DEPARTMENT OF THE IN TEH I OK

BULLETIN

OF TUB

UNITED STATES

GEOLOGICAL SURVEY

No. 195

SERIES C, SYSTEMATIC GEOLOGY^ AND PALEONTOLOGY, 57

WASHINGTON

GOVERNMENT PRINTING OFFICE

L902

UNITED STATES GEOLOGICAL SURVEY

CHARLES D. WALCOTT, DIRECTOR

IN THE

GREEN MOUNTAIN REGION

AND IN

EASTERN NEW YORK

(SECOND PAPER)

BY

T. NELSON DALE

WASHINGTON

GOVERNMENT PRINTING OFFICE

1902

CONTENTS.

Page.

Introduction............................................................. 9

Horizontal isoclines ....................... .............................. 9

Pinched folds..............................................J...........: 10

Faulting in a zone of intense folding....................................... 10

Elongation in folding..................................................... 11

Caves formed by tension and rupture....................... i. .............. 11

Shear indicated by annelid borings........................ ................ 12

Plano-convex limestone concretions ....................................... 12

Interbedded crystalline calcite and dolomite ............................... 13

Lines of rock bedding crossing feldspar crystals............................. 16

Divergent strikes in unconformable deposits................................ 18

Conclusions.............................................................. 20

. ILLUSTRATIONS.

Page. 

PLATE 1. A, Elongated qnart/ite fold, Williamstown, Mass.; H, Pinched fold in

marble, Dorset, Vt............................................ 10

II. Overturned syncline of quart/ite adjacent to an overthrust, Boardman Hill, Clarendon, Vt...................................... 12

III. Overturned syncline of Cambrian schist 50 feet east of the overthrust on Boardman Hill, Clarendon, Vt........................ 14

IV. Contact of pre-Cambrian gneiss, and Cambrian quartzite, The Dome,

Pownal, Vt...........^....................................... 16

FIG. 1. Isoclinal fold with horizontal axial plane, Dorset, Vt................ 9

2. General relations at overthrust on Boardman Hill, Clarendon, Vt.... 10

3. Plano-convex limestone concretions, Nassau, N. Y.................. 13

4. Interbeddecl calcite and dolomite marble.......................... 15

5. Plicated schist with feldspars in bedding plane, Dorset, Vt.......... 16

6. Albite twins containing traces of rock bedding, Dorset, Vt.......... 17

7. Contactof pre-Cambrian and Cambrian, Downer Glen, Manchester, \rt. 18

8. Structure in pre-Cambrian gneiss at its contact with Cambrian on The

Dome in Pownal, Vt............................................ 19

STRUCTURAL'DETAILS IN THE GREEN MOUNTAIN 

REGION AND IN EASTERN NEW YORK.

(SECOND PAPER.)

Bv T. NELSON DALE.

INTRODUCTION.

1 Since the publication of the first paper with the above title, in 1896,"

a few other details have been collected in the course of further explorations, and these are here presented with the same intent and in a similar

manner.*

HORIZONTAL ISOCLINES.

The southern part of the Dorset Mountain mass in Dorset, Vt., 

called Mount Eolus by President Hitchcock, but known locally as 

Green Peak, is a mass of marble and calcareous rocks in almost horizontal position, capped by schists, and it probably forms the center of 

a very open syncline. Scarcely half a mile west of the schist cap the

FIG. 1. (A) Diagram sketch of horizontal isoclinal fold in white marble; old quarry seven-eighths 

mile west of the Owl's Head, Dorset, Vt. (J3) diagram showing the probable general structure of (A )

strike suddenly changes from N. or N. 15° E. to N. 50° E., and a 

similar strike recurs in the marble across the valley, a mile and a half 

to the southwest. The folds at the former place are most intricate, 

and include a horizontal isocline 45 feet long, as shown in fig. l. c

ii Sixteenth Ann. Kept. U. S. Geol. Survey, Pt. 1,1896, pp. 549-570.

6 This paper, however, does not include the structural details of the New York and Vermont slate 

belt, which will be found on pp. 199-217 of the Nineteenth Ann. Kept., Pt. Ill, 1899. 

c A rough diagram of this appears in Hitchcock and Hager's Geological Keport of Vermont, 1861.

' 9
10 STRUCTURAL DETAILS IN GREEN MOUNTAIN REGION. [BULL. 195.

Such completely recumbent isoclines are exceptional in this region, 

and their presence at this locality in the midst of a gently undulating 

structure is yet to be accounted for."

PINCHED FOLDS.

Readers of the great masterpieces of Swiss geologists are familiar 

with those effects of extreme compression shown in the pinching of 

folds and in the tearing off of their cores or apices and their envelopment by the adjacent rock mass. Some of the earlier stages in this 

process are seen on a small scale at the locality in Dorset, Vt., just 

referred to. In PI. I, J?, the pinching off of the apex of the fold has 

just begun.

Sometimes this constriction occurs lower down on the limbs of the 

fold and results in the formation of more or less complete circles when 

the beds are seen in vertical cross section. Such a complete circle, a 

few inches in diameter, was observed by Mr. C. W. Hayes and the 

writer at a new quarry in banded marble opened in Pittsf ord, Vt., 

in 1901.

FAULTING IN A ZONE OF INTENSE FOLDING.

, Tangential compression, after having partly expended itself in 

intense folding, may then result in faulting. The overthrust which 

begins in the town of Pittsf ord, Vt., and extends more than 10 miles

FIG. 2. Diagram showing the probable general relations of the ledges (Pis. II and III) to theoverthrust at the top of Boardman Hill, Clarendon, Vt.

into the town of Clarendon, and which nas brought the Lower Cambrian of the eastern part of a ridge onto the Upper Ordovician of its 

western side, passes through Pine Hill in Proctor and Boardman Hill 

in Clarendon. Some of the characteristics of this fault have been 

published. 6

At the top of Boardman Hill the Cambrian quartzite and schist come 

within a few inches of the Upper Ordovician schist. The former are 

intensely folded, as shown in Pis. 11 and III, and similar folding marks 

a large area east of the fault. In PL II is shown an overturned syncline of quartzite, the axial plane of which passed through the now dislocated piece at the left In the inner part of the fold the pressure

a For similar folds see Dr. Wolff's section of Hoosac Mountain in Mon. U. S. Geol. Survey Vol. XXIII, 

PI. VI, and the writer's section on Lake Bomoseen in Nineteenth Ann. Kept., Pt. Ill, Pis. XVIII and 

XXXIV, N.

b On the structure of the ridge between the Taconic and Green Mountain ranges in Vermont, by 

T. Nelson Dale: Fourteenth Ann. Kept. U. S. Geol. Survey, Pt. II, 1895, pp. 525-549.
U. S. GEOLOGICAL SURVEY BULLETIN NO. 195 PL. I

A. ELONGATED QUARTZ1TE FOLD FROM HILLOCK AT WEST END OF MAIN 

STREET, WiLLIAMSTOWN, MASS.

i the lower part of specimen a quartz vein traverses the fold. The upper part shows the beginning of

another fold.

Jl. PINCHED FOLD IN WHITE MARBLE.

Hammer, 21 inches long. The lower part of the block consists of impure dolomite. Quarry dumps 

seven-eighths of a miie west of the Owl's Head, Dorset, Vt.
DALE.] EFFECTS OF FOLDING. 11

has found relief in the formation of secondary folds zigzagging vertically across the major fold and giving little idea at that point of 

the general character of the fold. The axes of the major and minor 

folds must originally have lain at a considerable angle to the direction 

of compression, and the overturn was an after effect. The more plastic Ordovician graphitic schists on the under side of the thrust are 

marked by the usual cleavage, which, judging from other localities, is 

probably approximately parallel to the fault plane." In fig. 2 the 

probable relations of the ledges in Pis. II and III to one another and to 

the fault are shown.

ELONGATION IN FOLDING.

Some of the more minute evidences of elongation were referred 

to in the first paper, but folding frequently' involves more or less 

elongation. Where the folding is intense the elongation may become 

apparent. There is thinning of the beds along the limbs of the 

folds and thickening of them at their apices. Many cross sections of 

plicated rock, whether in a cliff or a microscopic section, illustrate this 

familiar geological principle.

PI. I, A, represents a specimen of vitreous quartzite taken from a 

locality of intense and transverse folding,6 being one of a descending 

series of overturned folds along the limb of a major anticline exposed 

in cross section on a vertical joint plane. While the limb of the fold 

at its narrowest place measures li inches, its apex measures 11£ inches. 

If the bed be assumed to have measured 6£ inches prior to the folding i. e., the average of the thinnest and thickest parts there will 

have been a transfer of 77 per cent of material from the limb to the 

apex.

As the microscopic structure of the rocks of the vicinity indicates 

that metamorphism preceded as well as followed the folding, it may 

be assumed that this bed was quartzite prior to the folding. There is 

good evidence that the folding took place under a full half mile of 

other sediments.

The proportions of the specimen will be found repeated on a gigantic 

scale in many an Alpine section, but these proportions will, of course, 

vary with the intensity of the folding, and the limbs of the softer 

members of a fold may be so attenuated as to disappear altogether.

CAVES FORMED BY TENSION AND RUPTURE.

About a half mile northeast of the extreme southwest corner of the 

State of Vermont and of the town of Pownal, on the eastern side of 

the Taconic Range, the Ordovician schists are fissured in a direction

« Sec Fourteenth Ann. Kept., U. S. Geol. Survey, Pt. II, p. 537, fig. 56, and Sixteenth Ann. Kept., 

U. S. Geol. Survey, Pt. I, p. 565, fig.95.

6Sixteenth Ann. Kept., U. S. Geol. Survey, Pt. I, pp. 553, 554, figs. 77, 78.
12 STRUCTURAL DETAILS IN GREEN MOUNTAIN REGION. [BULL. 195.

parallel to the prevalent transverse jointing of the region. These 

fissures and joints strike N. 75°-80° E. and dip 90°. Between two of 

these a mass of rock 10 feet wide has been faulted down. In another 

the schists, which are noncalcareous and veined with quartz, have a 

cleavage striking N. 12° E. and dipping 45°-50° E. across a bedding 

dipping east at a much lower angle. These foliations have been parted 

in a N. 75° E. direction, leaving an irregular fissure from 1 to 5 feet 

wide and more than 130 feet deep across the schistosity and the veins 

which lie in it.

Such a.cave is evidently formed by tension of the schist mass in the 

direction of the strike, and is the result of some later movement than 

that which produced the synclinoria and anticlinoria of the region.

SHEAR INDICATED BY ANNELID BORINGS.

While Cambrian quartzite bowlders with annelid borings (Scolithus) 

are not infrequent in western Massachusetts and Vermont, the places 

where the borings may be studied in situ are few. One of these is in 

Arlington, Vt., on Roaring Brook, which has cut a ravine a thousand 

feet deep in the quartzite of the western flank of the Green Mountain range, and then, entering the Vermont Valle}r, flows into the 

Batten Kill.

At a point a half mile southeast of East Arlington, on the eastern 

side of the brook and road,.the quartzite crops out, filled with annelid 

borings and dipping about N. 20° W. at an angle of 10°, the direction of the dip being probably due to the northerly pitch of a fold 

with a N.-S. axis. As borings made by annelids in sand are vertical, 

they should remain vertical to the bedding, which here dips 10°. 

This would give the borings an inclination of 10° from the vertical, 

or 80° from the horizontal in the opposite direction, which is S. 20° 

E., but they actuallj^ dip at an angle of 45° about S. 65° E. The 

amount of shear has thus deflected them 35°, and the direction of 

the shear has also deflected them 45° farther eastward.

The relation of deflected annelid borings to shear is akin to that of 

distorted fossil shells, trilobites, etc.

PLANO-CONVEX LIMESTONE CONCRETIONS.

In the belt of little or no metamorphism which borders the Hudson 

River in Rensselaer County, N. Y., there are large areas of Cambrian 

shales containing here and there beds of limestone. In the town of 

Nassau, about 2 miles southeast of the village of that name, these 

shales contain plano-convex disks of limestone which afford no evidence 

of organic structure and also differ in form from ordinaiy concretions 

and septaria. Fig. 3 shows one in cross section. They measure from
U. S. GEOLOGICAL SURVEY 

OVERTURNED SYNCLINE OF CAMBRIAN QUARTZITE ADJACENT TO AN OVERTHRU

Looking N. 35° E, Hammer, 21 inches long
DALE.] INTEKJ3EDDED CALCITE AND DOLOMITE. 13

19 to 52 inches in diameter and about 5 inches in thickness and have 

a, slight depression on the under side. They occur in a bed of shale 

about 6 inches thick traversed by joints filled with calcite, and are 

separated from the overlying bed of limestone by a thin film of shale. 

As the beds dip at an angle of 50°, unless they have been overturned

FIG. 3. Diagram cross section of a plano-convex limestone concretion; Nassau, Rensselaer 

County, N. Y.; size of concretion, 42 toy 5 inches.

they represent one side of a syncline, and the convex side of the disks 

would thus be the under side.

It is suggested that they may have been formed by limestone sedimentation on an uneven clayey sea floor and have received their circular 

form from concretionary action.

INTERBEDDED CRYSTALLINE CALCITE AND DOLOMITE.

Vogt, in his exhaustive work on the marbles of Norway," gives a 

section of interbedded and slightly plicated calcite and dolomite 

marble, and describes them as occurring usualty in such association 

in that region. Although he does not undertake to account for their 

origin, he thinks that the dolomite is plainly not due to metasornatic 

processes.

In the Vermont marble belt there is an-extensive deposit of more or 

less 'microciystalline yellowish dolomite, which the painstaking local 

geologist Wing regarded as lying between the Cambrian quartzite 

and the Lower Ordovician calcite marble. There are, however, other 

minor beds of dolomite among the Ordovician marbles.

At the Green Peak quarries, already referred to, & there is an interbedding of the two rocks, like that described by Vogt in Norway, and 

they are also capped by schist. The dolomite, however, is .quartzose 

 and so very fine grained as to be properly termed microcrystalline, 

while the calcite marble is much coarser grained (see fig. 4). That 

this was calcite marble was determined by its strong effervescence 

under dilute hydrochloric acid. Mr. E. T. Alien, chemist of the Survejr, reports it as " effervescing strongly in the cold and containing 

only a very little magnesium. Not a dolomite." A chemical analysis

"Norsk marmor, by J. H. L. Vogt: Norges Geologiske Undersogelse, No. 22, Kristiania, 1897, p. 219, 

fig. 42. This work contains, on pp. 334-364, a resume in German. See also an abstract of it with 

several illustrations in Zeitschr. fur prak. Geol., Jan., 1898, p. 9, and fig. 5.

l>P. 9. 
14 STKUCTUEAL DETAILS IN GEEEN MOUNTAIN EEGION. [BULL. 195.

of the dolomite made by Mr. George Steiger in the laboratory of the 

United States Geological Survey yielded these results:

Chemical analysis of dolomite from Green Peak quarries, in Dorset, Vt.

Constituent.

Si02 ...... '............ ........... i .. ...............

AI n

Fe2 03 ....:..................................;....

FeO

MgO ..............................................

CaO....... '.................. ................I.-...

Na20 ...............:..............................

K20.......... .....................................

H.O-...... .......................................

H2 0+. .......................:....................

CO, ...............................................

S03 .............................................

Total. . ......................................

Soluble. ...........................................

Total. .......................................

Molecular 

weight.

60.4

102

160

72

40.3

56

62

94

18

44

80

Soluble in 

boiling 

HC1.

Per cent.

1.08

16. 44 

29.03

.03

41.66

88.24

88.24

12.15

100. 39

Insoluble 

in boiling 

HC1.

Per cent. 

8.36

1.77

.22

.24

.06

1.08

.42

12.15

A microscopic section of the same rock shows a granular mass of 

more or less angular but not interlocking transparent grains ranging 

from 0.025 to O.OT mm. in diameter, without twinning and rarely with

cleavage. The fineness of this grain appears by comparison: The 

West Rutland Statuary ranges from 0.07 to 0.3 mm., averaging about 

0.15 mm., while the finest European marbles, as given by Vogt, range 

from 0.02 to 0.03 mm.

This mass contains sparsely disseminated grains of quartz, and rarely 

a grain of microcline or plagioclase or a flake of muscovite.

The quartz, feldspar, and muscovite account for the SiO2 , A12O3 

FeO, Na2 0, K2 0, and H2O of the analysis. About 90 per cent of the 

rock is thus a dolomite in an extremely fine-grained crystalline condition. A piece of it was polished at the Survey laboratoiy.

The difference in the size of the grain of the two rocks is greater 

than at the* Norwegian quarries. As both beds, calcite and dolomite, 

must have been subjected to an almost identical amount of pressure, 

this difference in grain must be due either to a difference in the condition of the original sediments, which must account for similar differ-
U. S. GEOLOGICAL SURVEY 

OVERTURNED SYNCL1NE OF CAMBRIAN SCHIST, 50 FEET EAST OF OVERTHRUST

Looking N. 35 U E. Hammer, 21 inches long.
DALE.] INTEKBEDDED CALCITE AND DOLOMITE. 15

ences or grain in a continuous series of calcite marble beds, or to the 

different behavior of dolomite and calcite under equal pressure. Vogt 

points out that the greater tendency of bedded dolomite to complete 

its individual crystals than bedded calcite when subjected to the same 

regional rnetamorphism indicates that it possesses greater power of 

resistance to the effects of pressure. It is also harder than calcite.

In the section represented in fig. 4 there is a small quartzose bed 

between the dolomite and the calcite. This proves to be mainly finegrained dolomite, but with more quartz grains and jiiuscovite flakes 

and the same feldspars. It has also stringers of muscovite and lenses 

of coarsely crystalline calcite and quartz. This little bed probably 

represents a brief period of mingled argillaceous and calcareous sedimentation. As shown in the figure, there is also a lens of dolomite in 

one of the beds of calcite.

On the eastern side of Otter Creek Valley, !£ miles SSE. of Proctor

Scale 

10__ 20 feet

FIG. 4. Diagram sketch of interbedded crystalline calcite and dolomite. Length, 40 feet. The 

dolomite is veined with quartz. Old quarry three-fifths mile west of the Owls Head, Dorset, Vt.

and near the Pine Hill overthrust, the calcite marble contains angular 

fragments of dolomite. ' In weathering, the dolomite, being less soluble by CO2 , stands out from the calcite as would hard pebbles in a 

conglomerate with a soluble cement. Thin sections of this rock show 

the same differences between the matrix and the fragments that were 

described as existing between the interbedded dolomite and calcite. 

This is probably a breccia formed from alternating beds of dolomite 

and calcite, the calcite having become coarsely crystalline and the dolomite tnicrocrystalline in the process.

Do these alternations of dolomite and calcite correspond to alternations of chemical and organic sedimentation or to a difference in the 

calcium compounds extracted from the sea hy different classes of 

organisms?"

"Professor Van Hise, notwithstanding Vogt's opinion, regards it possible that the alternations are 

due to the dolomitization of some beds and not of others.

Thin sections of plicated interbedded dolomits and phyllite, in which the beds of each rock measure 

about a millimeter in thickness, have been recently described from the Alps. See G. Allenspach, 

DiinnschliffevongefalteltemRothidolomit-QuartenschieferamPizUrlaun: Vierteljahrs.Naturforsch. 

Gesells. Ziirick, 45. Jahrgang (1900), pp. 227-237. There can be little question as to the mechanical 

sedimentary origin of the phyllite beds. *
16 STEUCTUBAL DETAILS IN GEEEN MOUNTAIN EEQION. [BULL. 195.

LINES OF ROCK BEDDING CROSSING FELDSPAR CRYSTALS.

The frequent occurrence of albite feldspar in the rocks of the Green 

Mountain and Taconic ranges has been several times referred to in 

geological papers/'

In 1897 Mr. L. M. Prindle, then the writer's assistant, while working 

on Mount Anthony, in Bennington, came across a "prospect," about 

half a mile west of the top and 650 feet below it, known locally as 

Tibbetts mine. Here the Ordovician schists contain a vein, a few inches 

thick, of crj^stallized albite associated with quartz, chlorite, and pyrite.

FIG. S.^Diagram sketch of thin section of muscovite, chlorite, quartz, and albite schist from ledge 

three-fourths mile north of Green Peak, Dorset, Vt. ^4=quartz in bedding. .B=M.USCOVite beds. 

C=Feldspar zone. Ordinary light.

"The feldspar is also irregularly mixed with the schist over a width 

of 2 feet or more."

A specimen of schist of the same age which caps the marble at 

Green Peak, in Dorset, Vt., collected by the writer's assistant, Mr. Fred 

H. Moffit, in 1899, shows the usual speckling with albite. The cleavage foliation at the ledge is horizontal, and a quartz vein, 6 inches 

thick, lies in it. Thin sections made anywhere across the cleavage 

show some very exceptional features. The rock consists mainly of 

muscovite and quartz, with a little chlorite, a few tabular octahedra of 

magnetite, black streaks and dots, possibly graphitic (although the

« Geology, Second District of New York State, 1842, by Ebenezer Emmons, p. 158. The specimen 

there referred to is probably the one still preserved in the Williams College collection!"

Metamorphism of clastic feldspar in conglomerate schist, by J. E. Wolff: Bull. Mus. Comp. ZooL, 

Cambridge, Mass., Vol. XVI, No. 10, 1891.

Geology of the Green Mountains in Massachusetts, by Pumpelly, Wolff, and Dale: Mon. U. S. Geol. 

Survey Vol. XXIII, 1894, pp. 52, 60-62,183, 186-188, fig. 73, Pis. VII, A; VIII.
U. S. GEOLOGICAL SURVEY 

CONTACT OF PRE-CAMBRIAN GNEISS AND CAMBRIAN QUARTZITE, SOUTHWEST SID

MOUNTAIN RANGE, POWNAL, VT.

Looking N, 70° E. Sledge handle, 30 inches long. The head of the sledge rests on gneiss; its handle on quar

A secondary foliation in the gneiss is parallel to the bedding o
DALE.] BEDDING CROSSINC4 FELDSPAR CRYSTALS. 17

rock itself makes no black marks), limonite stain from the oxidation of 

the magnetite, and many albite feldspars. The bedding is extremely 

plicated and about at right angles to the slip cleavage. Some beds are 

more micaceous, others more quartzose (see tig*. 5). Parallel to the 

course of the bedding are irregular zones of albite grains without crystal boundaries traversed by undulating graphitic (?) streaks, and sometimes inclosing a magnetite crystal. Where the grains are massed the 

black streaks of the different grains are parallel to one another, and 

in some places to the black streaks in the micaceous matrix, and also 

to the plications in the muscovite folia. The black streaks are apparently, therefore, stratified material. Usually, however, these feldspar

FIG. 6. Sketches of albite feldspars from thin sections of rnuscovite-schist from ledge three-fourths 

mile north of Green Peak, Dorset, Vt. Polarized light. In A, B, C, D, inclusions of stratified material. 

In A, B, C, secondary enlargement beyond the ends of the bedding lines, also twinning across those 

lines, and inclusions of magnetite. The dark bands in the matrix are planes of slippage between 

libers of muscovite discolored by limonite. Dotted grains are quartz; the rest is muscovite.

grains have been so displaced by the shearing wjiich produced the slip 

cleavage that the inclosed bedding lines have lost their proper orientation and point in any direction, appearing then like scattered pebbles 

from an older stratified rock.

In most of these feldspars there has evidently been a secondary 

mineral growth, as a rim of clear feldspar extends beyond the termination of the bedding streaks, but the rim extinguishes with the core. 

Some of the albites are simple twins, the twinning plane being usually 

at right angles or diagonal to the included bedding lines (see fig. 6). 

Some of the feldspars are also irregularly disseminated in the cleavage 

foliation outside of the feldspathic zones. 

7991 No. 195 02 2
18 STKUCTURAL DETAILS IN GREEN MOUNTAIN. REGION. [BULL. 195.

This occurrence is a more complex phase of 'that described by Professor Wolff in the "White gneiss" of Hoosac Mountain,a in which 

feldspars are filled with, in elusions of epidote, biotite, muscovite, etc., 

lying in "planes parallel to the arrangement of the minerals outside the feldspar, and entirely independent of the crystallographic 

directions in the feldspar." Such feldspars are necessarily secondaiy.

The history of the Green Peak schist appears to be this: Argillaceous and carbonaceous (?) sediment metamorphosed into muscovite, 

quartz, chlorite, magnetite, graphite (?), and feldspar schist; continued 

pressure produced plications; slip cleavage ensued, and displacement of 

feldspars, followed by their secondary growth. Whether the albite has 

replaced some other feldspar is uncertain.

DIVERGENT STRIKES I3ST TJNCONFORMABLE DEPOSITS.

In a drift-bestrewn and forested region satisfactory contacts between 

distinct geological formations must needs be few. In the many miles 

of boundary between the pre-Cambrian gneisses and the Cambrian 

quartzites and schists and conglomerates of northwestern MassachuMAP 

Pre-Camb. gneiss Cambrian

SECTION 

Pre-Cemb. gneiss 7fr. /Sff. Cambrian

DIAGRAMMATIC SKETCH 

Course of brook

b. schist. tfle slrihe

FIG. 7. Diagrams illustrating the contact between pre-Cambrian gneiss and Cambrian schist in 

Downer Glen, Manchester, Vt.

setts and western Vermont there are but few such points, and at these 

both formations are in apparent conformit}'.

There are some localities, however, where these formations are 

clearly unconformable. One of these is in a deep transverse incision 

in the western flank of the Green Mountain range, 3i miles ESE. of

aMon. U. S. Geol. Survey Vol. XXIII, 1894, p. 52, P). VII, A.
DALE.] DIVERGENT STBIKES IN" UNCONEORMABLE DEPOSITS. 19

Manchester village, known as Downer Glen. The pie-Cambrian 

gneisses form the bed of Bourn Brook, striking almost with its course. 

In looking up the stream the parallelism between the foliation lines 

of the gneiss, the course of the stream, and the sides of the ravine is 

apparent, but in looking downstream the micaceous quartzites of the 

Cambrian are seen striking at right angles to the bed of the brook and 

parallel.to the trend of the range. The two formations approach within 

50 feet of each other, the interval in the brook bed being covered with 

gravel and bowlders, but on the southern side of the ravine the actual 

contact can be observed, and a hand specimen, partly Cambrian, partly 

pro-Cambrian, could be secured. The relations are shown in fig. 7.

Another locality of like purport occurs about 26 miles SSW., on the 

southwestern side of The Dome, in Pownal, 2,200 feet above sea level

F.IG. 8. Structure in pre-Cambrian gneiss at its contact with Cambrian quartzite on The Dome in 

Pownal, Vt., as shown in PI. IV. The axes of the plications strike about E.-W. and clip across the 

cleavage foliation; they are also nnconformable to the bedding of the overlying quartzite. From a 

photograph.

(see PI. IV). Here a mass of gneiss with a plicated foliation striking 

about E.-W. (N. 70° E. to N. 80° W.), underlies a bed of quartzite 

strikipg N. 45° W. and dipping 40° NE. This NE. dip is probably due 

to the local pitch of a fold striking NNE., as a half mile south the same 

rock dips gently SW. and the prevalent strike of the quartzite a little 

east of these localities is NNE. Between the gneiss and the quartzite 

is a bed, 1 foot thick, of muscovite-schist with some microcline. It 

appears to belong to the Cambrian, although it may belong to the 

gneiss, its original foliation having been obliterated in the shearing. 

The details of the gneiss are shown in fig 8.

On the western slope of Harmon Hill, in Woodford, Vt., about 19 

miles SSW. of the first locality and 7 miles NNE. of The Dome, the 

pre-Cambrian and the Cambrian come very near together, although
20 STRUCTURAL DETAILS IN GREEN MOUNTAIN REGION. [BULL. 195.

not in actual visible contact. The gneisses strike generally diagonal!}7 

to the Green Mountain a,xis, while the Cambrian quartzite and schist 

strike a little west of north.

The questions that such localities awaken when compared with the 

conformable contacts which Professor Wolff describes on Hoosac 

Mountain" are these: Which of these relations is the normal one? Do 

the strikes of the gneiss which are at right angles to the Green Mountain range represent the result of the Algonkian erogenic movement 

unaffected by the Ordovician one? In that case the conformable strikes 

would be due to the powerful Ordovician movement having affected 

both Algonkian and Ordovician masses, turning the Algonkian axes 

about into conformity to the Ordovician axes. Or are these preCambrian strikes transverse to the Green Mountain range purely local 

and exceptional? These few localities ma}7 be insufficient to decide so 

important a matter. Further investigations in Vermont and Canada 

ought to throw light upon it. The question whether we have here 

indications of the probable course of the first hills and valleys of 

Algonkian time is an interesting one. 6

Michel Lev}7 , in a sketch map of central France collated mostly 

from official maps of the French geological survey, 0 gives, in different colors the directions of two superposed orographic systems, one 

dating to Carboniferous time and meandering from east to west, the 

other to Tertiary time with axes nearly north and south. Could such 

a map be constructed for any considerable area in North America 

showing the difference between the directions of compression in Algonkian and Ordovician time?

CONCLUSIONS.

Notwithstanding the somewhat heterogeneous character of these 

local geological details, they all point to the complex interaction of

mechanical stratigraphical processes with more obscure chemical molecular processes, and in one case at least with biological processes; and 

they also indicate what far-reaching inductions may be drawn or questions awakened by the study of phenomena occupying but an insignificant amount of space.

«Mon. U. S. Geol. Survey, Vol. XXIII, 1894, fig. 27, p. 73.

&See, on the effect of a secondary horizontal movement on beds previously folded, Die Dislocationen der Erdrinde, by Emm. de Margerie and Albert Heim, Zurich, 18SS, p. 87, and references on 

p. 118.

( Bull. Soc. geol. France, ser. 3, Vol. XVIII, 1SS9-90, Situation stratigraphique des regions volcaviques de 1'Auvergne, pp. 692-693, PI. XXII,.iig. 1.
INDEX.

Pago.

Albite feldspar in Green Mountain and

Tuconic Range rocks ............... 16

in muscovitc-schist, sketches of, from

thin sections ...-................... 17

Algonkinn and Ordovician orogenic movements in Green Mountain region, 

questions concerning............... 20

Alien, E. T., report on ca-lcitc by........... IS

Allcnspacili, G., cited........................ 15

 Annelid boring?, rhear indicated by....... 12

Arlington, Vt., annelid borings showing

shear in quortzite at................ 12

Bcnnington, Vt., albito associated with

qnartz,chlorite,and pyrite found at. 1C

Boardman Hill, Clarendon, Vt., Cambrian 

schist on, plate showing overturned 

syneline of*......................... 14

qnartzitc at top of, plate showing overturned syneline of.................. 12

relations of ledges to ovcrthrust at, diagram showing...................... 10

Borings, annelid, shear indicated by....... 12

Calcito and dolomite, interbedding of...... 13-15

Cambrian qnartzite at The Dome, Pownal, 

Vt., contact of prc-Cambrian gneiss 

with, plate showing................ 16

Oaves in schists near Pownal,Vt., formation

of.................................... 11-12

Clarendon, Vt., overturned syneline in

Cambrian schist at, plate showing . ' 14 

quartzite at, plate showing overturned

syneline of......................... 12

DC Margerie, E., and Heim, A., cited ...... 20

Dolomite from Green Peak quarries, Dorset, Vt., analysis of................. 14

fineness of grain of..................... 14

Dolomite and calcitc, interbedding of...... 13-15

Dolomite and phyllite, interbedded, reference to ............................. 15

Dome (The), Pownal Vt., unconformablc

deposits on, divergent strikes in.... 19

Dorset, Vt., fold, isoclinal, in marble near,

diagram showing................... 9

folds, pinched, at, description of and

plate showing...................... 10

interbedded calcite and dolomite near,

diagram sketch showing........... 15

 schist from ledge nc:ir, description of.. 16-17

Downer Glen, Mar.chcrter, Vt., contact between prc-Ciimbrian gneiss and 

Cambrian schist in, d'a?rams Illustrating ............................. 18

Page.

Downer Glen, Manchester, Vt., divergent 

strikes of unconformablc deposits 

at................................... 18-19

Emmons, E., cited ......................... 16

Faulting in a zone of folding, occurrence

Of................................... 10-11

Feldspar, albite, In Green Mountain and

Taconic Range rocKs............... in

in muscovite-schist, sketches of, from

thin sections ....................... 17

Feldspar crystals, rock bedding crossing... 16-18

Fold a.t Pitteford, Vt ....................... 10

Fold, elongated, in' qnartzitc at Williamstown, Mass., plate showing ........ 10

Fold, isoclinal, in marble near Owls Bead,

Dorset, Vt., diagram showing...... 9

Fold, pinched, at Dorset, Vt., description of

and plate showing................. 10

Folding, effects of.......................... 10-11

elongation in........................... 11

Gneiss, prc-Cambrian, at contact with Cambrian quartzito, diagram showing . 

structure in ........... 1............ 19

at The Dome, Pownal, Vt., plate showing contact of quartzite with....... 16

diagrams illustrating contact of Cambrian schist with ................... 18

Green Peak, Dorset, Vt., isoclines (horizontal) at .......................... 9-10

schist at, history of..................... 18

schist from ledge near, description of.. 16-17

Green Peak quarries, Dorset, Vt., dolomite

from, analysis of ................... 14

interbedding of ^calcite and dolomite 

at................................... 13

Harmon Hill, Woodford, Vt., unconformablc deposits at, divergent strikes in. 19-20

Hayes, C. W., pinched fold at Plttsford, Vt.,

observed by ........................ 10

Heim, A., and Dc Margcrie, E., cited....... 20

Isoclines, horizontal, at Green Peak, Vermont, occurrence of................ 9-10

L6vy, Alichcl, sketch map of central

France by, reference to............. 20

Limestone concretions, plano-cunvex, ut

Nassau, X. Y..................:..... 12-18

cross section of......................... 18

Monchcstcr, Vt., diagrams illustrating contact between pre-Cambrian gneiss 

and Cambrian schist at............. .IS

divergent strikes in unconformablo deposits near.......................... 1& 19

21
22 INDEX.

Page.

Marble (calcite and dolomite), interbedding of.............................. 13-15

pinched fold in, plate showing......... 10

Moffit, F. H., reference to work of.......... 16

Mount Anthony, Bennington, Vt., albite 

associated with quartz, chloritc, and 

pyrite found on .................... 16

Mount Eolus, Vermont, isoclines (horizontal) at .......................... 9-10

Muscovite-schist, albite feldspars in,

sketches of, from thin sections .... 17

Nassau, N. Y,, plano-convex limestone concretions at..........................-12-13

Ordovician erogenic movement affecting 

Algonkian and Or lovician masses, 

questions concerning .............. 20

Otter Creek Valley, Vermont, dolomite in

calcite marble in................... 15

Owls Head, Dorset, Vt., isoclinal fold in

marble near, diagram showing..... 9

Phyllite and dolomite, interbedded, reference to............................. 15

Pittsford, Vt., pinched folds forming circles at.............................. 10

Pownal, Vt., divergent strikes in unconformable deposits at................ 19

gneiss and quartzite at, plate showing

contact of ...... v .................. 16

Ordovician schists near, fissures and

joints in............................ 11-12

Pre-Cambrian and Cambrian deposits, uuconformable, divergent strikes in.. 18-20

Prindle, L. M., cited........................ 16

Proctor, Vt., dolomite in calcite marble

near ................................ . 15

Pumpelly, R., Wolff; J. E., and Dale, T. N.,

- cited ............... ............... 16

Page. 

Quartzite, annelid borings in, near East

Arlington, Vt....................... 12

contact of pre-Cambrian gneiss with, 

at The Dome, Pownal, Vt., plate 

showing............................ 16

fold (elongated) in, at Williamstown,

Mass., plate showing ............... 10

syncline (overturned) in, at Clarendon,

Vt., plate showing.................. 12

Rensselaer County, N. Y., limestone beds

in Cambrian shales in.............. 12

Schist at Green" Peak, Dorset, Vt., history of. 18 

from ledge near Green Peak, Dorset,

Vt., description of.................. 16-17

thin section of.......................... 16

Schist, Cambrian, on Boardman Hill, Clarendon, Vt., syncline (overturned) 

in,map showing.................... 14

Schist, Cambrian, and gneiss, pre-Cambrian, 

contact between, diagrams illustrating ............................. 18

Schists near Pownal, Vt., fissures and joints

in................................... 11-12

Scolithus borings, shear indicated by ...... 12

Steiger, G., analysis of dolomite by ........ 14

Tibbets mine, Bennington, Vt., albite associated, with quartz, chlorite, and 

pyrite found at..................... 16

.Van Hise, C. R., cited ............ ......... 15

Vogt, J. H. L., cited ........................ 13,15

Williamstown, Mass., fold (elongated) in

quartzite at, plate showing......... 10

Wolff, J. E., cited........................... 10,18

Wolff, J. E., Pumpelly, R., and Dale, T. N.,

cited ............................... 16

Woodford, Vt., uiicouformable deposits at,

divergent strikes in ................ 19-20

o