DEPARTMENT OP THE INTERIOR

BULLETIN

lf THE

GEOLOGICAL SURVEY

No. 59

WASHINGTON

PRINTING OFFICE

UNITED STATES GEOLOGICAL SUEVEY

J. W. POWELL, DIRECTOR

THE GABBKOS

AND

ASSOCIATED ROCKS IN DELAWARE

BY

FREDERICK D. CHESTER

WASHINGTON

FEINTING OFFICE 

1890

CONTENTS.

Page.

Introduction................................................................ 7

General petrographical considerations .................. ..................... 8

Hypersthene-gabbro .......................... .............................. 10

Gabbro-diorite ......................................................!....... 15

Series illustrating transformations....................................... 18

Gabbro-grauite............................................................. 19

Series illustrating transformations ....................................... 20

Norite ......................................*................................ 21

The Iron Hill gabbros and gabbro-diorites................................... 22

Diorite .........................'............................................ 29

Series illustrating transformations....................................... 30

Gabbro-diorite and hornblende-gneiss........................................ 31

Structural relations......................................................... 36

The gabbro belt......................................................... 36

Stratigraphy ........................................................... 38

The origin and genetic relationship of the gabbros and their associated hornblende rocks.............................................................. 40

ILLUSTRATIONS.

PLATE I. Map of gabbro area in Delaware.................................... 7

FIG. 1. Inclusions within feldspar of hypersthene-gabbro .................... 14

2. Section illustrating the alteration of hypersthene into tremolite and

fibrous green hornblende........................................... 23

3. Section illustrating the alteration of diallage into compact green hornblende..........................................:.................. 26

4. Section illustrating the alteration of fibrous into compact green hornblende..-.-. ....................................................... 29

5. Inclusions within feldspar of Iron Hill gabbro........................ 30

5

U S.GEOLOGICAL SURVEY. BULLETIN NO.59 PL.I.

L.,iifjilncle cast fi-oiii Washington.

LAWARE

F.D.CHESTER.

THE GABBROS AND ASSOCIATED ROCKS IN DELAWARE.

By FREDERICK D. CHESTER.

INTRODUCTION.

 The following investigation upon the gabbros and their associated 

rocks in Delaware has occupied my attention for over two years past, 

and has been treated thus in detail, first, because the massive gabbro 

is the most prominent formation in the northern part of the State; and 

second, because a thorough study of it embraces some interesting questions in petrography and their bearing upon structural phenomena of 

great magnitude.

In the order of treatment the petrography has preceded the discussion of structural phenomena, for the reason that many of the problems under the latter head must depend for their solution upon petrographies! facts. In view of the magnitude and importance of the 

material under investigation, reflection upon the nature of the results 

herein set forth compels the confession of a certain timidity of statement in this paper. The evidence at hand has been carefully weighed 

aud opinions have been recast, until the results embodied finally possess at least the virtue of having been patiently evolved.

It would be unjust to prior authorship not to take note of the admirable and exhaustive memoir of Dr. G. H. Williams 1 upon the Baltimore gabbros. So great is the similarity between these rocks and those 

of Delaware that our present descriptions must in some cases involve 

a repetition of the work of Dr. Williams. In another sense, however, 

the gabbros of Delaware merge into types so entirely distinct and 

exhibit phases of paramorphic change so novel, as to render the present results, perhaps, of some additional interest.

My thanks are especially due to Dr. Williams for much encouragement and assistance, and also to Prof. R. D. Irving and to Mr. Whitman 

Cross for the examination of thin sections involving difficult points.

1 The Gabbros and Associated Hornblende Eocks in the Neighborhood of Baltimore. 

Bull. U. S. Geol. Survey, No. 26. 1886.

7
THE GABBROS OF DELAWARE. ' [BULL. 59.

GENERAL PETROGRAPHICAL CONSIDERATIONS.

The petrography of the Delaware traps is based upon an examination 

of numerous thin sections from all parts of a relatively small area, the 

form and extent of which are seen from the accompanying map. These 

sections show the different variations in our Delaware rocks, together 

with the various stages of paramorphic change from pyroxene to 

amphibole, so fully as to render additional material of no essential 

moment in establishing the conclusions adduced.

The Delaware traps, exclusive of the younger orthoclastic granites, 

may be designated by two terms, gabbros and gabbro-diorites. 1 These 

terms, however, have only the most general application, while a closer 

study reveals a variety of petrographical forms, not so much distinct in 

themselves as expressions of a wide variation from some arbitrary form 

which we call primary. This primary form is more clearly the common 

hypersthene-gabbro, or the so-called blue granite of the Delaware quarries. It is normally a finely granular mixture of basic plagioclase, 

hypersthene and diallage in nearly equal proportions, with accessory 

quartz, magnetite, apatite, hornblende, and biotite.

Eeference to the following analytical table will make clear the lines 

of variation proceeding from this primary hypersthene-gabbro:

f (B.) 

By addition of (original ?) brown hornblende ....... 5 GABMO-DIORITE &nd

I HORNBLENDE GNEISS.

(C.) HYPERSTHENE-GABBRO. . *..,. ,. > By addition of biotite as an important constituent; ? G.ABURO-GRANITE

nnnrf.7. in InrcrA n.mmiTit. ' .-*-* .

Plagioclase, hyper- . ,

quartz in large amount stheno diallage, with

accessory quartz, apatite, pyrite, magnetite, brown hornblende, biotite.

Highly feldspathic and quartzose, with a little hy- ? jjORJTE

persthene and much magnetite. ^ /T? ,

(&.)

By paramorpbism of pyroxene to green hornblende ? GABBRO-DIORTTE. 

(uralitic). * (Iron Hill.)

By Addition of compact green hornblende, either c GABBRO-DIOBITE and 

paramorphic or original. \ HORNBLENDE GNEI8B.

(G.) 

EPIDOTIC DIORITK.

In considering the Delaware gabbros as a whole we might say that 

their chief distinguishing feature is their highly acidic character, due 

to the large admixture of quartz. This is at once evident to the most 

casual observer, who not only sees quartz as a prominent constituent

1 This very convenient term has been used by Tb'rnebohm and G. H. Williams to 

indicate transitional types of rocks between gabbro and diorite, and to all uralitic 

diorites of. like origin.
CMEBTEE.) RELATIONSHIP OF DIVERSE TYPES. 9

in every hand specimen, but who observes in the faces of the rock in 

situ great seams and segregated masses of the same. It has been noticed that the rock which in a general sense we would regard as a normal hypersthene-gabbro is a most unstable form, running on the one 

hand into a rock possessing strongly the character of a granite, and on 

the other into those distinctly dioritic. Quartz, which is found in such 

large amounts in the fine-grained gabbros northeast of Wilmington, is 

absent or in but a small amount in the normal gabbro of which a number of types have been found. This more basic standard, however, is 

not long maintained; on the other hand, the Delaware gabbros tend by 

indistinct gradations to become more and more acidic from a gradual 

gain of quartz.

This last variation of gain in quartz is also characterized by an 

equally gradual gain of biotite, until, as in many of the Bellevue and 

Claymont rocks, we have a type which is more properly a granite, 

with pyroxene as only an accessory constituent.

The presence of a brown dichroic hornblende as a very variable constituent of the hypersthene-gabbro is a fact of the greatest interest, since 

by the gradual increase of this mineral from a mere trace to an amount 

sufficient to subordinate the pyroxene elements we are led to see an 

equally gradual transformation to a black, highly schistose, hornblende 

rock, which, structurally and macroscopically, seems at first sight to 

have no genetic relationship with the gray, compact gabbro with which 

it is associated. Eegardiug this brown hornblende as original, as I 

believe we must from present data, we can attribute' this change from 

a gabbro to a diorite only to the result of an original variation in the 

character of the eruptive material.

In the change, on the other hand, from gabbros to green hornblende 

rocks we have, to a large degree at least, the results of a paramorphic 

change affecting the pyroxene of the mother rock; the final product in 

all cases being a perfectly compact, green hornblende, whether this 

results directly from the alteration of diallage, or, as when hypersthene 

is the primary substance, from the intermediate fibrous amphibole, as 

described by Lehmann and by Williams.

Thus we may affirm that the whole result of a deliberate study of an 

extensive series of thin sections has been to teach most emphatically 

the principle that apparently diverse types can in reality possess an 

intimate genetic relationship. It is also believed that the sooner this 

is fully realized the sooner geologists will be defended from many errors 

of generalization based upon structural data alone. Nearly every section in our series is a transition stage from one type to the other, and 

so few are the clearly defined forms, that no sharp lines can be drawn 

or strong designations made. Those set forth in the accompanying 

table, it must be understood, are largely arbitrary, serving simply to 

indicate certain dominant tendencies in the line of variation.

It can not be otherwise than evident to the most casual observer that
10 THE GABBROS OF DELAWARE. [BULL. 59.

many of our former theories respecting the metamorphic rocks are undergoing radical alteration since the microscope hgis taken such a 

prominent place in geological investigation. This is particularly true 

as regards that great class of schistose, amphibole rocks which, until 

quite recently, have been generally regarded as altered sediments.

But since the investigations of Lossen in the Harz, J. H. Kloos in the 

Black Forest in Baden, F. Becke at Langeulois, Lower Austria, To'rnebohm in Sweden, Lehmann in Saxony, Teall and Phillips in England, 

Wads worth, Irving, and Williams in this country, an entirely new field, 

has been opened, and geologists are in a fair way to discriminate between crystallized sediments and metamorphosed igneous rocks. That 

highly foliated schists in many cases may have had an eruptive origin 

has already been demonstrated by a respectable body of workers, while 

what is more encouraging than all is the indication of future developments in this fertile field of research. The element of pressure which 

plays such an important role in the development of slaty cleavage in the 

sedimentary rocks and their derivatives must in the future be recognized in accounting for the secondary products of the massive traps, 

by producing great structural alterations, and possibly by inducing 

molecular rearrangements and consequent paramprphic changes.

HYPERSTHENE-aABBRO.1

This is taken as the primary and typical form of the trappean series. 

In color, it varies from a dark bluish gray to a black; in texture, from 

one distinctly crystalline to one quite aphanitic. Macroscopically the 

rock appears to be a finely granular mixture of striated feldspar, bluish 

quartz, rounded grains of black highly metallic hypersthene and a 

small amount of foliated greenish diallage. Under the microscope 

the several mineral constituents present characters which render their 

determination satisfactory.

The hypersthene occurs in irregular grains, never in crystals, which 

vary in size from lmm to 7mm, the majority approaching 4mm to 5mm , while 

others again are spreading branching individuals, which fill up the 

spaces between the plagioclase and inclose numerous rounded grains 

of the latter. The cleavage is well developed parallel to the prismatic 

face (QO P) and to the brachypinacoid (oo P o>).

The pleochroism of all sections is strong, offering a ready means of 

distinguishing this mineral from its associated diallage. Sections in 

which the doable prismatic cleavage is well developed, and which, with 

concentrated polarized light, show a biaxial interference cross, show 

also strong yellow and reddish pleochroism. Other sections with 

powerful greenish and yellow pleochroism were found, with which

l l have adopted the term Injpersthene-gabliro in view of its use by Dr. G. H. Williams, yet it remains a question whether, in consideration of the acidic character of 

many of our true hypersthene-gabbros, the term norite might not better express the 

position which our rocks bear to those in Europe.
CME8TEK.1 CONSTITUENTS OF HYPERSTHENE GABBRO. 11

equally clear interference crosses could be obtained. It is evident from 

this that in these two cases we have sections respectively parallel to 

the basal piuacoid (OP) and to the macropinacoid (oo P do), and thus 

the rhombic character of the mineral as well as the arrangement of 

the pleochroic colors is well established.

The trichroism is as follows : a copper-red; b yellow; c green.

All prismatic sections extinguish parallel to the cleavage. A number 

of cleavage leaflets detached by a penknife from the prismatic face of a, 

coarser fragment gave the same parallel extinction, and the color of the 

C ray, with a combination of the a and b rays for light vibrating at right 

angles to the cleavage.

The specific gravity of a number of these fragments, which, after examination with the microscope, were found to be pure, was determined 

by means of the Klein solution and found to be 3.38. This high specific 

gravity excludes the possibility that this mineral is bronzite.

The hypersthene seems to be entirely free of the brownish, leafy inclusions, so characteristic of many hypersthenes. and also of the hypersthene in the Iron Hill rocks, but beautiful syrnmetical needles or rods 

arranged parallel to the cleavage lines are common, associated with 

coarser, lenticular, black inclusions either of magnetite or of titanic 

iron ore.

The diallage can sometimes be found in the coarser hand specimens 

as a grass-green mineral with an eminent parting parallel to the orthopiuacoid and to the prismatic face. Under the microscope thin sections have a pale green color, and show no apparent pleochroism, while 

thicker cleavage fragments show only the slightest change of color on 

revolving the stage over the polarizer.

Most prismatic sections give high angles of extinction approaching 

45°. Cleavage leaflets parallel to the orthopinacoid extinguish parallel 

to the prismatic cleavage lines as would be expected, and give with 

concentrated polarized light the characteristic single optic axis. The 

specific gravity of a number of microscopically pure fragments of this 

mineral was determined by means of the Klein solution. While the 

diallage pieces remained freely suspended in the solution a fragment 

of green hornblende was dropped in, which easily rose to the surface 

and floated. The specific gravity of the solution, and therefore of the 

diallage, was 3.24. The diallage contains the same inclusions as the 

hypersthene, i. e., opaque needles and grains of magnetite, with frequently included fragments of hornblende and grains of plagioclase.

The feldspar occurs as granular particles, which vary in size from .lmm 

to .5mm in diameter. They show a fine polysyuthetic twinning, and 

quite commonly two systems of striation approximately at right angles 

to each other, according to the well-known albite and periline laws. 

The feldspars show those irregularities of twinning characteristic of 

many gabbros, i. e., lack of parallelism of the twinning lamellae, irregularity in the length of the same, and their frequent wedge-shaped
12 THE GABBfcOS OP DELAWARE. tstJLL.59.

character, showing under crossed nicols a beautiful interlocking of individual lamellae. The angles of extinction are invariably high, indicating a basic feldspar. The examination of cleavage fragments of 

the feldspar isolated from a typical gabbro gave angles of extinction 

answering to a more basic labradorite. The specific gravity of the same 

isolated feldspar was 2.689, confirming the optical determination.

The feldspar from a typical Brandy wine gabbro was separated by 

means of the Thoulet solution by Mr. J. S. Diller in the petrographical 

laboratory of the U. S. Geological Survey, and analyzed by Mr. E. B. 

Eiggs, with the following result:

SiC>2 = silica ........................................... 51.44

A1203 = alumina........................................ 30.05

Fe203 = ferric oxide (FeO not det.)...................... .96

CaO = lime........ .......... .......................... 13.19

MgO = magnesia....................................... trace.

NaaO=8oda ...................'.............'........... 4.07

K20 = potash .......................................... .21

Loss on ignition ....................................... .35

100.27 

Dried at 105° C.

This answers to the most basic member of the labradorite group 

and to a feldspar having the theoretical composition Abi An2 .

An examination of a number of cleavage fragments extracted from 

the feldspars of many of the coarser gabbros will show these feldspars 

to vary somewhat, which we could scarcely expect to be otherwise, in 

view of the great variation in the character of the iron-bearing silicates 

and in the proportion of quartz. This is illustrated most forcibly in 

the case of the more acid biotite-bearing gabbros, rocks similar to those 

which elsewhere have been styled norites. A typical sample of this 

rock was also treated by Mr. Diller, and the feldspar separated in the 

usual way from the ferruginous silicates. An analysis of the feldspar

gave the result shown below, in column I:

i. n.

Si02 = silica .................................. 70.37 64.12

A1203 = alumina .............................. 18.36 24.76

Fe203 = ferric oxide (FeO not det.) ............ .58 .....

MnO = manganese oxide ...................... trace. .....

CaO = lime. .................................. 5.08 5.44

MgO = magnesia ............................. .04 .....

Na20 = soda .................................. 4.32 5.68

K2O = potash................................. .63 .....

............................... .45 .....

99.83 100 

Dried at 105C C..

Column II gives theoretical composition of a feldspar having the for- 

.mula Ab3 An,, answering to oligoclase.

If in column I the .63 per cent, of potash be assumed as replacing 

some of the soda, the ratio between soda and lime becomes more accordant in I and II.
CHESTER.] ANALYSES OF FELDSPAKS. 13

In Column I the excess of silica above that required to form the feldspar molecule is represented by quartz which this specimen is known 

to contain, but which could not easily be separated from the feldspar 

owing to the little difference in the specific gravity of the two minerals.

The small amount of iron present is undoubtedly represented by 

inclusions of magnetite dust, which all the feldspars contain. On the 

supposition that the small amount of water in the analysis represents 

some kaolin, the amount of alumina in the analysis is thus slightly abnormal, but this will be fully counteracted by the opposite tendency of 

.foreign matter to lower the alumina percentage.

Neglecting these small amounts, and adding proportionately the excess of silica to the alumina, lime and soda in column I, we get a result 

nearly in accord with column II.

An optical examination of a number of cleavage leaflets detached by 

a penknife showed in all cases for sections parallel to 0 P, au extinction

so nearly parallel to the twinning bands as to make the less basic character of the feldspar plain. Sections parallel to co P o> gave with reference to the cleavage an extinction angle of +5°. The specific gravity 

of these fragments was 2.635.

Thus we are led to believe that the more acid biotite-bearing gabbros contain at least as an accessory constituent the species oligoclase. 

Upon the whole the facts seem to strengthen forcibly the presumption 

that as our gabbros become more acid the feldspars also assume the 

more acid types. We shall furthermore see as we proceed that the 

hornblendic gabbro-diorites have a tendency in the opposite direction, 

the feldspar in the Iron Hill rocks being clearly anorthite.

Notwithstanding these variations, a notable fact is the uniformity of 

micro-structure observed in all the feldspars from one end of the belt 

to the other. They are quite similar to those found so characteristically 

in other gabbro-feldspars.

These consist largely of black opaque needles of clear definition and 

of remarkable straightness, which, as they become wider, show generally 

an oblique termination at each end.

These run into elongated prisms or columns of a brownish color, 

similarly truncated, the longer axis of which, however, seems to bear 

no constant ratio to the shorter; the longer ones passing into those 

with nearly rhombic section. With these we find again similarly shaped 

prisms and rhombic sections of a black color and highly metallic character. The dust-like particles which everywhere occur as cloudy patches 

near the center of the feldspar individuals or arranged in bauds coincident with the twinning lamellae are made up of the shorter needles which 

merge into mere specks, or again of extremely minute crystals or irregular particles of a black, metallic character.

All the inclusions show a definite arrangement in parallel systems, 

of which there are commonly four, though sometimes only three. Of 

3, two StiQW an arrangement pa.ra.Uel fa flie edge QJ*; 09 p^ and. to
14 THE GABBROS OF DELAWARE. [BULL. 59.

the vertical axis. A section of triclinic feldspar, apparently parallel 

to the plane co P 06, showed distinct basal cleavage, approximately 

parallel to which was a definite arrangement of needles; but the system 

containing the greatest number of inclusions was that forming an angle 

of 110° with the cleavage, i. e., nearly parallel to the vertical axis. Sections more or less parallel to OP show an arrangement nearly parallel 

to the twinning traces, with another at right angles to the same. These 

two systems are again crossed by two other sets, perpendicular to each 

other, and forming with the first two sets angles from 20° to 25°. The 

character and arrangement of these inclusions are best shown in Fig. 1.

FIG. 1. Inclusions within feldspar of hypersthene-gabbro.

Besides these, tbe feldspars are everywhere full of inclosures of foreign 

minerals, including rectangular crystals and dust-like particles of magnetite, well formed crystals of brown hornblende, and traces of biotite and 

pyroxene. In a few cases they contain very minute elliptical or elongated inclusions, with darkly defined borders, arranged linearly, without bubbles, and similar to gas pores. These are scarcely more than 

,001mm in diameter; associated with them are elliptical or greatly elongated tapering glass inclusions of a light brown, sometimes greenish 

color.

Magnetite is a very constant constituent of all the Delaware rocks, 

occurring frequently in such large amounts as to nearly or quite equal 

the pyroxene. It occurs as grains and dust-like particles from .02 to 

.4mm in diameter. With reflected light it is found associated in many 

cases with pyrite. It shows no decomposition into leucoxene, while a 

careful test gave only a .slight reaction for titanium ; hence it is probably a true magnetite.

Apatite occurs as prismatic and cross-sections, more generally as inclusions within the feldspars,'also frequently within the hornblende and
CHESTER.) GABBRO-DIOEITE. 15

the pyroxene. It contains in many cases an abundance of inclusions. 

These consist (a) of long transparent needles or prisms, frequently etched 

or corroded, at other times intact, centrally arranged parallel to the 

longest development of the inclosing substance; (b) black, opaque 

.granules in elongated or irregular forms, such as so commonly occur 

within the feldspars, possibly ilmenite these occur fully within the 

apatite, at other times partly or nearly penetrating one end or side of 

the same. In a few cases the apatite crystals are corroded, owing apparently to the caustic action of the feldspathic liquid. In one case at 

hand the apatite prism has been cut entirely across in the center by a 

fissure, and the space has been filled with feldspathic matter; a little 

below this is another fissure which has so far eaten into the crystal as 

nearly to bisect it.

Quarts; occurs as an important element of all our rocks. In the thinnest sections it gives clear colors and feeble polarization, and is easily

recognized by its want of cleavage and by the appearance with converging polarized light of a uniaxial interference figure. It generally contains groups and lines of extremely minute, darkly denned inclusions, 

entirely free from bubbles, possibly gas pores. Less frequently are 

found inclusions with quiescent or feebly moving bubbles, which do 

not disappear on heating the stage to nearly 100° C.

Brown hornblende and biolite occur as accessories, by the increase of 

which the hypersthene-gabbro (A) merges indistinctly on the one hand 

into the gabbro-diorite (B) and on the other into the gabbro-granite 

(C). See table, p. 8.

Garnet is found as an accessory in rare instances.

The true gabbros occur most abundantly along the Brandywine and 

upon all the roads leading out from Wilraington. In these localities 

admirable exposures are found in numerous quarries, and upon the particularly rocky picturesque slopes of the Brandywine Valley. Northwest of the city, however, the rocks become distinctly hornblendic and 

exhibit the various varieties of gabbro-diorite; northeast in Brandywine Hundred the rocks are associated with those acid forms which we 

have here for convenience termed gabbro-granites, although the two 

are never separated by any closely drawn structural lines.

GABBRO-DIORITE.

This name is given to those rocks which represent an extreme stage 

in the variation of the normal gabbro. Brown hornblende is known 

to occur as an accessory constituent of true gabbro, but some phases of 

the Delaware rocks show a peculiarly large admixture of this mineral, 

which becomes even of such importance as to more than equal the 

pyroxenic matter.

In extreme cases, again, the rock consists mainly of a mixture of 

brown hornblende and pUigioclase, with pyroxene in but a trace, In
16 THE GABBKOS OF DELAWARE. [BULL. 59.

short, we find every stage between the massive gabbro and a rock 

which is apparently a hornblende gneiss, making, as a consequence, a 

proper designation of the rocks difficult. Many types described under 

this head might strictly be placed with gabbros, others might be placed 

with hornblende gneisses or amphibolites, the rock possessing upon 

the whole a union of gabbro and diorite characters, for which reason 

the term gabbro-diorite is adopted only as a convenient one in the 

present instance, without attempting to add to the already burdened 

rock nomenclature. In hardly a single case, so far discovered, has the 

progress to more and more highly dioritic forms resulted in an entire 

absence of pyroxene; on the other hand, most of the forms presenting 

in the field all the characters of a hornblende gneiss or an amphibolite, 

contain pyroxene in such an amount as to make the rocks from microscopic examination no more strictly amphibolites than gabbros, but 

rather standing intermediate between the two, and thus the term in 

many cases is a natural one.

The extreme type of the gabbro-diorite is a black rock with a satiny 

lustre, showing generally a high degree of foliation; other transitional 

types show an obscure foliation, which serves to connect structurally 

the foregoing rocks with the massive gabbros. The hypersthene and 

the diallage of this rock are identical with the preceding. The feldspar gives somewhat higher angles of extinction, in the more extreme 

forms of this rock, approaching anorthite. It contains, however, the 

same characteristic inclusions and is apparently identical.

The hornblende occurs in large irregular grains. They have a greenish brown color in ordinary light and show powerful dichroic properties. Sections transverse to the vertical axis with the prismatic cleavage well developed give for the a ray a deep yellow, and for the b ray a 

deep brown with powerful absorption. Many longitudinal sections show 

no perceptible change of color or absorption upon revolving the stage

over the polarizer; these are presumably sections approximately or quite 

parallel to the orthopinacoid (oo P 55), and give the color and absorption 

of the b and c rays. The mineral is therefore dichroic and of the variety' 

known as basaltic.

The absorption formula is, as usual, c = b > 3-

The mineral offers no notable characters except that it is unusually 

filled in many cases with inclusions of magnetite, which commonly in 

prismatic sections show an arrangement parallel to the cleavage. With 

these are found crystals of apatite, dust-like particles and symmetrical 

or irregularly shaped needles 'placed parallel to the vertical axis. A 

cleavage section of the brown hornblende parallel to the prismatic face 

gave an extinction angle c to c of 19°. Magnetite, pyrite, apatite, 

quartz and biotite occur as accessories of the gabbro-diorite (B, p. 8).

The relation which the hornblende bears to the pyroxene makes it 

probable that both consolidated out of a more or less molten magma; 

£ §eems ^h.a.t in majiy cases tUe pyroxene consolidate^ fet, it 1$
CHESTER.] HORNBLENDE AND PYROXENE. 17

equally true that pyroxene individuals in many cases must have formed 

around hornblende particles, since grains of both hypersthene and green, 

pyroxene, giving uniform optical orientation, are seen to inclose large 

and small grains of brown hornblende. If we can apceptthe well kuowu 

theory that hornblende and pyroxene are only two different crystalline 

forms of the same substance, determined by different physical conditions 

at the moment of consolidation, such as temperature and possibly rate of 

cooling, we might venture in this connection to suggest that the association of brown hornblende and pyroxene in these rocks might be accounted for by supposing these conditions to have been intermediate or 

oscillatory between those extremes, which in one case would produce a 

true gabbro and in another a true diorite. We have already intimated 

that the gradual increase of brown hornblende proceeding from a massive rock was also accompanied by the gradual assumption of a more 

schistose structure. That this structural change has been induced by

pressure is quite probable, hence there would seem to be some relationship between pressure and the relative proportion of hornblende to pyroxene. Could we h#ve abundant proof of the alteration of pyroxene 

to compact brown hornblende, which is possible in view of the late 

studies of Dr. G-. H. Williams on the rocks of the Cortlandt series1 and 

of Prof. E. D. Irving on the rocks of the northwest,2 we. would have a 

most natural explanation, especially since these rocks present similar 

structural changes from massive to schistose forms.

In an article by the late Dr. G. W. Hawes 3 we find a record of a 

rather doubtful instance of the alteration of pyroxene to compact brown 

hornblende in the occurrence of obscurely outlined iuclosures of brown 

hornblende within pyroxene, the author remarking in support of his 

opinion that paramorphic alterations of this character might originate 

inwardly as well as outwardly. Thus in a like way do we find in the 

class of gabbro-diorites under consideration inclusions of hornblende 

with indistinct borders; but cases of this character unaccompanied by 

other evidences of paramorphism can hardly be considered as at all 

conclusive, consequently until further facts come to our notice we must 

regard this brown hornblende as original.

The distribution of the rocks of this class is by no means a definite 

one. Many of the exposures along the Brandy wine show in places the 

imperfect development of a foliated character, these rocks in turn becoming more or less hornblendic. The northern and northwestern 

limits of the city of Wilmington are covered by eminently or obscurely 

foliated rocks containing both pyroxene and brown hornblende, and the 

same is true of all the exposures along the Concord and Lancaster turnpikes leading out from the city. Roughly speaking, a line drawn from 

Kiemensie station, on the Wilmington and Western Railroad, through

1 Am. Jour. Sci., 3d series, vol. 28.1884, pp. 259-268. 

"Fifth Ann. Rep., U. S. Geol. Survey, 1885, pp. 175-242. 

3 Am. Jour. Sci.,3d series, vol. W, 1876, p. 136. 

Bull. 59 2
18 THE GABBROS OF DELAWARE. [BULL. 59.

the northern limits of the city of Wilmington to the Brandywine will 

serve as a boundary north of which the outcrops are of black foliated 

rocks of this class. However distinct this area may at first sight seem 

to be, it is discovered that in all these rocks the genetic connection 

with the massive gray gabbros is steadily maintained, and into these 

the rocks of this area indistinctly merge. Since the most interesting 

feature of the brown hornblende gabbro-diorites is the position which 

they hold as connecting links between massive gabbros and highly 

schistose hornblende gneisses, it will be profitable to describe in order 

a series of slides which reveal most clearly these transformations.

SERIES ILLUSTRATING TRANSFORMATIONS.

Brandywine Creek, Wilmington. A dark gray rock of uniform structure, massive, 

and possessing in the field the characters of a true gabbro. Consists mainly of hypersthene and plagiociase nearly equally developed, with a small amount of brown hornblende. Diallage in but a trace. The hypersthene as irregular branching individuals .5mm to .6mm, which fill up the spaces between the feldspar and which inclose 

minute grains of plagiociase. Garnets in rectangular, isotropic crystals of a pinkish 

color. Quartz in a few small grains. Magnetite, apatite. The feldspars partly 

clouded by dust inclusions, and numerous minute crystals of brown hornblende.

Wilmington City Reservoir. Rock black, as massive bowlders with obscure foliation, 

consists mostly of plagiociase with a small amount of hypersthene and a few particles 

of diallage; brown hornblende somewhat subordinate. Magnetite; apatite rare. 

The feldspars much clouded by dust inclusions. Quartz in small amount.

Henry Clay P. 0. (Dupont'a). Rock black, with obscure foliation. Plagioclase 

the principal constituent, brown hornblende and hypersthene nearly equally represented. Diallage apparently absent. Quartz, in several large 'grains. Magnetite; 

apatite. '

Toll Crate, Eennot Turnpike. Rock black, partly foliated, consists mostly of plagioclase with much quartz. The next element is brown hornblende with a little biotite; hypersthene in greatly subordinate amount. Diallage absent. Magnetite, pyrite, apatite.

Bade of Old Alms House, Wilmington. Rock black, thinly bedded, schistose, offer^ 

ing in the field all the characters of an amphibolite. The bisilicates somewhat in 

excess of the feldspar. Brown hornblende the chief bisilicate constituent, with less 

pyroxene represented about equally by hypersthene and diallage. Magnetite and 

apatite. This rock has the schistose character more strongly developed than the 

preceding, which is due to the relatively large proportion of bisilicate.

Kiemensie Station, Wilmington and Western Railroad. A thinly bedded, highly foliated 

rock. Bisilicates considerably in excess of the feldspar, consisting of nearly equal 

parts of hypersthene and brown hornblende, and a small amount of characteristic 

diallage. Quartz in small amount. Magnetite scarce. The discovery of this rock 

is of particular interest, inasmuch as it forms a lenticular mass of highly foliated, 

thinly bedded hornblende rock between mica schists, along the north border of the 

main gabbro belt. A little farther north, and just below Braudywine Springs, is another outlying bed of gneiss, which is associated with a similar rock next to be described. Such facts as these enable us to assume with some probability that the 

outlying, lenticular masses of amphibolite within the mica schists are not to be separated in nature or in origin from the hornblende rocks of the larger area to the 

south.

Just Below Brandywine Springs Station. A black, aphanitic rock. Under crossed 

nicols the ground mass is seen to be made of minute quartz and plagiociase grains, 

1mm to 2 mm in diameter, in which are porphyritically developed large and small hy-
CHESTER.] GABBRO-GRANITE. 19

persthene individuals, and numerous crystals of garnet, several millimeters across. 

The hypersthene was of a yellowish color, visibly but not eminently pleochroic, yellow and copper red with distinct prismatic cleavage, many of these sections showing 

with concentrated polarized light a biaxial cross. The garnet occurs as rectangular 

pieces of a pinkish color, entirely isotropic and full of characteristic inclusions enclosed by individuals of hypersthene. Brown hornblende in small amount. Magnetite rare. The feldspar highly basic, approaching anorthite.

Three miles north of Newport. A black, serai-foliated rock, consisting of a 

nearly equal mixture of plagioclase and hornblende with but a trace of pyroxene, 

yet that little in well characterized fragments. The rock, therefore, represents 

the extreme stage of variation, and is properly a hornblende gneiss. The hornblende offers peculiarities; it is visibly trichroic, but not so strongly so as is 

characteristic of the green. The color of the C ray is a dark green; that of the b 

varies from a brown, quite like that of the basaltic variety to a greenish brown; 

and that of the a ray is a fine yellow. Absorption c > b ]> a. Sections approaching 

parallelism to the clinopinacoid gave extinction angles of about 15°. The hornblende, therefore, stands intermediate between the green and the brown, and is possibly indicative of an alteration of brown hornblende into the green variety. Magnetite in one large piece several millimeters across, full of beautiful included grains of 

plagioclase, also in smaller granules. The feldspars give high angles of extinction, 

and are probably either bytownite or anorthite.

Lancaster Pike and Wilmington and Northern Railroad. Black, fine-grained rocks in 

association with coarser varieties; thinly bedded and schistose, although less 

strongly so than the intimately related coarser rocks, showing the feeble effects of 

pressure in developing a schistose structure in such aphauitic rocks, notwithstanding 

the large amount of hornblende. The principal constituent of the rocks is brown 

hornblende with somewhat less plagioclase. Hypersthene and green pyroxene in but 

small amount. Magnetite, pyrite, apatite.

GABBRO-GRANITE.

Another accessory constituent of the normal gabbro (A) is biotite 

which gradually increases in amount so as nearly or quite to equal the 

pyroxene. This particular phase of the Delaware rocks, as already intimated, is furthermore accompanied by much quartz, so that the rocks 

become of an abnormal type, possessing in a strange degree a mixture 

of granitic and gabbro characters. So peculiar is this feature of Delaware petrography that it seems best, for want of a better term, to designate these extreme types as gabbro-granites. We .find all stages of 

transition from the typical form of this rock to those with but a trace 

of biotite, and in the other direction to those which are nearly true 

granites.

The biotite occurs generally as oblong blades, showing the usual 

basal cleavage and extinction of light parallel to the longest development 5 also as scales parallel to OP. All prismatic sections show strong 

pleochroism, i. e., a, light yellow color and weak absorption for rays 

vibrating perpendicular to the cleavage or parallel to a, and a deep 

brown with powerful absorption for rays parallel to the direction of 

cleavage. Sections parallel to OP give, as usual, but little variation of 

color or absorption.

The biotite frequently contains as inclusions apatite crystals and 

grains of magnetite; otherwise poor. Magnetite also generally in
20 THE GABBROS OF DELAWARE. [BULL. 59.

intimate association or contact with biotite, but never in borders around 

the latter. The biotite commonly shows those mechanical deformations 

so characteristic, i. e., a bending and twisting of the lainellse. The relation of the biotite to the other minerals shows that it must have 

consolidated before the other inagnesian silicates, but subsequent to 

the consolidation of the feldspar.

The plagioclase from several measurements on sections parallel to 

OP corresponds probably to labradorite, with it, and possibly replacing 

it in some cases. The less basic oligoclase can be proven to exist, as 

shown by examination of a number of cleavage fragments extracted 

from the coarser hand specimens. The feldspars in all cases contain the 

same characteristic inclusions already described. The highly micaceous rocks of Bellevue, which .offer notably distinct characters, contain a greasy gray, coarsely crystallized feldspar. A cleavage leaflet 

of this mineral parallel to M (oo P a>), as judged from the absence of 

twinning, gave with reference to the cleavage an extinction angle of 

 15°, and with concentrated polarized light the side appearance of 

an optic axis on the margin of the field. This would indicate a more 

acid labradorite in accord with the very acid character of these rocks. 

Quartz occurs in large amount in all the rocks of this class, several 

forms being largely a mixture of plagioclase and quartz, with little 

pyroxene or biotite; it is rich in liquid inclusions, with quiescent or 

feebly moving bubbles, which remain permanent at a temperature above 

100° C. Most of the highly quartzose rocks of this class show the results of mechanical movements in a partial or an entire crushing 

of the more brittle quartz granules, protruding angles being commonly broken off, or again an entire side reduced to a mass of 

granules from the pressure exerted by surrounding larger individuals. 

Again, smaller fragile granules of feldspar wedged between large, 

tough, quartz masses have been partly or completely crushed. The 

rocks of this class cover all that region east of the Brandy wine, and 

are found in such intimate association with the true gabbros as to render all attempts to accurately define their distribution quite ineffectual. 

As a rule, the gabbro-granites are massive, but assume a semi-foliated 

character whenever the biotite becomes an important constituent. In 

order to trace out the gradual passage from normal gabbro to those 

rocks of a highly granitic type it will be well to describe a few slides 

best illustrative of this point.

SERIES ILLUSTRATING TRANSFORMATIONS.

Characteristic Rock of the Brandytoine Quarries. Rock bluish black to gray, entirely 

massive. It consists primarily of a mixture of quartz 5 and plagioclase, the former in 

large grains several millimeters in diameter, the latter in smaller particles. Bisilicates consist of hypersthene and diallage nearly equally represented, with biotite 

in small amount. Magnetite; apatite; the feldspar rich in inclusions.

Walnut Hill, Brandywine Hundred. Rock bluish black, massive, much like the 

preceding, but contains a larger portion of bisilicates, which is mostly hypersthene
CHESTER.] NORITE. 21

with but a trace of diallage; biotite in email amount. Qnarfcz rich in liquid inclusions ; magnetite, apatite.

Concord Turnpike, two and one-fourth miles from Wilmington. Rock dark-gray, 

massive, of a peculiar acid type, consisting essentially of a mixture of plagioclase 

and quartz, with scarcely more than a trace of biotite and pyroxene. Magnetite 

and apatite in small amounts.

Episcopal Church, Concord Turnpike, North of Talleyville. Rock black, semi-foliated 

to massive, consists mostly of plagioclase with much quartz; hypersthene and diallage 

with a little biotite; magnetite.

Silver Brook Station, Wilmington and Northern llailroad. Rock may be taken as an 

average type of this class, presenting an equal combination of granitic and gabbro 

characters. Plagioclase is the principal constituent, with which is a smaller amount 

of hypersthene; diallage and biotite about equally represented. Quartz in a number of large grains; brown hornblende in small amountj magnetite, apatite.

Talleyville, Concord Turnpike. Rock much like preceding, except that it contains 

a little less pyroxenic matter than biotite. Quartz rich in inclusions of apatite and 

liquid cavities.

Belkvue, Dd. k black, thinly l)edded, schistose rock, presenting a strong contrast

to the massive gabbro-granites with which it is associated; it contains a much larger 

proportion of biotite and pyroxene to feldspar than any of the other rocks of this 

type. Upon a superficial examination in the field it might be mistaken for an amphibolite, but under the microscope it is not to be separated from the highly massive 

gabbro-granites already described, except in the relative proportion between plagioclase and the other constituents. The rock is of special interest in showing the 

effects of pressure in causing a schistose structure in highly basic rocks, aud its 

notably lesser effect upon the highly acidic rocks with which it is associated. It 

consists primarily of a mixture of hyperstheue aud biotite, and a somewhat less 

amount of plagioclase. Magnetite abundant; apatite, the feldspars unusually rich 

in characteristic inclusions.

In this locality, as in many others, the rocky faces of a light, coarse-grained trap 

appear streaked and beclouded by lenticular and most irregularly shaped segregations 

of a inuch darker fine-grained rock. These are found to be due to local variations in 

the relative proportion between feldspar and the iron-bearing silicates. This section, 

which is a type, is practically identical with the latter, but while the preceding rock 

covers a large area, the present type occurs as included masses whose dimensions are 

measured in inches; the cases, however, are parallel and show the great tendency of 

this rock to variation within very limited areas. The rock consists mainly of a mixture of hyperstheue and biotite, with a somewhat less amount of plagioclase; diallage 

in very small amount; magnetite. Long, characteristic nefidles of zoisiterichly developed in the feldspars; apatite in stont prisms; acicular inclusions abundant.

Characteristic Bock of the Bellevue Quarries. A coarsely crystalline rock of a bluish 

to greasy gray color, which in hand specimens appears to be a granite, consisting of 

a mixture of greasy, gray labradorite, bluish quartz and biotite. It is usually quite 

massive, but shows in places in the large quarry walls an imperfect foliation. Under 

the microscope plagioclase and quartz predominate; biotite insubordinate amount; 

hypersthene, scarcely more than a trace. Magnetite and apatite rare. The quartz 

 is rich in inclusions; these consist of darkly bordered cavities without bubbles, 

arranged linearly and in groups, with many reddish brown inclusions, probably of 

glass, elliptical or elongated with taperiug ends, acicular or imperfectly hexagonal. 

The quartz also contains numerous black hair-like needles, possibly rutile.

NOEITE.

Nothing is more variable than the exact shade of color which the 

massive rocks assume, dependent upon the relative proportion between 

feldspar and the bisilicates; thus the light gray uorites are found in all
22 THE GABBROS OF DELAWARE. [BULL.59.

stages of graduation into the bluish black gabbros, with which they are 

associated. As a rule the rock is highly quartzose, containing veins 

and lens-shaped masses of this substance developed within the body of 

the rock. In the field these are readily recognized by their light gray 

or dirty white colors, which, in extreme cases, stand in strong contrast 

to the dark gray or bluish black colors of their associated rocks.

A typical section under examination is composed mainly of a very 

fine-grained mixture of quartz and plagioclase, which, in hand specimens, 

appears quite homogeneous ; in this ground-mass are porphyritically 

developed larger grains of hypersthene and magnetite, with generally a 

few shreds of biotite.

The rocks are invariably massive, although in one prominent case 

along the Brandy wine, a true norite lies adjacent to a highly schistose 

hornblende gneiss, while the passage from one to the other through 

darker gabbros is so .gradual as to be indistinguishable. That these 

uorites have been subjected to great pressure at one time or another, is 

evidenced by the nearly complete crushing of the quartz granules and 

the associated plagioclase, no rocks exhibiting stronger evidence of 

mechanical action than these.

The norites are fouud in such intimate association with the darker 

gabbros that it is quite impossib]^ to restrict their distribution. Along 

the Brandywine are frequent alternations of lighter and darker trap, 

while over the whole of Brandywine Hundred one meets with outcropping bowlders truly characteristic of the rocks of this class.

THE IRON HILL GABBROS AND GABBRO DIORITES.

In macroscopical and structural characters the hypersthene rocks 

and diorites of Iron, Chestnut and Bed hills seem to be quite distinct 

from those already described. That .this difference, however, is more 

apparent than real, is shown by microscopic studies, which lead us to

regard those under consideration as but modified forms of the finer 

grained, more acid gabbros of Wilmington. The Iron Hill rocks are essentially coarse-grained mixtures of triclinic feldspar and hypersthene, 

with a greenish diallage in variable amounts. They seem to be nearly 

or entirely free from quartz, and, so far as studies have been carried, 

of the basaltic variety of hornblende, but they contain a large amount 

of a green, paramorphic hornblende which, by entirely replacing the 

pyroxene, causes the rock to run into true diorites, with which it is 

found in the most intimate association. In the field, the rock generally 

possesses a very characteristic purplish gray color, and is sometimes so 

coarse as to yield large paramorphic crystals of a grayish, fibrous mineral, several inches in length, which have the physical properties of 

fibrous hornblende, but the cleavage angle of hypersthene. Microscopic 

examination proves these secondary crystals to contain cores of hypersthene, with fibrous hornblende as borders around them. The process 

of alteration of hypersthene to green hornblende is by no means a 

uniform ones but the most common method is as follows: A single
CI1ESTEU.] HYPERSTHENE AND HORNBLENDE. 23

individual of hypersthene becomes surrounded by a fringe of colorless 

fibres, probably tremolitic, which, on the outer border becomes bluish 

green to green. These last'highly colored needles sometimes show but 

slight pleochroism, again, distinct bluish to yellowish colors. Under 

crossed nicols the whole fringe shows the most brilliant aggregate 

polarization, revealing fibres sometimes placed in radiating tufts, at 

other times .perpendicular to the periphery of the hyperstheue nucleus; while again, the white fibres of tremolite form a most confused 

arrangement. Later stages in the process show the gradual encroachment of this fibrous tremolite upon the hypersthene until finally it 

takes entire possession of the latter; thus we have in many sections 

cores consisting of a mass of tremolitic fibres surrounded by borders 

more or less deep of green hornblende needles.

Another stage in the parainprphism consists in the gradual deepening of the border of green hornblende aggregates until, finally, it 

takes entire possession of the secondary tremolite. Furthermore, there 

is a marked tendency of the fringes or cores of hornblende aggregates 

to assume a more compact nature, either upon the outer border or by 

the appearance of irregular nuclei within the body of the fibrous hornblende individuals. Thus we find in the last stage, individuals of perfectly compact green hornblende with only- the remains of small nuclei 

of green hornblende aggregates within the latter. So abundant and 

complete are the various stages of paramorphism in the relatively few 

thin sections which have been prepared that no link in the chain needs 

to be supplied by the imagination, and everything points to the gradual 

transformation of one mineral into another. Fig. 2 shows an early

FIG. 2. Section illustrating the alteration of hypersthene into tremolite and fibrous green hornblende.

stage in this process of paramorphism. In the large piece a core of 

copper red, highly pleochroic hyperstheue lies buried in a white sub-
24 THE GABBROS OF DELAWARE. [BULL. 59.

stance which, under crossed nicols, is shown to consist of a dense mass 

of tremolitic fibres, yielding brilliant polarization colors. The shaded 

portion surrounding the white tremolite is composed of dark green 

hornblende fibres, giving aggregate polarization and rather feeble 

pleochroisin. The large amount of magnetite, shown in the figure by 

the black granules, is a common associate of the secondary hornblende, being undoubtedly a separation product from the highly ferruginous hypersthene. In the paramorphic process just described the 

alteration has been entirely an outward one, the original hypersthene 

retaining its fresh appearance within the envelope of secondary matter; 

in many cases again, the hypersthene assumes a peculiarly fibrous character, while still retaining the characteristic pleochroism and parallel 

extinction of ordinary hypersthene. Between crossed nicols it is seen 

that this appearance is due to the development along the cleavage fissures of brightly polarizing fibres, which stand out. prominently in brilliant colors when the cleavage lines of the hypersthene coincide with 

the plane of vibration of the light, in which position the hypersthene 

is dark. The nature of these inwardly developing fibres is not so evident in the early stages as when the original hypersthene becomes 

nearly or entirely replaced by aggregates of parallel white fibres, which, 

by polarizing identically with those in the loosely aggregated fringes, 

are probably tremolitic. Furthermore, this change rarely stands alone, 

but is accompanied by the usual outward fringing, the confusedly 

placed fibres of the latter often penetrating the original hypersthene. 

Sometimes the cores of white tremolite, instead of being made up of 

aggregates of fine threads, become more compact, and are composed 

of coarser fibres or columns placed parallel, which give angles of extinction from nearly 0° up to 15°, and in converging polarized light 

show the appearance of one optic axis which lies parallel to the cleavage line.1 Furthermore, it roust be mentioned that the fringes of green 

hornblende aggregates which surround the tremolite and hypersthene 

cores do not always consist of confusedly placed needles, but very 

commonly the green hornblende possesses a finely fibrous character 

similar to uralite. This finely fibrous hornblende possesses rather distinct pleochroism, i. e., bluish green and yellow, and gives between 

crossed nicols rather brilliant aggregate polarization. Unfortunately, 

the term uralite has generally been used in a rather ambiguous sense 

to indicate a substance which possesses the external form of augite but 

the cleavage and the optical properties of green hornblende, yet showing in addition a few special characters that, in the particular cases 

from which the current descriptions have been framed, are clear and 

unmistakable; in others, not so. Thus it is difficult to use the term 

uralite with that confidence with which others have used it. Suffice it 

to say, however, that the green hornblende aggregates' which stand

'Rosenbusch; Mikroskopiscke Physiographic. Second edition. Vol. 1,1885, p. 465.
HYPERSTHENE AND HORNBLENDE. 25

between the original hypersthene and the final compact hornblende 

possess all the optical characters of green hornblende, and that the 

process is quite similar to that which is generally called uralitization.

The paramorphic process next to be described is one of particular interest, inasmuch as, in many respects, it is a novel one. In this case 

the hypersthene, instead of being changed into fine, loosely placed aggregates, passes into a more compact, green hornblende. -The properties 

of this hornblende are generally distinct its pleochroism is strong, with, 

the usual blue, yellow and green colors characteristic of ordinary green 

hornblende. The customary cleavage is well developed, but there is an 

entire absence of that finely fibrous habit characteristic of uralite. The 

optical orientation is uniform, and altogether the mineral appears perfectly compact. It forms borders around the hyperstheue with the usual 

intervention of a layer of white tremolite. These borders, which, with 

Ordinary light, appear to continuously surround the pyroxene cores, are, 

between crossed nicols, sometimes found to be made up of differently 

orienting areas of a compact nature, or again, very commonly a uniform 

extinction of light takes place throughout the whole border.

Notwithstanding some misgivings as to the secondary nature of these 

hornblende borders, as opposed to the idea of parallel growths of two 

original substances, the evidence still remains strong, and is based upon 

the following considerations: First, the process is identical with the 

ordinary alteration into uralite even to the intervention of tremolite, 

these two processes showing intermediate stages where the same individual is in one portion partly surrounded by a compact border and 

in another altered into loosely placed aggregates. Second, there are 

found all stages in the transformation, from large individuals of hypersthene with narrow fringes, to those with but traces of cores within individuals of apparently compact green hornblende. Bearing further 

upon this alteration of hypersthene to apparently compact green hornblende, it is necessary to consider the alteration of amonoclinic pyroxene, which proceeds in a like manner. The mineral which has been 

identified as a monoclinic pyroxene occurs in irregular grains of a greenish gray color, with an eminent cleavage, and frequently possessing 

the finely striated appearance characteristic of diallage. It is entirely 

unpleochroic, and gives high angles of extinction approaching 45°. 

These individuals of diallage are generally surrounded by borders of 

compact green hornblende, whose relations to the pyroxene cores are 

such as to be quite convincing regarding their secondary nature.

The contact between the diallage and the green hornblende borders 

is generally quite irregular, tongues and shreds of green hornblende 

continuous from the latter commonly penetrating far into the original 

pyroxene, and in one case under observation passing entirely through 

the core to the other side. The line of contact between the original 

and the secondary minerals is in some cases quite clearly defined and 

sharp, in others a liue of separation can scarcely be discovered with the
26 THE GABBROS OF DELAWARE. [BULL. 59.

highest powers. Accompanying this outward change there is observed 

the development within the pyroxene of numerous elongated inclusions 

of green hornblende, arranged generally parallel to the cleavage lines, 

and so covering the faces of the diallage sections as to give to the latter 

a peculiarly streaked appearance, often to the nearly complete replacement of the pyroxene. The outlines of these inclusions are indistinguishable with low powers (x 75), in most cases, and their forms are very 

irregular, which, together with their number, renders an accurate 

delineation difficult. We have already mentioned an extension of the 

outer green border in the form of narrow tongues into the pyroxene 

substance; in a similar way we often observe tongues or necks of green 

hornblende connecting the outer border with some of the more irregular 

of these inclusions which spread and branch throughout the central 

nucleus. In Fig. 3 we have a typical case of this alteration of pyroxene.

FIG. 3. Section illustrating the alteration of diallage into compact green hornblende.

The central core of pyroxene (BBB) is of a greenish gray color with 

eminent cleavage, as shown in the figure. It is entirely devoid of pleochroism, and gives an extinction of 33°. The border of green hornblende which surrounds it is entirely compact, its pleochroism is strong, 

with bluish and yellow colors. Between crossed nicols the portion 

marked AAA extinguished light uniformly at the same instant, while 

the portion DD deviated from the first position of extinction but a few 

degrees. With the very sensitive quartz plate the whole border showed 

uniform polarization colors, and was in the strictest sense compact. 

Within the body of the pyroxene cores are seen the characteristic inclusions of green hornblende, which is undistinguishable from that on 

the outer margin; two of these are continuous with the border by 

clearly defined necks. The outlines of these inclusions are somewhat 

indistinct and not so clearly defined as the drawing must show; they
CHESTER.] PARAMORPHISM OF DIALLAGE. 27

shade gradually into the lighter pyroxene substance and on their 

borders fail to extinguish the light, but in their centers extinguish uniformly with the border, strongly testifying to the crystallographic identity of the two. The case of the alteration of diallage to compact green 

hornblende, which has just been described and figured, may be taken 

as a type of many others as observed in a study of the Iron Hill gabbros 

and gabbro diorites. Exactly similar cases in the Baltimore area have 

been described by Dr. G-. JET. Williams, 1 who regards these hornblende 

fringes and intergrowths as secondary ; and in the Black Forest, by J. 

H. Kloos,2 who regards them as original. In the example before us a 

groat mass of evidence, as I believe, goes to favor the derivative 

nature of this compact hornblende, not alone because .of the favoring 

facts which a single typical case can furnish, but partly because the 

study of a long series of thin sections reveals all stages in the passage

from diallage to green hornbleude, and this, therefore, weakens to a 

large degree the assumption that all these cases can be explained by 

any theory which implies an original crystallization of both minerals.

A change like this, involving the diallage, being, as I believe, the 

result of a molecular rearrangement more favorable to the lower temperature, may take place within the body of a diallage particle as well 

as upon its surface, and hence we observe in the rocks under consideration these two processes going hand in hand to the complete replacement of the original substance. What has already been written regarding the alteration of diallage to compact green hornblende can be 

brought as a further evidence regarding the secondary nature of those 

hornblende borders which surround the hypersthene, as the two kinds 

of hornblende occur together and are identical. It must be observed, 

however, that the process of paramorphism in the two cases is distinct. 

There has as yet been found no substantial proof of the direct alteration of hypersthene to compact green hornblende without the intervention of a colorless tremolite zone, and there is furthermore, as was 

observed by Dr. G-. H. Williams, an entire absence in the body of the 

hypersthene of those obscurely defined green hornbleude inclusions 

which so characterize the diallage; on the other hand, the change is 

an outward one in all cases, and as the same writer has admirably 

demonstrated, the hornblende appears to be due to a reaction between 

the hypersthene and the accompanying feldspar, the basic plagioclase 

furnishing the lime necessary to this change.

Regarding the paramorphism of diallage, it must be mentioned that 

while the method just described seems to be the most general one, 

there is also an alteration into true uralite similar to that affecting 

the hypersthene in so many instances. This consists in the gradual replacement of diallage particles by aggregates of light fibres, either

1 Bull. U. S. Geol. Survey, No. 2-«, 1886, p. 41.

2 Neues Jahrbuch fur Mineral., Ill Beilage-Band, pp. 24-34, 1885.
28 THE GABBROS OF DELAWARE. [BULL. 59.

tremolite or actinolite, wliich in turn changes outwardly into a light 

green finely fibrous hornblende.

The feldspars of the Iron Hill gabbro-diorites are apparently somewhat more basic than those found in the more acid gabbros near Wilmington.

The feldspar in a typical sample of the rock was separated by Mr. 

Diller as follows: The iron magnesiau silicates were first separated by 

means of the Thoulet solution having a specific gravity of 2.877. The 

separated feldspar was then put into a solution of 2.592 specific gravity, 

in which an orthoclase indicator was slowly raised to the surface. The 

feldspar, thus cleaned of any suspected orthoclase, proved from microscopic examination to contain much included matter; to remove this a 

solution of 2.749 specific gravity was used. It barely lifted the feldspar, allowing portions containing the inclusions to remain behind. 

The feldspar thus obtained has a specific gravity between 2.749 and 

2.592, with most of it as heavy as 2.749.

An analysis of the feldspar, by Mr. Eiggs, is given below.

i. n.

SiO2 = sil5ca................................... 44.09 44.87

A12O:, = alumina .............................. 35.41 35.66

Fe203 =ferric oxide (FeO not det.) ............ .51 .....

MuO = manganese oxide ...................... trace. .....

CaO=lime................................... 18.47 18.61

MgO = magnesia .............................. none. .....

Na2O = soda ....'.............................. .99 .86

K2O=potasn..-............................... .19 .....

Loss on iguition............................... .35 .....

100. 01 100.00

Column No. II gives the theoretical composition of a feldspar with the 

formula Abi AnJ2. The mineral is therefore an intermediate member 

of the anorthite series, and is considerably more basic than the analyses 

of the feldspars separated from the acid gabbros indicate. A cleavage section of an Iron Hillgabbro-diorite feldspar was detached; it gave 

an extinction angle with the cleavage of 35° and had a specific gravity 

of 2.745, both answering again to anorthite. The development of inclusions within the feldspars is a notable feature of these rocks; these 

dust particles often so cover the twinning lamellae as to make them appear like a series of parallel black bands, separated by lines of light 

coincident with the twinning traces; again, short opaque needles occur 

in abundance, but there seems to be an absence of those brownish 

prisms so characteristic of other feldspars.

The hypersthene and the diallage show the same micro-structure 

already described, and frequently the former mineral contains a number of light brown inclusions arranged parallel to the vertical axis, 

similar to those which in other hypersthenes are regarded as brookite.
CHESTKR.1 ALTERATION OF HORNBLENDE.

DIOEITE.

29

To draw a distinct line between this type and the foregoing upon 

petrographical considerations is quite impossible; the distinction is 

more properly a structural one, standing as it does quite apart in macroscopical characters. The main mass of a series of elevations, of which 

Iron Hill is the most prominent, consists of a fine-grained, greenishblack rock, from which all traces of pyroxene have disappeared, the 

one bisilicate consisting of amphibole, 'either the colorless treinolite 

or the fibrous green hornblende. As we should expect, the rock is an 

extreme form of the one already described, in which the hypersthene 

has nearly or entirely disappeared, and has been replaced by secondary 

tremolite and green hornblende. The less advanced stages of the rock 

show cores of tremolite fibres, giving between crossed nicols brilliant 

aggregate polarization, with these cores in turn surrounded by borders

of greater or less thickness of green hornblende aggregates; other 

sections are entirely green, showing cores of green hornblende needles 

passing into the compact form. Fig. 4 represents an individual of

Fio. 4. Section illustrating tho alteration of fibrous into compact green hornblende.

hornblende which, with ordinary light, appears to be quite homogeneous; 

with the use of the polarizer alone the whole is distinctly pleochroic, 

but more uniformly so over the portions marked A A.. Between crossed 

nicols this last portion AA, including all the areas in the figure correspondingly shaded, extinguishes uniformly, and appears to be perfectly 

compact j the portions marked BB show brilliant aggregate polarization and appear to be composed of irregularly placed needles of 

green hornblende, which, from their marked pleochroism, are probably 

the latter and not actinolite. The irregularity and indistinctness of 

this compact border, which a drawing can not well portray, leaves
30 THE GABBROS OP DELAWARE. [Bl'LL. 59.

no doubt whatever of its secondary nature, and furnishes a typical instance of that final transformation which is everywhere observed. 

The feldspars of the typical diorites of this order are particularly notable for the number of colorless inclusions which they contain. These 

show an arrangement commonly parallel to the twinning traces, and 

again in patches which greatly obscure the feldspar; they have a 

definite crystalline form, and polarize strongly and distinctly from the 

feldspar which encloses them; in size they avenge .Olmm by .0075mm, 

many, however, measured near .005mm by .0025mm. The forms of these 

can be seen in Fig. 5. On account of their minuteness the extinction

FIG. 5. Inclusions within feldspar of Iron Hill gabbro.

could not be definitely determined, except that it was oblique to the 

prismatic edges. With these commonly occur numerous light green 

needles which, from their faint pleochroism, are probably actinolite. 

The feldspars of the highly paramorphosed diorites also show a notable alteration to epidote; the process, however, has seldom been carried beyond the formation of narrow fringes around ragged cores of 

plagioclase. In order, as before, to make still clearer the paramorphic 

tendencies already pointed out we will describe a series of sections.

SERIES ILLUSTRATING TRANSFORMATIONS.

.Bed! Hill. A. coarse-grained purplish gray rock, consisting of a mixture of basic 

plagioclase, hypersthene, and diallage, showing an early stage in the transition to 

diorite. The hypersthene individuals, which aro mostly large, and full of dark, 

acicular inclusions, are surrounded in the usual way by borders of tremolite and hornblende aggregates. In other cases individuals of hypersthene become surrounded by 

borders of perfectly compact green hornblendej with the usual intervention of a layer 

of white tremolite fibres. A light green diallage occurs in considerable amount, surrounded by the usual borders of compact hornblende, and full of obscurely defined 

green hornblende inclusions. Magnetite in large amount as a secondary product 

associated with the amphibole.
CHESTER.] GABBRO-DIORITE AND HORNBLENDE GNEISS. 31

Koad between Iron Hill and Sandy Bray. Much unaltered hypersthene, and a light 

green pyroxene, with less plagioclase rich in iuclusious. Both hypersthene and green 

pyroxene surrounded by borders of compact green hornblende with the*usual characteristics. There is, however an entire absence of anything like uralite. The hypersthene rich in inclusions; magnetite as usual.

J. W. Dyett's. Somewhat further paramorphosed than the latter. All the hypersthene in this section is much altered. The hyperstheue individuals show all stages 

ol passage into aggregates of whitish parallel fibers, yielding the brilliant aggregate 

polarization characteristic of tremolite. This change seems to take place along the 

cleavage fissures, as well as outwardly. Individuals of hyperstheue in this section, 

are found in all stages of passage into these tremolitic aggregates; these cores in 

turn altering outwardly into green hornblende needles, or into a finely fibrous green 

hornblende, the feldspars rich in dust-like inclusions; magnetite in small amount 

much altered to oxide of iron.

West end of Chestnut Hill. This section shows ragged remnants of hyperstheno imbedded in aggregates of tremolite fibers which are outwardly green. Green pyroxene 

individuals surrounded by borders of compact green hornblende, and much streaked

by inclusions of the latter; magnetite in large amount as dust particles within the

bisilicates; the feldspars much clouded by inclusions and partly decomposed.

Iron Hill. Shows a somewhat more advanced stage than the latter, with only traces 

of hypersthene embedded in the tremolitic fibers which outwardly become green. 

Inclusions in the feldspars abundant; magnetite.

Just beloiv Iron Hill Station. This rock shows no trace of hypersthene, but several 

pieces of a quite fresh light green diallage, which is found in all stages of replacement 

l>y aggregates of parallel fibers of light color, surrounded by borders of a finely 

fibrous, green hornblende, or by aggregates of green hornblende needles. The feldspars are notable for the dense clouds of colorless inclusions which obscure the twinning. The feldspars have their edges greatly corroded and fringed with epidote; 

magnetite with pyrite in small amount.

Iron Hill, road to Whitaker's. A still further stage in the process. The pyroxene 

has entirely disappeared, and been replaced by aggregates of white treraolite fibert 

which outwardly are green. The feldspar much clouded.

GABBRO-DIORITE AND HORNBLENDE GNEISS.

The normal gabbro is again found to merge by the addition of a 

green hornblende into the common hornblende rock of this region. The 

rock shows always a more or less perfect foliation, and has a black or 

greenish black color. Macroscopically it appears to consist entirely of 

hornblende, but thin sections show the presence of a considerable 

amount of basic plagioclase, also of either or both pyroxene elements; 

we have, therefore, adopted the term gabbro-diorite in this case, as in 

the others; where, however, the pyroxene becomes entirely replaced 

by green hornblende, the gabbro-diorites will merge into true hornblende gneisses. From massive gabbro to gneiss the same indistinct 

gradations are found, although in this case the structural change seems 

to more abrupt.

The hypersthene is identical with that in the normal gabbro; with it 

is associated a light green, nearly colorless, pyroxene, with the cleavage 

and other characters of diallage, occurring in large individuals several 

millimeters in diameter, of the same form as the green hornblende with 

which it is most intimately associated. It is the one pyroxene element
32 THE GABBKOS OF DELAWARE. [BULL. 59.

in this variety of gabbro-diorite, hypersthene being found in but small 

amount, rarely in more than a trace. The green hornblende which is 

such an important constituent of these rocks occurs in irregular grains 

and elongated individuals, showing the usual prismatic cleavage; its 

pleochroism is strong but distinct from that observed in the basaltic 

variety. Cross sections, in which the cleavage fissures intersect at 

about 124° and 56°, give for the a ray a light yellow, and for the b ray 

a green.

Sections which show an extinction parallel to the cleavage lines, and 

therefore sections parallel to the zone OP: oo P 6b; give the color of 

the c ray, which is a bluish green; and with concentrated polarized 

light the appearance of one optic axis. A cleavage leaflet detached 

with a pen-knife and parallel to a prismatic face ( oo P ) gave, with 

reference to the other prismatic cleavage fissures, an extinction between 

14° and 15°. The green hornblende shows no notable characters; it 

commonly is quite full of included grains of magnetite arranged parallel 

to the cleavage lines, and of black, opaque, acicular inclusions similar 

to those found in the basaltic variety. The feldspars of th.ese rocks 

give somewhat higher angles of extinction than the true gabbros, and 

probably upon the whole indicate a more basic form. Numerous measurements were made where alternate sets of twinning bands extinguish 

at equal angles on each side of the twinning traces, and the angles 

were all high, indicating a highly basic feldspar.

Cleavage fragments parallel to (0 P) were obtained from a typical 

Harmony Mill rock, and gave extinctions of -34° and -35°; the specific 

gravity of the fragments was 2.745. Similar cleavage fragments from a 

typical gabbro-diorite below Jessup and Moore's, on the Brandywine, 

gave extinction angles from -34° to -39° and a specific gravity of 2.737. 

Thus, in common with the Iron Hill gabbro-diorite, those of this class 

seem to contain, at least in the more extreme types, the basic anorthite. It will be well to repeat here that the more basic character of the 

feldspars is in accord with the fact that the rocks of this type always 

contain in addition to hornblende the lime-bearing pyroxene (diallage), almost to the exclusion of the magnesian silicate (hypersthene), 

and with the further fact that these rocks are poor in quartz. Magnetite and apatite occur in these rocks, but never in large amount; they 

are also less quartzose, although quartz is frequently found in small 

amount. The intimate association of diallage and green hornblende at 

once suggests the question whether the latter should be considered a 

secondary or an original substance. The evidence of the directpassage 

of diallage into compact green hornblende, which was so near at hand 

in the Iron Hill gabbros, is not so clear in this type of rocks. The 

study of particular thin sections might lead one to explain the intimate 

association of pyroxene and green hornblende on the supposition of 

parallel growth, while others would incline the observer to the view 

that the hornblende might be a secondary product from diallage; hence
CHESTER.] HOKNBLENDE-DIALLAGE INTERGROWTHS. 33

a most thorough comparison of many sections is the only safe ground 

for any conclusion. I am not therefore ready as yet to take any positive ground until further material shall have been collected and 

studied. It will be well, however, to weigh the evidence at hand and 

endeavor to explain the question in the light of all the facts.

In an examination of these rocks, after a study of the green hornblende gabbros, one is struck with the entire absence of continuous 

borders of green hornblende around the pyroxene. Inclusions of green 

hornblende within pyroxene, similar to those in the Iron Hill gabbrodiorites, are, however, common. These are, as a rule, very irregular in 

shape and so obscurely defined that even to high powers no well 

defined limit can be discovered. Their pleochroism is distinct, whereby 

they are placed in strong contrast to the inclosing unpleochroic diallage. 

At other times these included hornblende particles have such clearlycut boundaries as to leave little doubt of their primary origin.

Similar instauces of the iutergrowth of hornblende with diallage have 

been described at various times by Kloos, Streng, Macpherson, and 

Eohrbach, who have differently regarded these intergrowths either as 

original or as secondary. The analytical results of Bohrbach 1 Justify 

to a marked difference in composition after separation between the 

pyroxene and the hornblende, and he is led to believe in the original 

character of both substances. The studies of Williams 2 give somewhat 

different results, and render possible the existence of hornblende practically identical in composition with diall'age.

It was my intention to make a similar chemical- study upon some 

very promising material, but the press of work in the laboratory of the 

TJ. S. Geological Survey at Washington has hindered the forthcoming 

results. The true nature of the green hornblende iutergrowths as seen 

so commonly in this variety of gabbro-diorite can only at best be conjectured. Those of a more obscure outline are linked in the closest 

manner, as seen by a comparative study of sections, with those so common to the Iron Hill rocks, and appear to form such an indissoluble 

part of the diallage substance as to scarcely'be regarded as original 

crystallizations.

I have spoken of the absence of continuous borders around individuals of pyroxene; an outward alteration is, however, none the less 

apparent. In some cases a clearly defined isolated individual of diallage has one end replaced by compact, green hornblende. I was at 

first inclined to pass over these instances superficially, but as attention 

was more closely drawn to them the conviction became stronger that 

these were cases of alteration of pyroxene to hornblende. In a notable 

case in view an individual about lmm by 5inm is shown as a clearly de1 Ueber die Eruptivgesteine im Gebiete der Schlesisch-mahrischen Kriedeformation : 

Tschermak's Miueralogische und Petrographische Mittheilungen, n. a., vol. 7, 1886, 

p. 24.

8 Bull. U. S. Geol. Survey, No. 28,1886, p. 44. '

Bull, 59 Q
34 THE GABBROS OF DELAWARE. [BULL.59.

fined grain, which, crystallographically, can not be regarded as other 

than a unit. One end of this particle gives the color, pleochroism, etc., 

of the ordinary green hornblende, this hornblende substance extending 

also as a narrow, obscure border partly around the grain; the other 

end is of a lighter green color, totally unpleochroic, gives with reference to the cleavage a high angle of extinction, and is identical with 

the common green pyroxene of the section.

With a low power the two optically different portions shade indistinctly into each other; with the highest power no line of separation 

can be discovered. Between crossed nicols the green hornblende half 

gives uniform extinction where the color is deepest; where the two 

parts shade into each other and over the slightly pleochroic border of 

the diallage, no extinction occurs in any position; but at a point where 

the direction of vibration of the light makes a large angle about 38° 

with .the cleavage, the inner portion of the diallage half of the individual extinguishes the ray and reveals two irregular nuclei of unaltered pyroxene, which surround a light green, faintly pleochroic substance.

Sometimes individuals which with ordinary light have the appearance 

of the common diallage show with the polarizer a faint pleochroism on 

the outer border. Between crossed nicols the compact center extinguishes uniformly, but the outer faintly pleochroic portion fails in any 

position to extinguish the light. Thus it seems that the pyroxene 

outwardly changes first into an optically obscure substance, which, 

consisting of a mixture of hornblende and diallage matter, gives no 

definite optical orientation. Furthermore, when the diallage appears 

outwardly to be replaced by compact green hornblende, it is generally 

separated from the uniformly orienting diallage by an intermediate 

zone of this last substance.

The perfectly fresh, compact nature of all the green hornblende is 

the strongest proof in favor of its original nature; the boundary lines 

of the individual grains are clear and well defined, and have not that 

ragged form so characteristic of secondary minerals. This, however, 

is in conformity with the facts favoring a theory of paramorphisin. 

In all those cases where individuals of diallage show the hornblende 

pleochroism at one end or upon the border, the original boundaries of 

the diallage remain intact. This may be true of all paramorphic 

changes, which, as we understand, are merely molecular rearrangements, and imply none of those disintegrating influences which characterize pseudoinorphic alterations, as in the case of the alteration of 

the hypersthene to green hornblende, where such a change requires a 

reaction between the original pyroxene and its adjacent lime-bearing 

feldspar.

Epidote. A most important accessory of this variety of gabbro-diorite 

is the mineral epidote, which in certain instances occurs in such an 

amount as nearly to replace the feldspar, It occurs as eolprjess indi-
CHESTEK.] EPIDOTE IN GABBRO-DIOKITE. 35

viduals, generally elongated columnnar, also granular, which have a 

high index of refraction, as shown by their darkly marked relief. As 

would be expected from the absence of color in the thin section, the 

mineral is entirely unpleochroic. The elongated prisms, as is usual with 

epidote, extinguish the ray parallel to the longest development. The 

cleavage is most eminent in the direction of the base (0 P), also parallel 

to theorthodiagonal (txPco ). One section showed two directions of 

cleavage, crossing each other at an angle of 115°, over which the light 

is extinguished for rays vibrating nearly parallel to the vertical cleavage, and at an angle of 35° with reference to the basal cleavage lines.

Such a section as this can only be one of epidote nearly parallel to 

the clinopinacoid ( oo P so).

As in the case of the Baltimore rocks the epidote is secondary from 

feldspar, the microscopic peculiarities being the same as figured and 

described by Dr. GL H. Williams. Granular individuals of epidote occur, 

in which cores of striated feldspar are found, whose outlines are so 

irregular and ragged as to leave little doubt as to their relations to the 

surrounding epidote. The secondary nature of the epidote is still 

further evidenced by the continuity of the cleavage lines of the feldspar 

across the epidote borders both above and below, and the extension of 

many of the twinning lamellae with their marked interference colors for a 

certain distance beyond the limit of the feldspar cores, as determined 

with ordinary light. As shown by Williams, this alteration of feldspar 

to epidote is most frequent along the contact between a feldspar and a 

hornblende individual, the result of which is the corroding of both substances. A few slides show scarcely a grain of hornblende in its originally fresh condition, all the borders being highly etched and corroded 

into ragged forms, adjacent to which is invariably found the secondary 

epidote. This at once suggests the idea already advanced by Williams, 

that the epidote is due to a reaction between the feldspar and the hornblende, in which the hornblende took a far subordinate part, but a sufficient part to cause a considerable etching at points adjacent to the feldspar.

The gabbro-diorites and hornblende gneisses (F, p. 8) occupy a narrow 

belt which can be only roughly defined. Over the narrow tongue of 

rock which crosses the Brandy wine at Jessup and Moore's, colored dark 

on the map, we have black schistose rocks, which to the eye appear 

to consist entirely of green hornblende, but under the microscope show 

a large amount of both hyperstherie and diallage. Some hand specimens have a ground mass of hornblende and plagioclase, in which are 

porphyritically developed large bronze-colored crystals of hypersthene, 

and a light green foliated pyroxene. Farther to the west the samerocks outcrop back of the old almshouse in Wilmington. A section of 

this rock-is typical of the class, i. e., thinly bedded schistose, with a 

slightly less amount of diallage than of green hornblende, and but a 

trace pf hypersfchene. Thence, in a direction due west, the rock occurs
36 THE GABBROS OF DELAWARE [BULL. 59.

abundantly in the Wilmington and Northern Eailroad cuttings. Again, 

on both the eastern and the western sides of Harmony Mills a diallage* 

green hornblende rock is abundantly exposed; this rock contains a 

variable amount of epidote which give the same a characteristic greenish color. Farther west again, at White Clay Creek church, exposures 

of this same rock occur, but with less pyroxene in proportion to hornblende. From here to Newark and beyond the continuity of the rock 

is apparent, although the pyroxene seems to be entirely replaced by 

green hornblende, and the gabbro-diorite becomes a true hornblende 

gneiss. Between Harmony Mills and Iron Hill the outcrops are all of 

a greenish massive rock, in which there is much secondary epidote.

STRUCTURAL RELATIONS.

THE GABBRO BELT.

As will be seen from the map (PI. I) the great rock mass which forms 

the subject of the present study is club-shaped in general .outline, widest 

in its northeastern part and gradually tapering to the southwest. It 

forms an orographic part of the "syenitic areas" of southeastern JPeiinsylvania, already described as regards their structural habits by Mr. 

C. B. Hall. 1 It also continues with more or less breaks into Maryland, 

and is largely similar to the gabbro in the neighborhood of Baltimore, 

described by Dr. G. H. Williams. It will be seen that to the west of 

Newark the gabbro belt tapers quite rapidly and loses itself between the 

mica-schists j this, however, is the only point where the micaceous rocks 

are found upon both the north and the south sides of the gabbro mass. 

On the other hand, for the main distance, the plastic clays, of possible 

Jurassic age, rest upon the southern flank of the gabbro, with the micaschists either overlying or dipping below these rocks on the north. The 

topographical position of the rocks of this belt, especially westwardly, 

is such as in many cases to obscure their natural boundaries. Skirting 

the slope of the upland region of the State, which lies north of the horizontal Meaozoic clays, is a terrace of Quaternary gravel, already described by me, 2 which rises two hundred feet or more above tide, and 

from which the gravel spreads southward as a uniform sheet twenty-five 

feet in depth. This gravel deposit, coyering as it does all older formations, offers at points but a limited opportunity for the exposure of the 

rocks of this belt, consequently in a few places the line cannot be accurately drawn. The northern limit of this area from its western point 

to Newark can be clearly traced, .the gravels passing below these rocks. 

From Newark the line is traced close to White Clay Creek, by frequent 

outcrops, to Eoseville and Harmony Mills, on the Philadelphia and 

Baltimore Eailroad, where a fine exposure of an epidotic diorite occurs

'Second Geol. Survey Pa., reports C 4 on Chester County, 1883, and C 5 on Delaware County, 1885. 

«Ara- Jour, Sci., 3d series, vol, 27,1884, p. 189; vol. 29, p. 36.
CHESTER.] DISTRIBUTION OF GABBROS 37

in the railroad cut. From this point the gravels so cover the older formations that for a mile or more no outcrops occur until Mill town is reached, 

below which there is found a hornblende rock of this formation. From 

Milltown occasional outcrops of a greenish gabbro occur along the 

upper Wilmington road until at Green Bank and Kilmensie stations 

frequent exposures of gabbro-diorite are found as minor interstratifications between mica-schists. Green and brown hornblende rocks, all containing both of the pyroxene elements, are found again a short distance 

from the last place, with an abrupt turn of the line to the north, upon 

the Lancaster pike, near the " Oak Hill" school house. Then again the 

contact between schists and gabbro is well exposed just below Greenville station, on the Wilmington and Northern Railroad; thence the line 

turns abruptly north, the contact again occurring at Dupont station, 

on the same road; from here the line keeps close to the road from Dupont's to Eockland; then it passes abruptly northeastwardly, crossing 

the Concord pike near William T. Talley's place, above the Episcopal 

chapel, running into Pennsylvania, joining the gabbros of Delaware 

County, as delineated by Mr. C. E. Hall in his map of Delaware County. 

The southern boundary of the gabbro belt is made still more indefinite 

by the covering of gravel. From the western end to Harmony Mills 

the outcrops are confined to a narrow belt scarcely more than a quarter 

of a mile wide. From Harmony Mills a neck of gabbro, outlined by 

the outcrop of occasional bowlders of a green massive diallage-hornbleude rock, runs nearly due southwest, connecting this last place with 

the Iron Hill mass. The northern boundary of this rather obscure 

neck cuts the main road from Newark to the hill near J. W. Evans's 

place, whence it runs west by northwest along Christiana Creek to the 

Philadelphia, Wilmington and Baltimore Bailroad; thence it curves 

slightly to the southwest, keeping close to the railroad, crossing the 

railroad below Iron Hill station. Here it bends again to the south, or 

close to the railroad, to a point just north of. Elkton, where it curves 

around Bed Hill, returning again about one-half mile south of the old 

Philadelphia, Wilmington and Baltimore turnpike, gradually approaching nearer this road along the south slope of Iron Hill and Sandy Bray, 

to Cooche's Bridge, where the line again becomes the indefinite south 

line of the neck referred to. From Harmony Mills to beyond the Bed 

Clay Creek the southern limit of the belt is mainly outlined by tracing 

out the northern boundary of the Mesozoic clays, which appear in the 

railroad cut back of Stanton and again at Newport. Between Newport 

and the Wilmington and Northern Eailroad gabbro-diorites appear 

north of the turnpike and beyond the railroad to the south of the turnpike, where they run into Wilmington, crossing the center of the city, 

appearing most conspicuously at the Brandywine Bridge. From this 

point the line curves rapidly to the southeast, cutting the Delaware Eiver 

near Edgemoor station. The thick mantles of Mesozoic and Quaternary 

deposits probably deeply bury a portion of this gabbro mass, offering
38 THE GABBROS OF DELAWARE. [BULL.59.

no opportunity to study its structural relationship on the south with 

other rocks possibly contemporaneous. That the gabbro is bounded on 

the south by the same mica-schists which occur to the north can only 

be inferred from analogy. We know that to the west of Newark the 

schists do occur both north and south of the gabbro belt, this exposure being due to the fact that beyond Newark the northern limit of the 

Mesozoic bends abruptly to the south, bringing the micaceous rocks 

nearer the surface. Furthermore it must be noticed that a number of 

lenticular bodies of gabbro-diorite have been found along the northern 

border of the main belt as included masses, rather than independent 

formations, and thus there is reason to believe that the main gabbro 

body bears a similar relationship to the great formation of micaceous 

rocks.

STRATIGRAPHY.

We have learned from a study of the various rock species and varieties that they may be divided into two classes from a structural point 

of view the massive gabbros and the more or less schistose gabbrodiorites and hornblende gneisses. The portion more deeply colored 

upon the map is covered by a rock generally highly schistose, and containing a large amount of hornblende with a subordinate quantity of 

pyroxene, while the lighter portion is covered by the various varieties 

of massive gabbros, norites, and gabbro-granites. The boundary line 

between the two is of necessity general and somewhat arbitrary, since 

these two areas shade into each other quite indistinctly.

In this connection it must be understood that when one portion is 

spoken of as covered by schistose and the other as covered by massive 

rocks, reference is had only to their dominant features; in truth, we 

often find over the former area rocks perfectly massive including true

gabbros, and in the latter area the local development of more or less 

schistose hornblende rocks. To demonstrate that the rocks of these 

structurally distinct regions are genetically one has been the aim of 

former petrographical work, while, at the same time, these results are 

confirmed in the field by a study of the contact between the rocks of 

the two areas at the excellent Brandywine exposures, where there is 

revealed a most gradual passage of massive gabbro into black schistose gabbro-diorite, instead of a stratigraphical break as might be 

looked for. So true is this, that it is impossible to define where one 

begins and the other ends. Therefore, as has been repeatedly urged, 

we can not regard the foliated hornblende rocks as metamorphosed sediments and the massive gabbro as a body of intruded "trap" of later 

origin, but that the whole formation belongs to an important class of 

altered eruptive rocksx so prominently brought to our notice of late, is 

far more probable.

In the region covered by the hornblende rocks both strike and dip 

can be made out with considerable accuracy. The latter, from the 

extreme western point of the belt to one near Milltowu, is to the south-
CHKBTEU.] MODE OP OCCURRENCE OF THE GABBRO. 39

east at high angles. From Milltown onward the dip has been reversed 

to the northwest, the mica-schists in this case overlying the hornblende 

rocks and massive gabbros. The strike varies, but is generally within 

the limits of N. 40° to N. 60° E. In Delaware there is apparent conformity between the hornblende rocks and the 'mica-schists to the 

north. At Newark the hornblende gneiss dips with regular bedding to 

the south, the mica-schists pitching conformably beneath them; the 

strike for both is N. 55° to 60° E., dip 50° S. To the west of Newark 

the tongue of gneiss bears around slightly to the northwest, in conformity to which the inclosing mica schists have an average strike of 

N. 50° W. On both sides of the tongue the mica schists dip to the 

south, conformably inclosing the hornblende rocks. At Eoseville fche 

contact between the hornblende rock and the mica schist is not a distinct one as at Newark, but is marked by the occurrence of minor interstratifications of gneiss with schist. The same evident fusion of these 

two formations into each other occurs, as already pointed oat, at Kieinensie station and at Brandywiue Springs. Along the Wilmington 

and. Northern Kailroad, between Dupont and Adams stations, near 

Henry Dupont's place, the mica-schists strike N. 50° E., dip 55° N., 

and are deeply exposed in the cut. A short distance farther south is 

another fine exposure of the black hornblende rocks, which strike N. 

62° E., dip 40° N. Considering the great variability in the position of 

these rocks within very narrow limits, equally noticeable at this point, 

it is not at all probable that here any unconformity can be said to exist, 

notwithstanding that the actual contact of the two formations is not 

visible. Farther north, at Jessup & Moore's paper-mill, no evidence 

of unconformity can be discovered; on the other hand, the positions of 

the rocks of both formations are the same.

I have looked carefully for any other evidence of bedding except that 

coincident with the cleavage, but have found none. The so-called "bottoms W1 in Delaware County, Pa., described by Mr. C. E. Hall, could not 

distinctly be made out in Delaware. Planes which might be taken for 

these were irregular and not continuous, and correspond more to joints 

than to planes of bedding. The unconformity discovered by Mr. Hall in 

Delaware County between the "trappean" rocks and the mica schists 

has not the same chronological bearing, should we regard the gabbros 

and their associated hornblende rocks as intruded masses. Possibly, 

again, these so-called "bottoms," as Mr. Hall has suggested,2 may exist, 

but so obscurely defined as to be unrecognizable, in which event all 

that we have said regarding an apparent conformity must be changed, 

and our gabbros and gabbro-diorites must bear the same structural 

relation to the mica-schists that Mr. Hall claims they have in Pennsyl1 These "bottoms" are bedding planes, which cut across the cleavage of the micaachists, and consequently have a much lower angle of dip than that indicated by the 

cleavage.

3 Second Geological Survey, Pa., report 1835, Chap. 5, Part I, p. 3.
40 THE GABBROS OP DELAWARE. [BULL. 59.

vania, even to the borders of Delaware. But, however this may be, the 

real nature of the gabbros and their associated rocks, irrespective of 

chronological position, is not affected, and as no attempt is here made 

to determine the last point, the facts bearing upon it have no special 

relation to the aim of our investigation.

It must be noted, however, that if we consider the true planes of 

bedding of the mica schists to lie nearly horizontal, then the thin outlying lenticular masses of gabbro-diorite at Kiemensie and at Brandywine Springs, which are apparently interbedded with micaceous rock, 

must be regarded more strongly than ever as intruded bodies, cutting 

across the stratification. That these rocks are of. this character .the 

author is strongly inclined to believe.

The gabbros are devoid of all stratigraphical form, and it is only the 

development of local masses of hornblende gabbro, or the appearance of 

an obscure foliation in occasional cases that enables us to discern anything substantial. Everything learned from a study of the extensive 

quarries of bluestone along the Brandywine points to the conclusion 

that the rock before us is as truly eruptive as any ancient trap whose 

origin is indisputable. Huge jointage planes cut the rock into massive 

columns with rhombic cross-section. These continue downward at high 

angles indefinitely with persistent regularity, except when broken off 

by irregular cracks which, independent of the regular direction of the 

jointage planes, cut up the rock into massive pieces of various sizes and 

shapes. The jointage planes include two parallel sets those running 

normal to the direction of pressure and nearly parallel to the axis of 

the mass, and those whose north ends bear a little to the left of lines 

perpendicular to the first set.

THE ORIGIN AND GENETIC RELATIONSHIP OF THE GABBROS AND 

THEIR ASSOCIATED HORNBLENDE ROCKS.

Eegarding the origin of the. Delaware gabbros, two theories might 

be advanced that which would regard them as highly metamorphosed 

sediments, and that which would consider them as metamorphosed 

eruptive rocks.

In considering the former view but little need be said. The opinion 

which has been advanced that the gabbros, variously called u syenites" 

and "diorites," were but the so-called metamorphoic hornblende 

gneisses more highly metamorphosed, is, I believe, entirely irrelevant. Such a change as this would involve the alteration of hornblende into pyroxene, an idea which probably never would have been 

advanced, had the real nature of these massive rocks been understood. 

Pyroxene we are taught to regard as a product of direct igneous fusion, 

the hornblende arrangement of the molecules being more in accord 

with the conditions of ordinary metamorphic action: hence it is questionable whether, in any case, the more advanced stages of ordinary 

metamorphic action could ever result in the transformation of horn-
CHESTER.] ORIGIN OP THE GABBROS. 41 \

blende into pyroxene, or of a hornblende rock into a gabbro. On the 

other hand, we have had of late abundant and reliable instances 

where the alteration of pyroxene has occurred. The recent results 

from a large body of workers on both sides of the Atlantic, all go to 

explain the occurrence of schistose hornblende rocks in these instances 

to paramorphic or pseudomorphic changes affecting an original gabbro. 

Over the Delaware area the intimate association of pyroxene and hornblende rocks meets with an easy explanation from the facts set forth in 

previous pages, and the genetic unity of the two has been thoroughly 

revealed. ,

In the brown hornblende gabbro-diorites, which structurally possess all the characters of hornblende gneisses, we see this genetic unity 

admirably displayed in the gradual passage from massive to highly 

schistose forms, even a field study> without the use of a thin section, 

enabling us to trace forms which show all grades of semi-foliation, and 

which to the unaided eye show much hornblende in addition to pyroxene. In the Iron Hill gabbros we have seen the clear passage iuto dioritic forms through a two-fold alteration of hypersthene iuto uralite, 

and of monoclinic pyroxene into hornblende of the compact variety. In 

the Wilmington and the Harmony Mill green hornblende rocks an absolute proof of the origin of the hornblende from the green pyroxene is, 

as we have seen, impossible at this date; their genetic unity with the 

gabbros, however, is none the less evident from an equally gradual 

transformation from massive to schistose forms.

While it would be unwarrantable to make any detailed statement as 

to the exact origin of the Delaware gabbros, it is at least evident that 

they were at. one time in a more or less molten state. This is particularly evinced by the occurrence within the quartz and the feldspar of 

what appear to be gas pores and glassy particles, and from the caustic 

action which a few of the earliest-formed minerals have suffered under 

the influence of none other than a molten liquid.1 Thus we believe 

ourselves warranted in the assumption that all the rocks included in 

the scope of this paper were at one time molten,' whether we regard this 

original magma as one truly gabbro in its nature or strongly dioritic 

through the presence of original hornblende. Supposing, then, the matter entering into these schistose-hornblende rocks to have been at one 

time molten, i. e., a true dioritic magma, the structural habit which 

they show we must regard as the result of some external secondary influence. This influence many geologists in like cases of late have attributed to pressure. That a powerful pressure affected the whole gabbro body is shown in many instances. In thin sections of the schistose 

gabbro-diorites and hornblende gneisses, which lie transverse to the 

plane of foliation, the effect of pressure is seen in a flattening and an 

elongation of both pyroxene and hornblende individuals, producing a 

structure similar to the so-called "flasser structure" of the German

1 See observations on apatite crystal, op. cit., p. 13.
42 THE GABBROS OF DELAWARE. [BULL. 59.

petrographers. Thin sections cut in these directions show a definite 

arrangement of the bisilicates, which appear as elongated particles 

whose greater axes are commonly six times the length of the shorter, 

these greater axes lying parallel to the planes of foliation, and at right 

angles to the directions of pressure. .Not only have these individuals 

been pressed into flattened and elongated forms, but in some cases they 

appear bent, as shown by the curved positions of the cleavage lines 

and disturbances in the optical symmetry of the substance.

Quartz, wherever it occurs, commonly shows fracture a marginal 

crushing and sometimes an entire destruction, .while the smaller 

grains of feldspar wedged in between the rigid quartz granules show 

also the results of mechanical action in crushing phenomena. That the 

pressure which, as I believe, has resulted in the structural change 

above mentioned, must have affected the whole gabbro mass, is shown ^ 

by the occurrence within perfectly massive rocks of those highly schistose. The schistose forms of our gabbro derivatives may be grouped 

under two heads: First, including those varieties of gabbro-granite 

which contain a considerable amount of mica; and second, nearly all 

those rocks which contain a notable amount of hornblende, original 

or secondary, the degree of foliation bearing a direct ratio to the 

amount of hornblende.

It is a familiar fact that many truly eruptive granites merge into 

granitoid gneisses and into true gneisses by an increase of mica, the 

result of a normal variation. Exactly the same thing is observed in 

the study of the gabbro-granites, these rocks showing all degrees of 

semi-foliation. At Believue the prevailing rock is a coarse gabbrogranite containing about an equal mixture of hypersthene and mica. 

This rock is upon the whole quite massive, but shows in the quarry 

walls a continual tendency towards semi-foliation through increase of 

biotite with probably accompanying pressure. With it is associated a 

jet-black rock thinly bedded and highly schistose, which to general 

appearance seems to have no relationship with the former. A thin 

section, however, shows that this rock owes its black color to a large 

development of hypersthene and biotite, and its schistose structure to 

the relatively larger amount of biotite as contrasted with the highly 

quartzose and feldspathic rocks which surround them. That these 

massive rocks have been subjected to great pressure is shown by mechanical deformations in a crushing of the quartz and in an occasional bending of the mica laminae. We must recognize, then, that the 

reason why the more highly micaceous rocks are more strongly foliated 

is not alone one of difference of composition, but because a given 

pressure was more capable of producing a structural change in rocks 

of this character than in the more quartzose and feldspathic forms 

associated with them.

The intimate association of schistose hornblende rock with massive 

gabbro is admirably revealed in the Brandywine section at Jessup &
CHESTER.] EFFECTS OF PRESSURE. 43

Moore's paper-mill. A reference to the map will show a tapering of the 

area of schistose hornblende rocks running out into a tongue as it 

crosses the Brandywine. In the railroad cut below the paper-mill is 

an exposure of highly schistose thinly bedded green hornblende gabbrodiorite (F, p. 8), which upon either side gradually shades into a grayish 

perfectly massive gabbro without the least sign of any distinct line of 

separation between the two structurally different rocks. That these 

two structurally distinct rocks are genetically one is conclusively shown 

by an examination of a number of thin sections of each. How, then, 

are we to account for a schistose mass wedged between perfectly 

massive rocks'? Not from an effective pressure in one case and its 

absence in another, but rather, as we can well see, because of the different effects which a given pressure will have upon rocks of different 

mineralogical characters. It is not that pressure has not affected our 

liighly feldspathic and quartzose norites, but that pressure in these 

cases has been ineffective in .producing a structural change such rocks 

as these showing a crushing rather than a flattening of the mineral 

constituents. It would be difficult to prove conclusively whether the 

pressure which has resulted in the foliation of these rocks acted previous 

or subsequent to the consolidation. Writing of the Lizard gabbros of 

Pen Voose, near Landewednack,1 J. J. H. Teali finds that in those places 

where the foliation is most eminently developed he sees exhibitions of 

profound earth movements; and such seems to be the view of Lossen 

and Lehmann, who consider these changes the result of great orographic 

or mountain-making influences, such as have affected consolidated rocks 

of all formations. The reverse dip in the eastern .and in the western 

parts of the gabbro area, together with the twisted and contorted condition of the more highly foliated rocks in places, is proof that they 

have suffered the effects of orographic forces since consolidation.

The relation between pressure or foliation and paramorphic changes 

is not what would be looked for. Those hornblende rocks which show the 

foliation habit eminently developed, i. e., those which cover the darkly 

shaded area on the map, exclusive of the Iron Hill body, are the very 

ones in which we find an original brown hornblende or a green hornblende whose secondary nature can not be definitely established. The 

Iron Hill gabbro-diorites, in which the derivative nature of the hornblende is alone clear, show invariably a massive character. Many thin 

sections before the writer are taken from rocks strongly foliated, but 

with the hypersthene and diallage which they contain perfectly fresh 

and the brown* hornblende apparently original. Such facts as these 

indicate that pressure is not in all cases sufficient to produce paramorphic 

changes, even though this pressure be great enough to cause structural 

alteration; and again that paramorphism may be eminently revealed 

where we have no evidence of the action of these mountain-building 

forces.

'The metamorphosis of the Lizard Gabbros, J. J. H. Teall, Geol. Mag., decade 3, 

vol. 3, London, 1886, p. 488.

INDEX.

Page.

Analyses of feldspar..................... 12,28

Apatite.................................. 14,15

Areas of exposures of gabbro............ 36-38

Biotite................................... 15,19

Cross, Whitman, acknowledgments to.... 7

Diallage .................................. 11

Diller, J. S., analyses of feldspars by..... 12,28

Diorite................................... 29-31

Epidote.................................. 34,35

Feldspar................................. 11

analyses ..... .................... 12,28

Gabbro-diorite........................... 15-19

and hornblende gneiss............... 31,36

Gabbro-granite .......................... 19-21

Garnet................................... 15

Hall, C.E................................ 36-39

Hawes,G.W., cited....................... 17

Hornblende................. ......... 9,15

Hyperatene..................... ...... 10

Page. 

Hyperstene-gabbro...................... 10-15

Iron Hill gabbros and gabbro-diorites.... 22-29

Irving, K. D., acknowledgments to ....... 7

cited................................. 17

Kloos, J. H., cited........................ 27

Lizard gabbros .......................... 43

Magnetite ............................... 14

Norite...'................................ 21,22

Origin and relationship of gabbros....... 40-43

Pressure, effect of........................ 41-43

Quartz............. ....................8,9,15,42

Riggs, B. B., analyses of feldspars by .... 12,28

Rohrbach, cited.......................... 33

Stratigraphy of Delaware rocks.......... 38-40

Table showing variations of gabbro...... 8

Teall,J.J.H., cited ...................... 43

Tb'rnebohm,cited........................ 8

Williams^ G. H., acknowledgments to .... 7

cited...............................8,17,27,33

45