes of, definition and discussion of. Ill 

effects of, on flow cleavage........... 113-114

relations of, to elongation and shortening of rock masses............ 109-118

relations of flow cleavage and ... 109,112-118 

relations of fracture cleavage and.... 121-124

relation of rock flowage and flow cleavage to strain and ............. 112-118

relation of strain to .................... 112

Stress difference, definition of............... Ill

Structure, internal, of cleavable rocks, observations on.................. 96-101

Structures, original and secondary, cause,

names, and comparison of ..... 11

T.

Tait, P. G., on strain ................'..... 109,110

Talc, character and arrangement of........ 42

effect of ................................ 59

Temperature, effect of, on solutions........ 67

Terms pertaining to cleavage, application of 12 

Thompson, William, on strain............ 109,110

Page.

Tourmaline, arrangements and character of 43,60 

crystallization in....................... 94

effect of, on form of cleavage......:... 66,101

processes arranging .................... 91-92

Tremolite, crystallization in ............... 93-94

effect of, on form of cleavage ...... 59-60,101

occurrence, character, and arrangement of....................... 42,59-60

processes arranging .................... 90-91

Tyndall, John, on cleavage .............. 13,15,18

U.

Ultimate-structure cleavage, definition of.. 17 

V.

Van Hise, C. R., acknowledgments to...... 21-22

illustration from paper by............ 103,206

on chlorite ............................. 40

on cleavage ............ 14-19,120,123,124,140

on crystallization................... 67-69,141

on fissility.............................. 142

on granulation and slicing........... 141-142

on order of crystallization.............. 94

on recrystallization of calcite .......... 90

on rock deformation .......... 66,104,105,106

on strain.............................. 109,110

Volcanic textures, distortion of, relation of

flow cleavage to.............. 102,104

W. 

Water, work of, in crystallization ...:...... 67-68

Wausau, Wis., feldspars in rock from,

arrangement of................. 36-37

Weidman, Samuel, acknowledgments to... 22 

on feldspar rock........................ 36

photograph from ..................... 180,204

Wisconsin, University of, material supplied

by .............................. 20-21

Y. 

Young, Thomas, 011 strain ................. 109,110

0