BuUetin No. 209

Series t ^' descriptive Geology,
I D, Petrography and Mi

Mineralogy, 22

DEPARTMENT OF THE INTERIOR

UNITED STATES GEOLOGICAL SURVEY

CHARLES D. WALCOTT, Director

THE

BY

REGINALD ALDWORTH DALY

WASHINGTON

GOVERNMENT PRINTING OFFICE
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CONTENTS.

Page.

Introduction 7

Nature of the investigation 7

Acknowledgments 7

Chapter I. Physical geography 8

General topography of the area 8

Drainage 10

Glaciation of Mount Ascutney 13

Summary 13

Chapter II. General description of the schists in the area 14

Phyllitic series 14

Gneissic series 17

Geologic age of the schists and of the intrusive rocks 19

Summary „._ 20

Chapter III. Contact metamorphism 22

The metamorphic aureole . 22

Changes in limestones 23

Metamorphism of the phyllites 24

Series A of specimens from the metamorphic aureole 24

Series B of specimens from the metamorphic aureole 31

Series C of specimens from the metamorphic aureole 33

Comparison of the metamorphic effects 33

Summary ^ 35

Chapter IV. The eruptive rocks . 36

General table and correlation ^ 36

Gabbros, diorites, and related rocks : 38

Basic stock ; its gabbro and diorite phases 38

• Basic segregations 43

Dioritic dikes cutting the Basic stock 44

' ' Windsorite ' ' dikes cutting the Basic stock 45

Syenites 48

Main syenite stock; its phases 48

Basic segregations . . 64

Endomorphic zone of the syenite stock 68

Great syenite-porphyry dike of Little Ascutney Mountain 69

Syenite stock of Pierson Peak 70

Aplitic dikes cutting the syenites 70

Paisanite dike cutting the Main stock i 70

Basic segregations 72

Common muscovite-aplite of the Main stock 73

Paisanite dike on Little Ascutney Mountain 73

Breccia masses on Little Ascutney Mountain 77

Biotite-granite stock 79

Basic segregations in the granite 82

Endomorphic zone of the granite 84

3

4 CONTENTS.

Chapter IV — Contintied. Page.

Lampropliyres 85

Camptonites 86

Diabase dikes 87

Summary v 88

Chapter V. Theoretical conclusions 90

Manner of intrusion of the stocks 90

Application of existing theories to the Asctitney intrusions 91

Suggested hypothesis of the manner of intrusion 93

Abyssal assimilation 100

Evidences of differentiation 104

The petrogenic cycle 107

Stimmary and general application 108

The universal earth magma 110

Appendix. Tables, list of specimens, etc 115

Table showing mineralogical and structural constitution of the Ascut-

ney eruptives 115

Table of chemical analyses 118

List of the more iraportant specimens studied . 130

Index 121

ILLUSTRATIONS.

Page.
Plate I. A, General view of Asciitney Mountain, Pierson Peak, and Little
Asctitney Mountain; B, Ascutney Mountain and the terraces

of the Connecticut River 7

II. A, Unaltered phyllite, showing normal plane-parallel strvicture;

B, Phyllite, showing bent laminae and strain-slip cleavage 16

III. A, Hornfels containing abundant pleonaste and cornnduni in

a matrix composed chiefly of cordierite; B, Thin section of
quartz-bearing hornblende-biotite-diorite, showing an apatite
crystal inclosing a core of brown glass 32

IV. A, Typical thin section of the nordmarkite of the Main stock,

granitic phase; B, Pyroclastic feldspar (soda-orthoclase) siir-
rounded by a reaction rim rich in alkaline hornblende , from
the 'large paisanite dike on northwest slope of Ascutney

Mountain . . 58

V. A, Segregation of mica and hornblende concentrically arranged,
in paisanite; B, Basic segregations in nordmarkite at Crystal

Cascade 62

VI. Basic segregations and inclxisions of schist in nordmarkite at

Crystal Cascade 64

VII. Geologic map and section of Ascutney Mountain and vicinity 70

Pig. 1. Sketch x^lan of intrusive rocks on Little Ascutney Mountain 74

5

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U. S. GEOLOGICAL SURVE

BULLETIN NO. 209 PL

GENERAL VIEW OF ASCUTNEY MOUNTAIN, PIERbON PEAK, AND LITTLE
ASCUTNEY MOUNTAIN.
From a point on the New Hampshire side of the Connecticut River, looking northwest.

. '-Uii:Lr'',, , AND THE TERRACES OF THE CONNECTICUT RIVER.

Ascutneyville in the mid-distance, looking northwest across the Connecticut River terraces.

THE GEOLOGY OF ASCUTNEY MOUNTAIN, VERMONT.

By R. A. Daly.

INTRODUCTION.

Nature of the investigation. — The following pages embody the results
of an investigation of the lithology and geology of a plexus of eruptive
rocks and of the metamorphic aureole in schistose rocks surrounding
the igneous bodies. The field Avork was begun in 1893, but numerous
interruptions prevented the completion of the study until the present
year. In the meantime an elaborate series of chemical analyses was
made by Dr. Hillebrand (in 1896) and the results were published on
pages 68-70 of Bulletin 148 of the United States Geological Survey.
These analyses are here republished, with the oxides arranged in the
order recommended by Dr. H. S. Washington. «

Acknowledgments. — The writer's best thanks are due to Dr. Hille-
brand, for the completeness and accuracy of his analyses; to Prof.
J. E. Wolff, of Harvard University, who not only suggested this piece
of research, but also greatly assisted in the petrographical determina-
tions ; to Professor Rosenbusch, of Heidelberg, who likewise aided in
the laboratory study of the collected material; to Dr. F. P. Gulliver,
for the care he bestowed on the preparation of the topographical map;
and especially to Dr. T. A. Jaggar, of Harvard University, who, after
carrying on several weeks' field work in the area in collaboration,
placed his notes and rock collection at the disjposal of the writer. In
addition, Dr. Jaggar has done much in the microscopic investigation
of the specimens and in preparing the photographic illustrations for
this repoi't. He has also read the manuscript, which has been
improved both in form and contents by his valuable suggestions-

aAm. Jour. Sci., 4th series, Vol. X, 1900, p. 59,

CHAPTER L
PHYSICAL GEOGRAPHY.
GENERAL TOPOGRAPHY OF THE AREA.

Mount Aseiitney is the most conspicuous elevation seen by the
traveler in ascending the Connecticut River (see PL I, A). The moun-
tain, as well as the rest of the area considered in this paper, is sit-
uated on the right bank of the river and near the town of Windsor, in
southeastern Vermont. Though having an elevation of little more
than 3,000 feet (915 meters) above sea level, Ascutney is very prom-
inent as it rises from the floor of the deeply trenched master valley of
New England. The railway bridge over the Connecticut at Windsor
is but 301 feet (92 meters) above sea level; the summit of the moun-
tain is, according to Dr. Gulliver's determination, 3,114 feet (950
meters) above the same datum « and lies only 3 miles from the river
(see PI. I, B). Thus the mountain is considerably more imposing than
many other peaks in New England, wliich, although of the same or
even greater height, yet rise from a more elevated base. Additional
scenic importance attaches to Ascutney on account of its isolated
position. Among the nearer noteworthy elevations are Ludlow and
Shrewsbury mountains and Killington Peaks of the Green Mountain
Range; accordingly, for a distance of 20 miles in every direction,
the beautifully compact, broadly conical outline of Ascutney forms a
principal feature of the landscape. Largely for this reason the
mountain enjoys a special reputation for beauty among the inhab-
itants and tourists of New England.

The conditions for field work are good except in some parts of the
main mountain, where thick second-growth timber effectually conceals
the eruptive rocks. Two good paths to the summit, one from Browns-
ville, the other from the Windsor side, were open in 1898. The moun-
tain can, however, be easily climbed from any direction.

The softened j)rofiles of the mountain suggest, and a study of the
geological structure of the region proves, that Ascutney is a residual
of erosion (see PI. VII). It has been carved out of this part of the once
lofty Appalachian mountain system where the sedimentary rocks of
the range have been intruded by several stocks and thick dikes of
igneous rock. The relief features of the area discussed thus belong
to the same categorj^ as the very common sugarloaf peaks of Vermont,

«C. H. Hitchcock estimated the height to be "about 3,1G8 feet: " Geology of New Hampshire,
Vol. I, 1877, p. 180.

DALY.J TOPOGRAPHY. 9

located on intrusive granites and syenites. The geological map
(PL VII) indicates in an almost diagrammatic fashion the sympathy
between relief and rock composition. Ascntney itself owes its exist-
ence primarily to a great stock of qnartz-syenite. The picturesque
ridge of Little Ascntney is held up by a strong rib of intrusive syenite-
porphyry associated with other eruptives more resistant to the weather
than either the gabbro-diorite stock on the north or the gneisses on
the south. The shapely cone north of Little Ascntney, which, for
purposes of convenience in this report, has been named " Pierson
Peak " (after the hospitable owner of the farm at the base of the
hill), is strictly controlled in form by a small elliptical stock of alka-
line syenite cutting the softer diorites.

Apart from other more general considerations, the fact that these
eruptive rocks are more resistant to weather than the surrounding
schists is clear from the nature of the slopes and profiles at the con-
tacts. As a rule there is an abrupt steepening of the ascent along
the radiating spurs of the main mountain just above the contact
between the sedimentary and eruptive rocks. At the same line of
contact there is likewise a sudden change of gradient in the streams
draining the mountain, as if corrasion were considerably easier oa
the schists than on the eruptive rocks. An example is seen at the
beautiful "Crystal Cascade" on the southwest side of the main moun-
tain. This general feature of the residuals of erosion in our area may
be repeatedly and clearly seen in the granitic hills of New England
and might serve to dispel anj^ doubts which remain in the minds of
those students of erosion who are skeptical as to the prevailing theory
of New England reliefs, for it is doubtless true that the most illumi-
nating treatment of New England topography finds best explanatian
for its mountains and higher hills in the assumption of tlie supe-
rior strength of their component rocks — a strength, namely, superior
to that of the rock masses immediately surrounding. In a score or
two of instances in Vermont and New Hampshire striking differences
of relief are faithfully associated with equally striking differences of
lithologic composition. Plutonic eruptives compose the mountains
and generally weak schists underlie the encircling lowlands. In these
examples the greater height of the hills can scarcely be due to more
pronounced initial uplift during the original mountain building.
When, on the other hand, differential erosion is so clear for granitic
mountains like Ascntney, it seems legitimate to extend the idea to
many New England residuals of schistose composition, where, as yet,
full corroborative evidence as to the validity of the same assumption
is not obtainable.

The view that the rocks of the classic monadnock in New Hamp-
shire are harder than the somewhat similar rocks about that moun-
tain certainly wins most credibility from the general agreement of
that assumption with the most fruitful explanation of the New England
peneplain ; but it would be a matter of considerable satisfaction to

10 GEOLOGY OF ASCUTNEY MOUNTAIN, VERMONT. [bull. 209.

those who have to deal in theories of land sculpture if detailed
petrographic and field studies of the schist monadnocks were made
with intent to test the theory of differential hardness. Perhaps a
microscopic study of similarly exposed monadnock rocks and low-
land rocks would enable the investigator to ascertain the amount of
post-Glacial weathering which has occurred. If quantitative estimates
were likewise made as to the influence of jointage, rifting, cleavage,
etc., the result should be to give a more scientific basis for the dis-
cussion of the residuals of erosion than now exists. In a rough way
an analogous but incomplete study of the rocks of this area has sus-
tained the monadnock theory of the origin of Ascutney and Little
Ascutney. This theory, to be sure, here scarcelj^ needs other substan-
tiation than the facts of composition, structure, and present relief.

DEAII^AGE.

The CoEinecticut River flows along a belt of soft rocks parallel to
their strike, and is thus a typical longitudinal valley. In no psbrt of
its course is it more clearly "adjusted" to a relatively weak zone than
on the '' Oalciferous mica-schist" eastward of the mountain. Simi-
larly Mill Brook follows the strike of the rocks in that gorge-like part
of its course between the elbow south-southeast of Windsor and its
confluence with the Connecticut. Elsewhere Mill Brook, like the
stream entering the main river near Ascutney ville, belongs to the
class of "superposed" streams, having sunk its channel irregularly
through drift and terrace sands into the underlying schists. Short
but broad valleys, located partly on schists, partly on the compara-
tively soft diorite, separate Little Ascutney and Ascutney Mountain
proper. These valleys are also adjusted to weak zones in the rocks,
and belong to the now well-recognized class of ' ' subsequent " valleys.

But it is not eas}^ to place the radiating drainage of the main moun-
tain in the accepted classification of stream courses. There is nothing
to show that the eruptives of the area ever reached the surface to form
volcanic flows or cones; they seem rather to have consolidated in the
form of a complex stock-like boss. The structure of the region shows
that the radiating drainage is not the result of inheritance from the
surface of a dome in the overlying schists, in which a different pattern
of drainage would have predominated, namely, a more or less rectan-
gular network of stream courses. Such a dome would not likely be
able to alter seriously the directions of the streams originating either
in the folding of the schists or in the process by which newer valleys
would be worn out on weak belts parallel to the strike. These radi-
ating streams can not, thus, be regarded as "superposed" through
the schist blanket once overlying the stocks.

There is here, in fact, a kind of drainage which is controlled in its
development by constructional processes fundamentally different from
those usually considered in a systematic discussion of streams. Fold-
ing, faulting, and glacial and volcanic accumulation are examples of

DALY.] DEAINAGE. 11

processes leading to the formation of surfaces which are, in the initial
stage, exposed to erosion. But there is a kind of subterranean con-
struction to be found in the intrusion of large bodies of igneous rock,
which may, in the course of time, affect the relief and drainage of a
region much more conspicuously than the processes just mentioned.
The uncovering, by erosion, of a boss of igneous material harder than
the surrounding rock formations will necessitate either the true
"superposition" of streams in the manner just suggested, but excluded,
for good reasons, in the Ascutne}^ instance, or the formation of new
ones divergent, roughly speaking, from the center of the boss toward
the lowlands of the l6ss resistant formations. These latter streams
are logically consequent on the intrusion and, to a greater or less
extent, consequent in length and direction on the original contours
and ground plan of the irruptive body. This may be true in a large
way whatever the details of form in the upper surface of the igneous
mass. Whether it be a regular boss with smooth profiles, or one
irregularly terminated by apophyses into the overlying rock, the
superior hardness of the intrusive will, in the end, tend to cause its
projection, as a whole, above the soft-rock terrane; so that there will
be brought about an approximation to the average original profile of
the boss. There must in any case originate on its revealed surface
a number of streams divergent from the central region of the boss
and fiowing toward the surrounding lower land.

Such streams are seen to be analogous to those which drain the
retreating escarpments of tilted stratified beds — the class of " obse-
quent " streams as defined by Professor Davis. Obsequent streams
drain the scarped front of a hard member of the stratified series and.
are the result of the excavation of lowlands by the lengthening and
widening of valleys in an underlying softer formation. The radial
drainage here considered is similarly caused by the removal of rock
material less resistant to the weather than the intrusive igneous rock.
At the same time, that removal means the origination of drainage
" adjusted" to the soft encircling formation. The adjustment is here
circumferential and centrifugal with reference to the middle point of
the intrusive body, not longitudinal (parallel to the strike of a bedded
formation), as in the case of those " subsequent" streams into which
" obsequents " pour their waters.

The radial drainage of Ascutney is thus believed to owe its origin
to the degradation of the encircling schists — a centrifugal control
due to differential hardness. Located on a hard member, they are
to be associated with obsequent drainage, and share with obsequent
and subsequent drainage the characteristic of appearing only rela-
tively late in the whole geographical cycle of degradation. They are
also conditioned by the original form of the intrusive, and are thus
consequent. To express their composite nature they may be called
suhconsequent, using a term which was first proposed by Professor
Davis for what are generally coming to be called "subsequent"

12 ■ GEOLOGY OF ASCUTKEY MOUNTAIN, VERMONT. [bull. 209.

streams. His abandonment of the longer for the shorter term, which
was independently invented by him and by Jukes, leaves "subcon-
sequent " open to the special use to which we propose to attach it.
The prefix "sub" is especially appropriate, as it serves to indicate
the necessary lack of absolute and exact control possessed by the
constructional form of the intruded body over the trend of this class
of streams even where they run over the igneous rock. That control
will, to some extent, be imperfect on account of a variety of circum-
stances connected with the removal of the cover and the apophyses
penetrating the cover. While subconsequent drainage is always
divergent, it may be radial or elliptical where the intrusive has a
circular or elliptical ground plan; or bilateral, as in the case of
many batholiths and great dike-like intrusions; or, finally, irregularly
divergent.

GLACIATION OF MOUNT ASCTJTNEY.

The similarity of form between Mount Ascutney and other residuals
in the glaciated tract of New England, on the one hand, and the residuals
of Georgia and the Carolinas on the other, particularly in respect to the
systems of radiating drainage seen on all slopes of the northern moun-
tains, is suggestive of the fact, which seems borne out by many others,
that glacial erosion has very slightly affected the sliape of these greater
reliefs of New England. The accumulating evidence of intense glacial
erosion in alpine valleys, whereby hundreds or even thousands of feet
of fresh rock have been quarried away by master glaciers from their
rock floors, recalls the question, raised oftentimes a generation ago, as
to how much material was disturbed by the great Pleistocene glaciej'S
of North America. The answer seems again to be unequivocal tliat
such erosive work as that carried on during Pleistocene times in the
Norwegian fiords, for example, was not paralleled in New England.
If it had been, we should expect Mount Ascutney, once entirely over-
ridden by ice, to possess a somewhat definite stoss-and-lee form and
to have suffered a serious change in its drainage. The radiating
ravines are so deep and contain such clear evidences of glaciation in
their bottoms that they can not be ascribed to post- Glacial erosion.
They have not the appearance of cirques, and hence can not be
ascribed to the work of local glaciers, for which, indeed, on so small
a mountain, the required gathering ground is lacking. These ravines
and water courses must be pre-Glacial. This being the case, the con-
clusion lies near to hand that the Labrador ice sheet did not approach
in erosive activity the local glaciers of Switzerland, Norway, Labrador,
or Alaska. The northwest, north, and northeast slopes of Ascutney
would have borne the brunt of the glacial attack and perhaps suf-
fered a 7nore vigorous onslaught than the lowlands on account of the
projection of the mountain above the general glaciated floor, but these
slopes on the stoss side are as well provided with the usual radiating

DALY.] CHAKACTEE AND HISTOKY OF THE MOUNTAIN. 13

stream courses as those on the south. It is highly improbable that
such symmetry would persist if the Ascutney cone had beeu seriously
affected in volume by the glacier. The same patent observation can
be and has been made in many parts of northeastern America where
the apprppriate reliefs occur, but it is worthy of restatement in order
to point out once again the mysterious contrast between the excavating
power of i)resent and j)ast valley glaciers and of the incomparably
greater Pleistocene ice caps.

The mantle of glacial drift in the area discussed is much interrupted
and, in general, quite thin, so that outcrops of the bed rock are
numerous. The fine terraces of the Connecticut and of Mill Brook
cover some of the "Calciferous mica-schist" (PI. I, B). The highest
of these is 216 feet (66 meters) above low-water level of the river at
Windsor. It was used by J. D. Dana as important evidence of the
height to which the flooded Connecticut extended its banks in Cham-
plain times. '^^ The gneisses of the southwest portion of the area are
blanketed over with the alluvium of the brook at Greenbush.

SUMMARY.

Mount Ascutnej^ like most of the mountains of ISTew England, is a
residual of erosion, a monadnock overlooking a dissected, rolling
plateau. The relief as a whole and in its details is controlled by rock
composition in a specially definite manner. Proofs of differential
hardness are evident in the present topography, intrusive bodies
contrasting in this respect with one another and with tlie adjacent
schists. Tlie drainage of the area is that of an ancient mountain
system. There is clear adjustment of the streamways to soft struc-
tures, giving "longitudinal subsequent" streams and radially diver-
gent " subconsequents. " The latter occur on the main intrusive rock
body, which dominates all the others through its suiDerior strength
against weathering influences and through its relatively greater vol-
ume. The discussion of this mountain, which is but one of a numer-
ous family found in eastern North America, emphasizes once more the
need of recognizing deep-seated intrusion as a constructive process no
less important for certain regions than the faulting, folding, or some
other initiating deformation of the earth's surface which begins a new
cycle of erosion. The history of the Ascutney topography, including its
drainage, begins logically and chronologically with the date of the
intrusion of the Main syenite stock. The existing subconical form
and the radiating stream courses of the mountain may be said to be
"subcousequent" upon that constructional process of intrusion.

The general form of Ascutney was not essentially affected by the
Pleistocene giaciation. A veneer of pre-Glacial weathered rock was
removed and the rounding of minor points accomplished by the ice
invasion, but the pre-Glacial Ascutney had practically the form of
the present mountain.

a Am. Jour. Sci., 3d series, Vol. XXIII, 1882, p. 183.

CHAPTER II.

GENERAL DESCRIPTION OF THE SCHISTS IN THE AREA.

The fundamental rocks through which the eruptions took place
naturally first demand attention. The following account of them will,
however, be brief, as befits the main purpose of this paper. They
consist of two conformable members, a phyllitic and a gneissic.

PHXLLITIC SJERIES.

The numerous specimens collected from the phyllite indicate a
tolerable uniformity in the lithological character of that rock through-
out its whole extent in the neighborhood of Mount Ascutney. It
is composed essentially of quartz, sericite (often partially replaced
by biotite), argillaceous, chloritic, and carbonaceous material,
accompanied by notable amounts of iron sulphides and titaniferous
iron ore (PI. II, A). Rare crystals of orthoclase and of a triclinic
feldspar, equally rare grains of epidote, and perfect, minute crystals
of rutile and of titanite are sporadically developed. The quartz forms
interlocking grains between the sericite fibers and layers which pro-
duce the marked lamination of the rock. Straining in the quartz is
at times notable and seems to be correlated with microscopic faulting
in the rock as a whole. Along these incipient fault planes a further
development of sericite has taken place, thus giving the rock the
wavy appearance characteristic of strain-slip cleavage (PI. II, B).
Good examples of this phenomenon are to be found in the quarry
beside Mill Pond near Windsor. Lenses and laminae of milky quartz
are very abundant and have sometimes shared in the crumbling of
the phyllite, though generally they seem to have been formed posterior
to its folding.

Very often through the series the argillaceous material is nearly or
quite absent and we have a simi^le quartz-sericite-schist. An excep-
tionally fresh specimen of this phase, collected in the low cliffs just
west of Ascutneyville (spec. 24), has been analyzed (Table I, p. 15.)
It is practically a quartzite, which bears, in addition to the other
essential constituent, sericite, very small amounts of orthoclase, an
undetermined plagioclase, epidote, ilmenite, rutile, titanite, and a little
pyrite, with probably pyrrhotite.
14

DALY.J PHYLLITIC SERIES. 15

Table I. — Analysis of quartz-sericite-scMst {specivien 24) from cliffs ivest of

AsGutneyville.

Per cent.

SiO^ -- - 90.91

AlA 4.18

FeA 0.22

FeO 1.37

MgO 0.37

CaO 0.22

Na^O 0.77

K2O :--- 0.58

H2O above 110^ C 0.74

H2O below 110° C 0.06

CO2 0.18

TiO^ - 0.28

ZrO/ 0.02

PA .. 0.05

CI , Trace.

F Trace.

' FeSaandFe^Sg «0.11

MnO . - Faint trace.

BaO . Trace.

LiaO 1 strong trace.

0 '^O.IO

100. 06

Totals 0.056

Sp.gr V 2.678

The phyllites outcrop on the edge of the upper terrace of the Con-
necticut River with a strike of N. 5° E. and an average dip of 50" E.
(PI. VII.)

The planes of foliation here and all across the series to the gneissic
area are regarded as very closely coincident with the original bedding.
Many beds of limestone from 3 inches to 2 feet (8 to 60 centimeters)
in thickness are intercalated in the phyllites and are always, so far
as observed, conformable with their foliation planes. C. H. Hitchcock
used the foliation as expressive of true stratification and has remarked :
"I believe the strata on both sides of Ascutney are monoclinal and
dip easterly."^ Farther to the west, in the monoclinal structures of
the Green Mountains, patient and successful study of the actual
bedding has showed a close correspondence of schistosity and true
dip in the many folds overturned to the west.^ On the other hand,
there is no doubt that in other localites in this same phyllitic forma-
tion there is ' ' striking unconformity between the planes of deposition
and the fissility, " ^

a Note by Dr. Hillebrand, analyst: On boiling with dilute HCl, some HoS is given off, followed
by a strong and persistent odor of volatilizing sulpliui-, showing the decomposition of a sulphide
with the formation of H^S and the simultaneous deposition of sulphur. It is probable that
both pyrite and pyrrhotite are present.

b Another sample gave 0.06 per cent carbon and 0.42 per cent COo.

•^Geology of New Hampshire, Concord, 1877, Vol. II, p. 400.

dSee C.L. Whittle, The genei-al structure of the main axis of the Green Mountains: Am. Jour,
Sci., 3d series, Vol. XLVn, 1894, p. 347.

eC.H. Eichardson: Proc. Am. Assoc. Adv. Sci., Boston meeting, 1898, p. 296.

PLATE II.

A, Unaltered phyllite, showing normal plane-parallel strrictnre; ordinary light,
X20. (Seep. 14.)

B, Phyllite, showing bent laminae and strain-slip cleavage; ordinary light,
X 20. (See p. 14.)

16

U. S. GEOLOGICAL SURVEY

BULLETIN NO. 209, FL. II

(^)

(B)

HE MERIDEN GRAVUfiE CO.

DALY.] GNEISSIC SEEIES. 17

There is uo better summary of the writer's views, obtained from but
a limited stvidy of the series in the vicinity of Ascutney ville, than that
already given by Edward Hitchcock: "We have noticed no cases
where the stratification and schistose structure did not essentially
coincide, though often one or the other was obscure, ver}^ probably
because there was a discordance of this kind, which careful study
might have traced out."^'

Westward across the strike on the north side of the mountain the
dip of the phyllite is seen to steepen until the bedding shows an incli-
nation of 75° or more to the east. These high dips occur about in the
meridian passing through a point a half mile east of Brownsville,
where, in places, the dip is even vertical. The strike ranges from
N. 10° E. to ]Sr. 23° W., but is rarelj^ far from its average trend, which
is due north and south. The cross section on the south side of the
mountain indicates a variation in the strike of from IST. 15° E. to N.
20° W., with the average again practically north and south. The dip
averages 60° to the east, though a rapid steepening below the granite
quarries gives angles as high as 83°. The two sections of the phyllitic
series thus correspond to a dynamically metamorphosed integral mass
of sediments, deformed so as to present the appearance of a great
thickness of conformable tilted rocks with a high dip to the east.

GJ^EISSIC SERIES.

West of the meridian which passes through the diorite-syeuite
contact appears a group of medium-grained to coarse-grained crystal-
line schists more varied in composition and more complex in structure
than the phyllites. There is no distinct plane of junction between
the two sei'ies. Both north and south of the mountain, going west, the
phyllite simply assumes a more and more feldspathic character until
it merges into a conformable and typical gneiss. No attempt has
been made to unravel this complex, even so far as that is possible.
A qualitative treatment only has been deemed sufficient for present
purposes, and as a result only very slight differentiation of the gneisses
is to be noted on the map.

The most abundant member of the gneissic series is a muscovite-
biotite-epidote-gneiss of variable texture. It is often richly charged
with scapolite. Likewise abundant are biotite-muscovite-gneiss,
biotite-gneiss, muscovite-gneiss, epidote-gneiss, or mica-schists, all of
which seem to be transitional into the main gneissic type. Very often
the feldspars are large and the structure is that of a true augengneiss.
All these types may be garnetiferous. With them are associated thin
bands of beautifully crystallized hornblende-biotite-quartz-schists and
epidotic hornblende-schists. The finest types of these hornblende-
schists were found in a number of massive ledges on the west side of

1 Geology of Vermont, Claremont, 1861, Vol. I, p. 476.

Bull. 209—03 2

18 GEOLOGY OF ASCUTNEY MOUNTAIN, VERMONT. [bull. 209.

the road running through Greenbush, and thus outside the limits of
the area mapped, but excellent sill-like occurrences may be studied
just below the Crystal Cascade. Thick pods of coarsely crystalline
limestone and of marble, generally charged with nests of radiating
tremolite (the "wood-rock" of the quarryman), are included in the
gneissic area, but again outside the immediate region under considera-
tion. The nearest of these lenses of limestone has been rather exten-
sivel}^ quarried for the manufacture of quicklime at Amsden, about 2
miles southwest of Little Ascutney . Sheets of now greatly weathered
diabase are not uncommon in the gneisses, and the apophyses from
the diorites and syenites often assume the same form.

An intrusive sheet of composition and origin quite different from
that of any other rock yet studied in the area was found exposed for a
length of about 500 yards (458 meters) with a strike of N. 15° W. It is
terminated at the northern end by the younger rock mass of the Main
syenite stock at a point about one-half mile from Crystal Cascade.
The southern end of the sheet is concealed by a drift. This sheet
varies from 60 to 100 feet in thickness and dips, conformably with the
quartz-epidote-schist, 55° to the east. The inclusion within its mass
of horses of the schist and the appearance of apophyses from it within
the latter clearly prove its intrusive nature (see PL VII).

A gneissic structure is generally visible on weathered surfaces,
though it is sometimes quite absent. The sheet rock has evidently
been squeezed with the schist, and both have been broken by faults of
small throw. The light-gray hand specimen itself exhibits granula-
tion, and the extensive alteration of the old eruptive is indicated by the
presence of many irregular grayish to silvery- white blotches of mus-
covite (specimen 175). Dark-colored minerals are not visible macro-
scopically. Microscopic examination shows that the crushing has
been j)rofound. Abundant granulated quartz and orthoclase, greatly
bent lamellse of plagioclase and microcline ('?), and the abundant mus-
c-ovite present especially characteristic proofs. Besides epidote, which
appears to be a metamorphic derivative from the plagioclase, rare
zircon crystals and a few grains of an iron ore complete the list of
constituents. A muscovite-gneiss at the present time, this rock was
doubtless of the nature af an aplitic sill before the period of dynamic
metamorphism. It is of interest in representing something like the
condition to which the latter eruptives would have been reduced if,
since intrusion and consolidation, they had been affected by mountain
building on the scale indicated by the present attitude of the schists.

It has alreadj^ been noted that on both sides of the mountain the
phyllite assumes from east to west a more and more gneissic character
in a fairly broad north-south zone. The strike and dip in this transi-
tion zone are similar to those of the phyllites proper, and they are
retained in the gneisses on the south of the dioritic stock. On the
north side, however, while conformable with the phyllite near the

DALY.] AGE OF THE SCHISTS AND THE INTRUSIVE ROCKS. 19

transition zone, the gneisses vary in strike from north and south to
east and west. The average strike is about northwest to southeast.
As the section is followed westward the gneisses are seen to be greatly
contorted and to writhe about in the most irregular way, the angle of
dip changing considerably with the strike. These structural changes
are introduced so gradually in and west of the transition zone that
they do not preclude the idea that the gneiss underlies the phyllites
conformably. Such is believed to be the relation between the two
series.

In this instance the chronological treatment of the rocks of the
area has been deviated from. This has been done because the phyl-
lites have greater stratigraphic simplicity, and dynamic metamorphism
has affected them to a much smaller degree than the older crystalline
schists. At the same time, the amount of light thrown on the origin
of the gneisses by the brief and limited study of the phyllites is not
that which would accrue from an accurate and detailed mapping of
the schists far beyond the limits of the area mapped. The intimate
association of the two series and the occurrence of the limestone pods
in the gneiss render it highly probable that the gneiss is for the most
part composed of material that was originally sedimentary. Beyond
this general statement the facts obtained in the Ascutney area will
not permit us to go, nor for the immediate purpose of this paper is it
necessary to inquire further into the details of the history of the
metamorphism.

GEOLOGIC AGE OF THE SCHISTS AND OF THE INTRUSIVE

ROCKS.

Outside regions must be turned to for a solution of the difficult
problem as to the age of the schists and, inferentially, as to the maxi-
mum age that may be assigned to the eruptives. The Vermont Sur-
vey Report of 1861 includes the phyllitic series in the " Calciferous
mica schist," and states that this formation is overlain by clay slate
which a "strong presumption " would place in the Devonian; and
hence that the schist is at least Devonian, and may be older. « Speak-
ing of the underlying gneiss, Edward Hitchcock wrote, ' ' We have
already made it probable that the Calciferous mica schist has been
converted into gneiss from Ascutney southward. If so, whatever the
age of the schist may be, that of the gneiss is the same."^ Hitchcock
left the question of age open, though he seems to have given weight
to T. S. Hunt's conclusion, based on studies in the northern extension
of these schists into Canada, that they are of Niagara or, at any rate,
Upper Silurian age.'^ Emerson has proved that the Bernardston series
of Vermont and Massachusetts is of Hamilton and Chemung age.'^
That series overlies the Calciferous mica schist, so that the latter is
Lower Devonian or older.

« Vol. I, p. 485. c Am. Jour. Sci., 2d series, Vol. XVIII, 1854, p. 198.

b Geology of Vermont, 1861, Vol. I, p. 470. a Idem, Vol. XL, 1890, p. 263.

20 GEOLOGY OF ASCUTNEY^MOUNTAIlSr, VEEMONT. [bull.209.

Recently the detailed field observations of C. H. Richardson have
afforded more definite information. He divides the Calciferous mica
schist into a calcareous member, the Washington limestone, and a non-
calcareons member, the Bradford schist. The latter includes the
phyllitic series of the Ascutney region. Richardson correlates the
Bradford schist with the Goshen schist of Emerson in Massachusetts.
The Bradford schist is "flanked on the east by a band of clay slate
and on the west by the Washington limestone, which in turn is
flanked on the west by a band of clay slate. The two bands of slate,
the Bradford schist, and the Washingtoii limestone lie unconf ormably
both on the east and west on a synclinal trough of the hydromica-
schist, which is Hurouian." His discovery of fossils in the clay slate
has enabled Richardson to prove it to belong to the Lower Silurian.
"The Bradford schist and Washington limestone, which have oscil-
lated from the ' Primitive ' of Zadock Thompson to the Niagara of
Professor Dana, are Lower Silurian, and, more definitely. Lower
Trenton. "«

The schists had been flexed into essentially their present attitude
before any considerable eruption of igneous rock took place in this area.
The sheets of amphibolite and sills of aplitic material noted above
have been changed both in composition and structure by the dynamic
action of the tilting process. They are, however, of minor impor-
tance, and do not weaken the conclusion that the mountain-buiJding
forces had practically ceased acting before the eruptive masses of
Ascutney had appeared in the main conduit. Occasionally the feld-
spars and biotite crystals of the rock in the oldest (gabbro-diorite)
stock show considerable straining and bending, but such effects are
far inferior in degree to those which would result if the diorite had
undergone the enormous pressure necessitated in the folding of the
sediments. The still younger intrusives show even less evidence of
squeezing or dislocation since they were consolidated.

The principal intrusions are, accordingly, post-Trenton in age, and
probably, if we may judge from the analogy of other granitic intru-
sions in the Appalachian system, post-Carboniferous and pre-Creta-
ceous. Nearer than that they can not as yet be more definitely dated,

SUMMARY.

The irruptives of Mount Ascutney cut a series of tilted schists
assigned to horizons equivalent with that of the Bradford schist or
older; i. e., they are regarded as Trenton or pre-Trenton in age. The
overlying phyllites — the "Calciferous mica-schist" of the geological
survey of Vermont — belong to the Bradford schist proper. While
highly metamorphic and greatly deformed, the phyllites, with the
interbedded quartzitic and thin limestone bands, show an api^arent

'^ Proc. Am. Assoc. Adv. Sci., Vol. XL VII, 1898, p. 296.

DALY. J

AGE OF THE INTRUSIONS. 2l

parallelism between scliistosity planes and stratification planes.
Beneath this series is a conformable group of schistose rocks con-
sisting of common mica-gneisses, epidote-gneisses, amphibolites, and
crystalline limestone.

The intrusions are of later date than the last great period of rock
folding which has affected the Ascutney region, and the balance of
probability makes them post-Carboniferous and pre-Cretaceous in
age.

CHAPTER III.

CONTACT METAMORPHISM.
THE METAMORPHIC AUREOLE.

The metamorpliic aureole developed about the stocks is worthy of
detailed consideration. As one approaches the contact from any side
of the mountain he speedily notes plentiful evidences of the squeezing,
crumpling, and fracturing of the schists, which thus have a more com-
plex structure than they have at some distance away from the massive
rocks. Especially good examples of this may be seen on the cleared
spurs on the southeast side of Ascutney Mountain proper. There, as
elsewhere, further jjroof of the energetic nature of the intrusions is to
be found in the numerous apophyses sent into the bedded series and
in the disruption of blocks, great and small, from the latter, often
forming a "permeation area" in the eruptive masses; but the pur-
pose of the present chapter relates not so much to the dynamics of
the metamorpliic action as to the mineralogical changes which have
taken place in the schists.

The heat and the mineralizers accompanying the intrusions have pro-
duced alterations which are most important in the phyllites and asso-
ciated limestones. On account of the well-known mineralogical
stability of the gneisses and of the quartzitic bands the metamorphic
effects upon these rocks have been mechanical rather than chemical
or mineralogical.

The breadth of the aureole is not great in any part. Indisputable
contact metamorphism has not anywhere been recognized at much
over GOO feet (183 meters) from the contact, and may often be dis-
tinctly seen no more than 300 feet (91 meters) away from the same
line. • In the phyllitic series the metamorphic belt averages about 500
feet (154 meters) in width, and that irrespective of the attitude of the
schists and irrespective of the stock nearest to which the measure-
ment is made. The Main syenite stock has controlled the metamor-
phic action, although the Basic stock seems to have slightly intensified
the action at the triple contacts of syenite, phyllite, and diorite. It
is essentially correct to speak of the metamorphism of the phyllites
as resident in one aureole produced by the intrusion of the Main
stock.
22

daly] contact metamoephism. 28

cha:n^ges in^ the eimestoxes.

The interbedcled calcareous layers of the phjdlitic series are spe-
cially sensitive to this contact transformation. Two different rock
types result from the alteration of the fine-grained bluish-gray sili-
ceous limestone, which is composed simi3ly of ealcite, quartz, and
carbonaceous matter (spec. 114).

The first of these phases of recrystallization has resulted from an
abundant replacement of the ealcite by epidote, the other constituents
remaining unchanged. This variety was found at several points in
the aureole. Of the occurrences known, that farthest removed from
the syenite lies about 600 feet (183 meters) from the contact at the
base of the prominent 2,350-foot spur bearing east by north of the
summit. A very similar phase was discovered only 3 feet (0.9 meter)
from the contact at the Crystal Cascade. Here scapolite is also devel-
oped and rare grains of titanite and brownish-green hornblende are
accessory (spec. 100).

The second phase is illustrated in a specimen from a field just west
of the old road leading up to the quarries in the granite. The ledge
from which it was taken is 450 feet (137 meters) from the contact with
that intrusive. Attention was first called to it by the very noticeable
roughness of the weathered surface, which betrayed the jpresence of
some mineral in roundish masses more resistant to the weather than
the other constituents (spec. 5). The fresh specimen is light graj',
mottled with these subcircular, rather darker-colored, oily-looking
areas about 6 millimeters in diameter and at an average distance of
1 to 2 centimeters apart. Under the microscope the areas resolve
themselves into irregular aggregations of the colorless to yellowish
lime-garnet (grossularite), inclosing large amounts of ealcite. The
occurrence is thus similar to that described by Harker and Marr in
the metamorphosed limestone of the Shap Fell region in England"
and to many others of different localities.

The garnet never shows crystal form, nor, except in rare cases, any
distinct cleavage. The usual optical anomalies and zonal extinction
are present. The anisotropic propertj^ is very unevenly distributed
throughout the areas. It may be completely absent in one grain and
give a polarization tint like that of zoisite in a contiguous individual.
The double refraction shows that in SQpae instances the areas are
occupied by single poikilitic crystals of the grossularite as shown by
the uniform extinction on revolving the section between crossed nicols.
The same test would seem to indicate the comj)osite character of other
masses which are aggregates of small individuals. The only other
change from the normal limestone is the complete disapj)earance of
carbonaceous matter. Quartz remains in relatively high proportion
and occurs with ealcite as inclusions in the garnet.

a Quart. Jour. Geol. Soc, Vol. XL VII, 1891, p. 311.

24 GEOLOGY OF ASCUTNEY MOUNTAIN, VERMONT. [BnLL.209.

METAMORPHISM OF THE PHXELITES.

The average normal pliyllite is, as we have seen, considerably more
argillaceous than the quartzitic schist analyzed (see Table I, p. 15).
The minerals characterizing the rocks resulting from the alteration
are, as a rule, those which might be expected from mere recrystalliza-
tion of the original constituents, viz, quartz, sericite, chlorite, clayey
matter, iron ores, and sulphides.

There is no definite succession of zones of metamorphic action,
either of color, structure, or mineral aggregation, as in the classic
region of Barr Andlau. The macroscopic changes are simple and
uniform in all parts of the aureole. Quartz veins and eyes become
morf^ nujnerous as the contact is approached; the lamii .tion of the
schist becomes, at the same time, more and more lost, and the rock
takes on an increasingly compact and indurated look. Yet even at
the contact itself the presence of quartzose laminae in the original
phyllite often entails a partial preservation of the schistose structure.
Occasionally obscure spotted and knotted areas are found, but they
are not conspicuous nor are they arranged in any fixed order.

The general mineralogical changes may be summarized as compris-
ing a progressive disapi3earance of sericite, quartz, and argillaceous
substance and a corresponding development of biotite, red garnet,
cordierite, pleonaste, corundum, and sillimanite. These new minerals
naturally occur most abundantly and in larger crystals near the con-
tact than farther out in the aureole. In tracing these changes, the
attempt was made to collect specimens along the strike, thus inviting,
though, on account of the variability and disturbed character of the
schist and because of the lack of sufiicient outcrops, not entirely
securing, the maximum of certainty as to just how great has been the
influence of this local metamorphism on lithological units. Three
fairly representative sections of the contact zone were made in this
way ; perhaps no better means of describing the phenomena of the
zone can be adoj)ted than to consider each of the sections somewhat
in detail.

SERIES A OF SPECIMENS FROM THE METAMORPHIC AUREOLE.

The first of these sections in the aureole is noted on the geological
map as occurring at the syenite contact on the north side of the moun-
tain; the set of specimens collected there may be called "Series A."
For ease of reference each of the following ]3aragraphs relating to the
description of the specimens is preceded by a number indicating the
distance from the contact of the specimen to which that paragraph
refers.

500 feet {15 Jf 'meters). — Five hundred feet from the contact the phyl-
lite shows some crumpling and otlier evidences of disturbances; so
far as known, however, no new mineral has been developed at that
distance.

DALY.] SPECIMENS FEOM THE METAMORPHIC AUREOLE. 25

JfiO feet {19,'B meters). — One hundred feet nearer, the sericite is
largely replaced by an indeterminable chloritic snbstance, but it is
probable that this phase also is original.

SOO feet {91 meters). — Somewhere within the next 100 feet measured
toward the contact there is a comijaratively abrupt appearance of
true contact minerals, coupled with a decided loss of the original
fissility of the rock (spec. 129). At 300 feet from the syenite, there
is a partial replacement of the argillaceous and chloritic material
and of quartz by cordierite, while the whole rock is filled with a
swarm of extremely minute, light-green, isotro]3ic grains with high,
single refraction. An occasional grain of epidote lies with the
iron sulphides (pyrite and probably pyrrhotite) in the planes of
schistosity, which are still to be seen, both macroscopically and in tlie
thin section.

250 feet {77 meters). — Fifty feet nearer, the metamorphism has
affected the whole rock. Its original dark-gray color has now a bluish
cast (spec. 128) . A high degree of induration with a corresponding loss
of lamination and an increase in the specific gravity are characteristic.
A peculiar feature of this hand specimen is the presence of numer-
ous roundish and isolated areas of what can be discerned, even
macroscopically, to be granitic aggregates of quartz, feldspar, and
other minerals embedded in the general rock matrix. They bear a
relation to the transformed schist analogous to that of a miarole to
its igneous host, and, for lack of a better term, they vaay be called
"pseudo-miaroles." Microscopic examination shows that the basis
of the rock is now cordierite occurring in interlocking individuals
2 to 3 millimeters in diameter. It is always poikilitic either from
the mutual intergrowth of several crystals of its own substance
or from the swarms of mineral inclusions of different sorts (the
"sieve-structure" of Salomon). Those inclusions are the same as
the other constituents of the hornfels, viz, numerous small shreds
of intensely pleochroic brown biotite, abundant, irregular grains of
deep-green sj)inel, pyrite, pyrrhotite, ilmenite, tourmaline of brown
and yellow tones, quartz, and minute black, probably carbonaceous
particles.

These various inclusions cloud the whole thin section except in the
more quartzose laminse, which are doubtless residual from the origi-
nal rock, and in the pseudo-miaroles. They are also lacking in the
numerous stringers of quartz which traverse the schist. Probably
the recrystallization of the schist was complete before the quartz was
laid down in the veinules and the pseudo-miaroles were filled with
their granitic contents. The latter consists of the minerals charac-
teristic of the adjacent syenite — microperthite, cryptoperthite, quartz,
brown alkaline hornblende, and rare zircons. With them is often
associated a pale-reddish garnet averaging about 1 millimeter in
diameter. The quartz is so abundant as to give the hypidiomorphic-

26 GEOLOGY OF ASCUTNEY MOUNTAIN, VERMONT. [bull.209.

granular aggregate the composition of a true granite. These pseudo-
miaroles when round in outline measure from 3 to 5 millimeters in
diameter; when, less often, they are elongated in the section they
measure from 5 to 10 millimeters in length. They are not connected
with one another or with distinct apophysal veins from the syenite,
but are completely surrounded by the cordieritic matrix. It looks as
if there had been a shrinkage of volume in the schist during its
recrystallization and the resulting cavities were subse4uently filled
with the granitic substance by a pneumatolytic process. We have,
whatever be the explanation, a striking case of feldspathization of
schist by an intrusive granitic rock wherein the channels of approach
of the feldspathic material were of submicroscopic dimensions. The
deep-green spinel is pleonaste, which is now of increasing importance
as we go toward the eruptive rock. It was this mineral that war
seen at the 300-foot distance, where the very small grains were inde-
terminable. Similar fine material is present here, but it grades up
into larger individuals which are undoubtedly pleonaste. Here, too,
the pleonaste has an interesting localized distribution, being grouped
in roundish clusters composed of many crystals. In one case the
whole of one well-marked cluster about 0.3 of a millimeter in diam-
eter is included in a single crystal of cordierite. The spinel is
usually without crystal form and occurs as drop-like bodies. It is
worthy of note that not only in this case, but in all parts of the
aureole, pleonaste and the metamorphic biotite are in reciprocal
relation to each other ; where one is abundant the other, relatively, is
rare. This fact correlates with the observation of Lacroix that spinel
is a common product of the alteration of mica in inclusions caught
up in lavas," and with our own observation that, close to the contact,
where we should expect the more stable products of metamorphism,
the biotite is often completely replaced by pleonaste.

150 feet {J/-6 meters). — One hundred feet nearer the contact the schist
is macroscopically similar to the rock found at 250 feet in its compact-
ness, lack of pronounced schistosity, and bluish-gray color, but lacks
the pseudo-miaroles and is more strongly charged with the red garnet
(spec. 127). Mineralogically the most important difference is found in
the entrance of corundum as a new metamorphic constituent. This
occurs as irregular colorless grains, often grouped in clusters about
ilmenite in the form of a mantle. This new mineral is in small amount
and its description may be deferred for better occurrences at other
localities. Cordierite again composes most of the schist. It has here,
too, the pleochroic lialos found about inclusions, as well as other usual
features. Pleonaste, with a clustering habit, is comparatively abun-
dant, and biotite is rare. The garnets appear in large individuals,
showing characteristic cleavage and inclusions, and also in the form of
well-crystallized minute dodecahedrons without inclusions or cleavage.

« Les enclaves des roches volcaiiiqiies, Macon, 1893, p. 599.

DAiA-.] SPECIMENS FEOM THE METAMOEPHIC AUREOLE. 27

Manj^ of the larger crystals form the centers of eyes and are then
wrapped about by mica plates. Generally, however, the garnet is sur-
rounded by a clear zone of quartz and cordierite, unaccompanied by
iron compounds, which have either been used to build up the garnet
or are included in zones within that mineral. Though the pseudo-
miaroles are absent, there is evidence of some feldspathization of the
schist. Occasional crystals of microperthite may be discerned in the
thin section. They are products of late crystallization, as they are
intersertally related to the cordierite ; like the feldspars of the pseudo-
miaroles, they are free from inclusions of pleonaste, etc.

100 feet {31 meters.) — A specimen (No. 126) taken from a ledge 100
feet from the contact, seems in several respects to show a local excep-
tion to the general effect of metamorphism on the phyllites. Biotite is
once more developed in profusion, while pleonasteis quite subordinate.
Corundum is absent. Cordierite is again the principal constituent
and acts as host to the pleonaste clusters. The iron sulphides are
conspicuous in the hand specimen. Dr. Hillebrand remarks :

On boiling the powdered rock with dilute hydrochloric acid considerable H2S
is evolved and sulphur is set free, as can be plainly perceived by the strong smell of
volatilizing sulphur accompanying that of HjS. The decomposition begins at a
moderate heat. After a time the evolution of the HjS ceases and the smell of
sulphur is no longer noticeable, and there is then found only one-third of the total
sulphur left in the residue, presumably wholly as pyrite. The rest of the sulphide
is magnetic and, dissolving readily in HCl with the evolution of HjS, may be
considered as quite certainly pyrrhotite.

The feldspar is much increased in amount, is notably mieroperthitic,
and occurs in the same relations as in the last specimen described.
For the first time, the feldspar shows the Carlsbad twins characteristic
of the microperthite of the syenites. A chemical analysis has been
made of this phase (Table II, col. 1). As a whole it corresponds to
the analysis of a phyllite rich in argillaceous material. The soda is
to be ascribed mainly to the feldspar, and is thus believed to have
been introduced by hydrothermal action. On the supposition that all
tlie sulphide exists as pyrite, the proportions of the iron compounds
would be —

Per cent.

FejOg 0.03

FeO 6.41

FeS^ 0.58

If two-thirds of the sulphide is pyrrhotite, these compounds should
be recalculated to the following proportions :

Per cent.

Fe,03 0.30

FeO -----. 6.00

FeS^ • 0.19

Fe7S8 0.53

28 GEOLOGY OF ASCUTNEY MOUNTAm, VERMONT. [bull. 209.

The latter proportions are, for tlie reason already noted, believed
to be more nearly correct and are accordingly entered in the total
analysis. The carbon percentage is extremely variable, correspond-
ing to the irregular distribution of the coaly matter in the schist. One
independent determination gave 0.03 per cent carbon instead of 0.40
per cent, as found for the rock fragment analyzed completely. Two
analyses of typical cordierite-mica-hornfels from southern Carinthia
(Table II, cols. 3 and 4), show a rather strong similarit}^ to that of the
Ascutney rock.

50 feet {15.5 meters). — Fifty feet from the contact a specimen (No.
125) was collected that showed a reversion to the normal sequence of
the mineral occurrences as the contact is approached. The pleonaste
once more largely supijlants the biotite and is, in fact, more abundant
than ever. It clouds the whole of the thin section, though there is a
tendency toward a grouping along planes apparently representing the
original schistosity. Corundum and tourmaline, like the biotite,
occur but sparingly. The schist is strongly impregnated with small
quartz veins and with lenses 1 to 2 millimeters in thickness, composed
of quartz, microperthite, and brown biotite. The evidence is not so
clear as in the case of the pseudo-miaroles that these granite lenses
are not actually connected with one another and with the stock rock,
but it is highly probable that both types of the granitic aggregates
are to be referred to the same pneumatolytic origin.

25 feet {8 meters). — Halfwaj^ to the contact the rock is still the
massive dark bluish-gray heavy hornfels, hardly to be distinguished
in the hand specimen from the altered schist collected from the 200
or more feet of section over which we have just iDassed (spec. 124).
A notable difference from the hornfels at 50 feet ■ consists in the
abundance of corund«um, which is even more plentiful than the pleo-
naste. Cordierite is here again the chief constituent. While it pre-
sents the usual poikilitic interlocking habit, it sometimes has definite
crystal form, with the common hexagonal sections from the base and
rectangular sections from the prism. Biotite is an accessory, but
tourmaline has disappeared and does not reappear between this point
and the syenite. Microperthite occurs in intersertal contact with the
cordierite, but is only a rather rare accessory. Besides ilmenite or
titaniferous magnetite, the iron compounds include both pyrite and
pyrrhotite. The test for the occurrence of the latter mineral and the
method of its determination in amount are the same as has been out-
lined above. The strong basicity of the hornfels is noteworthy, as
well as the high content of alumina. The latter easily explains the
richness of the rock in corundum'* (see Table II, col. 2).

a Cf J. Morozewicz, Experimentelle Untersuchiingen iiber die Bildung der Minerale im Magma:
Tscher. Mm und Petrog. Mitth., Vol XVIII, ib98, p. 5?.

DALY.] ^ SPECIMENS FROM THE METAMORPHIC AUREOLE. 29

Table II. — Analyses of cordierite-hornf els.

].

2.

3.

4.

SiOa __._^_..,-_.

TiO^

AI2O3 - -

58. 35
0.87

21. 30
0.30
6.00
2.10
0.85
1.60
5.63

« 0. 86
0.81

None.
0.18

None.
0.03

Undet.
0.19
0.53
0.03
0.13
0.05

Trace.
Strong tr.

Trace.

&0.40

99.71

45.30
1.48

30.51
0.24
8.80
3.11
0.90
1.65
4.84

«1.05
0.26

Trace?
0.12
0.04
0.04
0.04
0.36
0.96
0.02
0.20
0.03

Trace.

Strong tr.

?

CO. 17

1 55.68

21.91
2.63
6.90
3.57
0.89
1.01
6.34

«1.41

56.88
20.86

FegOs

2.66

FeO

MgO

CaO - - -

4.54
3.15
1.29

NajO -

0.91

K2O

7.49

H2O above 110= C

H2O below 110° C.

« 2. 36

CO2

P„0, . -

SC

CI

F

FeSj - - -

Fe,SR

NiO, CoO

MnO .

BaO

SrO

LioO -- -... . .

CnO

•

C

0-F,Cl

100. 12
- 0.02

100. 34

100. 14

0.31
2.673

Total S -

100. 10
0.19
2.835

Sp.gr _.■

a Loss on ignition.

b Another sample gave 0.03 per cent carbon and no OOo-

0 Another sample gave 0.03 per cent carbon and 0.04 per cent COo.

1. Cordierite-biotite-microperthite-hornfels, a phase of the exomorphic zone
100 feet (31 meters) from the contact, north side, Ascutney Mountain; analysis by
Hillebrand.

2. Cordierite-corundum-pleonaste-hor:afels, taken from the same cross section
of the exomorphic zone as No. 1, 25 feet OS meters) frora the contact; analysis py
Hillebrand.

3. Cordierite-biotite-orthoclase-hornfels, Schaida, S. Carinthia; analysis by
Graber, Jahrb. der K.-k. geol. Reichsanst., 1897, Vol. XLVII, p. 290.

4. Cordierite-biotite-plagioclase-hornfels, M. Doja, S. Carinthia; analysis by
Von Zeynek. See Graber, ibid., p. 290.

30

GEOLOGY OF ASCUTNEY MOUNTAIN, VERMONT. [bull.209.

6 feet and 1 foot {1.8''meters and 0.3 meter). — A specimen taken at a
point 6 feet from the contact (spec. 123), and another taken from a
point only 1 foot from it (spec. 122), are very similar to each other
and to the phase just described. There is, however, a decrease in
corundnm and an increase in pleonaste, which now possesses perfect
crystal form. The octahedra are excellently developed, and are
furthermore interesting, as they show the octahedral cleavage, which
is seldom seen in rock-forming occurrences. The clustering habit of
the pleonaste is strongly marked in both specimens. Corundum often
appears as a core, about which the concentration of pleonaste took
place. In other cases, the grouping is wholly in quartz crystals in a
manner similar to that already noted for the clusters in cordierite.

This study of the cross section of the aureole may be summarized
in tabular form as follows:

Summary of cross section of series A of vietamorphic aureole.

Distance from
the contact.

Compound name of aureole phase,
showing its essential constitution.

Accessory and subordinate mineral
constituents.

Feet.

Meters.

600

183

Unaltered argillaceous phyllite.

Pyrite, pyrrhotite (?), ilmenite,
carbonaceous matter.

500

154

The same, crumpled

Do.

400

122

Crumpled chloritic phyllite

Do.

300

91

Cordierite-quartz-hornf els

Biotite, pleonaste, epidote, pyr-
rhotite, pyrite, ilmenite, car-
bon.

250

77

Pseudo - miarolitic cordieri:;e-

Pyrite, pyrrhotite, ilmenite, car-

biotite-quartz-microperthite-

bon, tourmaline, garnet, horn-

hornfels.

blende, zircon.

150

46

Cordierite - garnet - pleonaste-

Pyrite, pyrrhotite, corundum, il-

quartz-hornfels.

menite, carbon.

100

81

Cordierite - biotite - microperth-

Pyrite, pyrrhotite, pleonaste.

ite-hornfels.

carbon (graphite ?), ilmenite.

50

15.5

Cordierite - pleonaste - micro-

Pyrrhotite, pyrite. biotite, ilme-

perthite-hornf els .

nite, carbon (graphite Y) , tour-
maline.

25

8

Cordierite-corundum-pleonaste-

Biotite, microperthite, pyrrho-

hornfels.

tite, pyrite, ilmenite, carbon.

6
and

1

1.8
and
0.3

Cordierite-pleonaste-corundum-
hornfels.

Do.

DALY.] METAMOKPHISM OF THE PHYLLITES. 31

SERIES B OF SPECIMENS FROM THE METAMORPHIC AUREOLE.

A second suite of specimens was collected from a section across the
metamorphosed belt just south of Brownsville. The aureole is here
at least 600 feet wide, the relatively greater breadth being probably
due to the proximity of the diorite as well as the syenite. The effects
produced by the intrusives are practically the same as in Series A.

600 feet {183 ineters). — Six hundred feet from the contact along the
strike and 500 feet across it (spec. 136), garnets, tourmaline, corundum,
and a little cordierite are already found in the j)hyllite, which, in its
unaltered phase, is more quai'tzose than in Series A. The quartz
preserves much of its original imjjortance ; it is charged to a remark-
able degree with liquid inclusions, often containing gas bubbles.
Biotite is present, and is also doubtless inherited from the original
schist. Rare epidote is accessory. No feldspar is recognizable. Cer-
tain colorless grains with high single and low double refraction have
the appearance of andalusite, but its presence could not be proved
on account of the small size of the individuals.

300 feet {91 meters). — Three hundred feet from the contact pleonaste
appears as small disseminated grains for the first time (spec. 135).

100 feet {31 meters). — One hundred feet from the contact the schis-
tose structure is no longer visible macroscopically, though it appears
in the thin section (spec. 134). The biotite assumes a concretionary
rather than a plane-parallel arrangement. The increased importance
of cordierite and the entrance of a few needles of sillimanite are the
other chief points of difference from the last locality.

Jf5 feet {llf. meters). — At 45 feet pleonaste and corundum are quite
prominent, both in abundance and in size of the individuals (spec.
133). Sillimanite increases in quantity. Andalusite is, as before,
doubtfully present.

15 feet {6 meters). — At 15 feet, tourmaline is added to the list of
essentials (spec. 132). The pleonaste has two types of aggregation.
Besides appearing on the familiar clusters, it accumulates in long
strings, which reach 2 millimeters or more in length and have a uni-
form breadth of about 0.1 millimeter, reminding one of the linear
development of chlorite in the classic desmosite (PI. Ill, A). This
second kind of aggregation does not seem to have any fixed relation
to single individuals of other constituents, and the masses of pleonaste
pierce the rock in all directions. Again, this mineral is inclosed by
the cordierite in a way not observed elsewhere in the aureole. Numer-
ous rectangular grains of spinel may be arranged with their longer
axes parallel to the chief axis of the cordierite host. Idiomorphic
cordierite also incloses prisms of tourmaline with a similar orienta-
tion. Corunduui is first seen here to assume a crystal form. Silli-
manite is a common accessory.

5 feet {1.6 meters). — A specimen (No. 131) taken only 5 feet from the,
contact shows a rarity of metamorphic minerals which can only be

PLATE III.

A, Hornfels containing abundant pleonaste (black, of high relief) and coriin-
dtini (white, of high relief) in a matrix composed chiefly of cordierite (a charac-
teristic linear aggregate of pleonaste is conspicnons) ; ordinary light, X 12. (See
p. 39.) Compare with PI. II, A and B, illustrating the same phyllite from which
this hornfels has been j^roduced by contact metamorphism.

B, Thin section of quartz-bearing hornblende-biotite diorite, showing an apatite
crystal inclosing a core of brown glass; ordinary light, X 50.

32

U. S. GEOLOGICAL SURVEY

BULLETIN NO. 209, PL.

m;

(B)

THE MERIDEN GRAVURE CO.

DALY. J COMPARISON OF MET AMORPHIC EFFECTS. 33

»

explained as characterizing a phase of the schist which was originally
less richly charged with the argillaceous material that has elsewhere
yielded, under the metamorphic process, the minerals above mentioned.
The schistose structure is reverted to; garnet, pleonaste, corundum,
and tourmaline are present, but are very subordinate and confined to
the structure planes. Intersertal microperthite forms an accessory
it shows occasionally the Carlsbad twinning.

At the contact there Is the same relative poverty in metamorphic
products and probably for a similar reason to that adduced for the
last phase (spec. 130). The rock is a compact mass of quartz, chlori-
tized biotite, and pleonaste, with ilmenite, tourmaline, cordierite, and
corundum as accessories.

SERIES C OF SPECIMENS FROM THE METAMORPHIC AUREOLE.

A third suite of specimens illustrating the contact zone was col-
lected on the south side of the mountain. The phenomena are essen-
tially similar to those in Series A. It would be unprofitable to give a
detailed description of them, as it would be in the nature of repetition.
The perfection of the crystallization of corundum is, however, worthy
of special note. Within the belt 10 feet (3 meters) or more, measured
out from the contact, it is an abundant essential constituent of the
rock. Both basal and prismatic sections of the Idiomorphic and gran-
ular crystals exhibit cores of vivid blue in the otherwise colorless min-
eral. Each of these cores is oriented with the longer axis parallel to the
chief axis of the corundum individual. Rotated over the polarizer
alone, the basal section shows no change of color; in other sections
a striking pleochroism, from deep blue to colorless, is characteristic.

COMPARISON OF THE METAMORPHIC EFFECTS.

The constant nature of the metamorphism is indicated not only by
the correspondence of these three series of specimens, but also by
many specimens collected from points isolated in the contact zone and
not connected in the serial arrangement of a cross section. Wherever
the comparison could be fairly instituted, the alteration of the phyl-
Utic rocks was seen to be of the same character, whether produced by
syenite, granite, or gabbro-diorite. It is another illustration of a now
familiar fact — that so widely divergent intrusive types as granite,
syenite, diorite, diabases, and peridotite may form similar types of
hornfels.'*

The general order of the metamorphic effects to be observed as one
approaches the contact may be stated as follows :

At 500 feet or less from the contact there begins to be apparent a
distinct loss of schistosity in the phyllite. The rock gains in massive-
ness and in specific gravity. The extent of these changes, as of all

«Cf. Lacroix, Comptes Rendus, 18 fevr., 1895.

Bull. 209—03 3

34 GEOLOGY OF ASCUTNEY MOUNTAIN, VERMONT.^ [bull.209.

the others, is manifestly controlled by the nature of the particular
phyllitic band studied. The dominant mineralogical essential of the
aureole is cordierite, which appears suddenly and abundantly in the
outer part of the aureole. Second in importance to that mineral is
j)leonaste, assuming greater quantitative importance and greater size
and perfection of crystal form in its individuals as the contact is
neared. Metamorphic biotite is likewise abundant. Its increase
means, as a rule, a decrease of pleonaste in that particular phase.
Corundum behaves like the spinel in its progressive development
toward the contact, but appears later in the section. Sillimanite is
confined to the inner part of the zone, but is never abundant. Garnet,
tourmaline, and andalusite (?) are sporadic, appearing and disappear-
ing irregularly in the cross section, though more likely to appear at
its inner extremity.

Feldspathization characterizes the aureole as far, at least, as 300
feet (91 meters) from the intrusive. It is higlil}^ probable, however,
from the evidence of the hornfels analyses and of the microscopic
examination, that the transfer of material from the stock magmas to
their country rock is but subordinate in quantity. The mere heat of
the intrusion would doubtless have been sufficient to iDroduce some
of the more important new minerals. Cordierite and spinel in abun-
dance have been formed in coal-bearing mica-slates (schists) through
the melting up of the slates during the combustion of the coal.*
These minerals have also been observed to be the result of the altera-
tion of the micaceous inclusions caught up by volcanic flows. The
question as to just how much material has been added to the schists,
either as alkaline silicate or in other form, can, however, not be satis-
factorily and finally discussed until the same phyllitic band has been
followed across the aureole and analyses been made from that band
where it is unaltered as well as where it has been strongl}^ metamor-
phosed. So far it has proved apparently impossible to follow any one
band across the whole zone. The only analysis j^et made of the unal-
tered schist relates to a quartzitic phase outcropping at a distance
from the mountain (see Table I). 11" all tlie soda and potash in even
that i^hase were to enter into the jn'oper combinations with the alumina
and silica as much as ten jyev cent or thereabouts of alkaline feldspar
would result. The belief that feldspathization has really occurred
would thus be more strongly upheld by the peculiar nature of the
actual microperthitic intergrowth, so similar in every respect with the
feldspar of the adjacent syenite, and by the nonoccurrence of that
intergrowth in the phjdlite outside the aureole, rather than by the evi-
dence derived from the analysis of an unaltered phyllite more argil-
laceous than the quartzitic phase.

A list of the metamorphic constituents of the aureole, arranged in
the relative order of abundance as nearly as maj^ be by mere inspection
of thin sections, may complete this brief summary.

wLacroix, Les enclaves des roches volcaniques, Macon, 1893, p. 577.

DALY.] SUMMARY OF METAMORPHIC EFFECTS. 35

List of metamorphic constituents of the aureole.

In the limestones.
Epidote.
Grosstilarite.
Scapolite.
Hornblende.
Titanite.

In the phyllites.
Cordierite.
Biotite.
Pleonaste.
Corundum.
Lime-iron garnet.
Sillimanite.
Pyrrhotite.
Pyrite.
Tourmaline.
Andalnsite (?).
Graphite (?).

Microperthite

Quartz

Brown hornblende

Biotite

Introduced sub-
stance of the "pseu-
do-miaroles " and
intersertal areas.

There are strong resemblances between this list and those made
out by G. H. Williams at the Cruger's section (diorites of the Cort-
landt series metamorphosing biotite-muscovite schists),* and by Teller
and Von John in the Tyrol (norites and diorites cutting phyllites and

gneisses).^

SUMMARY.

The schists display unequal effects of contact metamorphism where
they lie in contact with the intrusive bodies. As was to be expected,
the gneisses are much the more stable and exhibit little mineralogical
change even close to the eruptive contacts, but the abundant argil-
lacous material of the phyllites has been extensively recrystallized
into a well-defined zone of hornfels. Cordierite, pleonaste, biotite,
garnet, corundum, and epidote form the chief secondary minerals thus
developed. The limestone bands have richl}' yielded grossularite and
epidote in the same contact zone. Repeated occurrences of intersti-
tial microperthitic feldspar lead to the conclusion that, during the
intrusion of the syenites and granite, feldspathization of the phyllitic
country rock has taken place.

« Am. Jour. Sci., 3d series, Vol. XXXVI. 1888, p. 254.

6 Jahrbuch K.-k. geol. Reichsanstalt, Vol. XXXII, 1883, pp. 655 and S.

CHAPTER IV.

THE ERUPTIVE ROCKS.

GENERAL TABLE AND CORRELATION.

It was thus into a series of tilted metamorphosed sediments that the
irruptions with whicli we are here particulai-ly concerned began (see
PL VII). The variety, as well as the relative ages of the resulting
rock bodies, is indicated in the accompanying table, which gives a sum-
mary statement of the succession, from the oldest to the youngest
intrusive:

Table III. — Rock bodies resulting from the irruptions.
(From oldest to youngest.)

A. Basic stock of five chief phases, viz:

a. Augite-gabbro.

b. Hornblende-biotite-augite-gabbro.

c. Biotite-hornblende-diorite.

d. Biotite-aiigite-hornblende-diorite.

e. Orthoclase-microperthite-bearinghornblende-biotite-diorite (containing

basic segregations) = acid essexite.
This stock is cut by —

1. Reticulate intrusions (forming intrusion-breccias) of augite-biotite-diorite

with and without essential hornblende.

2. Dikes of " windsorite."' the alkaline equivalent of granodiorite.

3. Nordmarkite porphyry stock-like dike of Little Ascutney (bearing basic

segregations and cut by Nos. 6 and 7).

4. Main stock (B) of Ascutney Mountain and its apophyses.

5. Pulaskite (quartzless biotite-nordinarkite) stock of Pierson Peak.

6. Hornblende-ijaisanite dike (cut by camptonites).

7. Caniptonite dikes.

8. Diabase diiies (?) .

B. Main stock of Ascutney Mountain, of four chief phases, viz:

f. Hornblende-biotite-nordmarkite of granitic structure (bearing basic

segregations) .

g. Hornblende-biotite-atigite-nordmarkite of porphyritic structure (bear-

ing basic segregations),
h. Alkaline granites without essential bisilicates.
i. Monzonite.
This stock is cut by—

9. Hornblende paisanite dikes.

10. Camptonite dikes.

11. Diabase dikes.

12. Common muscovite aplite.

13. Stock C.

C. Stock of alkaline biotite-granite (bearing basic segregations) .

36

DALY.] CORRELATION OF THE INTRUSIVE ROCKS. 37

A general correlation of these Intrusives in terms of eruptive periods
and composition maj^ be made in the form of a second table :

First eruptive period A (a. b, c, d, e) ( r^ ■,-, -,■ ...

^■^,. .., '-, h Gabbro-dioritic magma, «
Second eruptive period 1 and 2 )

Third eruptive period B (f. g, h, i), 3 and 5 ) c. ■^^

-r^ ,. ,. ■ ■, „ ., ^ - Sy em tic magma.

Fourth eruptive period 6 and 9 )

Fifth eruptive period C and 12 Granitic magma and aplite.

Sixth eruptive period _ 7 and 10, 8 and 11 Lamprophyres.

The reference of the individual intrusives to the different magmas
or their derivatives is indisputable except in the case of No. 2, which
is intermediate both in composition and in geological age between the
Basic stock and the Main syenite stock. The reference to the differ-
ent eruptive periods as stated in the table is to some extent arbitrary.
The stocks A, B, and C certainly followed one another in the order
named. As the map shows, the conduit through which the substan-
tial contributions to the whole mass were made migrated from west
to east. The intimate field relations of A, 1 and 2, seem to show that
all three antedate the syenites. Nos. 1 and 2 cut A and are probably
older than B or its equivalent. Nos. 3 and 5 are con-elated with B
on the ground of close mineralogical and structural similarity; they
are clearly older than Nos. 6 and 9, which were probably not strictly
contemporaneous. Though stock C also cuts B, it is probably not
contemporaneous with No. 6 or No. 9. It is probable, though not
proved, that stock B is older than the lamprophyres, which are cer-
tainly younger than all the syenitic intrusions.

In all the stocks and in many dikes of the area, nodular masses of
segregational basic materials occur. They are most common in the
syenites, less so in the alkaline granite stock, and comparatively rare
in the diorities. These nodules were early noted and described by
Edward Hitchcock and others, and remarked for their extreme abun-
dance. They will be treated of in some detail in the following pages
in connection with the petrographical description of their parent
rocks;

It will be seen that among the petrographical methods emploj^ed,
those for the determination of the feldspars have been in the most
constant requisition. In order to avoid repetition and a certain degree
of monotony in the description, the actual readings on which the
determinations of the various species have been based are, as a rule,
not given in the text. In general, several independent methods have
been used for each determination. Examples of these are noted in the
discussion of the rocks comj)osing the largest stocks examined. In
the comi^ound names of some of the rock types, as well as in the tables
of mineralogical composition, the mineral constituents are entered in
the order of decreasing importance in their resi)ective rocks.

«A magma intermediate in average composition between gabbi'o and diorite. The adjective
is not derived from the term " gabbro-diorite," as iised by G. H. Williams or by Tornebohm.

38 GEOLOGY OF ASCUTNEY MOUNTAIN, VERMONT. [Bri.L.209.

GABBROS, DIORITES, ANB REI.ATED ROCKS.

BASIC STOCK; GABBRO AND DIORITE PHASES.

The oldest of the intrusives illustrates the common characteristic
of basic stocks in exhibiting considerable variation of composition,
structure, color, and texture in its rock types. A large number of
thin sections were made from specimens collected in all parts of the
stock. They show that its material occurs in the form of five differ-
ent phases, repeatedly occurring in more or less typical form, and
connected with one another by transitions. All five seem to have been
differentiated from the product of a single intrusion. That Ave have
here to do with an eruptive of a truly exotic nature is abundantly
proved at almost any point of the schist contact. Both to the north
and to the south of Little Ascutney excellent examples of intrusive
dikes and sills, plainly apophysal from the massive rock, are to be
found cutting the gneisses. The horses of schistose rock are so abun-
dant at many contacts as to make up veritable flow breccias, and frag-
ments are occasionally found several hundred yards from the contact.

Phase d is the one rock type from the Basic stock which has been
chemically analyzed. It is somewhat more acid than the average
type, but it was selected on account of its relative freshness. The
specimen analyzed (Table IV, col. 1) was taken from some blasted
ledges about 100 yards north of Mr. Pierson's house, on the notch road
between the two Ascutneys. It is a fairl}^ coarse-grained dark bluish-
gray quartz-diorite of tj^pical hypidiomorphic granular structure, in
which the essential dark-colored silicates are biotite and a diopsidic
augite with subordinate brown hornblende (spec. 35),

The feldspar of the rock is almost alwaj^s multiple-twinned, follow-
ing the albite law, more rarely the pericline law. The ver}^ common
association of albite and Carlsbad twinning in the slide makes it pos-
sible to determine the feldspar with a high degree of accuracy. In
addition, Becke's method of differential single refraction, the reading
of extinctions on cleavage pieces and on sections cut parallel to
the bissectrices, and the principle of equal illumination in the zoned
individuals agreed well in establishing the average mixture of the
soda and lime molecules as one slightly more acid than the basic oli-
goclase. Aba -^^i- ^^^ single individuals maj^ vary from the acid
oligoclase Abj Aiij to bj'townite Ab, An4. Zoned crystals are com-
mon. The cores range from Abj An4 to Abj Auj, with an average close
to the andesine Ab4 Aiig, while the outer zones seem to be invariably
more acid, with an upper limit at the acid oligoclase Ab^ Auj.

The tolerabl}^ high percentage of potash in the analysis leads one to
suspect a potash feldspar, but diligent search has so far failed to
establish the i^resence of either ortlioclase or microcline. The refrac-
tion of the rare untwinned individuals was carefully compared in a
number of slides with the refraction of quartz and undoubted plagio-

DALY.] PHASES OF THE BASIC STOCK. 39

clase, and showed, often with the corroboration of convergent light,
that these untwinned individuals are likewise soda-lime felds^jars
averaging basic oligoclase. The determination of orthoclase in the
rock powder is impossible because the basic oligoclases and acid
andesines occur in such profusion. The potash of the analysis must,
then, be referred primarily to the biotite, and, in a notable degree, to
isomorphic mixture with the soda-lime feldspar. That there must be
some potash outside of the biotite is not to be doubted, for, granting
the high proportion of 10 per cent of potash in the biotite, and credit-
ing that mineral with all the oxide, there must be at least 23.4 ]3er
cent biotite in the rock. Inspection shows that even tliat minimum
proportion of the mica is not rex3resented, although it is next the feld-
spar in abundance. It has the usual properties of biotite from normal
granitic rocks — powerful pleochroism and absorption in brown and
yellow tones, extremelj^ small optical angle, and parallel extinction.

The augite is crystallized both alone and in the form of intergrowths
with hornblende. In both cases the habit of the mineral is that of
common diopsidic, colorless to pale greenish, allotriomorphic indi-
viduals from 1 to o millimeters in diameter. A third cleavage parallel
to (100) occurs in a few sections. Twinning parallel to (001) is not
rare. Pleochroism is absent. The alteration is largely confined to
chloritization, but paramorphic changes to a uralitic amj)hibole are
common.

The brown hornblende almost invariabl}^ forms intergrowths, either
irregular or oriented in parallel fashion with augite. In all cases the
mineral is doubtless X)rimary. The pleochroism is as follows :

a. Pale grayish j'ellow.

b. Greenish brown with a tinge of olive (medium absorption).

c. Brown (medium to strong absorption).
c>b>a.

The prismatic angle is 55° o2' (average of measurements ou eight
cleavage pieces). The extinction on (110) is al)out 12° 30', and on
(010) about 15°. It is seen that the mineral belongs to a common
variety of araphibole.

Quartz forms allotriomorphic, cementing grains which represent
the last stage in the crystallization of the rock. It is never present
in large amount. Gas and liquid inclusions, often simulating nega-
tive crystals in form, are common, particularly in the coarser-grained
specimens of the rock.

Apatite needles and larger crj^stals are abundant. A characteris-
tic feature of this accessory is the common inclusion, in the form of
elongated cores, of isotropic, probably glass, bodies of a deep-brown
color (PI. Hi, ^.)

Ilmenite or titaniferous magnetite and iDrimary titanite with weak
pleochroism are important accessories. Large zircon crystals and
occasional grains of pyrite complete the list of accessories.

The structure is hydiomorphic-granular. The feldspar is often

40 GEOLOGY OF ASCUTNEY MOUNfAUSt, VEEMONT. [bull. 209.

idiom Orphic against both augite and quartz. The biotite is always
idiomorphic against quartz, rarely against feldspar, and may inclose
all the other constituents in poikilitic fashion. The augite is usually
intersertal with reference to the feldspar and incloses all the accesso-
ries. The sequence of crystallization apj)ears to have been (in order
from the oldest to the youngest mineral) as follows :

Apatite.

Titanite, zircon and ilmenite.

Feldspar. |

Angite (and hornblende) . [ Nearly contemporaneotis.

Biotite. J

Qnartz.

The composition of the biotite, augite, and hornblende not being
known, it is not possible to calculate the analysis. The analysis
agrees with the microscopic examination in placing this rock decid-
edly in the class of true diorites, as a hiotiie-augite-hornblende-diorite.
For ease of reference, columns 2 and 3 are entered in Table IV in
order to show the similarity between our rock and a fair average
diorite and also the limits of chemical variation which can be found
in a list of typical analyses from that rock group. From its field
associations one might expect the Basic stock to have given a higher
proportion of alkalies in its average phase. For this reason column
4 has been added to point the dissimilarity between the Ascutnej^
rock and essexite, the alkaline type nearest to it in general habit.
The absence of monoclinic feldspar and of olivine among the con-
stituents, and the relatively low proportion of soda and of ferric iron
compared to the essexite, serve to dispel the a priori notion that the
alkaline stocks on the east should be accompanied, in their common
conduit, by an intrusive of essexitic or allied alkaline habit if that
intrusive were to be a plagioclase rock of basic character.

Phase c. — A second phase intimately allied to tlie first, both in field
relations and lithological characters, is represented in a long series of
outcrops lying southeast of Mr. Pierson's house on the Notch road.
Two sj)ecimens of the brownish-gray, medium-grained rock were
selected at a point 500 yards distant from the house, and proved to
be a normal hiotUe-hornblende-diorite, with structure and accessories
similar to those in phase d (spec 32). The feldspars generally show
two zones of growth ; the cores average a labradorite between Ab^ Auj
and Abj Aug, the narrower mantling zone averaging the oligoclase Abg
Auj. The average soda-lime feldspar is probably close to that of the
analyzed phase. The chief difference from the latter rock lies in the
replacement of the augite by an idiomorphic brown hornblende of
much deeper absorption than that characterizing the intergrown horn-
blende of phase d. Here the scheme of absorption is : a. Yellow, b.
Deep brown, with a suggestion of olive-green, c. Deep chestnut-
brown. c>b>aorc = b>a.

One or two large subidiomorphic individuals of nearly colorless

DALY.]

PHASES OF THE BASIC STOCK.

41

augite have been found in the slides. In each case a broad mantle of
the deep-brown hornblende surrounds the augite in x)arallel inter-
growth. The biotite is of the same nature as in phase d, but much
less abundant and seldom poikilitic.

Table IV. — Analyses of diorites and essexite.

1.

■ -i

3.

4.

SiOj-. _ -

53.13

16.35
3.68
6.03
4.14
7.35
3.65
2.34
0.88
0.35
0.07'
3.10
0.03
0.89
0.09
0.03
0.34

Trace.
0.17
0.04

Trace?

Trace.

56. 53
16.31
4.38
5.93
4.33
6.94
3.43
1.44

1 1.03

53.00-63.80
13. 41-18. 00
0.77- 7.43
2.41-12.84
2.03- 8.03
4. 99- 8. 98
3.31- 4.65
0.44- 2.37

0.16- 2.24

47.94

ALO,

17.44

FeaOs

6.84

FeO

6.51

MgO

2.02

CaO

7.47

Na^O:

5.63

K2O .

2.79

H^O above 110" C

H2O below 110' C---.

2.04

CO2

TiO.

0.35

0.03- 1.10

0.20

ZrO^- - _._ _

P2O5

0.40

0.17- 1.06

1.04

CI

F

FeS^

NiO, CoO

MnO

0.14

BaO

SrO

Li,0

100. 33
0.03

100. 98

99.93

0=F, C1-- -

Totals

100. 30
0.13
3.936

Sp. ^T _

1. Biotite-augite-liornblende-diorite, Basic stock, Ascutney Mountain. Analysis
by Hillebrand.

2. Average of a series of 16 typical diorites, compiled by Brogger, Die Eruptiv-
gesteine des Christianiagebietes, Vol. II, 1895, p. 37.

3. Limits of variation in the above-mentioned 16 analyses.

4. Classic essexite, Salem Neck, Salem, Mass. Analysis by Dittrich.

Phase b. — At various points in the stock, especially west and south
of Pierson Peak and near the crest of Little Ascutne^^ ridge (spec. 61),

42 GEOLOGY OV ASCUTNEY MOUNTAIN, VERMONT. [buu.209.

the rock becomes coarser than in either of the two phases just
described and shows a distinct difference of composition from either.
At the same time, repeated examination of ledes in the field could
discover no difference of age among* the three. This third phase is
dark colored and remarkable for its richness in bisilicates, which
have a strong poikilitie habit. Augite and biotite of the general char-
acter of those minerals in jjhase d, and large independent crystals
with augite intergrowths of brown hornblende similar to that in phase c,
are the essential dark-colored constituents. The biotite has, how-
ever, an optical angle considerably greater than elsewhere observed
in the stock; it was measured and found to be a few minutes more
than r.

A long series of feldspar determinations accorded with one's first
impression of the rock in stnd^'ing the hand specimen, that it belongs
to a phase much more basic than c and d. The prevalence and large
size of the Carlsbad albite twins and the uniform behavior of the
feldspar enables us to state conclusively that the average feldspar in
this phase is close to basic labradorite, Ab.jAug, with a narrow range
above and below the acidity of that mixture. Primary quartz is
entirely absent. Nearly colorless titanite is especially abundant, both
alone and surrounding the large ilmenites after the manner of
leueoxene.

The basicity of this phase unquestionably places it among the gab-
bros, and it may be called a liornblende-hiotite-augife-gabbro, not-
withstanding the absence of true diallage among the constituents.'*
It probably rivals i)hase d in the amount of surface covered in the
stock.

Phase a. — In the fields west-southwest of Pierson Peak a fourth
variant of the rock outcrops in the form of an unusirally coarse-
grained type (spec. 112). It is more feldspathic than phase h and
hence of a lighter color. Biotite and hornblende have almost com-
pletely disappeared, the former being a rare accessory, the latter
forming occasionally a mantle about augite. The light reddish-brown
feldspar is again very uniform and averages the basic labradorite,
AbiAuj. The usual accessories ai-e present excepting quartz. The
structure is the hypidiomorphic granular, but on account of the inter-
sertal relation of the augite to the feldspar, it assumes the special
habit of diabase. The poikilitie nature of the augite is very striking.
The phase has the composition and other characters of a typical
diabase excepting irr its geological occurrence. It may be called an
auyde-gabhro, though diallage is here, too, wanting.

Plidse e. — [•"'inally, a fifth j)hase remains to be noted, which is not
important on account of the amount of area covered by it in the stock
as a whole, but which merits particular attention on account of its

"We can not biitagree with Judd (Quart. Jour. Geol. Soc.Vol XLII,1886) and Lacroix (Bull.
Sferv cartt g^eol France, No. 67, Vol. X, 18S9, p. 27) in regarding it a.s indifferent, for purposes
of nomenclature, whether the ijyioxene of a gabbro possess the diallagic structure or not.

DALY.] BASIC SEGEEGATIONS. 48

forming a transitional rock type between the true gabbros and dio-
rites on the one hand and the alkaline rocks of the region on the
other. This phase was discovered just north of the contact between
the stock and the great syenite-porphyry dike of Little Ascutney,
and opposite the middle of that dike. The rock is fairly fresh and
tolerably coarse grained, and is rich in feldspar, brown hornblende,
and biotite (spec. 59). Augite forms in a few rare instances small
cores of hornblende intergrowths. The accessories common to all the
phases are present, and, in addition, some free interstitial quartz.
But the feldspars are in great contrast to those so far noted as occur-
ring in the stock. Plagioclase, averaging near the andesine AbgAug
is dominant; oligoclase ranging between AbgAn^ and AbjAnj is com-
mon. The plagioclase is sometimes surrounded by a mantle of ori-
ented microperthite. What is still more noteworthy is the existence
of much free orthoclase and microperthite alongside the triclinic
feldspar. The order of crystallization is as follows:

Apatite.

Titanite, zircon, and ilmenite.

Angite, hornblende, and biotite.

Oligoclase-andesine.

Orthoclase and microperthite.

Quartz.

The structure of the rock is the usual hypidiomorphic granular.
Its true relation to the diorites is indicated if we call it an orthoclase-
microperthite - hearing lioriiblende - biotite - diorite. Yet the rock is
clearly allied to a somewhat acid form of essexite.

BASIC SEGREGATIONS.

In phase e, at the locality indicated, basic segregations were spar-
ingly found (spec.59a). These are of distinctly darker color than the
parent rock, and both macroscopically and microscopically are seen
to be finer grained. They are roundish in form and average about 1
inch (2. 6 mm. ) in diameter. The boundary between nodule and parent
rock is not definite ; they merge into each other in a gradual way. The
nodules are essentially composed of j)lagioclase, hornblende, and bio-
tite grouped with a panallotriomorphic structure. The feldspar is
zoned; the most acid zone is the oligoclase, Ab^Auj, and the average
feldspar is near labradorite, AbjAuj. No certain orthoclase or micro-
perthitic feldspar could be identified in the slide. The dark constit-
uents have the usual properties of those minerals m this stock. The
augite is more abundant here than in the parent rock, though again
generally it occurs in the form of intergrowths with the hornblende.
Zircon is very rare, but apatite unusually abundant. A little inter-
stitial quartz is accessory.

44 GEOLOGY OF ASCUTNEY MOUNTAIN, VERMONT. [bull. 209.

Table V. — Analysis (by Hillehrand) of hornblende-biotite-diorite nodule.

Per cent.

SiO^ - 55.28

AI2O3 17.33

Fe^Og 1.54

FeO 6.23

MgO 2.69

CaO 5.60

Na^O 5.42

K^O 2.12

H^OabovellO" C 0.71

H,ObelowllO°C _• 0.20

CO2 0.04

TiO, 1.64

ZrO, Trace.

PA : 0.73

CI 0.07

F 0.28

FeS^ - 0.07

MnO 0.24

BaO :^ 0.06

SrO Faint trace.

LijO Trace.

100. 15
0=F,C1 --- 0.13

100. 02

Totals 0.038

Sp.gr . 2.822

The analysis of one of these nodules agrees with the microscopic
diagnosis (except in the matter of structure) in placing it in classi-
fication among the hornblende-biotite-diorites (Table V). The high
soda relates the nodule to essexite. It is more basic than its host,
though more acid than the diorite analyzed (j)hase d.) The high
fluorine is again noteworthy.

DIORITIC DIKES CUTTING THE BASIC STOCK.

Following the consolidation of the Basic stock the same magma
which it represents seems to have been erupted a second time, and as a
result we have networks of interlacing dikes in various parts of the
stock. These are oftentimes so numerous as to give the bare ledges
the appearance of mosaics on a large scale. Occasionally the younger
intrusive has so extensively displaced the older as to form miniature
stock-like bodies sending out apophyses into the coarser rock and
inclosing horses of the latter. The mosaics are thus intrusion-brec-
cias or flow-breccias. The younger intrusives cut with apparent
indifference both the gabbroitic and the dioritic phases of the stock.

In color, mineralogical and chemical composition, and even in

DALY.] DIKES CUTTING THE BASIC STOCK. 45

structure, the dikes are closely allied to the diorite analyzed. In
the fields southeast of Pierson Peak both the younger and older
rocks are augite-biotite-hornblende-diorites (spec. 147). Within a
hundred yards a second network of dikes is characterized by the com-
plete absence of hornblende and by a remarkably perfect zonal struc-
ture in its feldspar (spec. 14oa). The great range in acidity of these
feldspars was not found equaled in any other rock of the whole region.
By the method of equal illumination, checked by the behavior of each
zone in convergent light, it could be proved that the core of such a
feldspar may be a true anorthite. Outside the core basic labradorite
near AbjAug is succeeded by a third zone of oligoclase near AbgAnj,
and outside of all there comes a narrow zone of the albite Ab^gAn^. The
average of several determinations on Carlsbad albite twins gave basic
oligoclase, Ab^Anj, as the average feldspar of the rock. The usual
accessories are present. This rock is a typical augite-biotite-diorite.
A very similar type occurs in the form of a series of parallel dikes
on the northern slope of Little Ascutney and at its eastern end (spec.
184). Here a significant amount of orthoclase was discovered among
the accessories. In immediate association with the first group of
reticulate dikes mentioned, another group of a lighter color but of
similar structure showed in the microscopic examination a still greater
proportion of orthoclase, which is accompanied by microperthite. The
bulk of the feldspar is still, however, near the basic oligoclase AbgAn^.
Biotite is the only other essential. Augite fails and brown hornblende
is a rare accessory. The type may be called a hiotite-diorite bearing
accessory orthoclase and microperthite.

"WINDSORITE" DIKES CUTTING THE BASIC STOCK.

Potash-feldspar finally becomes of nearly equal importance among
the essential minerals of these rocks in a set of light-colored, pinkish-
gray dikes 1 to 3 feet (0.3 m. to 0.9 m.) in width; traversing the stock in
the notch just northeast of the easterii end of Little Ascutney (spec.
77), and again on the notch road near Mr. Pierson's house. The
dikes at the former locality seem to have been cut off by the jjorphyry
occurring on Little Ascutney. The plagioclase varies from andesine,
AbgAug, to oligoclase, AbgAuj, the average mixture being probably
basic oligoclase, AbaAn^. The triclinic feldspar is often surrounded
by mantles of orthoclase or microperthite. Orthoclase, microperthite,
and, probably, soda orthoclase, especially the first two named, are the
alkaline feldspars, the abundance of which is reflected in the chemical
analysis of this rock type (Table VI, column 1).^' Shreds, irregular
plates, and, rarely, idiomorphic crystals of biotite represent the only
other essential. Rare grains of augite and still rarer bleached indi-

a Through a mistake, a fragment of diorite from the younger dikes was included in the sample
of this rock sent to Washington for analysis. This unfortunate fact explains the difference
between column K (the vitiated analysis) and column L on page 69 of Bulletin 148 and on page
g5 of Bulletin 168 of this Survey.

46 GEOLOGY OF ASOUTNEY MOUNTAIN, VERMONT. [bull. 209.

viduals of hornblende with ilmenite, apatite, zircon, and (doubtfully)
titanite, compose the list of accessories. Py rite developed secondarily
on the joint planes explains the sulphide of the analysis. The mica
must be rich in magnesia and is probably a meroxene. Quartz occurs
interstitially in comparatively large amount. The bisilicate and biotite
exhibit a great amount of magmatic resorption, and it is therefore
difficult to be certain of the order of crystallization. It is ]3robably
as follows :

Apatite.

Zircon.

Ilmenite and titanite.

Biotite, aiigite, and hornblende

Andesine and oligoclase.

Micropertliite, orthoclase, and soda orthoclase.

Qnartz.

No analysis of the mica has been made. On account of its small
amount in the rock, no serious error in the calculation of the other
and more important essentials will be made if we assume that in the
biotite there is 20 per cent MgO, 40 per cent SiOg, and 8 per cent K2O.
On this supposition, the quantitative mineralogical composition of the
rock was calculated as follows:

Mineralogical composition of ivindsorite.

Per cent.

Albite molecule 38. 5

Orthoclase molecule . - 28. 5

Anorthite molecule 11.5

Quartz 13.0

Biotite 5.0

Magnetite and ilmenite 2.5

Diopside, apatite, and zircon 1.0

If the average plagioclase = AbjAn^, the rock contains 34.5 j)er cent
soda-lime feldspar and 44 per cent alkali feldspar.

DALY.] DIKES CUTTING THE BASIC STOCK.

Table VI. — Analyses of windsorite and other rocks.

47

1.

2.

3.

4.

.5.

SiO,

ALOo

64.62
16.46
1.82
2.14
1.10
2.39
4.57

67.14
15.37
2.24
1.93
1.36
3.60
3.29
4.06
0.59
0.07

65. 00
16.00
1.50
3.00
2.00
5.00
3.50
2.25

59. 00-68. 50

14. 00-17. 00

■ 1.50-2.25

65.65

16.84

Fe^O, . _

1

FeO

!^ 4 01
1.50- 4.50 1

MgO

CaO

Na^O

1.00- 2.50 0.13
3.00- 6,50 : 2.47
2. 50- 4. 50 ' 5. 04

K2O

5.21
0.39

1.00- 3.50 1 5.27

H,0 above 110° C

1 0.30

H,0 below 110° C

CO2

0.13
0.11
0.81
0.03
0.21
0.05
0.19
0.12
0.03

Trace.

Trace.

Trace.

.

TiOa

ZrOj

.

P,0=

CI

FeS^

MnO

BaO

SrO

Li^O

CuO

Remainder.

0.35

1.75

100. 38
0.01

100.00

100. 00

99.71

0— F,C1

100. 37
0.10

Total S

Sp.gr

2. 666

1. Dike of windsorite, Little Asciitney Mountain; analysis by Hillebrand.

2. Average of two typical quartz-monzonites, from the Sierra Nevada. Tnrner,
Jour. Geol.. Vol. VII, 1899, p. 152.

3. Average composition of granodiorite, according to Lindgren. Seventeenth
Ann. Rept. U. S. Geol. Survey, Pt. II, 1896, p. 35.

4. Limits of variation in granodiorite, according to Lindgren. Ibid., p. 35.

5. Alkaline augite-hornblende-syenite (nordmarkite), Diana, New York; ana-
lyzed by C. H. Smyth, Bull. Geol. Soc. Am., Vol. VI, 1895, p. 274.

Table VI represents type analyses of related rocks. Of these and
of the Ascutney dike the essential mineralogical composition is as
follows:

1. Basic oligoclase, microperthite, orthoclase, quartz, biotite.

2. Oligoclase, quartz, orthoclase, biotite, amphibole.

48 GEOLOGY OF ASOUTNEY MOUNTAIN, VERMONT. [bull. 209.

3. Oligoclase-andesine (usually andesine) , quartz, orthoclase, biotite, green horn-
blende.
5. Microperthite, albite, augite, hornblende.

This rock belongs to another type intermediate between the ortho-
clase rocks and the plagioclase rocks. It is almost identical in chem-
ical com]30sition with certain nordmarkites (cf. col. 5), but the lime
is practically all in the highly important essential, basic oligoclase.
This character definitively removes the rock from the nordmarkites.
We can not, on account of the high alkalies and relatively low lime,
place it in the group of the granodiorites (cf. cols. 3 and 4), nor, for
the same reason, in the group of the quartz- monzonites (cf. col. 2),
though in general the affinities are stronger with the last-named group
than with any other already well-defined type. The soda and the
combined alkalies are too high to characterize a normal lime-alkali
quartz-syenite. The rock is, in reality, a leukocratic analogue of the
quartz-monzonites in which augite is replaced by biotite. It may also
be considered as the alkaline equivalent of granodiorite. Standing in
a class by itself, both with respect to the other Ascutney intrusives
and with respect to the types now recognized in our rock classifica-
tions, the name windsorite is proposed for the rock in order to fix this
type and to facilitate reference to it. The name is taken from that
of the neighboring town northeast of the main mountain. Windsor-
ite may be defined as a leukocratic, hypidiomorphic-granular rock,
composed essentially of alkaline feldspar (microperthite and ortho-
clase), basic oligoclase, quartz, and biotite, and characterized by high
alkalies (potash slightly in excess of the soda), relatively low lime
(contained essentially in the plagioclase), low iron, and low magnesia.

SYENITES.

The Basic stock is cut by several large independent bodies of syenitic
habit. Their rocks are so similar in compositon that, as in the case of
the dioritic rocks, a detailed ]3etrographical description of a chief
phase in one of the bodies will suffice to illustrate the larger part of
what may be said in description of the other phases and related intru-
sives. In this way some repetition may be avoided.

MAIN SYENITE STOCK; ITS PHASES.

As we have already seen, Mount Ascutney owes its strong relief
to the largest intrusive mass in the area discussed — the Main syenite
stock covering about 4 square miles (10. 5 square kilometers) . The intru-
sive character of the rock is plainly indicated at almost any j)art of the
contacts with the diorites, gneisses, or phyllites (see PL VI). The walls
of the conduit appear to be usually nearly vertical, inasmuch as the line
of contact in all but two or three cases runs straight across the radiat-
ing gulches and does not turn up or down the corresponding brook

DALY.j MAIN SYENITE STOCK. 49

beds. The latter rule is departed from at three of the largest ravines in
the mouiitain. At Crystal Cascade the schists stand vertical or dip at
high angles to the east-northeast. They form a blunt projection into
the igneous body, and, on account of their relative softness, the strong
gulch below the cascade has been worn out. The actual surface of
contact between schist and syenite is exposed for a vertical distance of
100 feet. That sample contact is nearly vertical. A second deep
ravine, 1 mile east of the cascade, may be explained as located on
a similar broad tongue of schist less resistant to the weather than the
syenite to right and left of the ravine. At only one point does the sur-
face of contact seem to depart from verticality, namely, at the pictur-
esque ravine south of Brownsville. There the schists dip under the
syenite as if the latter had, during intrusion, followed the planes of
schistosity after the manner of a sill or laccolith. But this observa-
tion stands alone, and such a structural relation must be regarded as
exceptional and very local.

The syenite showed a general independence of the structure of the
invaded rocks as it found its way up from its deep-ljang source. Both
to north and to south of the mountain the schists strike steadily toward
the stock. They are not essentially displaced from their original tilted
position, except as a result of some relativelj^ slight crumpling in
the contact zone, but are cut squarelj^ off by the syenite. This is true
at the eastern contact as well. At several points the contact plane
was observed to cut across the structural plane of the phyllites. If,
however, the intrusion had been controlled hy the latter, we should
expect the surface of contact to dip eastward and the zone of meta-
morphic change in the schists to be broader there, at the easteim end,
than elsewhere. The facts do not agree with either conclusion.

The syenite thus constitutes a pipe-like stock of roundish outline,
the cylindrical form being modified by a few large projections of
schist in place and b}'^ the irregular stock of the younger Ascutney-
ville granite. -

A notable characteristic of the Main stock, as of the older one,
is the variability of the rocks composing it. Though they are every-
where related to the group of the alkaline syenites, thej'^ exhibit
important mineralogical, chemical, and structural differences. Four
chief types of the variations in color, grain, structure, proportion of
dark-colored to light-colored constituents, and the distribution of
inclosed basic segregations, are to be distinguished. In the field the
transition of these types into one another is so complete that they
must be regarded as the differentiated product of one body of magma.
As yet there is no certain observation forthcoming to show that there
was more than one eruptive period for all four, even in the sense of
the intimately associated diorite and reticulate dikes of the Basic
stock, or in the sense of Brogger's hypothesis of the cutting of still
unconsolidated augite-syenites by elseolite-syenite. It will be remem-
Bull. 209—03 4

50 GEOLOGY OF ASCUTNEY MOUNTAIN, VERMONT. [bull, 209.

bered that Brogger introduced that hypothesis in order to explain the
field association of the Christiania rocks.'*

Like the still younger granite, the syenite has, in general, a finer
texture than the gabbro-diorites. This contrast is to be related to
the greater basicity of the latter rather than to any essential differ-
ence of physical conditions under which the intrusion of the basic
and acid stocks occurred.

Phase f, nordmarkite of granitic habit. — The only quarry that has
recently been worked in the Ascutney area is situated within a few
hundred feet of the contact with the schists in the first of the four
phases of the Main syenite. Various attempts have been made to use
the stone for monuments and for ornamental purposes generally, but,
for a reason which will be noted further on, a market could not be
permanently secured by the owners. The quarry seems to have been
practically abandoned. The finest blocks yet taken out are doubtless
those which are to be seen in the large columns of the library build-
ing at Columbia University, New York City.

The rock, as represented in the quarr}^, is a handsome, dark-green
sj'enite, in this iDlace characterized hy medium to coarse grain and a
typical eugranitic structure (spec. 42) ; elsewhere this i^hase grades
into one possessing a trachytic structure. It is a syenite with variable
amounts of free quartz and a low percentage of colored constituents.
Primary veins or flow streaks are common; they are usually finer-
grained than the average rock, and are even more poorly ]3rovided
with bisilicates. In addition to the feldspars and accessory quartz,
the list of minerals includes, in the order of their abundance, a horn-
blende, biotite, a pyroxene, allanite, titaniferous magnetite, apatite,
pyrite, zircon, monazite, and a lime-iron garnet. The order of their
crystallization seems to have been as follows:

Apatite.

Zircon.

Magnetite, pyrite, garnet.

Monazite and allanite.

Augite, hornblende, and biotite.

Oligoclase.

Alkaline feldspars.

Quartz.

The feldspars. — The constituents which determine the structure,
texture, and color of the syenite are the feldspars (PI. IV, A.) Of
these, microperthite is by far the most abundant, and with it are
associated orthoclase, soda-orthoclase, microcline, and a plagioclase.
There is no observable difference in the macroscopic habit of these
feldspars, and it was only by the careful study of slides and rock
powder that all the species could be determined. All of them are
undoubtedly the product of primary crystallization.

oZeit. fur Kryst., Vol. XVI, 1890, p. 281.

DALY.] MAIN SYEmTE STOCK. 51

The microperthite is especially interesting on account of its typical
development. The usual intergrowth is that of orthoclase with a
plagioclase varjang from albite to an acid oligoclase near AbgAn^,
but on many cleavage pieces it was easily proved that the triclinic
feldspar was intergrown with a monoclinic, itself strongly charged
with the soda molecule. Such individuals gave extinctions of 12° and
19° on (010) for the two kinds of lamellae, thus indicating the associa-
tion of nearly pure albite with an orthoclase that stands at the extreme
soda end of the series, generally designated by the name "soda-
orthoclase." In all cases the intergrowth follows the law whereby
the triclinic lamellae lie in the monoclinic feldspar parallel to a steep
orthodome; the angle of 72° between the albite lamellse and the basal
cleavage on (010) indicates that this dome may be (801), the one noted
in this relation to intergrowth by Brogger. From the normal micro-
perthite there are all transitions to what would appear to be true
cryptoperthite. Both ends of the series sometimes show the mui-chi-
sonite parting, which is unusually clean and definite.

The tabular crystals of well-lamellated microperthite from a highly
feldspathic phase on the Brownsville slope of the mountain represent
a very high proportion of soda in the mixture, as shown by the polar-
ization phenomena and the specific gravity of from 2.610 to 2.611 at
17° C. Generally, however, the proportion of potash to soda is about
1:1, corresponding to a specific gravity of from 2.584 to 2.595 at the
same temperature. The same average ratio is believed to character-
ize the feldspars of: the rock as a whole. It is true that there is a not
unimportant amount of orthoclase and soda-orthoclase in most of the
slides, yet this lowering of the otherwise high percentage of soda is
occasionally counterbalanced by a little free oligoclase and always by
a microperthite which is richer in the triclinic component than the
average stated.

The pure potash feldspar is relativelj^ rare. It occurs as orthoclase
and as microcline, both contemporaneous_with the microperthite in
their period of crystallization.

The plagioclase is no more than accessory. The usual optical
methods of determination agreed in showing that it belongs to a series
from practically pure albite to the oligoclase AbgAuj. An orthoclase
could not be demonstrated in any of the Ascutney rocks, nor should
it, on account of the lowness of lime, be expected. Barium oxide
doubtless occurs in isomorphic relation with the soda and potash of
the feldspars. No hyalophane has been discovered in the rock.

Rapid tarnishing on exposure to air. — One of the most remarkable
properties of this rock consists of the unstable character of its color.
When broken out of the quarry a fresh specimen is uniformly, on the
surface of fracture, a light bluish gray. In the course of twenty-four
hours, under atmospheric conditions, this tint changes to one with a
greenish tinge, and after an exposure to the air of ab®ut thirty days

52 GEOLOGY OF ASCUTISTEY MOUNTAIN, VERMONT. [bull. 209.

it has become a deep brownish green — the color we have noted for the
numerous blocks of the quarry. This green color is in its turn lost
when the rock has suffered more pronounced weathering after many
years of exposure. The final change gives the familiar yellows and
browns of a decomposed ferruginous rock. In this stock the rapid
change from gray to green was observed only in phase /, as exposed
on the north and northwest slopes of the mountain.

Examination quickly showed that the color change of the rock is
conditioned by the feldspar and that it is altogether a superficial
phenomenon, taking place only where the air has access to the min-
eral. The question has naturally arisen as to the cause of this pecul-
iar instability of color, and a number of experiments were carried
out which have thrown light on the problem. To show that one or
more of the principal atmospheric gases were essential to the reac-
tion, a gray piece of the fresh rock was immersed in a stream of
carbon dioxide gas for twenty minutes and then kept in an atmosphere
of that gas for twenty-four hours. No appreciable change was noted
in the original gray tint, showing that in all probability the carbonati-
zation of some unknown element in the feldspar could not explain
the alteration of tint. The inference was ready to hand that it was
rather due to oxidation. A gray fragment of the rock was accord-
ingly placed in an atmosphere of purified oxygen over niglit. A per-
ceptible change to the green color resulted. The same piece was
then changed to -an intense green by an exposure of thirty minutes to
a stream of oxygen, while the fragment was kept at a temperature of
about 150° C.

The further question remained: What oxidizable substance pres-
ent in the feldspar would, on uniting with the oxygen of the air,
furnish the required color? That it is not organic was shown by
the fact that before the blowpipe the green tint was not only
not destroyed, but, on the contrary, was deepened in the oxidizing
flame — another testimony to the fact of oxidation as the true cause.
Partial decolorization resulted from the application of the reducing
flame. The probable explanation of the color change is found in
the oxidation of the ferrous oxide of the feldspars to the ferric,
thus giving a yellow which, in combination with the fundamental
blue-gray of the under layers of the crystal substance, affords the
green of the altered surface. In acids the mineral is decolorized to
the original bluish gray, which is permanent, and the filtrate gives a
strong reaction for iron. A iiigh power of the microscope shows that
the perfectly fresh feldspars are all crowded with myriads of extremely
minute blackish granules. It is possible that this dust is composed
of ferrous oxide, dating as to its period of formation from the time of
the original crystallization of the rock. If this be true, we can derive
the instability of color from one of the most familiar reactions in the
history of metasomatic processes.

Were the coloring substance uniformly distributed throughout the

DALY.] MAIN SYENITE STOCK. 53

body of the rock, this syenite would make a favorite material for
decorative purposes, for it is capable of- a fine polish; but the distri-
bution is, unfortunately, very uneven, and the consequence is that the
polished monument or shaft is often blemished with streaks of lighter
and darker hue than the average. Furthermore, as already implied,
the tint of any specimen can never be said to be permanent. As the
oxidation progresses, the bluish tone of the feldspar substance
beneath the surface will have less and less influence on the color
mixture and a more brownish tone will result. This is what has actu-
ally happened in the case of several tombstones which have stood for
some years in the cemeteries of Brownsville and Windsor.

A similarly rapid change of color — from a grayish green to a more
pronounced green — on exposure to the air, has been described by
Cushing as characterizing the squeezed augite-syenites near Loon
Lake, New York." He suggests staining from the oxidation of the
ferrous iron derived from decomposed hypersthene as a possible cause
of the rusty brown color, but leaves open the question of the causes
of the early stages of the color change. He points out that the
uncrushed crystals are alwaj^s less green than the feldspar granulated
by pressure. This would be expected as one result of the increased
ease with which oxidizing fluids would circulate in the rock after
crushing. The gray color of the Ascutney rock corresponds with its
other properties in showing that it has not been subjected to such
squeezing as that once suffered by the New York syenite, which in other
respects is strikingly similar to our rock. Types very close to botli
of these in nature and origin occur at Killington Peak in western Ver-
mont and at Shefford Mountain, Quebec, and possess the same pecu-
liar green color. At the latter locality the change from the fresh gray
to green has also been observed. It may be noted in passing that
green is a favorite hue for several species of alkaline rocks. Tin-
guaites are commonly green, like the groundmass of pantellerites,
and grorudite from the classic locality is green, the color in the last
mentioned rock being due, however, to the essential tegirine.

Hornblende. — The next most important constituent of the sj^enite is
a hornblende belonging to the alkali-iron group of amphiboles. Often
idiomorphic against the feldspars, it yet commonly possesses the fea-
ture characteristic of hornblendes that have grown in an alkaline
magma — namely the irregular outline due to resorption.* Within the
cavities thus formed by this magmatic solution, feldspar and quartz
have crystallized, and in section have the appearance of inclusions in
the hornblende. The color of the mineral varies through shades of
brown according to the following scheme :

a, light greenish brown to grayish yellow.

b, deep greenish brown to olive-brown.

c, grayish olive-green.
r-

o^

aBvLll. Geol. Soc. Am., Vol. X, 1899, p. 178.

bCt. Brogger, Zeitschr. fiir Kryst., Vol. XVl, p. 131.

54 GEOLOGY OF ASCUTNEY MOUNTAIN, VERMONT. [bull.209.

Tlie absorption parallel to b and c is very strong; b>c>a.

By the use of cleavage pieces mounted on the Fedorotf table, the
extinction was determined on (010) at 16°. The extinction on the cleav-
age plate itself was found to average 14° 39'. By turning the plate
about an axis at once coincident with the vertical axis of the crystal
and parallel with the principal section of the polarizer, it was possible to
test the curve of extinction in the vertical zone. Readings were taken
at positions of the cleavage plate where the plane of symmetry made
angles of 42° 30', 47° 30', and 77° 30' with the plane passing through
the crystal at right angles to the axis of the microscope. The corre-
sponding angles of extinction were found to be 16° 50', 17° 25', and
12° 0'. These results mean an angle of extinction on (010) of 16°, and
an optical angle of about 70° for the amphibole.** Etch figures on a
cleavage plate immediately oriented the crystal and therewith the
ellipsoid of optical elasticity.* The optical axis c lies in the obtuse
angle /i in Tschermak's orientation. These conclusions were checked
by the close study of rock slides, and chance sections of the hornblende
favorable to the rough measurement of the optical angle and to the
determination of c : c confirmed the results derived from the use of
cleavage pieces.

The usual twinning parallel to (100) was observed.

The angle of the cleavage prism was measured on about twenty
individuals and found to vary from 55° ]3' to 56° 0', with an average
of 55° 32'. This great variation from the mean is not to be explained
by poor reflexes or by the personal equation of the observer, but must
be conditioned by some as yet unknown cause or causes. '' The specific
gravity was taken with the Klein solution; it averaged 3.272 at 17°C.,
varying in a suite of ten specimens from 3.266 to 3.278. A thin splin-
ter of the mineral melts quietly in the Bunsen burner with a strong
soda flame. In view of such ];)roperties we can place this hornblende
near barkevikite, in the alkali-iron series developed by Brogger.*^
Poikilitic intergrowths with biotite and allanite and parallel inter-
growth with augite make it impossible to separate the hornblende and
thus permit a chemical anal^-sis of it being made, but it is plainly rich
in ferric oxide and soda.

Augite. — Compared with the amphibole, the pyroxene is present in
very subordinate amount. It almost invariably occurs in the cores of
parallel intergrowths with the hornblende, which there is every rea-
son to believe is j)rimary and has thus not been derived from the
pyroxene either b}^ magmatic or by metasomatic changes. The augite
has the usual diopsidic habit of most augite-syenites; the optical angle

(iProc. Am. Acad. Arts and Sciences, Vol. XXXIV, 1899, p. 311.

b Ibid., p. 373.

c There is need for a thorough investigation of the whole amphibole group for the purpose of
fixing the series of prismatic angles, as they undoubtedly vary with the chemical com.position,
and the student of the amphiboles would probably be repaid if he set about the task of finding
the possible causes for the noteworthy variation in the angle for the same species from one
locality.

f' Ge.steine der Groi-udit-Tinguait Serie, p. SS.

BALY.] MAIN SYENITE STOCK. 55

is, however, remarkably small, 45° 15' being measured in oil. The
extinction angle was not found on account of the lack of favorable
material.

Biotite. — In about equal proportion with the pyroxene is a deep-
brown primary mica characterized by normal properties. The optical
plane and the plane of symmetry are coincident. From the low per-
centage of magnesia in the total analj^sis it appears that the mica is a
true lepidomelane and not a meroxene. The formation of skeleton
crystals by magmatic resorption is here also very striking.

Allanite. — The rock of the Windsor quarry contains an important
accessory not recognized in any other part of the syenite stock. It
occurs in the form of elongated anhedral grains, either independent
or associated as irregular intergrowths with the hornblende. In the
hand specimen the mineral can be readily made out by its black
color, waxy to lustrous apjjearance, and by the presence of only one
good cleavage. In many cases the individuals are as much as half a
centimeter long. The most striking microscopic property is the
extremely strong pleochroism and absorption. The colors varj^ from
cinnamon -brown to deep walnut-brown in some individuals; in others
chestnut-brown and purplish brown appeared, while in the thicker
slides the more powerful absorption gave almost absolute black-
ness. The single refraction seemed to be higher than that of horn-
blende, but the double refraction was weaker. In addition to the good
cleavage visible macroscopically, there was also present a less jjerfect
cleavage transverse to the former at a high angle. Before the blow-
pipe the cleavage pieces fused with intumescence to a black magnetic
glass. Such an association of pro^Derties seemed to indicate allanite,
and an examination of some material from Suhl (orthite) confirmed
the close similarity with that mineral. The conviction became a prac-
tical certainty when some fragments were dissolved in hydrofluoric
acid, and from the solution an excellent test for cerium was obtained
by precipitating with ammonium oxalate.

The allanite is (on account of the strong magmatic resorption) never
idiomorphic. Yet it must be one of the oldest constituents of the rock,
as it is inclosed by the hornblende, in which it often forms lively
pleochroic halos. The two minerals are sometimes intergrown, but
the allanite never incloses the other. Apatite, zircon, and magnetite
antedate both in the order of crystallization.

We have here, then, one more example showing the importance of
allanite in eruptive rocks. As early as 1885 Iddings and Cross noted
the occurrence of the mineral at 28 localities and in 9 rock-types,
including granite, gneiss, granite-porphyry, quartz-porphyry, diorite,
dacite, and rhyolite.^ Since then it has been discovered at many other
localities, including some where the rock is alkaline and related in
character to the Ascutney syenite, e. g., the hornblende granite of

aAm. Jour. Sci., 3d series, Vol. XXX, 1885, p. 108.

56 GEOLOGY OF ASCUTISTEY MOUNTAIN, VERMONT. [bull. 209.

Essex County, Mass./* and tlie quartz-syenite of Loon Lake, New
York.^

Monazite. — A. second accessory which was attended with consider-
able difficulty in its determination occurs in some amount in the quarry
rock. It has never been observed macroscopically, but only in the
slide, where it is found in the form of roundish grains reaching- 1 milli-
meter in diameter. These are nearly colorless, with a grayish-yellow
tint, and are characterized by high single refraction and by high double
refraction, giving polarization colors of the third order. Crystal form
is always lacking, but optical tests showed the mineral to be biaxial
and monoclinic or triclinic. The cleavages, about at right angles to
each other, were seen in the section of one small individual. The min-
eral was found to be difficultly soluble in nitric acid and more easily
in hydrochloric acid. From the solution a precipitate with molybdate
of ammonia was obtained, one too abundant to be explained by the
associated apatite, and thus showed the grains to belong, without
doubt, to a i)hosphate. The quantity of the solution was so small as
to render impossible the sure determination of the rare earths which
should be expected if the mineral be really monazite. Yet it may best
be ascribed to that species as the phosphate nearest in optical proper-
ties to the one with which we are dealing.

The grains inclose numerous apatite needles of great minuteness and
a few square sections of magnetite. All three minerals seem to have
crystallized 'before the essential constituents. The monazite furtl^r
shows an interesting paragenesis with the allanite, the latter sometimes
appearing as a mantle about the former. Such an intimate associa-
tion of a phosphate with a member of the epidote f amilj^ is rather sur-
prising, but from the study of the material in hand both minerals
seem to be primary.

The magnetite is titaniferous. It is inclosed as a primary mineral
by all the other constituents except apatite. Its habit is the usual
one of granitic eruptives.

Titanite is rather less common than in the diorites, but possesses
the same features as in the older rock. It incloses apatite; its rela-
tion to zircon is indeterminable as to the period of crj^stallization.

Apatite is, as usual, most abundant in the vicinity of the bisilicates,
and is accordingly here, as in the f eldspathic phases of the stock as a
whole, very rare.

Zircon is more common than in the phases of the Basic stock. Its
habit is, however, the same, excepting that it here shows a pronounced
color and pleochroism.

E, pigment irregularly distributed— pale violet and colorless.

0, solid color — paler violet.

The zircon is younger than the apatite and seems to have accom-
panied the titaniferous magetite in its crystallization. Irregular

« Jour. Geol., Vol. VI, 1898, p. 792.

&H. P. Gushing, Bull. Geol. Soc. Am., Vol. X, 1899, p. 180.

DALY.] MAIK SYENITE STOCK. 57

roundish inclusions with wide margins of total reflection are ascribed
to imprisoned gas.

Quartz is uniformlj^ allotriomorphic and interstitial. Fluid cavities
and negative crj^stals are very numerous. The filling material of the
latter could be well studied here on account of the remarkable per-
fection of the forms. In many cases double bubbles, that unite on
heating the preparation, indicate carbonic acid gas in a saturated
solution of water. The usual orientation of the negative crystals with
their chief axes parallel to that of their host is easily demonstrable,
especially in the isotropic sections of the quartz; in them the fluids
lie in six-sided cavities, whose sharp outlines are of exceptionally
clear definition.

The extremely few grains of reddish common garnet were found in
this phase only in those thin sections made from specimens collected
near the schist contact and are doubtless to be referred to slight
endomorphic influence exercised bj^ the country rock on the eruptive.

Basic nodules, from 1 to 2 inches (2.(3 to 5.2 centimeters) in diameter
are occasionally seen in the quarry rock. They are differentiated min-
eralogically from their parent rock simply by a greater richness in
hornblende, which is here, too, strikingly corroded. One can not be
sure that the poikilitic habit of the mineral is anything more than
apparent; primarj^ inclusion of quartz and feldspar might give the
same appearance in thin section as that due to extensive embaying of
the hornblende by the caustic feldspathic magma.

PLATE IV.

A. Typical thin section of the nordmarkite of the Main stock, granitic phase,
composed ahnost entirely of microperthite and qixartz; crossed nicols, X 30. (See
p. 50.)

B. Pyroclastic feldspar (soda-orthoclase) surrounded by a reaction rim rich in
alkaline hornblende, from the large paisanite dike on northwest slope of Ascutney
Mountain; crossed nicols, X 7. (See p. 71.)

58

U. S. GEOLOGICAL SURVEY

BULLETIN NO. 209, PL. IV

(B)

HE MERIDEN GRAVURE CO.

DALY.] MAIN SYENITE STOCK.

Table YII.— Analyses of nordmarkite and other rocks.

59

1.

2.

3.

4.

5.

60.03

20.76

j 4.01

1 0.75

0.80

2.62

5.96

5.48

a 0. 59

6.

7.

8.

SiOa --

65.43
16.11
1,15
2.85
0.40
1.49
5.00
5.97
0.39
0.19
Trace?
0.50
0.11
0.13
None.
0.05
0.08
0.07
0.03
0.23
Trace.

64.88

16.24
1.37
2.70
0.89
1.92
5.00
5.61
0.46
0.19

None.
0.69
0.13
0.13

None.
0.04
0.08

64.04
17.92
0.96
2.08
0.59
1.00
6.67
6.08

i 1.18

60.45
20.14

[ 3. 80

1.27
1.68
7.23
5.12

0.71

60. 5-67. 0
20. 0-17. 5

U. 0- 3.0

1.0- 0.5
2.0- 1.5
7.0- 6.5
5.0- 6.0

61.49
16.14

5.81

0.99
1.67
6.19
5.70

«1.17

65.43

16.96

( 1.55

1 1.53

0 22

Al.Oa

Fe-A

FeO

MgO-

CaO

1 36

Na,0

5 95

K^O-

H.,0 above 110° C.

5.36

0.82

HjO below 110° C_-
COo

None.

TiOa

1 0.62

0.16

ZrO^

P2O,

0.07

0.53

0.02

SO3

0.06

CI

0.04

F

FeSa

BaO

0.06
0.14

Faint

trace.

Trace.

None.

MnO

0.23

Trace.

0.28

0.40

SrO

LijO Stronsr

trace.

100. 18
0.04

100. 53
0.04

101.37

100.40

100. 07

99.97

99.86

0— F CI

100. 14
0.036
2.659

100. 49

Totals

-

Sp.gr

2.683

a Loss on ignition.

1. Hornblende-biotite-nordmarkite of granitic structure, Ascutney Mountain
(phase /) ; analysis by Hillebrand.

2. Hornblende-biotite-augite-nordmarkite of porphyritic structure, Ascutney
Mountain (phase g) ; analysis by Hillebrand.

3. Classic nordmarkite, Tonsenas, Norway: Brogger, Zeitschr. fiir Kryst., Vol.
XVI, 1890, p. 54.

4. Classic nordmarkite, Auerod, Norway: Brogger, ibid., p. 54.

5. Classic pulaskite, Fourche Mountain: Williams, Arkansas Geol. Surv. , Ann.
Rept. for 1890, Vol. II, p. 70.

6. Limits of variation in nordmarkites and related quartz-syenites, according
to Brogger, op. cit.. p. 81.

7. Average analysis of three syenite-porphyry dikes from the northern Adiron-
dacks; Cushing. Bull. Geol. Soc. Am., Vol. IX, 1898, p. 248.

8. Nordmarkite of Shefford Mountain; Dresser, Am, Geol., Vol. XXVIII, 1901,
p. 209.

60

GEOLOGY OF ASCUTNEY MOUNTAIN, VERMONT.

[BULL. 209.

From the analysis of the fresh quarry rock (Table VII, col. 1) the
table of molecular proportions was calculated as follows :

Anal-
ysis.

Molecular
proportions.

SiOs -■--.

65.43
16.11
1.15
2.85
0.23
0.40
1.49
5.00
5.97
0.50
0.11
0.13
0.05

1.0905
0. 1579
0. 0079
0. 0396
0. 0032
0.0100
0. 0266
0. 0806
0. 0635
0.0061
0.0011
0. 0009
0. 0014

AI2O3 -- ---

FeaOg -- -- - ---...

FeO --

MnO

MgO -

CaO

NaaO - -

K.,0

TiO.,

ZrO., - -

P.,0, -

CI

A partial determination shows that zircon forms 0.2 per cent of the
rock; apatite, 0.4 per cent; magnetite (crediting it with all the FegOg),
1.8 per cent. A careful mechanical separation of the hornblende per-
mitted a rough estimation of its total amount; slightly impure from
included and intermixed allanite, biotite, and magnetite, it composed
5.2 per cent by weight of the total powder. Allowing for its impurity,
we shall not be far from the truth in regarding 5 per cent as the pro-
portion of hornblende. Arbitrarily estimating the lime content of
the hornblende as 10 per cent (near barkevikite), the proportion of
the anorthite molecule, after allowing also for the lime in the apatite,
was calculated at 4 per cent. On the supposition that all the soda
occurs in the albite molecule and all the potash in the ortlioclase
molecule, they would respectively compose 42 and 35.3 per cent of
the rock. Both these figures must be slightly too high. The result
of the whole calculation shows the following approximate composition:

Per cent.

Albite molecule 41. 0

Orthoclase molecule 35. 0

Quartz 11.0

Hornblende -■ - 5.0

Anorthite molecule 4.0

Magnetite . 1-8

Apatite 0.4

Zircon J 0.2

Biotite, titanite, diopside, and allanite 1.6

100.0
Three determinations of the specific gravity of the rock gave an

average of 2.659 at 17° C.

DALY.] MAIN SYENITE STOCK. 61

The geological relations, structure, and constitution of this phase
clearly place the rock among the alkaline quartz-syenites, closelj^
allied to the nordmarkites of TonsenS^s and other localities in the
Christiania region (compare cols. 1, 3, and 6). Brogger's table gives
the limiting values in the percentage composition of nordmarkite.
Two other American examples are noted in columns 7 and 8.

Phase g. — The porphyritic phase of the stock is widespread, espe-
cially on the east and southeast sides of the mountain. The rock is
structurally, but neither chemically nor mineralogically, except as
regards some of the accessories, to be distinguished from the normal
equigranular type. This second phase is exhibited on a large scale
on the prominent bald knob east of the main summit.

The color of the rock is always a light gray or pinkish gray, which
is stable and does not change to green on exposure (spec. 115). The
phenocrysts are almost always roundish feldspars which may reach
the diameter of one centimeter or more; much more rarely a horn-
blende or augite individual will approach the same dimension. The
phenocrystic feldspars are microperthite, orthoclase, albite, and oligo-
clase, often arranged in groups of two or more large individuals. The
first named is probably the most abundant, but is much less predomi-
nant than in the granitic phase on account of the greater amount of
free albite and oligoclase. The orthoclase, to judge from its typical
specific gravity (2.594 at 1G° C. ), must contain considerable soda in
intimate mixture. The acid oligoclase and albite are often surrounded
by a thin mantle of orthoclase, which is thus later in origin. Micro-
cline is probablj^ jjresent among the phenocrysts, but is quite rare.

The groundmass is a hypidiomorphic pepper-and-salt mixture of the
same essential minerals as in phase /. The diopsidic augite, browni
hornblende, and the biotite are more abundant than in that phase,
causing the specific gravit}^ of the rock to be higher (here 2. 685 at
17° C). The augite, as in the phenocrysts, seems always to occur as
cores in intergrowths with the jDrimary hornblende. Corrosion of
the dark-colored silicates is much less pronounced than in the granitic
phase; they exhibit, correspondingly more often, idiomorphic outlines.
Free quartz in the form of small interstitial grains occurs, but is not
so prominent an accessory as in phase /. Titanite is here more
abundant, and explains the somewhat higher percentage of TiO^ in
the analysis. The higher MgO is ascribed to the more abundant
biotite.

The rock shows no indications of crushing; we can not, therefore,
attribute the porphyritic structure to cataclastic processes. The
groundmass has unquestionably crystallized in its present form from
an igneous magma. The order of crystallization of its component
minerals is the same as in the green rock. Several facts favor the
view that the feldspar phenocrysts belong to an earlier stage in the
crystallization than that which produced the groundmass.

PLATE V.

A, Segregation of mica and hornblende concentrically arranged, in paisanite
(the same section of the segregation also appears in the lower right-hand quad-
rant of the micrograph represented in PI. IV, B) ; ordinary light, X 24. (See p. 73.)

B, Basic segregations in nordmarkite at Crystal Cascade; one-half natural size.
(Seep. 64.)

62

U. S. GEOLOGICAL SURVEY

BULLETIN NO. 209, PL. V

m;

H <^

(B)

THE MERIDEN GRAVURE CO.

DALY] MAIN SYENITE STOCK. 63

Feldsijars of similar nature and size are abundant in the basic
segregations with which this phase is richly charged, and with them
are granitic groupings of several individuals separated by interstitial
quartz. The segregations are mostly composed of those minerals of
the groundmass which crj^stallize out in an early stage of consolida-
tion. The large feldspars and the groups would thus antedate that
groundmass. The feldspar phenocrysts which are mantled with ortho-
clase very often present the appearance of having been extensively
corroded by the magma before the mantles grew about them. It is
probable, also, that there were two generations of the bisilicates.
The granitic groupings of large individuals suggest that the porphy-
ritic structure may be largely due to protoclastic action breaking up
a coarse-grained granitic rock already more or less completely solidi-
fied in the conduit when the somewhat later magma of the "ground-
mass" was erupted. On the other hand, we can not exclude the
possibility that this phase is the result of chilling, developing a
porphyritic structure equivalent to that which may be seen in the
endomorphic zone and in the aj^ophyses of the granitic phase; for it
is often impossible to distinguish hand specimens of the latter rocks
from typical specimens of ]3hase g. The problem thus merits further
inquirj^

One of the most peculiar features of the s^^enites which may be seen
in all the phases, but is best exemplified in this particular phase, is
the presence in the rock of numerous dark, roundish spots or kernels.
These vary from 1 millimeter to 1 centimeter in diameter. They occur
in all parts of the rock, but are specially abundant in the basic segre-
gations, which will be more fully described hereafter. The kernels
belong to two classes, which show the common characteristic of a core
and mantle structure. Within a relativel}^ thin black outer covering of
felted, often radiall}^ arranged, biotite (and less conspicuously horn-
blende) there is a core of variable composition. The latter maj^ be
composed entirely of chlorite and magnetite ; of chlorite, magnetite,
biotite, and a uralitic amphibole; or entirely of a light-green pleo-
chroic actinolitic hornblende. The last mentioned is the commonest
type of core.

The mantle is to be regarded as a reaction rim. The chloritic cores
are the product of the alteration of augite, probably in consequence of
metasomatic action. The actinolitic kernels are likewise plainly
derived, but in no one of some twenty-five slides could there be found
a remnant of the original mineral at the heart of the kernel. The
similarity in size and general relations between these and the chloritic
kernels suggest that augite was here, too, the original material from
which the hornblende felt was constructed. The freshness of the
biotite rim, the absence of secondary ore and chlorite, and the com-
plete freshness of the hornblende core lead to the conclusion that the
alteration took place before or during the consolidation of the rock.

64 GEOLOGY OF ASCUTNEY MOUNTAIIST, VEEMONT. [bull. 209.

This class of kernels would thus fall into the class of magmatic j *
domorphs after pyroxene. One is reminded of the analogous oce "f
alteration of olivine into hornblende.

Chemically this phase is practically identical with the green gran-
ular I'ock (cf. columns 1 and 2 in Table VII). Mineralogically the
similarity is almost as close. The only important difference is in the
structure. Phase g may then be classified as a nordmarkite with a
porphyritic habit.

BASIC SEGREGATIONS.

Every observant visitor to Ascutney is struck by the extreme rich-
ness of the Main syenite stock in basic inclosures of generally a
nodular form, and he might also note that they are more abun-
dant in the porphyritic phase than elsewhere (PI. V, B and PL VI).
They are distributed with great irregularity. Sometimes they occupy
as much as one-half of the volume of the rock, if one may judge from
the appearance of even broad ledges. At other times the nodules are
separated by many feet or yards of the normal rock. Partly on
account of their abundance in erratics won from the mountains a
well-defined glacial bowlder train has been shown by C. H. Hitch-
cock to exist in the lee of Mount Ascutney." The nodules are dark
gray to dark greenish gray in color, spheroidal or ellipsoidal in
shape as a rule, and of all sizes up to those occupying several cubic
feet. The section of one of them, outcropping near the contact with
the granite at the southeast end of the mountain, was found to meas-
ure 2 by 10 feet. While the much darker color causes the nodules to
be in striking contrast with the normal rock in hand specimen or in
ledge, microscopic study proves an intimate dovetailing and interlock-
ing of the minerals between the two. The nodule has not been
enriched in the bisilicates by the special impoverishment of the
matrix immediately surrounding, for in no case could there be found
a zone about the nodule distinctly lighter in color than the normal
rock. This is the more difficult to understand because of the very
evident lack of flow structure in the rock as a whole. The nodules
seem to have formed quietly in the magma after it had suffered its
"mise en place" and not to have been disturbed in position since.
Even those which are decidedly elongated do not shoAv the degree of
common orientation which we should expect if they had floated in a
streaming fluid matrix.

The nodular masses are themselves porphyritic. Large, irregularly
bounded crystals of microperthite, cryptoperthite, microcline, ortho-
clase and plagioclase (averaging acid labradorite AbiAnJ, green horn-
blende, and the usual diopsidic augite, with or without a hornblende
mantle, form the phenocrysts (see PL V, B). The dark matrix is a
fine-grained granular, panallotriomorphic mass of hornblende, oligo-

aBuU. Geol. Soc. Am., Vol. I, 1890, p. 30.

DALY.] BASIC SEGEEGATIONS IN MAIN SYENITE. 65

clase, biotite, and quartz, with abundant grains or idioraorphic crys-
tals of titanite, ilmenite, apatite, and zircon. The microperthite and
labradorite of the phenocrysts and the hornblende, biotite, and oligo-
clase of the groundmass are really the essential constituents. The
pleochroism of the hornblende seems to indicate that it is less alka-
ine than the amphibole of the matrix.

a, pale yellowish green.

b, grayish green (absorption medium to strong).

c, grass-green to leek-green (absorption medium to strong).
b = c > a.

As already noted above, these segregations characteristically contain
light-colored, coarsely crystalline areas from 1 to 2 centimeters in
diameter, similar in composition to the normal syenite of phase /, i. e.
equigranular aggregates of alkali-feldspar and bisilicates. These
have the appearance of having functioned as centers of crystallization
during the growth of the nodule, although there is a complete absence
of both radial and concentric structure in the nodules.'^' In no case
was there observed an approach to the "Kngelstruktur" of the rock
at Virvik or at the well-known Corsican localit3\

The specific gravity of the average segregation is near 2.850, and is
thus considerably higher than that of the matrix, and still higher than
that of the molten magma which represented the yet uncrystallized
matrix. Unless that matrix possessed a high degree of viscosity dur-
ing the formation of the nodules, they must have sunk down in the
magma, and we might expect to find them concentrated in the
lower part of the conduit ; yet they appear to be distributed in about
equal average proportion in all parts of the stock where the jiorphy-
ritic phase was found, whether at the summit or 2,000 feet vertically
below. This fact agrees with the absence of flow structure in the
rock in forcing us to take the view that the segregations do not belong
to the preemptive i)eriod, as advocated bj'^ Lacroix, Michael Levy,
Graber, and others for other occurrences. The nodules had best be
referred to an early stage in the actual consolidation of the syenite
alread}'^ occupying its conduit.

The accompanying Table VIII (col. 1) shows the analysis of the
average segregation from phase g (spec. 66). Columns 3, 4, and 5
give, for purposes of comparison, the analyses of classic essexite,
classic monzonite, and an average diorite.

The essential mineralogical composition of these rocks is as follows:

1. Oligoclase, microperthite. cryptoperthite, acid labradorite, microcline, ortho-
clase. hornblende, biotite, augite.

2. Microperthite, hornblende, ortlioclase.

3. Labradorite, orthoclase (nepheline), augite, biotite, barkevikitic hornblende
(olivine).

4. Orthoclase, oligoclase, andesine, labradorite, augite, green hornblende,
biotite.

aCf. Chrustsehoff, Mem. Acad. imp. sci. St. Petersboui-g, Ser. VII, Vol. XLII, No. 3, 1891, p 86.

Bull. 209—03 5

66 GEOLOGY OF ASCUTNEY MOUNTAIN, VEBMONT.

Table VIII. — Analyses of basic segregations, etc.

[BULL. 209.

1.

2.

3.

4.

5.

SiOj

56. 51
16. 59
1.35
6.59
2.52
4.96
5.15
3.05
0.71
0.21
0.33
1.20
0.04
0.41
0.07
0.24
0.06
Trace?
0.24
0.03
Trace
Trace

56.53

16.47
1.58
5.40
2.67
4.90
5.59
3.80
0.60
0.23
0.05
1.40
0.03
0.27
0.07
0.19
Trace
Trace
0.20
Trace
Trace
Trace

47. 94
17.44
6.84
6.51
2.02
7.47
5.63
2.79

1 2.04

55.88

18.77

[ «8.20

2.01
7.00
3.17
3.67

1.25

56.52

AI2O3 - -

16.31

Fe„Oo

t 4.28
1 5. 92

FeO

MgO

4.32

CaO

6.94

ISTajO

3.43

K2O

1.44

H2O above 110° C

H2O below 110° C ----

CO,

1.03

TiO, - -

0.20

0.25

ZrO.,-

P„0= - - - -

1.04

0.40

CI ---- -

F

FeS,

NiO, CoO .

MnO -

BaO -

SrO

LiaO

0-F,Cl-

100.26
0.11

99.98
0.09

99.92

99.95

100. 98

100. 15
0.03
2.849

99.89
Trace

2. 756

Total S

Sp. gr

o With MnO.

1. Basic segregation in phase g of Main nordmarkite stock; analysis by Hille-
brand.

2. Basic segregation in dike of liornblende-paisanite, Ascutney Mountain;
analysis by Hillebrand.

3. Classic essexite, Salem Neck, Salem, Mass.; analysis by Dittrich.

4. Average analysis of monzonite, according to Brogger, Die Eriiptivgesteine
des Kristianiagebietes, Vol. II, 1895, p. 39.

5. Average analysis of sixteen typical diorites, according to Brogger, ibid.,
p. 37.

Tliere is a relationship of the segregation with each of these types,
though that with monzonite is the closest. The writer has seen in
the laboratory of M. Fouque, in Paris, thin sections of monzonite from

DALY.] BASIC SEGKEGATIONS IN MAIN SYENITE. 67

Predazzo, made from a contact phase of that rock. The structure and
the small proportion of monoclinic feldspar showed close similarity
of this phase of the monzonite with the Ascutney segregations.

Two other phases of the Main stock may be noted, not only on
account of their importance in the field but also because they repre-
sent interesting extremes in the differentiation of the syenitic magma.

Phase li outcrops extensively in a belt about 400 yards (366 meters)
wide and adjacent to the contacts with diorite and gneiss on the north-
west side of the mountain. It is probably a special part of the endo-
morphic zone of the stock, as the phase has not been found anywhere
else than in the belt specified (spec. 34). This member of the rock
body is composed essentially of microperthite, usually in Carlsbad
twins, and an amount of quartz sufficient to place the rock among the
granites. Biotite, diopside, zircon, and magnetite are the accessories,
but make up probably no more than 1 per cent of the rock. One grain
of garnet, another of what is doubtless corundum, two individuals of
a brown hornblende, and a few needles of apatite were discovered in
the two sections that have been prepared from this iDhase.

The original color of the rock is due to the feldspar and is a striking
dark oil green, which is permanent in the hand specimen, and doubt-
less represents a late stage in the series of color changes already
described for the quarry rock. The structure is often fluidal or
trachytic as governed by the tabular feldspars.

To form an idea of the relative proportions of the soda and potash
molecules in the rock the specific gravity of some thirty cleavage pieces
of the feldspar was determined. Specific gravity could be safely relied
upon on account of the freshness of the rock and on account of the lack
of inclusions in the feldspar. The average for the thirtj^ X)i6ces was
2.594 at 22° C. ; the range of specific gravity was from 2.582 to 2.612.
The extinction angles showed that the albite of the intergrowth is
nearly pure and has only a very small intermixture with the lime mole-
cule. Accepting Brogger's values for the-specific gravities of pure
albite and pure orthoclase the average for this rock corresponds to a
microperthite in which the two silicates occur in about equal propor-
tion, with the albite the more abundant. The specific gravity of the
rock is 2.616 at the same temperature. If we assume that the ratio
Ab: Or=41: 35 as in the granitic phase, that the lime is 1 per cent of
the rock and the accessories 1 per cent, there would be about 20
per cent quartz in the rock. This rough estimate agrees with that
made by inspection, of the thin sections. This phase is thus a true
alkaline granite at the extreme end of the series which leads to a rock
with the composition of an aplite while preserving the hypidiomorphic-
granular structure. A very similar rock occurs near Stratford, N. H. ,
Albany, N. H., and Stark, N. H. These are illustrated in the collec-
tion of Professor Rosenbusch at Heidelberg. They all possess a
higher prox3ortion of bisilicates than phase h.

68 GEOLOGY OF ASCUTNEY MOUNTAIN, VEEMONT. [bull. 209.

Another variety of true granite, into which the syenite is transi-
tional, outcrops at the main summit of the mountain. It has the
ordinary pinkish color of the average syenite of the stock. The com-
position is essentially the same as that just described for phase li.

Phase i. — Near the most westerly triple contact of granite stock,
syenite stock, and phyllites, a fourth phase of the syenite was speedily
noted in the field as unlike all the others in bearing an unusual
amount of dark-colored minerals (spec. 111). The light-gray feld-
spars still give the dominant tone of color to this phase, which is als©
alkaline. The structure is that of phase/, equigranular; the grain
is somewhat coarser. Basic segregations fail altogether or are very
rare. The chief mineralogical difference between this phase and the
others is found in the character of the feldspars. Triclinic feldspar
is now one of the chief essentials. It varies in composition from the
labradorite Ab3An4 to the andesine Ab4Ang. Microperthite and ortho-
clase, hornblende, diopside, and biotite, with the properties of these
minerals in phase /, are the other essentials ; the same accessories are
found here excepting allanite, monazite, and garnet. In addition
there is a second hornblende among the essentials, with the following
scheme of pleochroism and absorption:

a, Yellowish brown.

h, Deep brown to black, with specially strong absorp-
tion.

c, Deep brown, with a trace of olive-green.
b > c > a.

The augite occurs only as cores in intergrowth with hornblende.
Quartz is very subordinate among the accessories.

The basic character of this i^hase, its richness in bisilicate, the
presence of much essential andesine-labradorite, coupled with the
alkaline habit of the rock, are properties which relate the rock closely
to nionzonite. The resemblance of the Ascutney hand specimens and
those from the classic locality of Mount Mulatto is ver^^ striking.

ENDOMORPHIC ZONE OF THE SYENITE STOCK.

All four phases of the stock are habitually more acid near the contact
than elsewhere. Free quartz is even macroscopically so dominant that
the contact rock stamps itself as a true granite. The usual chilling
phenomena occur within a narrow zone not more than 20 feet across.
Three feet from the contact the feldspar and quartz become idiomori^hic
and are embedded in a microcrystalline, often granophyric, ground-
mass, and a granite-porphyry is thus developed. Apophyses fail to
show as much bisilicate as the parent rock body, have more free quartz,
and tend toward the structure of a typical aplite. Excellent examples
may be studied among some thick sheets on the north slope of the
mountain, below the quarry. This endomorphic increase of silica is

DALY.] DIKE AT LITTLE ASCUTNEY MOUNTAIISr. 69

paralleled in the Christiania region, where angite granite occurs as
the contact phase of angite syenite rich in nepheline.'*-

GREAT SYENITE-PORPHYRY DIKE OF LITTLE ASCUTNEY

MOUNTAIN.

The stock-like dike to which Little Ascutney chiefly owes its exist-
ence is remarkable for following in its general east-west course the
zone of contact between the diorite and gneiss (PI. VII and fig. 1).
Throughout its whole extent the south wall of the dike is schist and the
north wall is either diorite or the green dike mapped as "' paisanite."
This replacement of the zone of contact rocks by a later intrusive is
undoubtedly due to the weakness of that zone, which is elsewhere
evident in the extensive brecciation and crumpling of the schistose
rock. Small apophyses from the dike into both schist and diorite
clearly prove the syenite-porphyry to be the younger rock.

The dike is quite uniform in composition from end to end, though
there is a coarsening of grain from the walls toward the center. The
rock is almost the exact equivalent of the average porphyritic phase
of the Main syenite stock, and a detailed description is therefore
unnecessarj^ (spec. 76). Hornblende and biotite are the essential
dark-colored minerals. They and the feldspars again display a great
amount of magmatic corrosion; this caustic action is here, as so often
elsewhere in alkaline rocks, responsible for the rarity of idiomorphic
boundaries among the phenocrysts, as, indeed, it may be responsible
for the general rarity of i^orphyritic dike representatives of the alka-
line magmas as a whole. Plagioclase seems to be entirely absent
from this rock except in the form of a few rare phenocrysts of an
acid oligoclase. Microperthite is not so abundant, either in the
groundmass or among the phenocrysts, as it is in the porphyritic
phase of the Main syenite stock.

The dike is to be classified as a nordmarkite-porphyry .

Dark patches of basic material are very common in this dike.
They are roundish in form and vary from a fraction of an inch to 3
or 4 inches in diameter. In color and general nnacroscopic appear-
ance they are similar to the nodules from phase g in the Main stock.
The correspondence is more fully shown in the thin section. The
same constituents are present as in the typical segregation of the
Main stock and in the same relative amounts. That exception is
significant. Both in the phenocrystic constituents and in the ground-
mass of the nodule microperthite is not so abundant as in the nodules
from phase g. This fact indicates another proof of close sympathy
between nodule and host and of the indigenous origin of the latter.
The nodules themselves add another evidence to the community' of
origin between this dike and the Main stock. There can be little
doubt that the two intrusions were products of essentially contem-
poraneous eruptions from a common magma.

o Zeit. fiir Kryst., Vol. XVI, 1890, p 327.

70 GEOLOGY OF ASCUTNEY MOUNTAIN, VERMONT. [bull. 209.

SYENITE STOCK OF PIERSON PEAK.

The same may be said of the small stock of Pierson Peak. The
plan of tills rock body is elliptical. The longer axis measures 400
yards (366 meters), running about N 70° E; the minor axis measures
175 yards (160 meters). Coarse-grained apophyses prove the intru-
sive origin of the rock. It is uniformly a coarse-grained, light gray
to light pinkish-gray alkaline syenite, with a proportion of dark
colored constituents which is low even in comparison with the phases
of the Main stock (spec. 62). The structure is the typical hypidio-
morphic-granular, the order of crystallization that of phase/ of the
Main stock. Microperthite, orthoclase, and biotite are the essentials.
Quartz, hornblende, augite, apatite, and zircon are all notably rare
accessories; more important are titaniferous magnetite and titanite,
the latter being unusually abundant. Basic segregations fail alto-
gether or are extremelj^ rare. The endomorphic zone is characterized
by an almost complete lack of colored constituents and of quartz.

All the minerals have the same characters as in the granular phases
of the Main stock. The rock is a nearly quartzless biotite-nord-
markite, or pulaskite. The writer proposes that the existing diffi-
culty of differentiating these two rock types (compare cols. 4 and 5,
Table VII, p. 59) be obviated by confining the name "pulaskite" to a
rock which is in all other respects the equivalent of the nordmarkitas
except in the absence or subordination of accessory free quartz among
the constituents. Excepting for a higher proportion of bisilicates in
the Arkansas syenite, it would be hard to distinguish macroscopically
this Ascutne}^ rock from the classic pulaskite of Fourche Mountain;
there is similar close parallelism with a geographically remote occur-
rence of the same type — that of Portella des Eiras at Monchique,
Portugal.

APIjITIC dikes CUTTIIS^G THJE SYEIflTES.

Three kinds of acid dikes have been found cutting the various sye-
nites of the area. Two of these are intimately related to the stock
phases; the third has variant features. It may be noted that there
is an unusual lack of pegmatite veins both in the syenites and else-
where about Ascutney,

PAISANITE DIKE CUTTING THE MAIN STOCK.

On the logging road running up from a sawmill on the northwest
slope of Ascutney Mountain proper, toward the main summit, a dike
was discovered in the dark-green granular phase /of the Main stock
at about the 1,600-foot contour (see PI. VII). The general trend of the
dike is northeast-southwest, but at the road it bifurcates into two
branches^ — ^one, 40 feet (12 meters) wide, striking N, 40"" E. ; the other,
50 feet (15 meters) wide, striking N. 25" E. The dike, as a whole, is
visible only for about 100 yards (91 meters); at each end its continua-
tion is lost in the underbrush and talus of the steep mountain side.

^|- llXi-i-

111

iD-|[jiCiUfflLiart|fBiii

DALY.] PAISANITE DIKE CUTTING MAtN STOCK. 71

The rock is alight-tinted, pinkish-gray, pepper-and-salt, fine-grained,
somewhat porphyritic aggregate of microperthite, soda-orthoclase,
quartz, and alkaline hornblende, abundantly charged with basic seg-
regations, with kernels of biotite and hornblende, and with pyroclastic
feldspars won from the coarse syenite through which the dike passed
during intrusion (spec. 139). The general habit is suggestively like
that of the porphyritic phase g of the Main stock, and we must believe
that the two are products of the same magma. Yet, as we have seen,
the implication that phase / and phase g are of different ages (the
former being cut by the latter) does not agree with the fact of obser-
vation in the field. It is probable that there was not a great interval
of time between the intrusion of the Main stock and that of this dike.

The dike is characterized by a conspicuous platy structure due to
jointing, and, near its walls, exhibits, for the space of a foot or two
from the contacts, a strong fluidal character which is the more pro-
nounced as the basic segregations have shared in the movement and
are pulled out in long, dark-colored streaks in the dike.

The phenocrysts are either microperthite or, more rarely, ortho-
clase; they are specially abundant and are difficult to distinguish
from the pyroclastic feldspars. The texture of the rock is really con-
trolled by the groundmass, the structure of which is aplitic or panal-
lotriomorphic. It is composed of microperthite, quartz, and brown
hornblende, with properties identical with those of phase / in tlie
Main stock. The hornblende always occurs in the form of small,
poikilitic, and greatly resorbed grains. No biotite, diopside, or pla-
gioclase were discoverable. Titanite, ilmenite, zircon, and apatite
are, as usual, the accessories.

The pyroclastic feldspars are of special interest. They occur as
single individuals or as groups (with interstitial quartz) of the same
structure and grain as the country rock of the dike itself. Close study
in the field showed conclusively that thej^ are of pyroclastic origin.
Their presence in the still unconsolidated dike affected its crystalli-
zation, so that many of these foreign feldspars are surrounded by typi-
cal reaction rims of material considerably more basic than the aver-
age groundmass of the dike. The usual appearance of the feldspar
inclosure with its basic aureole is illustrated in PL IV, B (p. 58). The
feldspar in this case is soda-orthoclase, and in the reaction of the exten-
sively corroded feldspar with the matrix, the bisilicate of the reaction
rim is even more strongly charged with soda than the brown horn-
blende of the average groundmass. The rim is an interlocking aggre-
gate of microperthite and amphibole, with abundant magnetite and
some zircon and apatite. The amphibole has bluish tones, as indica-
ted by the scheme of pleochroism and absorption.

a, brownish yellow.

b, deep blue-green.

c, deep chestnut-brown, with a tinge of blue on the edges.
l)>c>a, or perhaps b=c>a.

72 GEOLOGY OF ASCUTNEY MOUNTAIN, VERMONT. [bull. 209.

The extinction c: c is about 19°.

This same hornblende forms minute basic segregations varying in
size up to 1 or 2 millimeters in diameter. These lie in the general
groundmass of the dike, and are not directly connected with the pyro-
clastic feldspars. Other segregations or replacements which recall
the "kernels" of the Main syenite, and yet show somewhat different
composition and structure, also occur in the groundmass. One of
these is illustrated in PI. V, A (p. 62). It is composed entirely of a
faintly pleochroic, yellow to light-brown, biotitic mica arranged in
alternating concentric zones with a blue hornblende. The pleochro-
ism of the latter is :

a, pale straw-yellow.

b, deepish green-blue.

c, blue, with a trace of green.
c about=b>a.

The origin of these concentrically arranged mica-hornblende aggre-
gations has not yet been determined. Their resemblance to the ker-
nels of the Main syenite, which have been interpreted as magmatic
pseudomorphs, is only partial. Especially difficult of understanding
is the recurrence of the zones of mica and hornblende.

BASIC SEGREGATIONS.

The usual basic segregation of the dike is very similar to that in the
porjDhyritic phavse of the Main syenite (spec. 141). It is a dark-gray,
mottled aggregate of phenocrysts and p}. roclastic feldspars surrounded
by a dense, granular groundmass of panallotriomorphic brown horn-
blende, microperthite, and orthoclase. Here there can be no doubt
that the segregation grew under the directing influence of the large
feldspars now seen within their mass, for the segregation of basic
material is decidedly more pronounced in the immediate vicinity of
the feldspars than elsewhere in the nodules. The nodules vary in
size up to 2 or 3 inches (5.2 to 7.8 cm.) in diameter. In one slide a
large crystal of pale-green augite with a mantle of green hornblende
was found, suggesting that, after all, the kernels of this rock may
have been derived from that mineral through magmatic influences.
The hornblende of these larger segregations has a bluish cast, and
seems to belong to a variety of amphibole intermediate between the
hornblende of the reaction rim described above and the normal horn-
blende of the Main syenite. The specific gravity of a typical segre-
gation was found to be 2.756; that of the parent rock, 2.633.

The chemical analysis of the average matrix in which the basic
segregations lie is given in the Table IX, col. 1, p. 75; that of an aver-
age segregation from the dike is entered in Table VIII, col. 2, p. 66.
The structure and composition, both mineralogical and chemical,
relate the dike most intimately with paisanite, as described by Osann

DALY] PAISANITE DIKE ON LITTLE ASCUTNEY MOUNTAIN. 73

(see Table IX, column 5). If we assume that there is 5 per cent of
soda in the hornblende, and that that mineral makes up 3 per cent of
the rock (a fair estimate after insijection of the slide), the mineral com-
position of the rock can be thus roughly determined as the following:

Per ceut.

Albite molecule 36. 4

Orthoclase molecule 29. 6

Quartz 29. 5

Hornblende 3.0

Titanite , : .5

Other accessories 1.0

100

The analysis of the segregation does not lend itself to calculation
on account of the abundance of the hornblende, the constitution of
which is unknown. The essential equivalence of this analysis and
that of the basic segregation from phase g of the Main syenite is
striking. Again, among normal rocks, we must go to the monzo-
nites for the nearest allies, chemicallj' speaking, to these nodular
masses. Mineralogically, the greatest difference between segregation
and its matrix is found in the absence of triclinic feldspar in the
former.

COMMON MUSCOVITE-APLITE OF THE MAIN STOCK.

Due south of the Windsor quarry, at the 2,350-foot contour, a dike
about 1 foot (0.3 meter) in diameter traverses the syenite at a point
where the latter is porphyritic and full of segregations. No other
dike of the same composition has been discovered in the area, but it
is highly probable that others exist. This dike is a typical aplite, a
panallotriomorphic sugary mixture of quartz, orthoclase, and albite,
with a little micro]3erthitic feldspar and a few shreds of muscovite
(spec. 191). The last mentioned mineral occujpies certain areas in
the thin section as if it is secondarj^ after feldspar; in other cases,
patches of matted quartz and muscovite represent the filling of small
miaroles which are common in the rock. Numerous miarolitic cavi-
ties bearing terminated quartz crystals and muscovite plates are
visible in the hand specimen.

PAISANITE DIKE ON LITTLE ASCUTNEY MOUNTAIN.

On referring to the plan of Little Ascutney intrusives (fig. 1) it
will be seen that there is intercalated between the great syenite-
porphyry dike of that ridge and the diorites on the north a second
interrupted dike, which thus entered the same zone of weakness at
the schist-contact as that earlier followed by the porphyry. This
second dike is much smaller than the first, but measures, neverthe-
less, about 50 yards (46 meters) in width at the broadest part. It isj
probably this rock that was referred toby Hawes as the "granitell

74 GEOLOGY OF ASCUTNEY MOUNTAIJSr, VERMONT. [bull. 209.

of Little Ascutney."** It sends apophysal tongues into the diorite
and is similarly believed to be younger than the porphyry, although
only on account of the chilling phenomena observed in hand speci-
men and slide from the smaller dike where it is in contact with the
other.

Hints as to the relationship of this dike are to be found in the ledge
and hand specimen. When quite fresh the rock is a fine pale gray
with a blue tone; in a few days it changes color to the same hand-
some olive-gray green which has been described as characteristic of
the Windsor quarry rock. The resemblance between the two rocks is
also manifest in the way in which they fracture, and in the peculiarly
vibrant musical note given out when a large fragment is struck with
the hammer. Certain of the finer-grained streaks in the quarry can
hardly be distinguished from the green dike in the hand specimen.

?"^?3^

CamptonitediKes Paisanite Nordmarkite-porphyry Diorite Breccia

Fig. 1. — Sketcli plan of intrusive rocks on Little Ascutney Mountain.

The rock is a very fine-grained typical aplite with a sugary, panal-
lotriomorphic structure (spec. 60). The essential minerals are nearly
the same as in the paisanite just described from the main mountain.
Quartz is, however, quite prominent among the phenocrystic indi-
viduals which are otherwise composed of microperthite, either in
separate crystals or in groups. The same constituents, with crypto-
perthite and an alkali-iron hornblende identical in characters with
that of the Windsor quarry rock, are the essentials in the groundmass.
The quartz and feldspar " phenocrysts " are connected through all
stages of transition with the same minerals of the groundmass, and it
is probable that there has been but one generation of these essentials.
The hornblende is strikingly poikilitic, as if corroded in the extreme.
Biotite, oligoclase, magnetite, apatite, and zircon occur as accessories.

n Geology of New Hampshire, Vol. Ill, part 4, 1878, p. 302.

DALY. I

PAISANITE.

75

Table IX. — Analyses of paisanites and other- rocks.

1.

2.

3.

4.

.5.

6.

SiO^ __._

AI2O3

FcsOg

73.69

12.46

1.21

1.75
0.17
0.36
4.47
4.92
0.24
0.14
Trace
0.28
0.14
0.04
0.02
0.05

73. 03
13.43
0.40
1.49
0.14
0.79
4.91
4.54
0.35
0.18
Trace?
0.30
0.06
0.06
0.03
0.08
0.09
0.15
Trace
Trace
Trace
?

77.14
12.24
0.39
1.04
0.06
0.35
4.64
4.47

\ «0.14

66.50
16.25
2.04
0.19
0.18
0.85
7.52
5.53

a 0.50

73.35
14. 38
1.96
0.34
0.09
0.26
4.33
5.66

70.19

11.96

4.94

FeO

1.18

MgO

CaO -- .

0.16
0.65

NasO

5.73

K2O.

4.06

H2O above 110° C

H2O below 110° C-

CO2

Ti02

0.39

0.70

ZrOg

p,o.

Trace

CI

F

FeSa

MnO

0.15

Trace

0.20

0.48

BaO

SrO

Faint tr.
Trace?
Trace

LijO

CuO

0-F,Cl

100. 09
0.02

100.03
0.04

100.66

100. 46

100. 37

99. 94

100. 07

99.99
0.05
3.628

Total S - - -

Sp. er

2. 633

a Loss on ignition.

1. Hornblende-paisanite dike cutting Main syenite, Ascutney Mountain; analy-
sis by Hillebrand.

2. Hornblende-paisanite dike cutting nor dmar kite-porphyry, Little Ascutney;
analysis by Hillebrand.

3. Lestivarite, Bass rocks. Gloucester, Essex County, Mass.; analysis by Wash-
ington, Jour. GeoL. Vol. VII, 1899. p. 107.

4. Classic lestivarite, Brogger, Die Eruptivgesteine des Kristianiagebietes,
Vol. Ill, 1898, p. 216; analysis by V. Schmelck.

5. Classic paisanite, Osann, Tscher. Miner, n. Petrog. Mitth., Vol. XV, 1896,
p. 439.

6. Average grorudite, according to Brogger, Die Eruptivgesteine des Kristiania-
gebietes, Vol. I, 1894, p. 63.

76

GEOLOGT OV ASCUTNEY MOUNTAIN, VERMONT. [bull. 209.

The chemical analysis of this rock is given in Table IX, column 2,
along with that of other rocks of related types. Their corresponding
essential mineralogical composition is as follows :

1. Microperthite, soda-orthoclase. qviartz, alkaline hornblende.

2. Microperthite, qnartz, soda-orthoclase, alkaline hornblende.

3. Microperthite. other alkaline feldspar, quartz, hornblende, biotite.

4. Cryptopertliite, segirine.

5. Microperthite, cryptoperthite, quartz, riebeckite.

6. Quartz, microperthite, microcline, albite, soda-orthoclase, segirine, catoforite.

The molecular jjroportions for the Ascutney rock have been calcu-
lated as follows:

Analysis.

Molecular
proportions.

SiOa

73.03
13.43
0.40
1.49
0.14
0.79
4.91
4.54
0.30
0.06
0.06

1.2165

A1203 . _ ..

.1313

FCaOg

.0025

FeO

.0207

MgO - - -

.0035

CaO _ _.......__

.0141

Na,>0 . _

.0792

K2O .

.0484

TiO, .__

.0036

ZrOs

. 0005

P2O5----

.0004

If we suppose that the hornblende has 10 per cent lime, 3 per cent
soda, and 40 per cent silica (not far from the proportions of those
oxides in barkevikite), we can get an aj)proximate idea of the quan-
titative mineral composition of the rock. On these su^jpositions the
albite molecule would make up 40 per cent of the rock. The propor-
tion of the same molecule would be 38 per cent if the hornblende were
5 per cent soda, and 41.5 per cent if all the soda were in the feldspar.
The results of the calculation, based on an accurate knowledge of the
composition of all the minerals, would not be far from the following:

Per cent.

Albite molecule 40

Orthoclase molecule 27

Quartz 27

Anorthite molecule 2

Hornblende. -.'. 3

Accessories 1 1

100.0

A comparison of columns 1, 2, and 5 in Table IX shows at once the
thorough similarity of this rock to classic paisanite and to the great

DALY.] BUECCIA OK LITTLE ASCUTNEY MOUNTAIN. 77

aplite dikes on the northwest side of the main monntain. The last
mentioned we have seen to be an acid representative of the porphy-
ritic phase of the Main stock; in the same way the Little Ascutney
paisanite, in its composition, evanescent color, and freedom from
basic segregations, is closely allied to the granitic phase of the same
stock. Both of the Ascutney paisanites are allied to grorudite (column
6) and to the " lestivarite " of Essex County (column 3) which, how-
ever, is a type considerably divergent from classic lestivarite
(column 4).

BRECCIA MASSES ON LITTI.E ASCUTNEY MOUNTAIN.

Inclosed in the green paisanite dike are 2 horses of breccia, the
larger measuring in plan 25 feet (7.5 meters) by 8 feet (2.4 meters).
In the older adjoining sj^enite-porphj-ry dike tliere are at least 16
similar horses exposed on the crest of the ridge, as indicated on the
sketch map (fig. 1, p. 74). The largest of the horses in the porphj^ry
is 180 feet (55 meters) in length by 55 feet (16.5 meters) in breadth.
The smallest one mapped covers 8 (2.4 meters) or 10 (3 meters) feet
square, though many smaller fragments of the same rock occur
scattered through the porphyry.

These interesting bodies were first described by the geological sur-
vey of Vermont. In its final report Edward Hitchcock developed a
theory of the Ascutney eruptives which is founded on the discovery
of the breccia. It can best be expressed in his own words :

3t- * * If yf^Q ascend Little Ascutney, near its west end, on the top, just where
the southern slope begins, masses of a conglomerate of a decided character, several
feet and even rod's wide, appear on the side of the porphyry and granite. All traces
of stratification in the conglomerate are lost and it passes first into an imperfect
porphyry, and this into granite without hornblende, in the same continuous mass,
without any kind of divisional plane between them. Where the conglomerate is
least altered it is made up almost entirely of quartz pebbles and a larger amount
of laminated grits and slate, the fragments rounded somewhat and the cement in
small quantity. It is easy to see that a metaniorphism has taken place in all the
conglomerate and some of the pebbles might even be called mica-schist. In the
cement also we sometimes see facets of feldspar. In short, it is easy to believe
that the process of change need only be carried further to produce syenite, por-
phyry, or granite. One can not resist the conviction that the granite rocks of the
mountain are nothing more than conglomerate melted down and crystallized, or
at least that such was the origin of part of them."

Van Hise has dissented from this view and briefly stated his opinion
that these "pseudo-conglomerates " are flow breccias. He says : "The
matrices of these rocks are thoroughly crystalline granular granite,
syenite, or porphyry. Thus they are eruptives which have caught
within them fragments of the rocks through which they have passed."*

The observations of the writer do not agree with Hitchcock's deter-
mination of the relation between the horses and the porphyry. Tliere

a Geology of Vermont, 1861, Vol. II, pp. 565-566.
&Bull. Geol. Soc. Am., Vol. I, p. 236, footnote.

78 GEOLOGY OF ASCUTNEY MOUNTAIN, VEEMONT. [bull. 209.

is no transition between the two, but instead a very clean-cut contact,
which is just as distinct as that between the porphyry and the schists.
The facts that lead to the rejection of Van Hise's theory of the breccia
will also be briefly noted.

The horses do present in the field the general appearance of flow
breccias (spec. 36). In thin section, however, the compact, dark
greenish-gray cement resolves itself into an aggregate of clastic grains
of quartz and feldspar in a secondary groundmass of argillaceous
material, chlorite, biotite, and quartz. The biotite looks metamor-
phic and is concentrated about magnetite, which is comparatively
abundant. Small garnets are also interspersed in the cement in great
numbers, and, like the biotite, were, in all probability, formed in the
partial alteration of the cement by the heat and mineralizers of the
porphyry intrusion.

There can be no doubt that such a matrix is clastic, and the shape
and nature of the inclosed fragments agree with that interpretation.
They are subangular or angular and without visible stratification.
In size they vary from those of microscopic dimensions to others hav-
ing an area of a square foot or more. Usually the corners are sharp
and plainly indicate that the fragments have not been worked over
by water. As Hitchcock pointed out, these "pebbles" are of many
different sorts.

The great majority of the fragments belong to the schists. A
phyllite composed of quartz and sericite (occasionally with metam Or-
phic biotite) as essentials, and of graphitic and iron ore with zircons
as accessories, is very common. It is the slightly metamorphosed
equivalent of the phyllite in the eastern half of the Ascutney area.
The rocks of the contact aureole of the main mountain are also rep-
resented by manj^ fragments that are still more altered forms of the
phyllites than the type just mentioned. Certain of the dark, fine-
grained blocks are made up essentially of cordierite rendered turbid
by numerous microlitic inclusions, probably sillimanite. The gneissic
fragments are usually of the varieties found in the fundamental for-
mation at the foot of Little Ascutney. They are typical biotite-
gneisses, often garnetiferous and sometimes charged with epidote.
The mica-schist of the basal crystallines has a place in the breccia as
a biotite-quartz-schist with little accessory material. An ainphibo-
lite, identical in composition with that described from the locality at
the foot of Crystal Cascade, is likewise x)resent among the blocks.

Quartz occurs as large, angular jjieces from single crystals, as com-
pact quartzite, and as a chalcedonic variety. Granular epidote with
some quartz forms smaller fragments up to 1 inch (2.6 centimeters) in
diameter, suggesting the equivalent of the metamorphosed limestone
of the Ascutney contact zone. Large broken crystals of orthoclase are
also common and are unquestionably fragmental in the same sense as
the cement. So far as known, there is only one igneous rock among

DALY.] BIOTITE-GEANITE STOCK. . 79

the breccia fragments, and it is one of the least abundant kinds. It
is a tyjpical granite-porplijny with an ideal development of idiomor-
pliic quartz and feldspar phenocrysts. The only dark-colored con-
stituent is biotite, the lamellae of which are always grouped after the
manner of segregations and never seem to form true phenocrysts.
This is the only component of the breccia which is not to be found in
crystalline schists surrounding Ascutney. The breccia cement itself
may be regarded as the comminuted remains of the broken-up schists.
Each of these great horselike inclusions is now seen to have the
composition, structure, and possible field relations of a true fault
breccia. The most satisfactory^ explanation of them would attribute
them to the disrupting action of vigorous and long-continued differ-
ential movements in an ancient zone of dislocation. That zone was
perhaps nearly coincident with the present course of the two large
dikes in which the horses are embedded. The faulting gradually
prepared the material and recemented it into a new, tough, solid rock.
Later, the invading eruptives carried off large masses of it as they
forced their way along the old zone of dislocation and consequent
weakness. The average specific gravity of the horses is about 2.79;
that of the sj^enite porphj^ry, 2.(346. It is reasonable to suppose that
the immersed blocks of breccia sank to their present position rather
than that they were carried up from below.

BIOTITE-GRANITE STOCK.

The second of the stocks which go to make up the main mountain
is much the smaller of the two. The total area is about 1 square
mile (2.6 sq. km.). The highly irregular contact line touches both
syenite and phyllites (see PI. VII).

That this rock (an alkaline Ijiotite-granite) must be referred to a
period of intrusion different from that of the Main stock was a matter
of somewhat prolonged stud}^ in the field. An early examination of
specimens and outcrops indicated that on the southeast side of the
mountain along the contact with the schists there were two points
where the igneous rock changed from a quartzless hornblende-bearing
phase to a highly quartzose phase, devoid of all dark-colored essen-
tials save an occasional shred or plate of biotite. This change was in
all cases sudden, and each phase held its character for a long distance
from their common contact with the schists. A second visit to the
same localities explained the true relations. It showed that the gran-
ite is not of the nature of an acid flow streak on a large scale in
the syenite, but that the former was intruded after the syenite had
consolidated. A characteristic endomorphic zone that developed in
the granite will be described below. A slight amount of alteration
in the syenite itself may be detected. It is of the nature of the
formation of a secondary granophyric structure among the feldspars

80 GEOLOGY OF ASCUTNEY MOUNTAIN, VERMONT. [bull. 209.

of the once thoroughly granular syenite, but it is sufficient to empha-
size here the granitic apophyses in the nordmarkite as a proof of the
younger date of the more acid rock.

In one of these apophyses about 6 inches (15.2 centimeters) in width
the walls of syenite must have been already solid when the granite
was intruded, since out from them have been developed a large num-
ber of quartz prisms standing squarely on the walls and terminated
with the usual planes at the free ends, which, of course, point toward
the middles of the apophysis. The prisms average 3 or 4 centimeters
in length by 1 centimeter in thickness. Other large crystals similar
to these sessile ones are to be seen completely surrounded by the
granitic matrix; they are doubly terminated. This apophysis, while
thus allied to the pegmatites, is yet believed to be a true offshoot of
the granite because of the identity of composition existing between
its matrix and the material of the more normal apophyses. An anal-
ogy is to be found in a syenite dike cutting the classic laurvikite,
wherein cryptoperthite crystals take the place of the large quartzes of
the Ascutney dike.*

The granite has been mapped with a fairly close degree of accuracy.
The very dense second-growth timber and a lack of outcrops pre-
vented the discovery of the contact line in some places.

In three abandoned quarries situated in the southern lobe of the
stock, about 250 yards (225 meters) from the schist contact, a more
or less well- developed master jointage is to be seen. Its chief interest
consists in the evident independence of this jointage and the present
topography of the deep valley in which the quarries are situated.
The latter lie on an east-west line. At the main quarr}', in the mid-
dle, the joint planes dip about 15" south, or a few degrees east of
south. At the eastern and western quarries they are found to swing
into an easterly and westerlj;- direction of dij) respectively. In the
western quarry the ground slopes east by south; in the eastern, west
by south. In other words, the jointage is quaquaversal in the bottom
of a steep- walled ravine or gorge. It must be said that such jointing
can hardly be explained by the formation of concentric rift-planes
through atmospheric changes of temperature. The conditions seem
rather to confirm the more general view that the structure is an orig-
inal phenomenon due to cooling.

The granite, excepting in a narrow contact zone, remains quite uni-
form in character throughout the stock. It is a typical pseudo-
porphyritic alkaline biotite-granite (spec. 2). The color is a light
grayish X3ink, the grain medium to coarse, the structure liypidiomor-
phic granular in the groundmass. The phenocryst-like constituents
are quartz, microperthite. orthoclase, and soda-orthoclase (with a spe-
cific gravity of 2.584 at 16"^ C), accompanied by a small proportion of
biotite individuals. These minerals have the same features as in

a^rogger, Zeit fur Kryst., Vol. XVI, 1890, p. 193-

DALY.]

BIOTITE-GBANITE STOCK.

81

the nordmarkites. They compose the groundmass in which a con-
siderable amount of multiple-twinned albite (near Abg An,) and
titanite with some magnetite, zircon, apatite also occur. The pheno-
crysts are not sharply separated from the groundmass in size, and it is
likely that there has been only one generation of those constituents.
The biotite is nearly uniaxial. The large amount of FeO in the analy-
sis suggests that it is a lepidomelane. Titanite is beautifully crystal-
lized with the ordinary rhombic outlines and well-developed prismatic
cleavage. Its pleochroism is strong:

a, pale yellow.

b, yellow.

c, reddish yellow.
c>b>a.

Pseudomorphs of magnetite (ilmenite?) after titanite are not uncom-
mon. In one out of seven slides made from this rock a single small
individual of pale-green amphibole was discovered. Augite fails
entirely. Here, as in the other stock rocks of the area, the quartz is
rich in liquid and gaseous inclusions and in negative crystals arranged
in lines and also provided with double bubbles of gas immersed in
liquid.

The order of crystallization is the normal one:

1. Titanite, apatite, zircon, and magnetite.

2. Lepidomelane.

3. Albite and ortlioclase.

4. Microperthite.

5. Quartz.

The essential oxides (see Table X, col. 1, p. 84) and their molecular
proportions are noted in the following table:

SiO^.

AI2O3

Fe^Oa

FeO-

MgO

CaO.

Na,0

K2O-

TiO.,

ZrO,

P,0.

Molecular
proportions.

1.1980
0. 1382
0. 0070
0.0120
0. 0082
0. 0201
0. 0723
0.0511
0. 0043
0. 0003
0. 0008

If all the Ti02 be ascribed to titanite, and if we assume that the
mica contains 10 per cent MgO, 8 per cent KjO, and 40 per cent SiOg,
Bull. 209—03 6

82 GEOLOGY OF ASCUTNEY MOUNTAIN, VERMONT. [bull. 209.

the analysis may be calculated and the quantitative inineralogical
composition determined, with small degree of error, as follows:

Per cent.

Albite molecule 37. 9

Orthoclase molecule 27. 0

Quartz . 25.0

Anorthite molecule _ _ . 3. 9

Biotite - . - 3.8

Magnetite . 1.6

Titanite 0.9

Apatite 0.3

Zircon 0.1

100. 0

Chemically, this rock is an ideal equivalent, among the alkaline
rocks, of granite among the nonalkaline eruptives, a biotite-granite
characterized by a high total of alkalies with the soda and potash in
nearly equal proportion. Iron, lime, and magnesia are all low. It is
again to the Christiania region that we must go for the already
described type nearest to this one. The "granitite" of Lier affords
an analj^sis which is noted in column 4, Table X. In the Norwegian
field, as at Ascutnej^, the biotite-granite is the youngest eruptive
excepting the lamprophyres. A close and interesting correspondence
between these two rocks is further illustrated in the character of the
endomorphic contact phase. It is notably granophyric and miarolitic
in the Norwegian occurrence, and, as we shall see, is in these respects
similar to the Ascutney rock.

BASIC SEGREGATIONS IN THE GRANITE.

The homogeneity of the biotite-granite is affected by the presence
of nodular basic segregations which, while not nearly so abundant as
in the nordmarkites, are characteristic of the rock. They vary in
color, composition, and size. Three classes may be distinguished, not
only from each other, but, as well, from the metamorphosed schist
inclusions, which occasionally appear within the mass of the stock.

The commonest segregation is of a more basic character than the
other two kinds (spec. la). It is dark greenish gray in color, spher-
ical, oval, or lenticular in form, and in the hand specimen sharply
outlined against its host. In thin section, however, it is once more
seen that this macroscopically sharp outline of a segregation does not
forbid a very intimate interlocking of its minei-als with those of the
host. The size may vary from that of a pea to nodules as large as a
man's fist. Under the microscope the nodule is seen to be a panallo-
triomorphic aggregate of much biotite, hornblende, and triclinic feld-
spar always close to and averaging the oligoclase, AbjAuj, together
with smaller amounts of microperthite and orthoclase. Intei'stitial
quartz, much titanite and apatite, and a remarkably small amount of

DALY.] BASIC SEGREGATIONS IN THE GRANITE. 88

magnetite comprise tlie accessories. The hornblende is not identical
with that of the syenite nodules, as is shown by the j)leocln'oism :

a, pale yellowish green.

b, olive-green (medium to strong absorption).

c, olive-green with a strong bluish cast (medium to strong
absorption).

b>c>a.

The other constituents have the same projjerties as the parent rock.
Often in this class of segregation there are small secondary nodules
of nearly pure biotite and hornblende, some of which, by the concen-
tration of the mica around the j)eriphery, recall the kernels of the
nordmarkites. On the other hand, the general continuity of the main
segregation may be interruj^ted by light-colored spots composed chiefly
of oligoclase, quartz, and idiomorphic hornblende.

But one example of the second type of segregation has been found.
This is a gray, roundish mass about 7 feet (2.1 meters) in diameter
occurring near the 2,100-foot contour close to the northern contact
(with the syenite) of the great southwestern tongue of the granite
(spec. 113). This large nodule is strongly alkaline, the predominant
feldspars being microperthite, albite (pure or charged with the anor-
thite molecule up to the limit, AbgAuj), microcline, and probably
cryptoperthite — named in the order of their abundance. Free quartz
makes up probably as much as one-third of the rock. A hornblende
that has not been observed in any other rock of the area is the
remaining essential. It forms long, narrow, microlitic, irregularly
terminated blades. It is pleochroic according to the scheme :

a, light greenish yellow.

b, deep brownish green.

c, deep brownish green with a strong bluish tinge.
b=c>a.

Much idiomorphic titanite, a few rare corroded plates of biotite,
many crystals of magnetite, considerable apatite, and very rare zircons
form the list of accessories. The structure is here hypidiomorphic
granular.

Allied to the second type is a third class of the segregations, type
analyses of which are given (Table X, cols. 2 and 3, p. 84). Here the
biotite is much more common than the hornblende, the feldspars and
accessories remaining the same in nature and relative abundance
(spec. lb). Quartz is not so abundant. The structure is the hypidio-
morphic granular. The feldspars are somewhat altered, as is indi-
cated in the chemical analysis. Calcite and a little muscovite are the
secondary products. The microscopic diagnosis and the analysis
agree in putting these segregations among the alkaline quartz syenites
(akerites). Again, it will be observed that there is an especially large
amount of the mineralizer, fluorine, in the segregation.

84 GEOLOGY OF ASCUTNEY MOUNTAIN, VEEMONT. [bull. 209.

Table X — Analysis of biotite-granite.

3.

SiO,

AI2O3

FeA

FeO

MgO

CaO

Na^O

K2O

H2O above 110° C.
H2O below 110° C.

CO2

TiO^

ZrO,

PA

CL_.. J

F

FeS,

NiO, CoO

MnO

BaO

SrO

LLO

0=F,CL

Total S- -
Sp. gr...

71.90
14.12
1.20
0.86
0.33
1.13
4.52
4.81
0.42
0.18
0.21
0.35
0.04
0.11
0.02
0.06
Trace.

0.05
0.04

Trace.

Trace.

59.27
15.76
2.07
8.57
3.04
3.69
5.63
3.33
0.74
0.23
0.30
1.12
0.04
0.42
0.03
0.42
0.07

Trace.
0.37

Trace?
Faint tr.

Trace.

56.01
8 15.19
2.34
4.89
4.67
4.85
5.66
2.16
0.36
0.90
Undet.
1.13

100. 85
0.03

100. 32
Trace.
2.616

100.10
0.19

0.53

Undet.

Undet.

0.09

0.03

0.40

Trace?

99.91
0.037
2.661

99.21

2.720

75. 74
13.71

0.55

Trace.
1.26
8.72
4.69

0.46

0.17

100. 30

ft "With ZrOo.

1. Biotite-granite, Asciitneyville quarries; analysis by Hillebrand.

2. Basic segregation in the biotite-granite: analysis by Hillebrand.

3. Another sample of the last, containing more hornblende; analysis by Hille-
brand.

4. '"Granitite" from Lier, Christiania region; Brogger, Zeit. ftir Kryst., Vol.
XVI, 1890, p. 72.

ENDOMORPHIC ZONE OP THE GRANITE.

The detection in the field of the actual plane of contact of granite
and syenite is rendered comparatively easy on account of a con-
spicuous structural variation whicli characterizes the endoniorphic
zone. At the average distance of 20 feet (6.1 meters) from the con-

DALY.] LAMPEOPHYEES. 85

tact the normal granite becomes much more pori)hyritic in appear-
ance (spec. 105). The phenocrysts are chiefly quartz, which may
either retain its dimensions in the normal rock or may form much
larger doubly terminated crystals. The groundmass is much finer
grained, though always holocrystalline, and is either granophyric or
identical in general structure with the normal granite. Large ter-
minated crystals of quartz may sometimes be seen projecting from
the syenite into the granite in the same way as in the apophyses
already described. Especially marked on the ledges of the contact
zone are abundant roundish miaroles from 1 inch (2.6 centimeters)
to 3 inches (7.8 centimeters) in diameter, either completely filled with
crystalline matter or presenting cavities lined with well-formed
crystals (spec. 106).

The usual occupants of the miaroles are quartz crystals showing
the common terminal and prismatic i3lanes and crystals of the same
feldspars as occur in the granite proper. The feldspars are flesh col-
ored or light brownish and, in thin section, turbid. They bear the
planes (001) (010) (110) (101) (201) (021) and (111); (010) and (001)
are especially wall developed. The commonest of these feldspars
seems to be a genuine microperthite. It illustrates in excellent
fashion the rare Manebacher law of twinning and the murchisonite
cleavage. The triclinic feldspar of the intergrowth is pure albite,
giving an extinction angle of 19° on (010). Microcline and orthoclase
crystals also occur in the miaroles. The latter has the small optical
angle of sanidine. Finally, pseudomorphs of limonite after siderite
completes the list of the minerals which have been found in the mia-
roles. Biotite seems to be absent. A close parallel to this endo-
morphic zone is furnished in the aplitic granophyre described by
Brogger as the contact phase of the alkaline biotite-granite of Lier,<^
which in other respects resembles the Ascutnej^ rock.

The endomorpliic zone has been enriched by the incorporation of a
certain amount of basic material evidently-derived from the syenite.
In the granophyric groundmass there are sporadic irregular granular
areas impregnated with an alkaline hornblende near barkevikite and
biotite. These are not found where the granite is in contact with the
phyllites.

LAMPROPHYRES.

A number of dikes of lamprophyric habit cut the syenites at various
points, and rocks of the same character intersect the Basic stock and
the schists. No such dike has been discovered in the granite stoclc,
but it is probable that they are all younger than that stock. They
belong either to the class of camptonites or to the class of diabases.
A similar association of the two groups in the same region has been
described by Kemp as occurring on the Maine coast,*

aZeit. fiii- Kryst., Vol. XVI, 1890, p. 7■^. 6Bull. Geol. Soc. Am., Vol. I, 1890, p. 32.

86 GEOLOGY OF ASCUTNEY MOUNTAIN, VEEMONT. [bull. 209.

CAMPTONITES.

At the top of Little Ascutney a camptonitic dike cuts the nord-
markite-porphyry dike, the paisanite dike, a horse of the breccia,
and the diorite. The rock is a very compact grayish-black mass, in
which here and there a hornblende crystal and, more rarely, a feld-
spar appear as phenoerysts (spec. 57). In thin section it is seen to be
an acid camjDtonite of the usual structure and composition. The
hornblende phenoerysts are idiomorphic and measure from 1 to 3
millimeters in length. The pleochroism and absorption are those of
a common basaltic hornblende:

a, pale brownish yellow.

b, deep, rich brown.

c, deep, rich brown.
c>b>a

The extinction on (010) is about 15° 30'.

The plagioclase is apparently very uniform in composition and
averages the basic labradorite, AbgAng, both in the rare phenoerysts
and in the groundmass.

The rock is greatly altered, and this characteristic adheres to all
the camptonites of the area. Chlorite, epidote, calcite, secondary
quartz, and kaolin are the products of decomposition.

Analysis 1, in Table XI, represents the approximate composition of
the average camptonite of the area (spec. 74). It was made from a
type differing from that just described in containing a small i^ropor-
tion of augite in the groundmass and, more rarely, among the pheno-
erysts. The feldspar is here again the labradorite Ab^Aug. The
augite is much altered (into uralitic amphibole, chlorite, and the
ores) from its original diopsidic condition. Dikes corresponding to
this analysis were found cutting the syenite on the Windsor trail
about 500 yards (457 meters) from the main summit of Ascutney
Mountain, cutting coarse diorite in the saddle at the east end of
Little Ascutney, and cutting gneiss east of the notch road and south-
west of Brownsville.

While there is a noteworthy difference between this analysis and
that of Hawes's classic camptonite, the former agrees well with the
average analysis of camptonite as calculated by Brogger (cf. Table
XL)

DALY.]

OAMPTONITE.

Table XI. — Analysis of camptonite.

87

1.

2.

3.

SiO^

48.22

14.27
2.46
9.00
6.24
8.45
2.90
1.93
1.66
0.28
0.15
2.79
0.03
0.64
0.10
0.05
0.36
0.03
0.20
0.04

Trace.

Trace.

Trace.

41.94
15.36
3.27
9.89
5.01
9.47
5.15
0.19

} 3.29

2.47
4.15

43.65

AI2O3 . .

16 29

FeA

FeO

14 76

MgO...

5 96

CaO

10 16

NaaO

3 05

K2O

1 50

H2O above 110° C .

H2O below 110° C

CO2

TiO^

4 63

ZrO^

P2O5

CI

F.

FeSj

NiO,CoO .

MnO

0. 25

BaO .

SrO

LijO

CtiO

0=F,C1

99.80
0.04

100.44

100.00

99.76

0.19

2. 810-2. 869

\

Totals. - -

Sp.gr

1. Camptonite dike, Ascutney Mountain; analysis by Hillebrand.

2. Classic camptonite, Campton Falls, N. H.; Rosenbnsch: Elem. der Gesteins-
lehre, 2ded., 1901, p. 244.

3. Average analysis of eight camptonite dikes; Brogger: Quart. Jour. Geol.
Soc, Vol. L. 1894, p. 26.

The hornblende has not been analyzed; it is, hence, not possible to
calculate the analj^sis. The specific gravity of these dikes varies
from 2.810 to 2.869; on account of alteration, no two pieces, even
from the same hand specimen, will agree in specific gravity.

DIABASE DIKES.

The second class of melanocratic dikes comprises compact, equi-
granular or porphyritic diabases of normal composition and structure,

88 GEOLOGY OF ASCUTNEY MOUNTAIN, VERMONT. [bull. 209.

thus not differing essentially from the common dikes of the same rock
occurring so abundantly up and down the Connecticut Valley
(spec. 120). The feldspar is here also near the basic labradorite
AbgAug. Like the pale-green intersertal augite, it is much affected
by weathering ; the products of the change are the same as in the
camptonites. A sulphide, probably pyrite, is visible in notable
amount, even in the hand specimen. The magnetite is strongly titan-
iferous. Zircon and titanite are absent, and there is comparatively
little apatite. The specific gravity was measured at 2.922. The
total analj^sis is given in Table XII. It corresponds to that of a
common, somewhat weathered diabase.

Table XII. — Analysis of diabase (by Hillebrand)

Si02._--. .- 49.63

AI2O3 . 14.40

FeA . 2.85

FeO 8.06

MgO 7.25

CaO 9.28

Na^O 2.47

K26 0.70

H3O above 110° C 1.47

H2O below 110° C 0.27

CO2 1.36

TiO^ 1.68

ZrOa - Trace?

PA 0.25

CI 0.07

F Trace

FeS., 0.22

NiO,CoO 0.04

MnO - 0.17

BaO Trace?

SrO Trace?

LiaO Trace

100. 17
0=F, CI 0.02

100. 15

Totals 0.12

Sp.gr 2.922

SUMINIARY.

The list of eruptive rocks in Mount Ascutney includes the Basic
stock of gabbros and diorites transitional into one another and into
an acid essexitic phase, ramifying dikes of younger diorite, dikes of
a rock type not heretofore described and called "wiudsorite" from
this Ascutney occurrence, the nordmarkite-porphyry dike-like stock
of Little Ascutney, the pulaskite stock of Pierson Peak, the great
paisanite dike of Little Ascutney, the variable nordmarkites of the
Main syenite stock with granitic and monzonitic phases, the homo-
geneous alkaline biotite granite of the Ascutneyville stock, and the

DALY.] GENERAL FEATURES OF THE ERUPTIVES. 89

aplites and lamproph^yres (diabase and cami3tonites) cutting nearly
all the other bodies.

Chemically, the series of eriiptives as a whole is characterized by
normal silica, high alkalies, the potash slightly predominating, nor-
mal alumina, medium iron, low magnesia, low lime, and high titanic
oxide. Mineralogically, they are rich in feldspar which is generally
microiperthitic, and are poor in biotite and bisilicates. Especially
noteworthj^ is the extraordinary development of indigenous basic
nodules of segregations and of ' ' schlieren " in the different rock bodies,
including both dikes and stocks. The abnormal abundance of the
segregations is i)robably to be connected with the smallness of the
conduit through which each irruption took place. Variations of tem-
perature, abundance of foreign fragments, and the repetition of intru-
sion in the same conduit have all played their part in disturbing the
normal process of crystallization.

Considering the small area occupied by the Ascutney intrusives,
they must be considered as having an unusually wide range of com-
position (see list on p. 36). While dioritic types fail in the allied
petrographical iDrovince of the Christiania region and nordmarkites
and related alkaline rocks are absent in the Monzoni region, both of
these classes are represented at Ascutney. The intimate association
of independent bodies of such nonalkaline rocks as the gabbros and
diorites of the oldest stock and of the older basic dikes, with the
several bodies of typical alkaline syenites and granite, is to be par-
tieularlj^ emphasized. That such an association is not rare, in America
at least, is shown by its repeated occurrence in New York State '^ and
in Essex County, Mass.^ Not the least significant fact concerning the
Ascutney eruptive group is the occurrence of rock types transitional
between the nonalkaline and alkaline irruptives. Thus there is not
only the most striking consanguinity among the respective members
of each of these classes, but the two classes are themselves allied by a
family relationship which is reflected also in many details of miner-
alogical and chemical composition.

Among the details described in connection with the irruptives, we
may recall some which, while of greater or less importance to the gen-
eral geology or mineralog}^ of the area, are not implied in the forego-
ing resume of the intrusions. These include the evidence for the
cylindrical character of the main controlling stock of Mount Ascutney
itself, the remarkable tarnishing which slight exposure produces in
the nordmarkite of the Windsor quarry and in the related paisanite
of Little Ascutney, the great masses of breccia in the nordmarkite
porphyry and paisanite dikes, and the interesting endomorphic phases,
especially of the biotite-granite. Finally, the considerations relative
to the mode in which the conduit has become occupied by the different
intrusives will be summarized in the following chapter.

aCushing, Bull. Geol. Soc. Am. Vol X, 1899, p. 177. Smyth, Ibid., Vol. VI, 1895. p 263. Eakle,
Am. Geol.,Vol. Xll, 1893, p. 35.
6 Washington, Jour. Geol., Vol. VI, 1898 p. 799.

CHAPTER V.

THEORETICAL CONCLUSIONS.
MA:N^]SrER OF IXTRUSIOK OF THE STOCKS.

Probably the most important question in connection with the dy-
namical geologj'' of the mountain centers about the actual method of
injection followed by each of the five largest eruptive bodies.

Hitherto it has been assumed that the Ascutney rocks belong to the
category of genuine intrusives and that, for example, we have not
here to do with a deeply eroded volcanic neck or pipe. This view
will be still held in the further discussion of the rock bodies. The
evidence, however, that the granitic rocks now exj)osed do not, after
all, represent the deep-seated equivalents of magmas which, in the
eruptive periods, escaped at the earth's surface as lava flows, is largelj^
but negative. To exclude this volcanic hypothesis it is clearly not
sufficient that the igneous bodies have all the characteristics of a plu-
tonie origin and that there is in the vicinity, at present, lack of explo-
sion breccias or lavas in any form. The complete removal of such
products is only a question of the time allowed and the depth of
denudation since the early period of the eruptions. Nor, again, and
just as surely, is the general lack of flow structure showing a rela-
tively rapid upward movement of the magma of distinct help in decid-
ing the alternative.

The ]3eculiar arrangement and form of the stocks, especially the,
lobate plan of the granite, seem to give greater satisfaction intheplu-
tonic hypothesis. But a much stronger reason, affording cumulative
evidence for its adojjtion, is doubtless to be found in the analogy of
Ascutney Mountain with a score of other granitic areas in Vermont
alone. There appears to be no evidence at all for the volcanic nature
of these igneous bodies. Although the geological conditions are often,
particularly in the smaller areas, very similar to those at Ascutney,
extrusive rocks seem never to be organically associated with a single
granitic mass. So far as known, there is no more reason to attribute
a volcanic origin to any one of these smaller granitic bodies in Ver-
mont than to the others; nor, indeed, than to the great massifs typi-
fied by the Barre granite. The transition in size from the smallest to
the largest area is gradual, and mere size and shape will not suffice as
a criterion of volcanic origin for any one. It is, perhaps, not too much
to say that, if the Ascutney bodies occupy the vent of an ancient
greatly eroded volcanic neck, the Barre granite itself, contrary to the
90

DALY.] THEOEIES APPLICABLE TO THE INTEUSIONS. 91

received opinion of geologists, may be regarded as possibly of the
same origin. In other words, it is reasonable to believe that the vol-
canic hypothesis for Ascutney Mountain has no stronger foundation
in fact than it has for normal granitic areas the world over. In this
view, the igneous rocks of Ascutney are all Irruptive and each irrup-
tive body assumed its present position and full volume only after
some process had prepared the corresponding space within the coun-
trj^ rock^. The problem before us relates to the manner in which that
preparation was carried out.

APPLICATION OF EXISTING THEORIES TO THE ASCUTNEY

INTRUSIONS.

It is held by some of our ablest masters in petrological science —
perhaps by most of them — that stocks, sills, laccoliths, and dikes are,
from the point of view of dynamical geology, of the same nature — i. e.,
that they vary only in size and form. Each is composed of a con-
solidated rock magma which has been injected into the country rock
because of a previous or accompanying oj)ening of cavities within the
earth's crust. Displacements of folded and faulted rocks in mountain
massifs are made responsible for the cavities or chambers. The lat-
ter may be supposed to antedate the eruption (plainly an inadmissible
premise for the larger eruptive bodies) or to have been magmatically
filled pari passu with the dislocation. In either case the adherents
of this theory believe that no important assimilation of the country
rock by the invading magma takes jjlace, and that, therefore, the
composition of the magma is not affected by such assimilation.

It is clear from the foregoing discussion of the Ascutney intrusives,
as well as from the inspection of the map (PI. VII), that none of the
larger ones is laccolithic in character. The syenite-porphyry of Little
Ascutney has apparently followed a zone of weakness, the contact of
the gneiss and Basic stock. Though for this reason dike-like in its
geological relations, it has so far enlarged its conduit as to assume
the proportions of a stock. The other four bodies are true stocks.
Each eruptive cuts across the schists, so that the igneous contact gen-
erally stands at a high angle to the strike of the schists. Only at the
eastern end of the mountain are the two parallel, and it has already
been noted that the surface of contact of the Main syenite stands
nearly vertical, and is thus not coincident with the dip plane of the
schists there any more than on the other slopes of the mountain.
The facts show that a laccolithic origin can not be hypothecated for
the Main syenite, much less for the granite.

The east- west elongation of the igneous area as a whole might sug-
gest that the intrusions occupy a zone of dip faulting in the schists.
But the transitional contact belt of the phyllitic and gneissic series is
easily recognized on both the north and the south side of the moun-

"The term "country rock" is used in this report as a convenient expression denoting the ter-
rane invaded by and thus in contact with an irruptive body.

92 GEOLOGY OF ASCUTNEY MOUlSfTAIN, VERMONT. (bull. 209.

tain, and it is quite as easily seen in the field that the belt has not
been appreciably offset by a fault transverse to the strike. It is like-
wise in the highest degree improbable that displacements iiarallel to
the strike of the schists can be safely called upon to explain the spaces
now filled with the solidified magmas.

The facts derived from field study also speak strongly against the
idea that all or any of these intrusions took jDlace in consequence of
the removal of the country rock en bloc bj^ faulting. It might be
conceived that faulting could have led to the transfer of the displaced
schists upward, as if punched out by a huge die,^' or to the founder-
ing of the corresponding blocks, which would thus become buried
deeply by the magma entering the resulting chamber. If such circling
faults had occurred we should expect some evidence of them to be yet
decipherable in the country rocks. Yet the latter show no sign of
disturbance that can be traced to those particular movements. Even
if the intrusion had taken place after this manner in one instance, it
is in the highest degree improbable that so unusual a dynamic proc-
ess should have been repeated three times in this limited area. The
ground plan of the different stocks as expressed in the geological map
can scarcely be explained on the hypothesis of circling faults. We
must rather conclude, from a survey of the ground, that each of the
stock bodies has actively displaced its country rock so as to find room
for itself. The Basic stock has displaced the gneisses; the nordmark-
ite stock has displaced diorites and schists; the stock of Pierson
Peak has displaced gabbros and diorites; the nordmarkite-poriDhyry
of Little Ascutneyhas displaced gneisses and diorites, and the gran-
ite has displaced the syenite and a small amount of the phyllites.

A second commonly held view of many stocks and of many "batho-
liths" is that they have undergone their "mise en place" as a result
chiefly of the caustic and assimilating property of the igneous magma
in contact with the country rock. Thus, while Brogger regards the
Predazzo-Monzoni area as illustrating in a thoroughgoing manner the
process of differentiation in deep-seated chambers prepared by crustal
movements, Fouque finds in the same province an almost typical
example of assimilation. It is here, as elsewhere, a question of the
degree to which the process produces its effect, as every field geologist
is bound to credit the assimilation of small foreign fragments caught
in a molten magma and, as well, the local and subordinate digestion of
the walls at certain of the eruptive contacts already described in geo-
logic literature.

Stated in its usual form, the assimilation hypothesis also will hardly
fit the facts recorded for the Ascutney eruptives. The oldest stock is
quite basic, though it cuts a series of acid gneisses. The Main syenite
shows no basification at its contact with the diorites. Its endomorphic
contact phase is, on the contrary, there, as elsewhere, more stronglj^^
quartzose and has often even a smaller proportion of bisilicate than

"Recalling the "bysmalith" of Iddings, Mon. U. S. Geol. Survey, Vol. XXXII, Pt.II,1899,p.l6.

DALY] SUGGESTED HYPOTHESIS AS TO THE INTEUSION. 93

the average syenite. The biotite-granite has, it is true, a hornblende
among the constituents of its basic segregations; but the endomorphic
zone, where the granite comes in contact with the older syenite, does
not exhibit any special sympathy with that rock. Still more impor-
tant is the complete lack of basification in the pulaskite of Pierson
Peak which cuts the diorite and associated gabbro. That stock is of
small dimensions. It is alkaline, and hence is composed of material
which in its magmatic form must have been of specially caustic nature.
The crystalline rock through which the magma found its way has a
markedly different composition. There, if anywhere, we should, on
the assimilation hypothesis, look for an endomorphic zone sensibly
affected by the country rock. The failure of such a zone is as unques-
tionable as in the notable case of the shonkinite laccolith of Square
Butte, Montana.^' The Pierson Peak stock is made up of a homogeneous
syenite which is indistinguishable, save for the absence of free quartz
and the disappearance of much of the essential bisilicate, from an
abundant phase of the Main stock of Ascutney Mountain proper.
Both stocks are syngenetic with the stock-like dike of Little Ascutney.
The occurrence of all three with essentially similar mineralogical and
chemical properties, but with essentially diverse country rocks, seems
to prove that they came from a single magma which persisted in
nearly its pure form even after injection and notwithstanding the
well-known solvent power of alkaline magma on both acid and iron-
rich basic rocks.

SUGGESTED HYPOTHESIS OF THE MANNER OF INTRUSION.

Without considering other and less important views of the mechan-
ics of intrusion, which, suggestive as they are, must yet be regarded
as insufficiently supported by observations in nature, a somewhat
detailed statement may be made of a third hypothesis which has
forced itself upon the writer. It not only explains the facts as
far as Mount Ascutney is concerned, but meets as well all the tests
which have yet been ai^iDlied to it from the results of experimental
geology, from observations in other regions, and from the theory of
igneous bodies generally.

Most geologists are agreed that intrusion on a large scale is not a
sudden act, but occupies a period of time comparable to that required
for complex folding in a mountain massif. This conclusion has been
reached by those advocating the assimilation theory, as well as by
those holding the rival theory of laccolithic and allied crustal dis-
placement.^ While the conclusion of any investigator as to the time
required for a magmatic injection is itself in part a by-product of the
intrusion theory ruling in his mind, it is yet noteworthy that the
present exponents of the opposed theories accord in ascribing great
duration to the time required for granitic intrusions at least. It will,

"Weed and Pirsson, Bull. Geol. Soc. Am., Vol. VI, 1895, p. 389.

bW. C. Brogger, Die Eruptlvgesteine des Kristianiagebietes, Vol, II, 1895, p. 149.

94 GEOLOGY OF ASOUTNEY MOUNTAIN, VERMONT. [bull. 209.

then, be no antecedent objection to the hypothesis now to be proposed
that it is based on a process of integration of small effects, and that
the integration suffices for the geological work in hand only after the
lapse of a long period between the beginning and end of the process.

The starting point of the hypothesis is found in the consideration
of the phenomena of the contact belt belonging to each irruptive body.
The present assimilation theory of Michel Levy and others demands
a study of the same belt as a test, rather than as the basis of formu-
lation.

At many points in the internal contact zone of any one of the stocks
numerous fragments of the countr}^ rock may be seen in the erup-
tive (PI. VI). These are completely isolated, immersed in the crys-
tallized magma, though often they are seen to have moved only a few
feet or inches from the parent rock. As found in endomorphic zones
of plutonics generally, the fragments are further quite normal in
showing angular outlines and very sharp boundaries against the
eruptive rock. There is usually, indeed, plain indication that these
fragments have suffered little, if any, chemical solution by the
naagma. Recent experiments, however, have established beyond per-
adventure that, at temperatures but slightly above that of complete
fusion of any silicate mixture, every important rock-forming mineral
may be completely dissolved in that magma.* The conclusion seems
unavoidable that, at the moment when a given foreign fragment was
torn or floated off from its wall and thereafter, the immersing magma
was relativel^^ cool, and thus enfeebled in its solvent power. That its
metamorphosing power was likewise diminished is suggested by the
fact, borne out by microscopic study, that the recrystallization of the
fragment is generally no more advanced than that of the country
rock many feet from the irruptive.

But a still stronger proof of a comparatively low temperature at the
moment of isolation of any one of the fragments is the fact that it is
now to be seen floating, as it were, or, to be more accurate, suspended,
in the magma. A brief consideration of certain experimental deter-
minations shows that such suspension can occur in a normal magma
invading rocks of average specific gravity only on the condition that the
magma is highly viscous and near the point of consolidation. It has
been established that, for each class of hoioerystalline silicate rocks, the
specific gravity of the corresponding glass is considerably lower than
that of the natural rock, and that the specific gravity of the same rock
when completely melted is still lower than that of the glass. No inves-
tigation has been made on these points for any of the Ascutney rocks,
l)ut it is fair to use the results for similar rocks from other parts of
the world.

The most important case for consideration is evidently the relation

" Amoug other papers, cf. C. Doelter, Die Schmelzbarkeit der Mineralien und ihre Loslichkeit
in Magmeu. Tscher, Mm. u Petrog Mitth.. Vol XX, laoi, p 30?; and Ueber emige petrogene-
tische Fragen: Centralbl. t. Mm., Geoi. und Pal., i90'«i, p 545

DALY.]

SUGGESTED HYPOTHESIS AS TO THE INTRUSION.

95

between the specific gravity of the rocks in the Gneissic series and
that of the diorite-gabbro magma; therein we must have the closest
approximation in density between the material of any one stock and
its staple inclusion. The specific gravity of the chemically analyzed
diorite is 2.936. Similar determinations were made for the more basic
gabbro jjhases collected at three different j)arts of the same stock ; the
values here ran from 2.95 to 3.19. The average for the gabbro is 3.08.
We may take 3.10 as the approximate average specific gravity of
the more basic parts of the oldest stock.

The most thorough and careful experiment bearing on this question
is that made by Bar us in the fusion of diabase.^' He found that a
sample of diabase at 20° C. had a specific gravity of 3.0178 and the
glass produced by the dry fusion of the same rock had, at the same
temperature, a specific gravity of 2.717. The density was much less
in the molten state. Thus, at 1,400° C. the specific gravity was only
2.523, corresponding to an increase of volume of about 20 per cent. A
critical discussion of many fusion experiments by Delesse and Cossa
along the same line shows a close agreement in the behavior of the
basic rocks treated in the older researches, as compared with that of
the diabase of Barus's refined experiment.^ One phase of the corre-
spondence is shown in the following table :

Rock type.

1.

Sp.gr. of
rock at
ca 20° C.

Sp.gr. of
glass at
ca 20° C.

3.

Net de-
crease in
density,
rock to
glass.

4.

Net in-
crease in
volume,
rock to
glass.

5.
Sp. gr.of
rock molten
at 1,400° C,
calculated
from Ba-
rus's fusion
curve.

Diabase of Bams

3.0178

2. 999

2.859

2.667

2.710

2.684

2.717
2.652
2.657
2.403
2. 430
2.438

Percent.

10.00

11.57

7.07

9.90

10.33

9.16

Per cent.
11.2
13.1
7.6
11.1
11.5
10.0

2. 523

Average gabbro of Delesse- _
Average diorite of Delesse . -

Quartz-diorite of Cossa

Syenite of Cossa

2.507
2.390
2.229
2.266

Average granite of Delesse .

2.243

Average of above

9.67
6.95

10.7

7,5

Gneiss of Delesse -

2.821

2.625

2.358

It is seen that these various independent investigations establish a
tolerably constant ratio for the i-elative volumes of a holocrystallme,
plutonic rock and of the glass produced by its fusion. Of special
interest are the small differences among the results of Barns, Delesse,
and Cossa on diabase, gabbro, and quartz-diorite. These are rocks
related to various facies of the Basic stock at Ascutney Mountain.

aPhilos. Mag., Ser. V, Vol. XXXV, 1893, p. 173; and Bull. U. S Geol, Survey No, 103, 1»93. Cf.
Joly, on fusion of basalt, Trans Roy. Dublin Soc, Ser. II, Vol. VI, 1897, p. 298

6 Delesse, Bull. Soc, Geol France, Ser, II, Vol. IV, 1847, p. 1380; Cossa, ref. by Zirkel, Lehrbuch
der Petrographie, Vol. 1, 1893, p 681

96 GEOLOGY OF ASCUTKEY MOUNTAIN, VEEMONT. [bull. 209.

The behavior of all basic rocks iindei- fusion has, unfortunately, not
been tested for high temperatures, but, for reasons well established by
Barus and derivable from a survey of this particular field of research,
it is admissible to apply the fusion-curve of Barus to any rock of
allied composition. On this supposition, at one atmosphere of pres-
sure, the specific gravity of the Aseutney gabbro would fall from 3.10
to 2.59 at 1,400° C. and that of the average diorite from 2.94 to 2.46.
At this temperature the rock would remain highlj^ fluid even at the
depth of 5 miles in the earth's crust. The specific gravity of the nor-
mal gneisses occurring near the Basic stock ranges from about 2.69 to
about 2.76, with a probable average of 2.73.

To determine what these would be if fragments with the correspond-
ing densities could be kept solid and obey the law of expansion for
solid rock, at 1,400° C, it is permissible to use Reade's expansion coeffi-
cient for granite without incurring serious errror.*

Barus has shown that pressure simplj^ elevates the melting point in
the normal type of fusion without interfering essentially with the
value of the coefficient determined at ordinar}^ temperatures and
pressures.* The average gneiss would have, as a result of the appli-
cation, a calculated specific gravity of 2.63. It must be remembered,
too, that contact metamorphism here, as generally elsewhere, would
raise this value still higher, and that any acidification of the magma
in contact with the gneiss would lower the density of the magma.
Now, the beautiful experiments of Barus in the fusion of various car-
bon compounds under varying pressures show that, in thermal expan-
sibility and in compressibility, they behave in a manner extremely
similar to the few silicates on which any studies in fusion have been
made. He has shown that naphthalene, a substance obejang, like
diabase, the normal law of fusion, is slightly more compressible as a
liquid than as a solid.'' The fusion curves indicate that, for the same
increase of pressure, liquid naphthalene gains in specific gravity about
twice as fast as solid naphthalene. The compressibility of a fused sili-
cate rock is probably, then, approximately twice that of the same rock
when solid. But his diabase curve demonstrates that the thermal
ex]3ansibility of the liquid rock is 1.9 as rapid as that of the solid rock.
Thus a block of cold solid gabbro immersed in a deep-seated molten
magma of the same chemical composition would be less condensed by
the pressure than the molten rock, but the effect on relative densities
would be partly compensated by the relative rate of expansion due
to any superheating of the magma. A block of gneiss would behave
in a manner closel}^ similar to that of a block of gabbro. It is believed
that the pressure of several thousand atmospheres would not affect
seriously the contrast in densities which experiment would lead us to
expect if a fragment of the Aseutney gneiss were completely immersed
in the fused gabbro at plutonic pressures. If this be true, only one

a Origin of Mountain Ranges, London, 1886, p. 110. h Philos. Mag., Vol. XXXV, 1893, p. 306.
c Am. Jour. Sci., 3d ser.. Vol. XLII, 1891, p. 140.

DALY.]

SUGGESTED HYPOTHESIS AS TO THE INTRUSION.

97

conclusion can be drawn. Since uniform pressure affected botli
gneissic fragment and magma when the former was j)arted from
the parent country rock, the difference of density of the two would
prevent the suspension of the fragment as a mere matter of flotation.
Further, the fragments, like the basic segregations, could remain in
the positions in which they may now be seen only if the magma
possessed a high viscosity at the time when they were rifted off.

If we are forced to this view of the conditions in the Basic stock,
still more surely may we have confidence in it as explaining the
presence of even more numerous schist fragments in the syenitic and
granitic stocks. The following table shows that, even in the holo-
crystalline state, each irruptive rock has a specific gravity lower than
its country rocks. It indicates further that the inequality increases
in the same sense the greater the degree of exomorphic change in the
invaded schist.

Eruptive rock.

Specific gravity.

Corresponding country rocks.

Approximate
specific
gravity.

Normal sericitic schist

2.70

Main stock

(2. 616 -2. 683'
laverage, 2. 65

Average of three specimens
from phyllite of contact
zone.

2.84

Average of Basic stock

3.05

Nordraarkite-p o r p h yr y
of Little Asciitney.

I 2. 633

(•Average of Basic stock

I Average of gneisses

3.05

2.73

Pulaskite, Pierson Peak.

about 2. 63

Average of Basic stock

3.05

rNormal sericitic schist

2.68

Granite stock .

2.616

Average of hornfels from
phyllitic contact.

2.84

-Average of Main syenite . _ .

2.65

Delesse found that, in melting down granite to a glass, the specific
gravity was lowered about 10 per cent on the average.'^' Accepting
his figure, the biotite-granite of Ascutney would afford a glass with a
specific gravity of about 2.35.^ A block containing 1,000 cubic feet of
the porphyritic phase of the Main syenite would tend to sink in a magma
of the latter specific gravity by virtue of a downward pull equal to
the weight of at least 5.3 tons of rock in the air, and evideutly, from
Barus's results, still faster in the tliinly fluid granite itself. It is in
the highest degree probable that this difference of density would not
be significantly altered by the great pressures reigning at the moment
when such a block would become detached from the wall of the granite
bodj^ Nothing less, then, than a very unyielding, highly viscous

«Annales des Mines, Ser. II, Vol. IV, 1847, p. 1380,
''Cf. granite, specific gravity 2.63; obsidian, 2.3 to 2.4.

Bull. 209—03 7

98 GEOLOQ-Y OF ASOUTNEY MOUNTAIN, VERMONT. [bull. 209.

condition of any one of the Ascutney magmas can account for the
presence of the foreign blocks in the immediate vicinity of their
homes in the invaded formations. The viscosity probably approached
that of the Archean granitic magmas, which, according to Lawson,
were capable, under enormous djmamic stresses, of shearing and atten-
uating foreign blocks suspended in those magmas near the moment of
consolidation of the latter. Lawson has also suggested that, although
the viscosity was so great, the temperatures may have been high
enough to melt up the more basic foreign fragments completely.^'
Whether solid or molten when sheared or pulled out, such blocks
could not sink in the magma, because of its thick, pasty condition.

At Ascutney Mountain, as elsewhere, the magmas that formed the
stocks were capable of forcing their way through fissures a few inches
or but a fraction of an inch in width, for distances of hundreds of feet
or yards from the respective main eruptive mass. These are clearly
offshoots from the stocks, though the junction with the latter may
not be seen in many instances. Each magma must have been very
fluid when it filled its own set of these narrow fissures.* That con-
clusion accords with the results of the recent careful experiments of
Doelter.^' He has shown that there are but comparatively small differ-
ences among the temperatures at which a granitic rock or an artificial
mixture of silicates is softened by heat, becomes thinly molten, or
solidifies from that molten condition. Thus he found that a foyaitic
mixture (of orthoclase, el?eolite, and tegirine) became soft at 1,070° C,
thinly fluid at 1,110-1,115°, and then solidified at 980-1,000° C. The
corresponding figures for a basaltic mixture (of labradorite, augite,
olivine, and magnetite) are 1,120-1,125°, 1,140-1,150°, and 980-1,000° C.
Predazzo granite and Remagen basalt became softened at respective
temperatures of 1,150° and 992° C. ; completely molten at, respectively,
1,240° and 1,060° C. But a slight restoration of heat, therefore, would
be necessary to reconvert a cooled and toughly viscous endomorphic
zone, yet hot enough to quarry blocks from the invaded formation,
into a highly mobile state. It can not be denied that there must
occur a loss of at least that small amount of heat in the closing stage
of stock intrusion. The magnitude of plutonic pressures puts no
difficulty in the way of accepting this conclusion as to high fluidity.
Oetling has proved, on the contrary, that the temperature point of
consolidation of melted rocks and silicate mixtures is lowered by
pressure. He, in fact, shares the view of Amagat, that, if the pressure
be sufficiently high, solidification can not occur at all.^^ Moreover, it

«Geol. Rainy Lake Region, Ann. Rept. Geol. and Nat. Hist. Survey Canada, 1887, Part F, pp.
131-2-3-8, etc.

''There is no contradiction between this statement and the previous one of high viscosity in
the main magma which isolated and suspended foreign blocks. As implied in a following para-
graph, it would simply mean that the apophysal tongues were injected before the magma had
come in contact with the present walls of its miain chamber.

c'Tscher. Min. u. Petrog. Mitth., Vol. XX, 1901, pp. 333 and 307

rflbid.. Vol. XVII, 1897, p. 370.

DALY.] SUGGESTED HYPOTHESIS AS TO THE INTRUSION. 99

is coming to be generally accepted that pressure induces mobility in
plntonic magmas by retaining water and mineralizers."

In summary, tlien : Field observation and experiment agree in at-
tributing a thinly fluid condition, except at the moment of final crys-
tallization, to such magnias as those from which the Ascutney stocks
were derived.^

High fluidity must have two important results: First, it would
facilitate the formation of ax)ophysal tongues, often intersecting;
secondly, it would entail a downward strain on any disjointed blocks
in the roof contact of the stock body. Following joints, planes of
stratification, schistosity, or slipping, the apophyses must seriously
impair the strength of the roof and walls. The same planes of weak-
ness, even without the aid of the irruptive wedge, already form a
menace to the integrity of the walls and roof, especially the latter;
this on account of the gravit}^ component already demonstrated as a
result of a difference of density between the solid rock and the
magma beneath. Moreover, a shattering of the country rock may be
expected by reason of the differential temperature strains induced by
the magma.

When, from these causes, a block becomes dislodged and completely
immersed in magma, it must sink, and sink rapidly.'^ The space
formerly occupied by the block is now filled mth magma. In the
same manner an indefinite number of blocks may be removed by this
natural stoping. New surfaces will continually be presented to the
invading magma, and so long as the stated conditions persist there will
be greater and greater destruction of the country rock. It is simply a
question of time whether the advance of the magma shall be so great
as to fashion the chamber of an Ascutney stock or of a great batholith.

A brief statement of this central idea of the stoping hypothesis
has been given by Lawson in a review of certain of Brogger's writ-
ings. So far as known to the present writer this noteworthy para-
graph contains the only clear enunciation of the doctrine to be found
in geological literature, and is worth j^ of quotation in full:

The essential features of tlie assimilation hypothesis were formulated by the
reviewers some years agO; before the publication of Michel Levy's views, and
urged as a satisfactory explanation of the remarkable relations which obtain
between the Laurentian granites and gneisses and the upper Archean or Ontarian
metamorphic rocks. These intrusive granites and gneisses occupy vast tracts of
the Canadian Archean plateau, and there seems to be no escape from the view
that they bear a bathohtic relation to the crust which they invaded from below.
Portions of the crust were absorbed, but there are two possibilities as to the
method of absorption, viz: 1, by fusion; 2, by sinking into the magma. The
numerous blocks of rock scattered through the granites lend much probability

aDoelter, Tscher. Min. u. Petrog. Mitth., Vol. XXI, 1902, p. 218.

&See the general statement by Brogger,Die Evuptivgesteine des Kristianiagebietes, Vol. Ill,
p. 338.

c Johnston-Lav is has seen a piece of compact lava sink quickly in a flowing lava stream from
Vesuvius. Proc. Quart. Jour. Geol. Soc, Vol. XXXVIII, 1882, p. 240.

'100 GEOLOaY OF ASCUTNEY MOUNTAIN, VERMONT. [bull. 209.

to the latter having played a part in the process. Such batholiteswere doubtless
accompanied by laccolitic satellites. «

The hypothesis of natural overhead " stoping " accords with the
facts known with regard to other kinds of igneous intrusion. Even
in the case of those great granitic massifs organically associated with
master lines or zones of dislocation (e. g., the tonalite and the "Judi-
carienlinie" of the Tyrol), the magma chamber may have been largely
opened by overhead stoping. The same process may similarly
greatly enlarge the deep-seated cross section of a volcanic neck.
Yet no one can deny its practical insignificance in the intrusion of
sheets or dikes, nor, for obvious reasons, does that fact injure the
strength of the proposed hypothesis when dealing with vastly larger
igneous bodies. The latter must be much longer molten by reason of
their size, and have more direct communication, through convection
and other currents, with the earth's interior. The same remark apr
plies in general to laccoliths, although it is possible that, in limited
degree, laccolithic magmas maj^ carry on independent stoping, and
therewith assimilation, in their hot interiors.

The hypothesis, it will be observed, is allied in one respect to
the assimilation theory of Kjerulf, Michel Levy, Lacroix, and others.
According to each of the two views, the plutonic chamber occupied
by stock or batholith has been formed by the activity of the magma
itself along the internal contact. But, in the older theory, the assimi-
lation at the contact is essentially caustic and chemical; in the newer
view the assimilation there is essentially mechanical. The former
attempts to explain in one step the opening of the space now filled
with eruptive material and the disappearance of the corresponding
mass of country rock ; the latter has still to give account of the multi-
tude of larger or smaller blocks sunken in the magma. What becomes
of them? How far will they sink? What is their fate when they

come to rest?

ABYSSAL ASSIMIIjATIOIS^.

It is at once evident that such questions are most difficult to answer
in detail ; perhaps the second is always destined to remain unanswered.
It is evident, too, that we are now many removes nearer the realm of
speculation than in any previous explanatory step. Yet it can not
be considered a fatal objection to any theory of intrusion that it must
refer ultimately to the unexplored interior of the earth. The attempts
to solve the plutonic problem with attention rigidly kept on the acces-
sible part of the earth's crust must have but partial success. If
experiment, analogy, and the considerations of cosmical physics can
aid in exx^lanation, they should be employed. The field geologist has
only the earth's outer skin to study; yet granite, with all its relatives,
is a product of physiological processes occurring, as it were, in the
vital organs of the earth.

The cardinal fact of fluidity in plutonic magmas needs to be viewed

"Science, new series, Vol. III., 1896, p. 637.

DALY.] ABYSSAL AbSIMTLATION. 101

iu relation to the equally certain fact of the earth's rigidity and to
the necessity of finding some mechanical explanation for the support
of the roof over the igneous body during intrusion. A complete dis-
cussion of the former topic would carry us farther afield than the
scope of the present report warrants. Suffice it here to note that the
same problem confronts every modern theory of intrusion.

In the case of the Ascutney stocks it is believed that the strength
of the roof over each irruptive mass was doubtless sufficient to pre-
vent its foundering en masse in the less dense magma. Other and
larger stocks and batholiths must be studied in this regard each by
itself. As the underpinning of the schist cover of the Ascutney
igneous area as a whole was demonstrably aided by a progressive con-
solidation of the partial magmas, so it is conceivable that there may
be a lateral progression of solidification in the homogeneous magma
of a much larger body with a corresponding strengthening of its roof.
In all such intrusions there will also be the continued presence of
country-rock buttresses still remaining unassimilated.

Whether a stoped-out block sinks in the magma but thousands of
feet or miles from its former position in roof or wall, that block must
undergo an increase of pressure, and, with the greatest probability,
an increase of temperature.*

The added pressure would have, according to the experiments and
field studies of Barns, Doelter, Daubree, Fouque, Michel Levy, and
others, the secondary effect of increasing in the magma the capacity
of retaining water and other solvents, even at very high temperatures.^
So important are other experiments in this connection that a brief
resume of certain results accruing from them must be given.

The solubility of rock-forming minerals in silicate magmas has been
shown by fusion experiments to depend on (a) the temperature of
the magma; (5) the chemical com]3osition and fluidity of the magma;
(c) the fusibility of the minerals, and {d) on jjressure. Doelter has
been able to prove that, under one atmosphere of pressure, all the com-
mon types of rock-forming minerals are completely soluble in certain
representative magmas at temperatures only slightly above those of
the respective consolidation points of the latter. These magmas were
made from granite, obsidian, common basalt, limburgite, phonolite,
foyaite, leucite-basalt, leucitite, hornblende-andesite, and nepheline-
basalt — a magmatic range so wide as to demonstrate the practical
certainty that all silicate^ magmas have similar solvent properties.
He further shows that the melting point of a silicate rock occurs at
about the average temperature of fusibility of its constituent min-
erals. Long before, Bischof easily dissolved clay- slate in fluid lava,
using a bellows furnace for fusion.^ These important deductions

"Perhaps the block would sink to the zone of pressure-solid magma.

6Among the more recent papers, cf. C. Barus, Am. Jour. Sci., Vol. XXXVIII, 1889, p. 408, and
Vol. XLI, 1891, p. 110; C. Doelter, Centralbl. f. Min., etc., 1903, p. 550, and Tscher. Min. u. Petrog.
Miith., Vol. XXI, 1902, p. 218.

'Chem. u. Phys. Geol., Supplement, 1871, p. 98.

102 GEOLOGY OF ASCUTNEY MOUNTAIN, VERMONT. [bull. 209.

from laboratory investigations correspond to the facts of outdoor
nature. Well-known practical examples may be found in the fused
and greatly corroded granite inclusions in the basalts of the Auvergne,
and again in the complete disapjDearance by fusion of the "floating
islands" in the caldera of Kilauea.^' The high fluidity of the normal
plutonic magma would likewise facilitate the complete solution of
foreign fragments, as experimentally proved by Doelter.

It is true that the direct influence of pressure is directed toward
elevating the melting points of silicate mixtures, though probablj^
not in a degree proportional to the amount of the pressure.* Yet
that effect on the solvent power of the magma may be much more
than counterbalanced by the indirect effect of pressure in retaining
water and other solvents. Once molten, pressure tends to keep sili-
cate magmas molten, since it lowers the temperature point of consoli-
dation.^ In determining the solvent power of a plutonic magma,
temperature furnishes here, as in fixing the melting point, the " coarse
adjustment," as i)ressure furnishes of itself the "fine adjustment."

In conclusion, then, it seems legitimate to regard the conditions of
the abyssal portions of plutonic magmas as conspiring toward the
perfect digestion of a submerged foreign rock fragment during all the
time of intrusion except during the short period preceding final con-
solidation. Even so uncompromising an opponent of the theory of
contact digestion by stock magmas as Brogger admits that such
assimilation can be, in the greater depths, exceedingly important,
' ' ausserordentlich bedeutend. " '^

Since it is ]3robable that magmas are more or less completely satu-
rated solutions,^ there would doubtless be a volumetric increase on
the fusion of each block at whatever depth it attained, an increase com-
parable to that demonstrated in fusion experiments at 1 atmosphere of
pressure. The question at once arises as to what compensation can
be made for the increased bulk of rock matter below the earth's sur-
face incident to abyssal assimilation on a large scale. Two possibil-
ities suggest themselves in the face of the hj^drostatic problem involved.
Either volcanic outflow elsewhere or secular uplieaval in the region
would satisfy the conditions. The latter would seem to be more
likely of fulflllment in regard to stocks and batholithic intrusions gen-
erally. It is to be noted that magmatic stoping would tend to weaken
the earth's crust immediately above the intruding body, and there
secular elevation of the surface would be particularly looked for.
There may, in this way, be found one cause of the huge buckles filled
with the "central granites " of Alpine mountain chains. This implies

«.J. D. Dana, Characteristics of Volcanoes, New York, 1891, p. 176.
b Doelter, Tscher. Min. ti. Petrog. Mitth., Vol. XXI, 1902, p. 221.
cOetling, op. cit., p. 370.

rtDie Eruptivgesteine des Kristianiagebietes, Vol. Ill, 1898, p. 350.

eLagorio, Tscher. Min. u. Petrog. Mitth., Vol. VIII, 1887, p. 504. Cf. Delesse, Bull. Soc. Geol.
France, ser. ii, Vol. IV, 1847, p. 1393.

DALY.] . ABYSSAL ASSIMILATION. 103

that the doming of the great intrusive masses of the Christiania
region, attributed by Brogger to laccolithic injection, may, in reality,
be due to this crustal weakening and buckling by magmas working
up from the "ewige Teufe," but at present it must remain only
the suggestion of a possibility, as the writer has no personal knowledge
of the region.

It is, moreover, worthy of inquiry whether this sort of live energy
of intruding granitic magma may be responsible for many of the
well-known cases where the secondary structure planes in the in-
vaded formations v/rap around their respective intrusive bodies.
Examples are seen in the highly developed peripheral cleavage and
schistosity parallel to the outlines of such magmas in the Rainj^ Lake
region '^ and in the Black Hills.* Such structures could certainly be
produced by the force of magmatic expansion, provided that force be
sufficient in amount, for it must be exerted alwaj^s normal to the
chamber walls.

If the foregoing reasoning is correct, the ]3reparation of the chambers
within which the stock bodies of Ascutney Mountain now rest was
carried out by mechanical, piecemeal disruption of each invaded ter-
rane by the attack of the magma on the main contacts. This physical
action was accompanied by chemical assimilation at greater depths.
Consequently, at those depths the magma must become more and more
mixed as the result of assimilation. Each successive eruption from
the magma basin beneath may be expected to show indications of the
gradual alteration of the magma by the incorporation of foreign sub-
stance. This important corollary has to do with the great question of
the origin of the igneous rocks, a subject which, in spite of all its com-
plex difficulties, must here be dwelt upon so far as to show agreement
or disagreement with the hypothesis just outlined. But a less impor-
tant, although significant, test of the hypothesis may first be noted.

The hypothesis of rifting not only gives adequate reason for the very
general sharpness of contact between an irruptive and its country
rock, but also goes far to explain the observed lack of enrichment of
the endomorphic zone with the material of the country rock. The
blocks would be likely to suffer most from solution in the magma after
they had begun their rapid downward journey. Thej'^ would yield up
their substance along the whole path. There would thus be a tendencj^
toward an equal distribution of the absorbed material throughout the
magma. In any case, there would be far less impregnation of the
endomorphic zone with the substance of the invaded formation than
that demanded by the supposition of the slow digestion of the latter
in place. In so fluid a magma convection currents would tend still
further to destroy any contrast of composition between the endo-
morphic zone and the body of the intrusive.

"Lawson, Ann. Rept. Geol. and Nat. Hist. Survey Canada, 188", Part F, map.
6 Van Hise, Sixteenth Ann. Rept. U. S. Geol. Survey, Part I, 1896, pp. 637 and 81.5.

104 GEOLOGY OF ASCUTNEY MOUNTAIN, VEEMONT. [bull.209.

EVIDEJfl^CES OF DIFFERElVTIATIOlSr.

In turning to the main problem still awaiting us, the relation of the
hypothesis of rifting, overhead stoping, and abyssal assimilation to
the sequence of the eruptive rocks at Ascutney Mountain, it must be
be stated in advance that differentiation in the usual sense of that
term has, it is believed, been operative in the production of these
rocks. This illuminating principle seems to win added credibility
every year, as the petrological facts concerning consanguinity, com-
plementary dikes, etc., become more numerous and more clearly
ascertained. Without entering further into the general question, the
course of our argument demands that some of the concrete evidences
for the value of the principle be noted as the result of a study of
Ascutney Mountain.

Direct witness to the fact of differentiation is found in the abundant
and remarkable basic segregations from most of the stocks and dikes.
Moreover, the ' ' blood relationship " in mineralogical and chemical
composition of the main rock bodies of Little Ascutney, Pierson Peak,
and Ascutney Mountain proper, occurring as they do in so strikingly
different geological associations, and the close agreement in composi-
tion with the distant syenitic rocks of Essex County, Mass., Killing-
ton Peak, the Adirondacks, Rigaud Mountain, Quebec, and the eastern
townships of Quebec, seem to indicate that strict chemical and phys-
ical laws, and not fortuitous similaritj^ in the products of assimilation,
govern the particular groupings of metals and oxides found in the
respective intrusives. The occurrence of nordmarkites in all of these
regions must be regarded as the result of the independent assertion in
each region of one and the same set of laws of attraction and concen-
tration in an originally more complex rock magma rather than the
result of multiplied consolidations of one great nordmarkitic magma
underlying all this part of North America.

Further, the conclusion that mere assimilation of the invaded
sedimentary terranes by a magma can not be used to exj)lain the
intrusives of this part of the world is rendered all the more probable
by a detailed comparison of the Ascutney eruptives with those of
Mount Shefford, as described by Dresser.* The Canadian intrusives
named in the order of injection are essexite, nordmarkite, pulaskite,
camptonite, and bostonite. The first of these is considerably more
alkaline than the Ascutney diorite analyzed, but is probably close
chemicallj^ to phase e of the Basic stock. Macroscopicall}^, the Ascut-
ney diorite and the Shefford essexite are remarkably alike in general
habit, and the writer has seen a coarser phase of the latter which has
the poikilitic bisilicates and other detailed features of the Ascutney
gabbros. As striking similarity characterizes the green nordmarkites
of the two mountains. These facts seem to prove conclusively that
definite chemical and physical laws have governed the formation of

a Am. Geol., 1901, Vol. XXVIII, p. 203.

DALY.] EVIDENCES OF DIFFERENTIATION". 105

each special magma which crystallized after irruption into the rock
bodies now exposed to view. There has been some post-eruptive
differentiation in the Shefford intrusions, as they possess basified
endomorphic zones. Dresser holds that the essexite, nordmarkite,
and pulaskite form the filling of a laccolithic space in the Lower
Silurian sediments of Shefford Mountain. Accepting his view of the
mode of intrusion, preemptive differentiation from a magma origi-
nally composed of a mixture of these special magmas, might be
credited with a full explanation of the Shefford rocks, though even
then the possibility is quite oiDen that the original complex magma
had been formed by the considerable digestion and assimilation by a
still earlier magma, of the Trenton slates and other sediments through
which the eruptions took place. On the other hand, the fact of some
kind of assimilation preparatory to differentiation at Ascutney can
hardly admit of doubt.

The differentiation of the alkaline rocks in the area, on the hypothesis
outlined for Ascutney, would be local and confined to a magma which
had been more or less strongly affected by the "mise en place" of
the Basic stock. If we have anywhere an igneous formation approxi-
mately representing the main magma whicli underlay the region
before the intrusion began, it must be found in that stock. All sub-
sequent intrusions might, on account of the intermixture of assimi-
lated schists, be exi^ected to show a divergence from the original
magma that would be the stronger the later the corresponding intru-
sive appeared in a series of eruptions. In other words, the windsor-
ite dikes, the nordmarkites, pulaskite, monzonite, paisanite, granites,
and aplites are, by the hypothesis, regarded as the product of the
deep-seated assimilation of the schists followed or accompanied by the
differentiation of these related magmatic types from the mixture due
to subcrustal digestion. The high silica, ijotash, and alumina of the
micaceous and quartzose phyllites and gneisses would explain the
increasing acidity, the alkalinity, and feldspathic character of these
differentiated products, though other features must be credited to
differentiation alone.

Just how differentiation takes place is still to be reckoned among
the mysteries of geology. There is no doubt that several determina-
tive factors must be taken into account. Without in any way wish-
ing to question the validity of the other causes, the writer will here
briefly instance one of them as seeming to be of more general appli-
cation. Rosenbusch has published the view that the separation of
differentiated products may be due in part to the gravitative effect,
whereby the more acid and lighter constituents of a complex magma
become segregated and float upon the more basic and heavier residue.'*
It is supported by the valuable observations of Morozewicz in
synthetic experiments and in the study of glass furnaces.^ Doelter

«Mikroskopische Physiographie d. Min. u. Gest., Vol. II, 1896, p. 552.
bTscher. Min. n. Petrog. Mitlh., Vol. XVIII, 1898, pp. 170 and 333.

106 GEOLOGY OF ASOUTNEY MOUKTAIN, VERMONT. [buil. 209.

has pointed out that such results adhere to excejjtional cases, both
in his own experiments and in those of the Russian investigator;
yet their significance is still great, since they agree with Gouy and
Chaperon's theoretically deduced principle of gravitative stratification
in saline solutions,^' as well as with some positive field observations.
For example, Sir A. Geikie describes the separation of a lower layer
of picrite and an overlying layer of olivine-basalt in the same lava
flow, and finds it probable that similar differentiation has taken place
in basic sills. ^ It is at least worth while to apply the gravitative
theory to the Ascutne}^ magmas, so far as to state briefiy the course
of events entailed.

By the separation of the differentiated products, the uppermost
layer would, by the antecedent addition of the abundant silica from
the digested schists, become more and more acid as the assimilation
progressed. The aplites and granite would appear as the latest
products (excepting the complementary dikes) of a differentiation
deiDcndent on the assimilation for its final expression.

Opposed to the hypothesis is the more usual view of simple differ-
entiation as explanatory of the eruptive sequence. The latter has
been well expressed by Brogger for the similar sequence in the Chris-
tiania region. He points out the general harmony existing between
the theoretical order of differentiation, the order of eruption in the
province, and the order of crystallization in the various rocks. '^ On
the same principle the oldest Ascutney stock would be regarded as
of the nature of a gigantic basic segregation which had absorbed into
itself the basic orthosilicates and metasilicates of lime, magnesia, and
iron before the crystallization, from the same original magma, of the
syenites and granite where the dark-colored constituents are so poorly
represented.'^ The possibility of mere differentiation (without assimi-
lation) producing the Christiania rock bodies is due, according to
Brogger, to the peculiar laccolithic nature of the intrusions in that
province. The preparation of free space for the play of chemical
reactions leading to differentiation is quite in contrast with that
hypothecated for the Ascutney area, though many of the rock types
of the two regions are extremely similar. Brogger has pronounced
against the assimilation theorj^ of Kjerulf and Michel Levy, largely
for the reason that h fails to meet the controlling test, the proof of
chemical sympathy between the formation invaded and the igneous
body supposed to have performed the digestion. Thus the granite of
the Christiania region contains scarcely 0.5 per cent of CaO, although
the Cambrian and Silurian beds through which the intrusions occurred
contain as large an average as 24.5 per cent of the same oxide. '^ The
objection does not, however, apply to the modified assimilation

aAnn. de Chimie et de physique, 6th ser., Vol. XII, 1887, p. 384.

b Ancient Volcanoes of Great Britain, London, 1897. Vol. I, pp. 419 and 443, and Vol. II, p. 310.

cDie Eriiptivgesteine des Kristianiagebietes, Vol. II, 1895, p. 175.

dCl. Zeit. fur Kryst., Vol. XVI, 1890, p. 86.

eDie Eruptivgesteine des Kristianiagebietes, Vol. II, 1895, p. 1^9.

PALY.] THE PETEOGENIC CYCLE. 107

hypothesis as outlined in this chapter. Brogger's own cross sections
would imply that the Cambrian and Silurian limestones were depos-
ited on Archean crystalline schists. The vertical thickness of this
formation is probably several times as great as the thickness of all
the Lower Paleozoic limestones combined. Differentiation working
on the magma produced by the mixture of the digested material of
both limestones and schists might very well give a granite with a low
content of lime. Be the method of intrusion what it will, the simi-
larity of the Norwegian and Vermont rocks seems to ijoint unmistak-
ably to the truth of the main principle of differentiation — the tendencj^
toward definite chemical and mineralogical segregation in a silicate
magma, irrespective of how that magma was prepared.

As the specific gravity of the acid magmas must in every case be
lower than that of the original basic magma, the latter would tend to
rid itself continually of the foreign substance being dissolved from
the sunken blocks. We have seen that the latter would sink deepl}^
Whether this gravitative cleansing be perfect or not at a moderate
depth of, perhaps, a mile or two below the original magma surface,
the magma might there still be quite basic. If we now imagine a i3ro-
longed period during which the overlying acid-alkaline intrusives were
completely crystallized and, afterwards, a limited fracturing of the
Avliole compound terrane, we can secure some explanation of the final
series of basic dikes. They would represent the product of renewed
eruptive activity from the deep-lying, still molten magma pressed
upward along the easy paths of the fractures. The common occur-
rence of the diabase dikes and lavas through the whole length of the
Connecticut Valley and in many parts of the Appalachian system
suggests correlation with this hypothetical explanation. Possibli'
the camptonites are nothing more than dikes of diabase which
have absorbed a small amount of ferrous iron and alkalies from the
syenites through which they have found their way. Nevertheless, in
spite of the diificulty of determining the place and exact manner of
the differentiation of complementary dikes in general, the possibility
that these youngest dikes correspond to the basic poles of secondary
differentiation can not be excluded. Nor is it necessary to the
hypothesis of abyssal assimilation that either alternative be estab-
lished, for the hypothesis must be linked with the belief in secondary
differentiation.

THE PETROGENIC CYCLE.

Finally, it should be observed that the whole series of events lead-
ing from the beginning of the invasion of the oldest stock to the
irruption of the youngest stock and dikes might, after the solidifica-
tion of the last of these, be followed by a resumption of plutonic
activity. There might thus be relocated the sequence of changes
memorialized in the existing rock bodies — basic to acid through inter-
mediate types. Or any part of the cycle might be repeated, whereby

108 GEOLOGY OF ASCUTNEY MOUNTAIN, VERMONT. [bull.209.

relatively basic irruptions into the schists would be followed by more
acid ones. Or, thirdly, the cycle represented in the unsqueezed igneous
rocks of the present mountain might have been preceded by an
older cycle, the records of which are still buried deep within the
schistose formations in the neighborhood. Such an earlier cycle
would account for the amphibolites and aplitic sheets which antedate
the last great period of folding and dynamometamorphism in the
schists.

SUMMARY AIN^D GEKERAI. APPLICATION.

In order to bring this hypothesis of overhead stoping, abyssal
assimilation, and differentiation into relation with the general prob-
lem of the -plutonic rocks, it will be expedient to recapitulate (I) the
essential facts of observation in the Ascutney area, (II) the results
of experimental investigation on the specific gravity of the Ascutney
rocks and on silicate magmas, and (III) the conclusions won from the
correlation of both groups of considerations.

(I) The Ascutney irruptive bodies exhibit the following character-
istics:

A series of true stocks ranging from the oldest, most basic, and
least alkaline to the highly alkaline, youngest, and most acid, followed
and accompanied by groups of aplitic and lamprophyric dikes.

Two of the stocks (Basic stock and Main stock) characterized by a
noteworthy heterogeneity; the other three by just as striking homo-
geneity.

An almost entire lack of sympathy between the structural planes
in the country rocks and the form of each intrusive body.

Conclusive evidence that the different magmatic chambers were not
prepared by circumferential faulting.

In each stock a decided lack of any enrichment of the endomorphic
zone by substance dissolved from the invaded formations; a general
freedom from foreign inclusions in the interior, with a characteristic
abundance of angular inclosures near the contacts; an exceedingly
sharp line of contact with the country rocks; equally sharp contacts
of the foreign fragments and their respective hosts; lack of direct
sympathy between the composition of the intrusive stocks and of their
respective country rocks.

The existence of many long and narrow, apophysal offshoots from
each stock, betokening their high fluidity at the time of intrusion.

The presence of many basic segregations in four out the five stocks.

The mineralogical and chemical characters of the stock rocks
which, compared among themselves and with the rocks of other
petrographical provinces, compel belief in some kind of differenti-
ation of the Ascutney igneous bodies from a common magma.

(II) The experiments of Barus, Delesse, Daubree, Doelter, Oetling,
Morozewicz and others have shown —

That representative natural or artificial silicate mixtures at ordinary

DALY.] SUMMAEY. 109

atmosplieric pressure become thinly molten at a temperature only
slightly above that of solidification.

That, in every instance, a great increase of volume characterizes
the change from the solid to the molten state.

That the corresponding difference of density is, no doubt, essentially
preserved under plutonic conditions.

That the chief rock-forming minerals are soluble in all of the melted
silicate mixtures yet investigated and at the temperatures ruling when
those mixtures are thinly molten.

That pressure aids the solubility indirectly by retaining water and
other mineralizers in the magma, but retards it, probably in much
less degree, by raising the temperature of fusion for silicate minerals.

That there is evidence of differentiation in molten silicate magmas
by gravitative effect.

Numerous specific gravity determinations on the solid Ascutney
rocks show that the lightest of these would, under the same conditions
of pressure as the densest of the magmas (that of the Basic stock),
sink on immersion in that magma.

(Ill) The conclusions necessitated, it is believed, by these facts are :

1. That the various chambers now occupied by the igneous bodies
were not opened by bodily movements in the earth's crust, but by
some kind of assimilation of the invaded formations.

2. That this assimilation did not take place, except in subordinate
degree, by caustic solution on the main contacts.

3. That, even in its relatively inactive state near the moment of
final consolidation, each magma was capable of rifting off numerous
large and small blocks from the walls wdth which it came in contact —
blocks now visible because the magma was then so toughly viscous as
to suj^port them in suspension.

4. That during the much longer period of high fluidity each magma
was capable of still more powerful rifting action.

5. That throughout that period there must have prevailed a more or
less steady rain of the rifted blocks downward into the lower depths
of the magma and a corresponding enlargement of the magma cham-
ber, the size of which would depend on the time during which the
action continued; independent testimony may be had of the high
probability that the time taken in all plutonic intrusion is very great.

6. That in the abyssal region the blocks must undergo active solu-
tion by the magma, which would thus become mixed and gradually
more complex.

7. That some compensation for the increased volume of the rock
digested must be made — suggesting either surface extrusion from
another part of the same magma basin or secular upheaval of the
earth's crust above the basin.

8. That the original magma was at least as basic as the gabbroitic
l)hase of the oldest stock.

110 GEOLOGY OF ASCUTNEY MOUNTAllST, VEEMONT. [bull.209.

9. That there would be a tendency for the mixed magma to become
more and more acid by reason, of the assimilation of the schistose ter-
ranes.

10. That this magma would be expected to difCerentiate by slow
gravitative action, through which the lighter, more acid submagm.as
would float on the heavier basic residues.

11. That such differentiation must be supplemented by other causes,
real and universal, though at present ill understood, leading to a
comparatively definite splitting of the main magma; thus homogeneous
rock bodies would be produced similar to those in other parts of eastern
North America and elsewhere.

12. That the Ascutney stocks are the crystallized product of such
differentiation from an ever-changing magma constantly enriched by
assimilation.

13. That the series of petrogenic events at Ascutney constitute a
cycle that might be repeated either as a whole or in part within the
same area.

14. That the later basic dikes may be explained as the beginning of
a second petrogenic cycle, or as the basic poles of a secondary
differentiation.

Now, the facts of field observation at Ascutney Mountain, with two
possible exceptions, correspond to possible characteristics of most of
the granitic intrusions of the world. The heterogeneity of the Basic
stock and of the Main stock is doubtless of a higher order, and the
basic segregations in the latter are more numerous than in the normal
granitic mass. Yet these contrasts may be largely explained by the
action of secondary differentiation. The experimental results of
investigation on melted silicate mixtures are manifestly capable of
general application. There is, accordingly, reason to believe that
the hypothesis summarized in the list of conclusions concerning the
Ascutney eruptives may be applied to most stocks and batholiths.

THE UIS^IVEESAIj EARTH INIAGMA.

If this hypothesis be accepted for stocks and batholiths generally,
and if dikes, sheets, and laccolithic intrusions (including all such as
have been conditioned by the action of hydrostatic pressure on a
magma entering spaces opened by bodily crustal movements) are the
results of the eruption of submagmas differentiated from the deeper- .
lying and greater magma produced bj^ the incorporation of invaded
formations, the further inquiry as to the original composition of such
assimilating magma thus becomes a matter of special interest. The
required space can not here be taken for a full discussion of this
question, even if only the limited number of facts now known con-
cerning the subject were given full statement. Special dilfidence
may be felt in approaching this most difficult theme. Yet certain
preliminary considerations are offered, primarily those which, in the
opinion of the writer, do not at the present time receive the full share

DALY.] THE UNIVERSAL EAKTH MAGMA. Ill

of attention that they should have in the problem of the earth's
interior; taken together, they seem to form, in a measure, a test of
the foregoing hypothesis.

The evidence is accumulating that the normal order for the erup-
tion of plutonic rocks is that of from most basic to most acid. That
the same order maj^ be preserved, on the large scale, in extrusions of
lava at volcanic cones is illustrated by Sir Archibald Geikie in his
treatise on The Ancient Volcanoes of Great Britain. « It seems
established, moreover, that the oldest eruptive in the majority of
petrographical provinces approximates a gabbro or basalt in compo-
sition. Yet the oldest intrusive, by the foregoing hypothesis, is that
one which should most nearly represent the original magma, modi-
fied as the latter tends to become by the assimilation of the more
siliceous crystalline schists and sedimentary terranes.

Again, in those conduits where escape of igneous rock from the
earth's interior to the surface takes place to such an extent as to
build large volcanoes, we should expect the sequence of eruption to
be completed by an effusion of lava more nearly representing the
original magma than the antecedent flows. This for the reasons,
first, that assimilation (deep-seated digestion of the overlying crust) in
the immediate vicinity of the vent would, in that late stage in the
development of the volcano, have progressed so far as to have
enlarged the conduit to a size suitable to the large cone; secondly,
that the vent would by the long continuance of the volcanic activity
have become freed from the products of such digestion, and, thirdly,
that the latest flows would be derived from the original magma prac-
tically unaffected hy assimilation. Now, it is a significant fact that
the latest extrusive product of our greatest volcanoes, such as Etna,
Fusiyama, Chimborazo, Cotopaxi, etc., is without known exception,
either basalt or andesite. The unnumbered lofty volcanoes which
spring from the floor of the deep Pacific and Indian oceans are, with
but few exceptions, capped with basalt or andesite. Indeed, such
basic lava seems to be the only igneous rock exposed in oceanic areas
making up at least one half of the whole surface of the globe.

Not less important is the equally indisputable fact that the great
fissure eruptions of the globe give birth to only one kind of lava,
again basaltic. The familiar examples in Iceland, Northwestern
Europe, India, the Northwestern United States of America, and the
Hawaiian Archipelago, tell no uncertain story concerning the nature
of the vast reservoir from which they have derived their enormous
volumes of lava. The more acid flows which occur in any one of
these regions are insignificant in bulk when compared to the total basic
output. The former could be explained, in accordance with the pres-
ent hypothesis, as the product of differentiation acting on the uni-
versal magma influenced by the assimilation of the continental rocks,
which are characteristically more acid than that magma. Further,

a Vol. II, 1897, p. 477.

112 GEOLOGY OF ASCUTNEY MOUNTAIN, VERMONT. [bull.209.

we should expect assimilation to be less active in determining the
composition of fissure eruptives than in preparing the secondary
magmas erupted in volcanic cones or injected in the intrusive form.
From the nature of the geological dynamics rendering possible the
rapid expulsion of the voluminous flows at great fissures, it is clear
that the corresponding magma had, in each case, relatively easy
access to the earth's surface, and had not to work its way through the
crust. The plateau lavas accordingly merit particular notice in the
search for the general earth magma. Too little attention has been
paid to the volume, relative abundance, and geological occurrence of
the different eruptive types in the extant discussions of the origin of
igneous rocks. Those questions must always be of prime importance
in deciding on the question of assimilation.

For different reasons, excepting that derived from the enormously
greater abundance of basaltic lavas on the earth, Dutton came to this
same conclusion as to the nature of the "primordial matter." lie has
rightly emphasized the importance of the fact that basalt is a "syn-
thetic or comprehensive type of rock." His theory of the derivation
of other igneous rocks by simple fusion of sedimentary formations,
derived in their turn by atmospheric agencies from this ' ' primordial
matter," takes insufficient account of the facts of differentiation
learned since 1880. Yet his theory has a suggestive relation to the
one proposed in these pages. '^^

Thus, partly by the induction of known facts, partly by the deduc-
tion of certain conclusions which are explanatory of a considerable
number of related phenomena, we have been led to the view that there
is, all ]-ound the earth and not far from its present surface, a single
fundamental magma of a composition allied to basalt. This magma
must probably be regarded as molten only potentially and to uncer-
tain depth bj' the local relief of pressure. It has been implied that
all other rocks may have been indirectly derived from such a magma,
though the possibility is not excluded that part of the normal conti-
nental intrusive (acid-alkaline) rocks may form the more or less pure
equivalent of primal matter differentiated at the surface of the origi-
nal crust of the earth. It is, of course, evident that we are now face
to face with other principal earth problems, most of which are nothing
more nor less than true riddles. The nature of the earth's original
crust, the antiquity of the ocean basins, the duration and geological
historj^ of the Archean era during which most of the siliceous material
of the crust was prepared in nearly its present form, the origin of the
crystalline schists, the preponderance of ]3otasli among the alkalies of
continental formations, the explanation of the high soda content of sea
water, are among those problems bearing on the hypothesis. It can
only be said that the writer has not yet met with insurmountable objec-
tions to the hypothesis in the partial solutions now attained for them.

a Of. C.E. Dutton, The High Plateaus of Utah: U. S. Geol. and Geog. Surv. Rocky .Mountain
Region, Washington, 1880, p. 125 et seq.

DALY.] THE UTSriVEESAL EARTH MAGMA. 113

The probability that the combined variety and tyi)e constancy of
the continental igneous rocks are due to both abyssal assimilation
and magmatic differentiation is' taught not only by a detailed study
of a small area like Ascutney Mountain, but as well bj^ a review of
the earth's igneous output as a whole. Perhaps the hypothesis founded
on this conviction may do something toward removing the difficulty
that is felt by most students of igneous rocks; it is the dilemma once
well described to the writer by a leading petrologist: "As a geologist,
one must believe in assimilation; as a petrographer, he must declare
against it."

Bull. 209—03 8

APPENDIX.

TABLES, LIST OF SPECIMENS, ETC.

Table XIII. — Mineralogical and structural constitution of the Ascutney eruptives.

Essential feldspars.

Other essential
constituents.

Accessory.

Structure.

Camptonite

Basic labra-

Hornblende

Titaniferous

Panidiomor-

dorite (Abj

Augite.

magnetite.

phicporphy-

Ans).

Pyrite.

Apatite.

Decomposi-
tion prod-
ucts: Chlo-
rite, epi-
dote, cal-
cite, sec-
o n d a r y
quartz,
kaolin.

ritic.

Augite-gabbro _

do

Augite.

Biotite.

Hornblende.

Pyrite.

Ilmenite.

Titanite.

Apatite.

Hypidiomor-
phic granu-
lar.

Diabase

do

do

Titaniferous
magnetite.

Ophitic.

Pyrite.

Apatite.

Decomposi-

tion prod-

ucts: Chlo-

rite, epi-

dote, cal-

cite, sec-
o n d a r y
quartz,
kaolin.

115

116

GEOLOGY OF ASCUTNEY MOUNTAIN, VERMONT.

[BULL. 209,

Table XIII. — 3Iineralogical and structural constitution of the Ascutney erup-

tives — Continued .

Essential feldspars.

other essential
constituents.

Accessory.

Structure.

Hornblende-

Basic labra-

Hornblende.

Ilmenite.

Hypidiomor-

biotite-an-

dorite (Abj

Biotite.

Pyrite.

phic granu-

gite gabbro.

Ans).

Augite.

Titanite.
Apatite.

lar.

Biotite - h o r n -

Av. basic oli-

Biotite.

Quartz.

Do.

blend e-dio-

goclase (Aba

Hornblende.

Ilmenite.

rite.

AnJ.

Pyrite.
Apatite.
Titanite.
Zircon.

Biotite -augi te-

do

Biotite.

Quartz.

Do.

hornblende-

Atigite.

Ilmenite.

diorite.

Hornblende.

Pyrite.
Apatite.
Titanite.
Zircon,

Essexite

Andesine (Abg

Hornblende.

Quartz.

Do,

Ang).

Biotite.

Augite.

Micropertliite.

Ilmenite.

Orthoclase.

Apatite.
Titanite.
Zircon.

Monzonite

Microperthite.

Hornblende.

. Quartz.

Do,

Ortlioclase.

Augite.

Titaniferous

Labradorite

Biotite.

magnetite.

(AbiAni).

Apatite.

Pyrite.

Zircon.

Windsorite

Micropertliite.
Orthoclase.

Biotite.

Quartz.
AiTgite, horn-

Do.

Basic oligo-

clase (Aba
Ani).

blende.
Ilmenite.
Apatite.
Zircon.
Titanite?

DALY.]

APPENDIX.

117

Table XIII. — Mineralogical and structtiral constiUition of the Ascutney erup-

tives — Continued.

Essential feldspars.

other essential
constituents.

Accessory.

Structure.

Pulaskite

Microperthite.

Biotite

Titaniferous

Hypidiom Or-

Orthoclase.

magnetite.

Quartz.

Titanite.

Hornblende.

Augite.

Apatite.

Zircon.

phic granu-
lar.

Nordmarkite _.

Microperthite

Hornblende.

Quartz.

Hypidiom Or-

(cryptoper-

Biotite.

(Allanite.)

phic granu-

thite).

Augite.

lar.

Orthoclase.

Quartz.

Tit anif erous

Porphyritic.

Microcline.

magnetite.

Trachytic.

Acid oligoclase.

Apatite.

Pyrite.

Zircon.

Monazite.
Garnet.

Biotite-granite _

Microperthite.

Biotite.

Magnetite.

Porphyritic.

Orthoclase.

Quartz.

Titanite.

Microcline.

Apatite.

Acid oligoclase.

Zircon.

Paisanite

Microperthite.

Quartz.

Titanite.

Panallotrio-

Soda -ortho-

Hornblende.

Ilmenite.

morphic por-

clase.

Pyrite.
Zircon.

Apatite.

phyritic.

Muscovite - aj)-

Orthoclase.

Quartz.

Microperthite.

Panallotrio-

lite.

Albite.

Muscovite.

niorphic.

118

GEOLOGY OF ASCUTNEY MOUNTAIN, VERMONT.

[BULL. 209.

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APPENDIX.

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120 GEOLOGY OF ASCUTNEY MOUNTAIN, VERMONT. [bull.209.

Table XV. — List of the more important specimens studied.

No. la and lb. Basic segregation in biotite-granite.

2. Biotite-granite.

5. Metamorphosed limestone of contact-zone, bearing grossularite.
24. Sericitic quartzite.
32. Biotite-hornblende-diorite.
34. G-ranite; phase h of Main syenite stock.
36. Breccia of Little Ascutney.

42. Green nor dmarkite; Main syenite stock (phase/).
57. Camptonite dike; Little Ascntney.

59. Microperthite-bearing hornblende-biotite-diorite,
59a. Basic segregation in 59.

60. Paisanite; Little Ascntney.

61. Hornblende-biotite-angite-gabbro.

62. Pulaskite; Pierson Peak.

66. Basic segregation in porphyritic phase g of Main syenite stock.
74. Angite-camptonite.

76. Nordmarkite-porphyry; Little Ascutney.

77. '" Windsorite"" dike; Little Ascutney.

100. Metamorphosed limestone of contact zone, bearing epidote, etc.

105. Endomorphic zone of biotite-granite.

106. Miarolitic phase of 105.

111. Monzonite; phase i of Main syenite stock.

113. Segregation in biotite-granite.

114. Unaltered siliceous limestone.

115. Pori^hyritic phase g of Main syenite stock.
120. Diabase dike.

122-136, inclusive. Metamorphosed phyllite of contact zone about the Main

stock.
139. Paisanite of Main stock.
141. Basic segregation in 139.

145a. Augite-biotite-diorite dike; Little Ascutney.
147. Augite-biotite-hornblende-diorite; Little Ascutney.
175. Altered aplite.
184. Augite-biotite-diorite.

191. Muscovite-aplite.

192. Augite-gabbro.

INDEX

Page.

Alkaline granite, occiirrence of. - - 67

Alkaline biotite-granite, basic segrega-
tions in - 83-84

endomorphic zone in _ _ 84-85

occurrence and character of 79-85

Aplitic dikes, occurrence of 70-77

Ascutney Mountain, drainage of 10-13

general form and character of 13

glaciation of 13-13

map and section of 70

topographic features of - - - 8-10

view of 7

Augite-biotite-diorite, occurrence of 45

Augite-gabbro, occurrence of _ _ - - 43

Augite-hornblende-syenite, analysis of - - 47

Barus, Carl, cited 95,96,101

Basic segregations, occurrence and char-
acter of _ _ 43-44

Biotite-augite-hornblehde-diorite, occur-
rence of _ 40

Biotite-diorite, occurrence of 45

Biotite-granite, analysis of 84

basic segregations in _ 83-85

analysis of 84

endomorphic zone in_ 84-85

mineral composition of 83

molecular proportions of 81

occurrence and character of 79-85

Biotite-hornblende-diorite, occurrence of 40

Biotite-nordmarkite, occurrence of - 70

Bischof, cited - 101

Breccia masses, Little Ascutney Moun-
tain _. 77-79

Brogger, W. C, analyses compiled by 41,

53,54,59,66,75,84,87

cited 85,93,99,103,106

Camptonite, analyses of 87

occuri-ence and character of _ _ 86-87

Chrustschoff, cited 65

Cossa, cited. 95

Oushing, H. P., analysis by _ 59

cited _ 53,89

Dana, J. D., cited_ - 13,103

Delesse, cited __ 95,97,103

Diabase, analysis of- -- 88

Diabase dikes, occurrence and character

of _._ 87-88

Diorite, analyses of. 41,66

occurrence and character of 38-44

Dioritic dikes, features of 44-45

Dikes, occurrence of - 69, 70-77, 86

Dittrich, analysis by 41

Page.

Doelter, C, cited 94,98,99,101,103,105-106

Doja, M., analysis by 39

Dresser, cited 104, 105

Dutton, C. E., cited 113

Earth magma, original, composition of. 110-113

Emerson, B. K., cited 19

Eruptive rocks, abyssal assimilation of

country rock by 100-103

character and occiirrence of 38-89

methods of differentiation of 104-107

table and correlation of . _ 36-37

Essesite, analyses of 41,66

Gabbro, occurrence and character of 38-44

Geikie, cited _-_ 106

Gneissic series of rocks, occurrence and

character of 17-19

Gouy and Chaperon, cited - . 106

Graber, analysis by.- 39

Granite, alkaline, occurrence of ;.. 67

basic segregations in 83-84

endomorphic zone in - _ 84-85

occiu'rence and character of 79-85

Granitite, analysis of 84

Granodiorite, analysis of _ 47

Grorudite, analysis of _ 75

Gulliver, F. P., acknowledgments to____. 7

Harker and Marr, cited.. -. -. 23

Hawes, cited 73-74

Hillebrand, "W. F., acknowledgments to . 7

analyses by 15, 37, 39,

41,44,47,59, 60,66, 75,76,81,83,84,87,88,119

cited 37

Hitchcock, C. H., cited .-- 8,15,64

Hitchcock, Edward, cited 17, 19

quoted 77

Hornblende-biotite-augite-gabbro, occur-
rence of 43

Hornblende-biotite-diorite, occurrence of 43

thin section of, plate showing 33

Hornblende-paisanite, analysis of 75

Hornf els, plate showing 33

Hunt, T. S., cited 19

Intrusion of the stocks, hypotheses con-
cerning 90-103

Intrusive rocks of the area, geologic age

of.._ - 19-30

Irruptive rocks of Ascutney Mountain,

characteristics of 108

date of intrusion of 21

specific gravities of .- 97

Jaggar,T.A., acknowledgments to 7

Johnston-Lavis, cited 99

131

122

INDEX.

Page.

Joly, cited .- __- 95

Judd, cited _ _ 48

Kemp, J. F., cited 85

Lacroix, cited 34,42

Lagorio, cited , 102

Lamprophyres, occurrence and charac-
ter of - ___. 85-88

Lawson, A. C, cited _ 98-103

quoted 99-100

Lestivarite, analyses of 75

Limestones, changes in 23

metamorphic constituents of 35

Lindgren, Waldeniar, cited on composi-
tion of granodiorite _ _ 47

Little Ascutney Mountain, breccia masses

on : -. 77-79

intrusive rocks on, plan showing 74

paisanite dike on 73-77

syenite-porphyry dike of 69

view of - - 7

Metamcjrphic aureole, features of _ _ 23

Metamorphism, effects of 33-35

Monzonite, analysis of 66

occurrence of 68

Morozewicz, J., cited 28,105

Mount Ascutney. See. Ascutney Moun-
tain.

Muscovite-aplite, occurrence of 73

Nordmarkite, analyses of _ . ^ 47, 59

basic segregations in, plate show-
ing - - . - 62

mineral composition of 60

molecular proportions of 60

occurrence of 53,61,64^69

Nordmarkite-porphyry, dikes of 69

Oetling, cited 98,102

Osann, analysis by ^ 75

Paisanite, analyses of 75

occurrence of 70-77

mineral composition of 73, 76

molecular proper tions of 76

thin section of, plate showing _ _ 62, 64

Phyllite, plate showing 16

metamorphism of 24-35

Phyllitic series of rocks, occurrence and

characterof - 14-17

Pierson Peak, location of 9

syenite stock of.. - 70

view of -- 7

PulasMte, analysis of _ 59

occurrence of ..- 70

Pyroclastic feldspar, thin section of, plate

showing 58

Quartz-diorite, occurrence of 38

Quartz-monzonite, analysis of 47

Quartz -sericite-schist, analysis of _ _ _ 15

Quartz-syenite, analysis of 59

Reade, cited _ 96

Richardson, C. H., cited 15,20

Rosenbusch, H., acknowledgments to ... 7

analysis by 87

Schists of the area, geologic age of 19-20

Smyth, C. H., analysis by 47

Specific gravity of eruptive rocks, table

showing 97

Specific gravity and temperatures of

rocks, relations of 95

Stock rocks, intrusions of, theories con-
cerning 90-100

Syenite, allanite in 55-56

apatite in 56

augite in _ 54

basic segregations in 64-68

biotite in . 55

dikes cutting 70-77

endomorphic zone of 08-69

feldspars in 50-51

garnet in 57

hornblende in 53-54

magnetite in _ _ 56

monazite in _ _ 56

occurrence and character of 48-70

quartz in 57

tarnishing of, on exposure to air 51-53

titanite in : 56

zircon in _ _ . . . . 58-57

Syenite-porphyry, analysis of 59

dike of, at Little Ascutney Mountain. 69

Teller and von John, cited 35

Temperature and specific gravity of

rocks, relations of __ 95

Turner, H. W. , analysis by 47

VanHise, C.R., cited 103

quoted _ _ 77

Washington, H. S., analy.sis by 75

cited .-.. 89

Weed and Pirsson, cited 93

Whittle, C. L., cited-. 15

Williams, G. H., cited 35

analysis by - 59

Windsorite, analysis of 47

definition of 48

mineralogical composition of 46

dikes of 45-48

Wolff, J. E., acknowledgments to 7

Zirkel, P., cited 95

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( D, Petrography and mineralogy, 22 I

Department of the interior | United States geological survey |
Charles D. Walcott, director | — | The | geology of Ascutney
Mountain, Vermont | by | Reginald Aid worth Daly | [Vi-
gnette] I

Washington | government printing office | 1908

8°. 123 pp., 7 pis.

Ill

BOSTON

COLLEGE

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