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MUSEUM OF COMPARATIVE ZOOLOGY

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"Tov. 11, 1892

Occasional Papers

Moston Society of Natural History.

IV.

GEOLOGY OF THE BOSTON BASIN

1

Hi
jJ

BY

|
| WILLIAM О. CROSBY.

i

е
| VOL. I.

| PART I.—NANTASKET AND COHASSET.
1 er
h

BOSTON :
BOSTON SOCIETY OF NATURAL HISTORY.
1893.

(Pccasional Papers

OF THE

Boston Society of Natural History.

GEOLOGY OF THE BOSTON BASIN.

BY

WILLIAM O. CROSBY.

WITH THIRTY-EIGHT PLATES.

BOSTON:
PRINTED FOR THE SOCIETY.

1893-1900.

This work, Geology of the Boston Basin," was originally planned to
comprise two volumes of several parts each. Only the first three parts
of the first volume, however, were completed by Prof. Crosby before his
death. Plate 13, designed to accompany Part II, was never issued.

Title-page issued April, 1927.

Part

Part

CONTENTS OF VOLUME IV.

I.—Nantasket and Cohasset. (6 plates.)

II.—Hingham.

(6 plates.)

October, 1894

Part III. -The Blue Hills Complex. (26 plates.)

April, 1893

September, 1900 .

GREAT HILL SEEN FROM THE NORTH. A CHARACTERISTIC EXAMPLE OF
NANTASKET TOPOGRAPHY.

GEOLOGY

OF THE

BOSTON BASIN.

WILLIAM O. CROSBY.

IN TWO VOLUMES.

VOL. I.

PART I.—NANTASKET AND COHASSET.

«P

BOSTON :
DOSTON SOCIETY OF NATURAL HISTORY
1893.

- The expense of publishing this part is defrayed

chiefly by Mr. Thomas A. Watson.

NANTASKET AND COHASSET.

INTRODUCTION.

lr is obvious upon the most casual observation that the town
of Hull, Mass., which is virtually an island, being joined to the
main land of Cohasset only by the narrow barrier beach con-
necting the Green Hill drumlin with the granite ledges along
the Jerusalem Road, is sharply divided into two districts which
are very strongly contrasted in their topographie and geologic
features. In the order of geologic age and interest these аге:
(1) The highly irregular, broken and rocky tract, commonly
known as Nantasket, which forms the southern extremity of the
town, adjoining the mainland, and is almost completely isolated
by Strait’s Pond, Weir River S Nantasket Harbor and. the
Atlantic. (2) The narrow, and, at some points, extremely
slender peninsula stretching from е Hill north-northwest
for more than three miles to Point Alle rton, whence, turning at
а right angle, it extends to the westward two miles further to
Pemberton and Windmill Point. This peninsula thus embraces
Nantasket Beach and the Vill: age of Hull; and, with the excep-
tion of the single ledge of slate on the south side of Thornbush
Hill, near the Village, it is composed entirely of rounded drift
hills or drumlins and the connec ting beaches of sand and shingle.
The div iding line between these two districts is where the sands
of the beach rest against the rocky northern base of Atlantic
Hil.

Although the main purpose of this paper is to set forth the
results of a detailed study of the intricate geologic structure

exhibited in the m: ienificent, rock exposures of the southern area

OCCAS. PAPERS B. 8. N. H. IV. 1

9
(Nantasket), the concluding pages, especially, relating to the
glacial and more recent phenomena and the non-lithified de-
posits of the distriet, will apply to the northern or beach area as
well. Hence the scope of the paper really embraces the entire
area of Hull, or Nantasket in the broad, original application of
the name.! The mouth of Weir River, or Weir River Bay,
forms, in the topographie sense, the natural western boundary of
the Nantasket area; yet, geologically, it is impossible to exclude
the promontory of Rocky Neck, in Hingham. This rocky head-
land is lithologically and. structurally identical with the district
ast of Weir River Day, while it is completely separated from
the other and dissimilar sedimentary areas of Hingham by the
ledges of granite on the south and southwest and the drumlins
of Planter’s Hill and World's End on the west and northwest.
The natural boundary line between the geologic areas of Nantas-
ket and northern Hingham appears, therefore, to be, not Weir
River Bay but the eastern shore of Hingham Harbor. The
southern boundary or limit of Nantasket is clearly marked by
the continuous depression formed by Straits Pond, Lyford's
Liking and Weir River Day, which is continued, as a narrow
strip Qf marsh, south of Rocky Neck to the bay which sharply
indents the eastern shore of Hingham Harbor. This topographic
trough defines with approximate accuracy the present border of
the Boston Basin, separating the sedimentary and volcanic rocks
from the somewhat more elevated granitic area on the south,

Although the line of depression just traced is a natural and
important geologic boundary, it has appeared best to give this
paper greater breadth and completeness by including in its scope
not only the sedimentary and volcanic rocks of Nantasket, and
the granite floor upon which they rest, but also a sufficient
breadth of the granitic border of the basin to make apparent

the marked contrast in geologic structure on opposite sides of

=

'The outer islands of Doston Harbor (the Brewsters, Calf Id., Green Id., etc.) belong
to the town of Hull. But the geology of these formed the subject of a previous paper
(Proc. В. S. N. IL, XXIII., 450-457); and will be more fully described in a later paper

of this series.

the boundary. The geology, so far as the hard rocks are con-
cerned, is as simple south of the line as it is complex north of it.
Hence, although, as the map shows, the addition of the whole
of Cohasset and a portion of Scituate much more than doubles
the area to be described, it extends but slightly the limits of this
paper, which is still, notwithstanding its title and areal scope,
chiefly a detailed account of the ledges of Nantasket.

The numerous islands of Boston Harbor, as any one may ob-
serve in going from Boston to Nantasket by water, are, with
unimportant exceptions, composed, like the Nantasket Beach
area, wholly or almost wholly of rounded drift hills and con-
necting beaches of sand and gravel; and the fine sections of
some of the drumlins due to marine erosion show that in most
cases they are pure drift accumulations, at least above sea level.
The Boston Dasin does not afford a stronger topographie contrast
than is presented when the Nantasket steamer, having rounded
Dumkin Island, the graceful drumlin outline of which is abruptly
terminated on the west by marine erosion, and followed the
winding channel past the beautiful drumlins of the World's End
and Planter' Hill, sweeps by the bold and massive ledges of
Rocky Neck and Nantasket. We pass in an instant from an
area which is deeply covered by drift and presents only the most
typieal drift-contours, to an essentially driftless area, where the
ice-sheet left hardly sufficient detritus to fill the narrow gorges
and chasms dividing the rocky hills.

The problem involved in this unequal distribution of the drift,
in this abrupt passage from an area in which the ice-sheet acted,
at least during its later stages, as an important agent of depo-
sition, to an area in which its action was purely erosive, lies
beyond the scope of this paper ; although, if the drift had not
been swept from the Nantasket area, there would have been little
occasion for a special study of its geologic features. We may
now simply accept the almost perfect exposures of these rock
masses as one of the best gifts of the ice-sheet to the student o(
our loeal geology ; for there is probably no equally limited area
in the Boston Basin more worthy of detailed and thorough in-

vestigation than this southeast corner.

The structure of other districts is, no doubt, equally varied
and intricate ; but Nantasket excels in the very favorable op-
portunities which its almost continuous rock surfaces present
for deciphering its structure lines. t is especially interesting,
in this connection, to observe that, while the rocky area occur-
ring next to the north of Nantasket, in the middle line or axis
of the Boston Basin—the outer islands of Boston Harbor—
affords the finest and most typical exposures of intrusive sheets
or interbedded dikes to be found in this region; the characters
of the contemporaneous beds or ancient lava flows of the Boston
Basin and their relations to the interstratified conglomerate can
be studied to the best advantage at Nantasket, on the border of
the basin.

At no other point is the evidence equally clear and conclusive
that the melaphyrs and porphyrites of the Boston Basin are, in
the main, true contemporaneous lavas which were poured out
on the sea-floor at different periods during the deposition of the
beds of conglomerate and sandstone. The Nantasket area also
shows more clearly than any other that in some parts, at least,
of the Boston Basin the conglomerate beds, as well as the as-
sociated melaphyrs rest directly upon the fundamental granite,
without the possible interposition of the slate series.

It may be pointed out in this conneetion that the main pur-
pose of the present investigation is simply to elucidate the
structure and explain the origin and sequence of the rocks
of this somewhat eomplicated area. In other words, this is a
structural rather than a lithological study; and I wish to ac-
reat obligation to Mr. Geo. P. Merrill of the

O1
=
U. S. National Museum for undertaking a microscopic examina-

knowledge my

tion of the newer eruptive rocks of Nantasket. His determina-
tions have prevented serious errors in my work and enabled me

to draw certain conclusions with a degree of confidence which it

would otherwise have been impossible to feel.

TOPOGRAPHY.

The rocky areas or ledges of Cohasset and Nantasket are
characterized, considering their diversified geologic structure,
by aremarkable uniformity of elevation, indicating that in pre-
glacial times this region was worn down nearly if not quite to
its base-level ; and the deeply incised and generally abrupt
character of the valleys now dividing this old peneplain shows
that it must have shared in the general and marked elevation of
the land at the dawn of the great ice age; while the fact that
the deeper valleys are now to a large extent occupied by the sea
or its deposits, 7. e., are or have been true fiords, is sufficient evi-
dence of a subsequent subsidence ; and, finally, the extensive
beach and marsh formations prove that the present level of
the land has been maintained for a very long time. The mod-
ern base-level is strongly accentuated, not alone through the
constructive action of the sea, but marine erosion has made ex-
tensive inroads upon the drumlins and other drift deposits,
especially of the Nantasket peninsula and adjacent islands, de-
veloping the prominent sea-eliffs of Telegraph Hill, Point Aller-
ton, Strawberry and Green Hills, etc.,as well as broad submarine
platforms or shoals, the outlines of which сап be traced on the
Coast Survey chart.

Since even the most typical peneplain must slope gently sea-
ward, the average elevation of the rock-surface of eastern Mass-
achusetts increases gradually as it recedes from the coast ; and
Nantasket and Cohasset are, therefore, one of the lower portions
of it. In the valleys, the rock-contours are, of course, carried
down to and below sea-level; but the ledges and rocky hills
Separating the depressions, the real remnants of the ancient
peneplain, are, even in the Nantasket area and alone the Co-
hasset shore, rarely below 50, and usually from 75 to 100 feet,
in height; and the elevation, increasing very slightly inland,
attains its maximum of 125 to, possibly, 150 feet in the broad

6

area of rocky woodland along the boundary between Cohasset
and Hingham. The moderate disparity of these figures does
not contradict the previous statement concerning the general
uniformity of the rocky elevations; for the main point is that
while there are hundreds of rocky hills and ridges or ledges rising
10 to 50 feet or more above the ground between them, there are
none rising to commanding heights, that is, there are no rock
elevations decidedly overlooking the surrounding country, as
do the drift hills or drumlins. It is of course this absence of
true rock hills that proves the ancient peneplain ; for the present
interstream surfaces, where not encumbered by drift, are essen-
tially ledgy plateaus. The absence of crags and pinacles of rock
is casily accounted for by the severe glaciation which this region
has suffered ;. but we cannot thus explain the fact that there are
no dominant rock hills of rounded or glaciated outline, like
the Blue Hills. That glacial erosion was not equal to the com-
plete obliteration of such reliefs is proved by the survival, even
when quite isolated, of the numerous hills and ledges which do
not rise above the surface of the dissected peneplain. Briefly
stated, then, the rock hills of this district have all been carved out
of the preglacial peneplain ; and the general equality of relief thus
determined must continue until an increased elevation of the
land, giving renewed energy to the ordinary agents of erosion,
permits a more general and unequal effacement of the original
interstream surfaces.

That the depressions or valleys which now interrupt the репе-
plain are not due chiefly to glacial, but to preglacial aqueous,
erosion is sufficiently obvious, in many cases, from their direc-
tions; and it is also seen in the fact that the rock hills and
ledges of Nantasket and Cohasset, through the influence of the
faults, dikes and joints by which they are bounded, are, in the
main, approximately rectangular in plan, with, frequently, very
steep or even precipitous slopes, and showing only secondarily,
and not as their primary form, the roche moutonée outline due
to glaciation. Hence, cliffs, sometimes rising abruptly from

the water or the salt marshes, and straight, narrow defiles are

characteristic topographic features of the district, although the
partial submergence of the valleys and the general plateau form
of the hills causes the elevations to appear incommensurate with
the otherwise rugged character of the topography. North of
Atlantic Hill we descend into the great central valley of the Bos-
ton Basin, now occupied by Boston Harbor; and the pene-
plain is lost. This is an area of much softer, sedimentary
rocks, which was deeply and broadly eroded, when the region
was more elevated, by the united Charles and Neponset Rivers
and their affluents.

The lower portions, especially, of the old peneplain, in Cos
hasset, are to a considerable extent emphasized by level
expanses of modified drift. The principal sand plain, through
which project both the drumlins and the higher ledges, ranges
from 40 to 60 feet in height and extends interruptedly over a
large part of the town, mantling but not greatly masking the
contours of the underlying rocks. Although distinctly recog-
nizable at many points, it has its finest development in and
about Cohasset Village, between Little Harbor and the railroad,
rising very abruptly 45 feet from the level marshes of Little
Harbor to the kettle-dimpled plain on which the village stands.
North of Strait's Pond and Weir River Day, in Nantasket and
northern Hull, the modified drift is very scantily developed and
of no topographic importance. But in the terraces or elevated
shore-lines traceable at corresponding heights on some of the
drumlins in Hull and also in Cohasset and Scituate, and aecu-
rately marking the varying levels of the sea at the time when
the sand plains were formed, we have a related feature of con-
siderable interest which will, in its proper sequence, be described
in detail.

While the modified drift is, at the best, only a minor factor
in the topography of this district, the unmodified drift or till, on
the other hand, occurring almost wholly in the form of drum-
lins, adds greatly to the topographic relief and diversity. The
drumlins constitute, virtually, the only elevations in Hull
north of Atlantic Hill; in fact it is doubtful if this part of the
peninsula would have any existence as dry land if it were not

8

for these solid nuclei of till about which the beach deposits have
gathered. As already noted, and as the map shows, the drum-
lins are not evenly distributed. In Hull, in the direct line of
the beach, like a string of beads, are Point Allerton and Straw-
berry Hills, White Head, Sagamore Head, Hampton Hill,
Rockland Hill (a lenticular slope of till) and Green Hill.
Parallel with this line, on the west, is a second, including the
drumlins of Hull Village — Thornbush and Telegraph Hills,
Little Hog Island and Bumkin Island, this line being continued
to the west of Nantasket in the double drumlin of the Worlds’
End, Planters’ Hill and Pine Hill. From either of these lines

we pass to an area, including the Nantasket ledges and all
that part of Cohasset and Hingham north of the railroad and
east of Hingham Harbor, in which drumlins, or any noteworthy
accumulations of till, are almost wholly wanting; the hard
granite surface of the peneplain standing forth clear and naked.
But south of the railroad, again, in the broad depressed or valley
portion of Cohasset, the drumlins are thickly planted, greatly
obscuring this portion of the peneplain. The series begins on
the northwest with Turkey Hill (181 feet) and Scituate Hill
(177 feet), two of the most typieal drumlins in the Boston
Dasin and the eulminating points of the area to which this pa-
per relates. Southeastward, as the map shows, the belt broad-
ens, but the drumlins, although more numerous, are also
smaller and very much flatter in form, being, where they reach
the level of the principal sand plain, as in Hoop Pole and Mann
Hills, in Seituate, not readily distinguished from it. Dut
farther south in Scituate, we find again, in Booth Hill, a drum-
lin of respectable height and great horizontal extent. West
of King Street, Scituate Pond and the head waters of Bound
Brook, and south of Turkey and Scituate Hills, no drumlins
have been observed in Cohasset, the peneplain being here, as
north of the railroad, but slightly encumbered by drift de-
posits,

The drainage of this district is a simple stury. No part of
Tull is more than one-fourth of a mile from the salt water, and

„!!... аана
7

there are no streams. The plateau of North Cohasset drains
directly by several short brooks, of which the most important
is Turkey Hill Run, rising in. the swamp between Turkey and
Scituate Hills, into the Strait’s Pond trough, the Atlantic and
Little Harbor. The remainder of Cohasset and northern
Scituate are tributary either to Cohasset Harbor and its ex-
tension, the Gulf, or to Bound Brook, which is the principal
affluent of the Gulf. Considerable attention has been given to
tracing out not only the actual drainage lines or streams, but
also the areas of obstructed drainage, the ponds, swamps and
marshes ; and it is believed that these features are represented
more accurately on the maps accompanying this paper than on
any earlier published maps.

Desides Strait's Pond, which is due to ап artificial barrier. at
its western end and is mainly salt water, the only important
pond in this entire distriet of Hull, Cohasset and northern
Scituate is Scituate Pond (sometimes called Lily Pond) in
the southern central part of Cohasset. — It is tributary to Bound
Drook; and it is also connected on the northeast with a swampy
tract from which a much smaller brook flows directly into the
Gulf. Scituate Pond is thus very nearly an example of a basin
having two distinct outlets at the ваше level or in a state of equi-
librium. It is probably due primarily to the accumulation of
modified drift in the valley of Bound Brook ; and the general
absence of ponds in this district may be attributed to the lack of
continuity of the sand plains. Even the small kettle ponds
of other districts, with a few exceptions, as in the vicinity
of Little Harbor where the principal plain has its best devel-
opment, are wanting here. Neither is the form of the ground
favorable to the view that any important ponds have been obliter-
ated by the cutting down of barriers or the growth of silt and bog:
deposits. If the valley of Bound Brook were cleared out, the
Gulf would doubtless be extended inland to Scituate Pond, if
hot considerably beyond ; and this main drainage channel and
its affluents are now so nearly on a base-level basis that they

are bordered throughout by broad swamps and meadows,

10

Besides Bound Brook and the Gulf, the principal submerged
or drowned rock valleys of this district are Weir River Bay
and Strait’s Pond, Little Harbor, and Cohasset Harbor. These
fiords, as remarked, are all far advanced in the process of silting
up to the present level of the sea. Even Cohasset Harbor is a
harbor in little more than name, presenting with the ebb of the
tide a broad level expanse of mud and sand of great but unde-
termined thickness ; while the widely extended marshes on the
east and south show how greatly the area as well as the depth
of the harbor has been reduced. The barrier beach bordering
and limiting the salt marshes on the north has nearly separated
the outer harbor from the Cove; and the soundings оп the
chart indicate that a similar bar is now forming across the mouth
of the outer harbor, from the Glades westward. In more ex-
posed situations, the sea has already closed by barrier beaches
two eastern entrances to Cohasset Harbor, the main entrance to
Little Harbor, the eastern end of Strait’s Pond, and connected
all the drumlins of the Nantasket peninsula by beaches from
fifty feet to nearly half a mile in width. The numerous rocks
and islets fringing this part of the coast are not the result of
marine erosion; but these, and also the ledges, sometimes of
great extent, now isolated by the salt marshes of Nantasket
and Cohasset, testify equally with the submerged valleys to the
subsidence of the land. Continued subsidence would sub-
merge these ledges and isolate others, leaving the general

aspect of the coast unchanged.

MAPS.

This paper is accompanied by two maps ( Plates I and IL). The
first, on a scale of 2400 feet to the inch, embraces the entire
peninsula of Hull, the whole of Cohasset, a portion of Scituate
and all but the southern end of Hingham, or, in other words,
the entire area to be deseribed not only in this paper but also in
the following paper on the geology of Hingham. Topographi—

cally, it is based primarily upon the Coast Survey chart of
a } | y:

cA

11

Boston Harbor and the atlases of Plymouth апа Norfolk Coun-
ties; but certain features, especially the marshes, drainage lines,
drumlins and other drift reliefs have been greatly modified in ac-
cordance with original observations. The aim has been to com-
pile the best obtainable data, rather than to preserve a uniform
degree of accuracy, and undoubtedly some parts of the map are
much more reliable than others. Even in the absence of con-
tour lines, it exhibits some of the more important relief features,
the kames and drumlins being clearly distinguished ; while the
hydography, including the shore-lines, ponds, streams, swamps
and marshes, is represented with unusual completeness and ac-
curacy ; and it shows in addition the general structure of the
hard rocks, and especially the relations of the granitic masses to
the sedimentary and voleanic deposits. The Nantasket area, the
elucidation of which is the special object in view, forms the central
part of this first or general map, which thus illustrates its topo-
graphic and geologic relations to contiguous districts ; while the
second or special map, on a scale of 600 feet to the inch, permits

of much greater detail.

No good or even approximately accurate topographic map of

all that large part of the Nantasket district west of the County
Road has heretofore been published, and it has been necessary to
devote considerable time and labor to the preparation of a suita-
ble basis for the representation of the ‘geologic features. I
am particularly indebted to Mr. Wm. M. Beaman for the sub-
stantial accuracy of the western area, between the railroad and
Weir River Bay, and for a general triangulation of the entire
district from Planter’s Hill to Green Hill and Black Rock ; and
to Mr. Gordon H. Taylor for the contour lines upon nearly the
entire map excepting the western area.!

The map of Rocky Neck is based upon the plan of a land
survey kindly furnished by Mr. John R. Brewer; and the

coastal area, from the steamboat wharf to Green Hill is re-

! Messrs. Beaman and "Taylor have made a careful study of the topography of this
section as students in the Department of Civil Engineering in the Massachusetts

Institute of "Technology.
t

duced, with some modifications, from the map of Nantasket in

the atlas of Plymouth County, 1879; while I am personally re-
sponsible for the outlines of the central area, between the ( bounty i
Road and railroad, and for the entire south shore of Weir River
Day, Lyford's Liking and Strait's Pond.

The general absence of roads or other artificial land-marks
west of Hull Street and the County Road, and especially west
of the railroad, and the fact that the hills and other natural
features in this section were still unnamed, caused it to appear
desirable, in the interests of concise and intelligible description,
to assign names to the more important rock masses, marshes,
ete. Of course it is not expected that many if any of the geo-
logie and descriptive designations which appear upon the map
and in the following pages will gain general acceptance ; and
this is in no wise essential to their present usefulness. The
aetual exposures of the hard rocks are so nearly continuous
over the areas where they are indicated by colors on this map "
that it has appeared unnecessary to represent the individual
ledges or outerops; although this is virtually done for all the
ledges in the marshes and below the high-tide line. And; ex-
cept in the case of some of the fault-lines; the map may be
fairly regarded as a plain record of actually observed facts, de-

vold of theory.

THE GRANITIC ROCKS OF COHASSET.

This heading, in its broadest application, covers all the geo- |
logical formations of Cohasset, except the dikes of diabase and |
porphyrite and the drift deposits. The dikes, being essentially |
similar in character and age to those of Nantasket will be most |
conveniently described in that connection ; and since the super- f
ficial geology forms naturally one continuous chapter for the en- | | 8
tire area to which this paper relates, the drumlins, sand plains,
ete., of Cohasset will not be taken up separately. But it appears
best to introduce a general account, lithological and structural,

of the granitic rocks of Cohasset at this point, because they form,

18

not only the border, but the floor of this part of the Boston Basin.
They are alike the foundation upon which the newer sediments
and lavas of Nantasket are piled, and the principal source from
which the material for building the Nantasket strata was de-
rived.

Among the granitic rocks of Cohasset are included chiefly
the diorite and the granite proper. The diorite is in every in-
stance clearly the older, as well as the less abundant and less
import: int, rock ; the relations of the two rocks bei ing essentially
the same here as elsewhere about the Boston Basin. To a large

extent they are quite intimately associated, the diorite occurring

very generally in the form of irregular fragments or masses, of

all sizes, enclosed in the granite; while in other cases the gran-
ite forms irregular, branching dikes in the diorite. In fact, the
granite, however massive it may be, is rarely entirely free from

inclusions of diorite ; and the diorite, even when farthest from a

main body of granite, almost invariably exhibits a net-work of

granite intrusions. In other words, the diorite has been very
generally fissured, and in large part completely shattered, and
then injected by the granite. Hence, although the two rocks
are always perfectly distinct in their Se relations, ob-
serving an invariable sequence, it is, for considerable areas, а
hopeless task to trace or define the кли of either sepa-
rately from the other; and it is for- this reason alone that they
are not distinguished on the m: ap. Any boundaries that. might.
be drawn upon the map would have but ve ry little significance,
since in no ease could they be either exclusive or inclusive. The
granite very largely predominates; and tho diorite, except ав
isolated inclusions in the granite, is quite restricted in its distri-
bution. It has been observed chiefly in the ledges south of Co-
hasset Harbor, north and west of Little Harbor and along the
shore between Little Harbor and Nantasket. The large ledge
south of the Cove and west of the Gulf — Kent Rocks —
18 mainly granite, but encloses considerable diorite, while the
pointed, rocky hill immediately east of the Gulf — Government

14

Rocks — is all granite, coarse and pinkish. Та the large ledgy
tract nearly half a square mile in extent between the Gulf and
the Scituate shore, diorite appears to be the prevailing rock 3

but the granite is always close at hand and forms some large

— —

masses. The ledges along the Scituate shore, east of Cohasset
Harbor and the Glades, and advaneing from the south, are,

except for an occasional inclusion of diorite, wholly composed

АЕ

of a beautiful, coarsely but uniformly erystalline and massive
pinkish granite, which also forms the adjacent islands, from
Darr's Rocks to the Osher Rocks. On reaching the prominent
point a short distance north of the Osher Rocks, however, we
pass abruptly from the granite to diorite with only oceasional
irregular dikes of granite breaking through it. The diorite forms
the north shore of this point; but, crossing a short shingle
beach, we find that the east shore of Strawberry Point is chiefly
coarse granite ; while across the north side there is much diorite

alternating with the granite. It is partly in solid, unbroken

masses, more commonly veined with granite, and to a consid-
erable extent completely morcellated, yielding a very coarse
breccia in which the diorite forms the fragments and the granite
the cement. Gull Island presents at low tide a broad flat sur-
face of diorite irregularly veined with granite; but Sheppard’s
Ledge and the ledges southwest of Gull Island are granite
with little or no diorite. These characteristic relations of
the two rocks are frequently repeated on the many other ledges l.
and islets between Cohasset Harbor and Minot’s Light, although
the granite usually predominates. Along the north side of Co-
hasset Harbor, between the railroad and White Head, the ledges,
so far as observed, are nearly all granite ; and following the
shore northward from White Head, around Sandy Cove, to the ‘
mouth of Little Harbor, the rock is all the most typical, coarsely

erystalline, light gray granite, weathering pink or reddish, very

massive in structure and with only very rarely a small inclusion
of diorite. North of the narrow mouth of Little Harbor, on
Beach Island, the granite continues, unchanged, to a point be-

ween 200 and 300 feet south of the artificial harbor on the

— —

15

headland opposite Brush Island. Here we pass very abruptly
from the coarse, massive granite to the mixed granite and diorite,
for the first hundred feet or so a very confused mixture, in which
the diorite seems to predominate; but west of that, along the
entire stretch of shore, as far as Green Hill and Nantasket, the
granite is the prevailing rock, with frequent inclusions of diorite.
This is, structurally, an extremely interesting section, and it will
be deseribed more fully a little farther on. The essentially
patehy distribution of the diorite in the granite makes the tra-
eing of this rock in the weathered and lichen-covered inland
ledges rather unsatisfactory, not to say unprofitable, and only
enough work has been done in this direction to show that what
we can see so clearly along the shore is really characteristic of
the whole town. Thus it must be evident to any one observing
the ledges along the west side of Little Harbor and on Forest
Avenue that the mixed granite and diorite extends inland a con-
siderable distance; while in other parts of the town it is equally
clear that coarse and massive granite covers large areas.

The diorite is always dark-colored and holocrystalline, but
usually rather fine-grained, varying in texture from compact or
aphanitic to distinctly but not coarsely crystalline, z. e., the dio-
rite is rarely coarse in the sense that the granite often is. On the
other hand, it rarely resembles diabase, except in the most com-
pact forms, the normal difference in crystalline structure or
habit being readily recognized in the macrocrystalline diorite.
Under the miscrocope it is usually seen to be composed chiefly
of plagioclase and hornblende. The feldspar is commonly
rather opaque ; but the hornblende is, in many cases, beauti-
fully clear and dichroic, although usually bordered by secondary
biotite, chlorite, ete. ; while in the more compact and highly
altered diorite the hornblendic element is very largely reduced
to hydrous silicates, which give the rock a dark greenish color.
Among the secondary minerals occurring in veinlets and irregu-
lar segregations epidote is most prominent, but it is frequently
accompanied by chlorite and quartz. The black oxides of

iron (magnetite and menaccanite ) are usually present but rarely

16

abundant, and the rock also often contains a small amount of
original quartz. The quartz diorite is especially interesting as L
indieating a gradual passage to the more basic granites. 4
The diorite is undoubtedly a plutonic rock ; and it is sufficient-
ly varied in character to suggest that it is possibly not all of the |
same age; but no facts have been observed which point to a |
definite conclusion, and all that can be regarded as well deter- |
mined is that its relations to the granite are always essentially
the same.
The granite, on the other hand, belongs very clearly to two,
and probably three, more or less distinet periods of igneous
activity, or successive phases of the same period. First in order
of time comes the granite which is most intimately associated
with the diorite. This is very abundant, and partakes of the
character of the diorite. It is on the average only a little more

coarsely crystalline than the diorite; and usually contains suffi-

cient hornblende or black mica to make it quite dark colored
for a granite. The quartz is often deficient, and the feld- |
spar is partly plagioclase, so that it would be easy to mistake |
a portion of the rock for either diorite or syenite. The horn-
blende shows, perhaps, even more alteration than in the diorite,
being very largely replaced by chlorite. Next in order comes
the light gray and pinkish granite, which is usually coarsely
. crystalline and massive and is as a rule comparatively free from
inclusions of diorite. This rock is rich in acid feldspar and
quartz ; but the hornblendic element is usually quite scantily de-

veloped ; and black mica (biotite) often partially, sometimes

wholly, replaces the hornblende, while both of these accessories
are, as arule, largely altered to chlorite or other hydrous species.
It can be seen breaking through the first granite and the diorite at
many points along the shore, especially between Little Harbor
and Nantasket ; and some of the smaller dikes of this granite are
quite fine grained and not easily distinguished from the third |
type. This coarse acid granite is the only rock in this district
of any particular economie interest ; and it has been quarried

only to a very limited extent. There is one small quarry on the

17 |

Scituate shore, near the Osher Rocks, and another on the Co-

hasset shore, north of Sandy Cove; but inland nothing deserv- |
b ing the name of a quarry has been observed. The third granite,
| in chronological sequence, forms small and irregular dikes
| from a fraction of an inch to several feet in width cutting all the
| older rocks, but occurring chiefly in the coarse, typical granite
| (No. 2), from which it appears to differ in composition only in
containing less hornblende or mica. These dikes are always
fine-grained, varying from a finely crystalline gray or pinkish
granite or micro-granite to a true felsite.

The miero-granite passes into an acid felsite of the same chem-
ical composition. But while the micro-granite is common, form-
ing hundreds of small dikes, true felsite, which does not reveal |
a holocrystalline ground-mass under the microscope, is rather I!
rare. The largest mass which has been observed is at the ||
northeast corner of the large ledge of granite on the south
shore of Lyford’s Liking, south of Round Hill. This is a

brown, thoroughly compact or felsitic, structureless rock: and

. —

forms a mass several yards across, just at the water’s edge.

The contacts with the granite are not clearly exposed; but it is
evidently a dike in the granite. Small dikes of a true quartz-
porphyry have also been observed on the Jerusalem Road, east
of Green Hill. The great abundance of pebbles of felsite in

the Nantasket conglomerates indicates that this rock was for- |
merly much more extensively developed in this district. It |
probably occurred chiefly in the form of broad surface flows, |

Such as still exist їп other parts of the Boston Basin; and

the dikes of both felsite and miero-granite probably date from |
these volcanic eruptions, being branches from the main fissures
or necks through which the felsite reached the surface. |

Although these three types of granite clearly reveal the |

chronological succession described above, it can not be shown |
that they are widely separated in time or belong to entirely
distinct periods of igneous activity ; and it is especially obvious

|
|

` .` . |

that the micro-granite and felsite, although cutting the coarse |

` OCCAS. PAPERS B. S. N. H. IV. 2

18

granite, should, on account of their resemblance to it in com-
position, be regarded as, in a general view, essentially syn-
chronous.

This coast abounds in instructive exposures of the granitic
rocks, but the nearly continuous belt of wave-washed ledges
along the Jerusalem Road between Nantasket and Beach Island
is particularly worthy of thorough study, illustrating, as it does,
the relations of all the rocks and presenting some features which
are not clearly exposed elsewhere. On the south side of Besch
Island, as already noted, there is a fine development of the
coarsely crystalline, massive granite ; but from the junction of
this with the mixed granite and diorite just south of the artificial
harbor to Nantasket the finer-grained, dark-colored, older granite
is the principal, and for a considerable portion of the distance,
excepting the dikes, almost the only, rock. This older granite
is characterized throughout the entire section by a remarkable
gneissoid structure due to the flowing of the material while in a
viscous condition. At many points this flow-structure is so
perfectly developed that the rock presents a distinctly gneissic
character even in small hand-specimens ; and where the fluidal
lines are least obvious, they are still traceable on the broad,
clean surfaces of the ledges. The trend of the flow-structure or
pseudo-stratification varies somewhat. It is about N. IZ. -S. W.
on Beach Island; while farther west it ranges usually between
N.-S. and N. W.—S.E., but becomes N. 60° W. as we approach
the beach leading to Green Hill. The dip is usually nearly
vertical, although sometimes as low as 60? or less.

This gneissoid granite encloses at most points numerous frag-
ments of diorite of varying lithological character. These exhibit
a great range in size; and they are usually more or less elongated
in form, being often distinctly lenticular and indicating in their
smooth and somewhat indefinite outlines a partial fusion of the
diorite in the melted granite. The elongated fragments coincide
in direction with the flow-structure of the granite, and this struct-
ure is always most perfect where the diorite is most abundant.

The form of the diorite fragments means, probably, that the

э

— -_

19

diorite also possessed a gneissic structure of some sort before the
eruption of the granite ; in fact this can frequently be observed
in the more lenticular masses of diorite. The diorite, then, has
been injected by the granite chiefly along its own original struct-
ure-planes ; and the flow-structure of the granite is probably due
largely to its having come up between these parallel walls of
diorite.

Breaking through this gneissoid granite and the enclosed dio-
rite at irregular intervals are many small and some large masses
of the coarse, light-colored granite. These usually conform in
direction, at least approximately, with the flow-structure of the
older granite, and exhibit in this direction a similar but, as a
rule, less distinct flow-structure. We thus not only have a
somewhat eneissoid diorite of probable igneous origin broken
through very profusely by, and enclosed in, a highly perfeet
gneissoid granite — a true eruptive rock which, but for its rela-
tions to the diorite, might readily be classed as of sedimentary
origin, a true gneiss; but both of these terranes are traversed
again and again in a way possible only to an igneous rock, by
à second and more acid pseudo-eneiss. Again, all the preceding
rocks are injected by occasional small and irregular dikes of micro-
granite and felsite, which, for the most part, are devoid of
gneissic structure. And, finally, this entire sequence of granitic
eruptions is divided by a well-defined series of porphyrite dikes
and no fewer than three distinct systems of diabase dikes : all
of which will be described in connection with the dikes of Nan-
tasket.

This shore undoubtedly presents the best general section of
the plutonic rocks of the Boston Basin. Certainly at no other
point do we find plutonic masses of so many different ages so
clearly exposed in their normal relations ; and in this connection
it should be noted that the passage (in natural sequence) from
the diorite through the more basic to the more acid granite
affords some indication that the diorite is not widely separated

in time from the granites, the entire series constituting but one

complete igneous cycle.

THE ROCKS OF NANTASKET.
GENERAL RELATIONS AND ORIGIN.

Although the surface exposures of the rocks of the Nantasket
area are completely isolated by drift deposits and the sea, these
strata are probably continuous to the north and west with the
great body of sediments occupying the Doston Dasin. And it
is certain, as will appear later, that they are terminated on the
south by profound dislocations; so that the sharply defined
boundary between the Nantasket sediments and the broad area
of granite can not be regarded as marking the true original
border or maximum extension of the Boston Basin in this direc-
tion. On the contrary, the facts point very plainly to the con-
clusion that the basin rocks formerly extended a considerable
but undetermined distance beyond this line; being here, how-
ever, on the upthrow side of the great faults, they were lifted
above the present plane of erosion. We may, nevertheless, en-
tertain the hope that future investigation will reveal upon the
granitic plateau outlying remnants of the sedimentary series,
and thus indicate more exactly the original limits of the basin.

The Nantasket rocks, above the fundamental granite, and
omitting the dikes, consist chiefly of the conglomerate (Roxbury
pudding stone) and the interbedded lavas and tuffs. The
incompleteness of the Nantasket section is plainly shown in the
entire absence of the great slate series, which elsewhere in the
Boston Basin overlies the conglomerate ; and it is probable, as
will appear when the facts are presented, that the upper members
of the conglomerate series are also wanting. The student will,
however, find compensation for these deficiencies in the magnifi-
cent development of the basal beds of the conglomerate. At no

other point are the floor of the Boston Basin and the strata rest-

—

— a

21

ing directly upon it so clearly and instructively exposed. On
the other hand, the eccentric position of Nantasket with reference
to the basin as a whole renders it a natural if not a necessary
inference that the beds which are here seen to repose upon the
granite are not the lowest or oldest sediments of this series;
but these must be sought farther north, along the axis
of the basin, which, according to the view of a progressive sub-
sidence during the deposition of the conglomerate and slate,
must be the deepest and oldest part of the trough.

Throughout the Nantasket area the true conglomerate, as
distinguished from the tuff, is largely composed of granitic
detritus ; but this material is especially prominent where the
lower beds of conglomerate lie immediately upon the granite.
The volcanic rocks intercalated in the sedimentary series include
three principal kinds: melaphyr, melaphyr-tuff and porphy-
rite; and, as will appear in the detailed descriptions, the evidence
for the contemporaneous origin of these igneous sheets is well
nigh perfeet at every point. — It has been found impracticable,
in the main, to determine satisfactorily the positions of the vol-
canic vents. It appears probable, however, that the eruptions
were chiefly submarine; while the tuffs would seem to indicate
that the lavas issued to some extent from craters rather than
fissures ; and we may well suppose that these ancient igneous
channels, of whatever form, are still buried beneath their own
ejectamenta. It will be shown, however, that the source of the
more acid lava, the porphyrite, is probably indicated in part at
least by the dikes of porphyrite along the Cohasset shore; and
of the oldest melaphyr by the great dike of melaphyr (2) in the
western area. s

Briefly stated, then, the history of the Nantasket strata is sub-
stantially as follows : The district now known as the Boston Basin,
which had previously been a land surface, experienced a gradual
but profound subsidence, during the progress of which the sea
slowly encroached upon what is now the Nantasket area. The
abundant detritus which must have resulted from the sub-aerial

chemical decay of the granitic and other crystalline rocks of this

region during long preceding ages — a thick sheet of seden-

22

tary soil such as may now be observed in low latitudes — was
rapidly worked over by the advancing tide to form extensive
beds of gravel and sand on the beaches and near the shore,
while the clayey constituent of the soil was carried far out into
the deeper waters of the bay.

Not long, apparently, after the formation of these strata
began, and as a result, probably, of the same agency that gave
rise to the subsidence, volcanic phenomena were instituted in
different parts of the Boston Basin ; and for a long time Mas-
sachusetts Bay must have borne some resemblance to the mod-
ern Bay of Naples. But since, as already stated, the eruptions
were largely submarine, this region doubtless exhibited a still
more perfect combination of aqueous and igneous activity.
Sheets of basic and sub-acid lavas of greatly varying thickness
and extent were poured out over the newly formed beds of gravel
and sand and were in turn covered by these ever-growing de-
posits, the overlying gravel enclosing many fragments worn
from the lava itself. The different flows of lava represented
in the same vertical section are usually separated by considera-
ble beds of conglomerate ; but sometimes the eruptions suc-
ceeded each other so rapidly as to produce composite beds of
lava, or lava and tuff, several hundred feet thick without any
interlarded sediments. Some dikes of porphyrite and at least two
of melaphyr probably date from this period and may, perhaps,
be regarded as channels through which the lava reached the sur-
face. But the great majority of the dikes traversing the conglom-
erate and melaphyr are diabase and appear to have been formed
long subsequently, when, after the upper conglomerate and a
great thickness of slate had been quietly deposited over this area,
the strata were tilted up and extensively faulted. The geologic
revolution marking the close of this period in the history of the
Boston Basin was not attended by severe plication in the Nan-
tasket area; but faulting is by far the more prominent structural
feature. The subsequent history of this area, during all the
long ages down to the glacial epoch, is recorded only in the
quiet, erosion which has swept away the great slate series and,
at some points, the entire thickness of the conglomerate and
interbedded lavas.

0990

لے

GENERAL. STRUCTURE OF THE NANTASKET AREA.

The main structural features are so clearly expressed on the
map that this topic may be treated very summarily. Nantasket
is a monoclinal area; and, although it has been completely
shattered by numerous faults and dikes, the strata exhibit only
low and little variable dips, being almost unique in this re-
spect among the sediments of the Boston Basin. Along the
northern margin, from Atlantie Hill to Rocky Neck, the pre-
vailing dip is south-southeast 10? to 20?. But southward, or
toward the granite border, the dip usually diminishes, and
becomes more easterly, the beds immediately adjoining the
granite being often horizontal or having a slight inclination
obliquely away from the granite. These gently inclined beds,
together with the granite floor on which they rest, are broken
by longitudinal and transverse faults into a series of blocks.
A glance at the map, on which the faults are represented by
broken black lines, shows that these blocks are extremely vari-
able in form and size; and it is somewhat surprising that the

unequal up and down movements or jostling of these numerous

earth-blocks could take place without greater disturbance of

their bedding-planes.

The principal longitudinal faults, such as those immediately
bordering the granite and the profound fraeture which skirts
the southern shore of Nantasket Harbor, traversing the entire
district from Straits Pond to Weir River Bay, downthrow to
the north; and the displacements are so great that, although
the bedded rocks dip toward the granite, in receding from the
granite northward we pass in general from older to newer
strata. The continuity of the strata is interrupted so frequently
and completely, both longitudinally and transversely, by the
numerous faults; and the conglomerate is, in the main, 8O
homogeneous and, the tuff so local; that we are obliged to
depend very largely upon the lithological characters of the

interbedded lavas in correlating adjacent ledges as well as the

more widely separated parts of the field. “The sequence of the

24

bedded roeks of the entire area can not be read off directly
from any more or less continuous or connected horizontal sec-
tions. But we find instead numerous isolated vertical sections,
which can only be correlated, if at all, by a critical ledge-to-
ledge study and comparison. The best results of such an inves-
tigation are presented in the subjoined general section of the
Nantasket strata; from which it appears that above the granite
floor there are no fewer than six beds of conglomerate, alterna-
ting with five sheets of mel: aphyr and tuff and one sheet of por-

phyrite.

Table of the Nantasket Strata.

Granite, bordering and underlying the bedded rocks.

First Conglomerate, basal, resting on granite. . . . . . 295-50 feet.
Rocky Neck, Granite Plateau, Cliff Plateau, cte.

First Melaphyr, compact and jaspery . . . . 25-40 feet.
Rocky Neck, East and West Porphyrite Hills ‘ia clin Баб

Second Conglomerate, jaspery . . * ^. « 20-00 feet.
Rocky Neck, East and West Rouri Hills, ( rreat Hill, etc.

Ро ДУ еу . 50-100 feet.
East and West Porphyrite Hills, Blac k ‘Rock: es

Third Conglomerate . . . 1 50-100 feet.
Conglomerate Hill and Pl. ibid, vicina 8 ШЕН Round Hill

and Green Hill.

Second Melaphyr, green and amygdaloidal . . . . 2080 feet,
Crescent Hill, Melaphyr Plateau, and South Sidra

Fourth Conglomerate . . ege wow %% „ NOR O
Crescent Hill, Marsh island, etc.

Third Melaphyr, green and amygdaioidal . . . . . 85-45 feet.
Rocky Neck, Melaphyr Peninsula and Plateau, C vesdelit Hill
and Marsh Island.

Fifth Conglomerate . . . WO Q bee
Rocky Neck and Long Bodd h Rod к.

Fourth Melaphyr and Tuff, green, compact or brecciated . 300-450 feet,
Atlantic and Centre Hills, Gun Rock, Little Black Rock, etc.

SUSU Gone sce VU UNI eec oe re ce CL vo O
Rocky Neck.

Fifth Melaphyr, green and amygdaloidal. , . . . . . . 185-20 fact,
Rocky Neck,

600—960

— EE ERE -

Or
25
a) ә, . .

The assigned thicknesses are in part actual measurements ;

but chiefly estimates and approximations. They are intended,

i
|
|
| also, to give some idea of the variations in thickness observed in
| tracing the individual beds from one part of the field to another.
| While it must be admitted that the data upon which this
| table rests are in some respeets much less conclusive and more

| ambiguous than could be desired, it may be fairly considered |

a conservative determination as regards the number of distinct
terranes involved; since in all cases where the indications
appeared to be evenly balanced that interpretation of the facts
has been preferred which avoided multiplying the alternations
of conglomerate and lava. It is quite possible, indeed, that in
some instances what have been regarded as parts of one continu-
ous sheet of lava are really entirely distinct and, perhaps, not |
even synchronous flows. This view would, however, seem to |
L carry the further supposition that some of the flows failed to |
extend over the entire field. This is not, « priori, improbable ;
| though few facts have been observed which seemed to demand
this explanation. Some of the tuff beds are unquestionably
very local, the underlying and overlying melaphyrs coming
P together abruptly at their margins; but, although carefully |

sought, no corresponding indications have anywhere been |
detected in the melaphyr, except in the case of the flow on |
Atlantic and Centre Hills which appears to be terminated on |
the west by the junction of the underlying and overlying tufts.

It is very clear, however, that, as already stated, the melaphyr

lowed each other in quick succession over the same area. This
|

|
|
flows are composite in some cases, several eruptions having fol- |
||
|
is conspicuously true of the fourth melaphyr, the great. sheet |
d |

forming Atlantic and Centre Hills, ete.
Although, as the map shows, few of the Nantasket faults |

san be classified as distinctly longitudinal or transverse, the |
d й |
greater number are either approximately east-west or north- |
H . Hy
south in direction. It is also noticeable that the blocks bounded

by the principal faults are in several instances broken into | |

quite narrow wedges by a series of parallel east-west faults,

| |
| |
Hl
|

26

The arrows pointing. away from the fault lines on the map
indicate the direction of the downthrow in each case; and the
accompanying figures, when present, express the amount of
the displacement in feet wherever it could be even approxi-
mately determined.

The map also brings out very clearly the principal facts con-
cerning the dikes of Nantasket — their sizes, trends, distribu-
tion and correlation in different outerops. Mr. Merrill has
shown that they consist almost exclusively of diabase, and are
all highly altered; although they exhibit some notable differ-
ences in texture and degrees of alteration. The dikes are
numbered on the special map, and the numbers are repeated
for each important outcrop, so as to express more clearly their
individual continuity. The most of the dikes are readily arranged
by their trends in three distinet systems. The dominant trend is
manifestly approximately east-west. But these are seen on closer
inspection to embrace two systems, diverging 20° to 80°. The

normal trend of the first or oldest system is N. 75° to 80° Му,

and of the second S. 75? to 80° E. Their relative ages are in-
dicated by a slight difference in texture and especially by a very
clear intersection on the shore east of Gun Rock and several inter-
sections on East Porphyrite Hill. The third system trends
due north-south and is the newest of all, as shown by two
intersections of dikes of the second system : Dike 62 cuts dikes
13, 14 and 15 of the second system in Green Hill Ledge very
clearly, indeed; and the composite dike (66) in the gorge
between East Porphyrite Hill and Cliff Plateau certainly cuts
31 of the second system which in turn cuts 22 and 24 of the
first system on East Porphyrite Hill. Notwithstanding the
regularity of their forms and trends, the dikes, with the partial
exception of the newest system, exhibit a remarkable independ-
ence of the best developed systems of joint-planes. This is
especially noticeable along the northern bases of Atlantic and
Centre Hills, indicating that the older dikes, at least, probably
antedate the joint-structure of the rocks to some extent. А

further inspection of the map will show, however, that' not only

—

27
do some of the dikes coincide in position, but they all coincide
in direction, with the fault-lines of the district. This general
coincidence in trend of the faults and dikes is a very significant
fact, indicating that the dikes date from the period of general
disturbance, when the rocks were tilted and faulted. The map
also brings out the fact that the east-west dikes occur in three
distinct zones or belts, marking the three principal dislocations
of the Nantasket area.

LITHOLOGY.

To avoid unnecessary repetition in the detailed descriptions
of the Nantasket ledges we may advantageously notice in
advance the general lithological characters of the principal
types of rocks. hese fall naturally into three classes, ав
follows :—

(1). Sedimentary rocks, consisting chiefly of the conglom-
erates.

(2). The older eruptive rocks, embracing the diorites,
granites and felsites.

(3). The newer eruptive rocks, including both dikes and
lava-flows.

The third division is by far the most important; and it is a
matter for sincere congratulation that it has been worked up

by a competent specialist.

The Sedimentary Rocks.

The Nantasket conglomerate is, with unimportant exceptions,

chiefly composed of well rounded or water worn fragments of

granite, felsite, melaphyr and porphyrite. On account of tho

predominance of the granite and felsite débris, the conglomer-

ate is, in general, light-colored, pinkish or reddish, It has in

8

the main the medium texture of the Roxbury puddingstone, the
pebbles not exceeding four inches in diameter; but at some
points, such as Great Hill, Green Hill Ledge and the small
area north of Crescent Hill, they are in part much larger, one
to two feet. The larger pebbles are chiefly granite and mela-
phyr, the felsite having been always, as now, too brittle and
too finely jointed to form many large pebbles. The conglom-
erate is usually a typical puddingstone, presenting distinctly
the character of a breccia only where it immediately overlies the
granite, and is largely composed of local granitic débris.

It is very noteworthy that the quartzite pebbles, which are
such a prominent feature of the conglomerate in most parts of
the Boston Basin, are almost entirely wanting in the Nantasket
ledges ; indicating that formerly, as at the present time, the
ancient quartzites had but a slight development on this side of
the basin. The same re: soning will not, however, explain the
general absence of pebbles of diorite in the conglomerate.
Next to granite, diorite is by far the most abundant’of the older
rocks bordering the Boston Basin, covering an aggregate area
of at least one hundred square miles within ten miles of the pres-
ent margin of the basin. Since the diorite is every where inter-

sected by, and therefore older than, the granites, it is unques-
tionably also older than the conglomerate ; and we are obliged
to suppose that, like the other of our older rocks, it was proba-
bly abundantly exposed over the surface of this region during
the formation of the conglomerate. A few pebbles in the con-
glomerate, in different parts of the basin, may be doubtfully
referred to the diorite; but, generally speaking, this rock has
contributed little or nothing to the composition of the conglom-
erate. Bowlders and finer débris of the diorite are extremely
abundant in the glacial drift of this section ; but it is noticeable
that the smaller masses, especially, are usually more or less
decomposed, showing much more alteration than similar frag-
ments of granite or felsite. If, therefore, as already suggested,
we may follow Mr. Douvé! in attributing the formation of the

early sediments of this region to the working over by the sea of

Proc. B. S. N. H., XXIII., 20-80,

—

29
a thick sheet of sedentary soil or chemical detritus, it is obvious
that the more basic rocks, like diorite and diabase, must have
been more deeply and thoroughly deeayed than the more acid
rocks, like granite and felsite, the latter being intermediate in
this respect between the former and the purely silicious rocks—
the quartzites. The débris of the diorite is, therefore, to be
sought in the slate rather than in the conglomerate ; and we
must recognize the principle that the occurrence of older rocks
in the conglomerate is not in the order of their abundance во
much as of their chemical stability.

Limited layers of finer sediment—grit and sandstone—by
reference to which the dip and strike may be determined, occur
rather sparingly in most of the Nantasket ledges, as in Long
Beach Rock, Green Hill Ledge and Great Hill; although they
are practically wanting in several of the largest masses, includ-
ing Conglomerate Plateau and Conglomerate Hill. The con-
glomerate is throughout very firm and thoroughly consolidated,
breaking, usually, without reference to the contours of the
almost adamantine pebbles. In fact, the paste, including the
intercalated sandstone layers, is often so intensely hard as to
suggest that the volcanic heat of the interbedded lavas has been
an important agent in lithifying these ancient gravels. This is
undoubtedly a true cause, but still it does not afford an entirely
adequate explanation of the phenomena, since the conglomerate
beds are usually well indurated through their entire thickness,
even the lower layers of gravel, which were deposited upon a

cold and eroded surface of lava, being sensibly as hard in most

cases as the upper layers, which were covered by a stream of

incandescent lava. There are, on the other hand, many facts
Which indicate that the intense lithifieation is largely due to the
deposition of secondary silica over and between the grains com-
posing the paste, thus virtually changing the arenaceous matrix
of the pebbles to quartzite. This interstitial silica is especially

obvious where it takes the form of distinct segregations of' red

Jasper, as in the western part of Nantasket and on Rocky Neck.

30

Its source is undoubtedly to be found chiefly in the alteration
of these basic lavas; and it occurs even more abundantly in the
melaphyr itself than in the conglomerate, in the form of irreg-
ular segregations and veins of jasper, chalcedony and vitreous
quartz.

The Older Eruptive or Granitic Rocks.

Doth the diorite and the older, dark colored granite of the
Cohasset shore are practically wanting in the Nantasket area;
the granitic rocks being limited to the coarsely crystalline, acid
type, excepting a few small, irregular and ill-defined dikes of
finely crystalline granite (micro-granite) and felsite. The ex-
traordinary abundance of pebbles of quartz-porphyry and other
varieties of felsite in the conglomerate is a plain indication that
these rocks were far more extensively developed over the
granitie area in early times than now, forming, probably, sur-
face flows as well as dikes. The coarsely crystalline, acid
granite of Nantasket is essentially similar to that of Cohasset
and the South Shore generally ; and little need be added to the
statements of the preceding pages. Over Granite Plateau and
Granite Point and on Rocky Neck the relations of the basal
conglomerate to the granite are very intimate and intricate ;
but absolutely no facts have been observed which indicate that
the granite is intrusive in the conglomerate, or that any of the
granitic rocks of the Nantasket region are more recent than the
sedimentary rocks. On the contrary, the relations of the two
series, as will appear later, are, at every point, consistent with
the view that the conglomerate was deposited over an uneven,
fissured surface of the granite, such as that rock presents to-day

at many points.

|
)

The Newer Eruptive Rocks.
BY GEORGE P. MERRILL.

The more recent eruptive rocks of Nantasket, including
all those of later date than the diorite, granite and felsite,
oceur in the form of (1) lava-flows and (2) dikes. "The lava-
flows include both melaphyrs and porphyrites and these vol-
canic types are also found among the dikes, although the
latter consist chiefly of diabase. The melaphyrs and dinbases
have, in many cases, undergone such complete alteration that
their correct identification is attended with the greatest difficulty.
Indeed, the slide often shows not a single recognizable original
constituent; but consists wholly of a compact agereg:
secondary minerals, among which are epidote, quartz, kaolin,
and sundry chloritie and ferruginous products for which there
are, perhaps, no better names than the terms viridite and
opacite, already in general use.

Perfectly satisfactory results from such materials could be
obtained only by prolonged and careful microscopie examina-
tions and chemical tests, accompanied by study in the field,
where the rocks could be observed in all their varying aspects
and in varying stages of alteration. Such study I have natur-
ally been unable to give them, and my main effort has been to
80 identify the rocks and note their essential differences, if any,
48 to aid Professor Crosby in his difficult task of ascertaining
their field relations.

Tum Lava-rLows om VOLCANIC Rocks.

The effusive rocks or true lavas of Nantasket embrace, as
Stated, two principal types — melaphyr and porphyrite. The
melaphyr is the predominant type and forms several distinet
flows or sheets, of varying lithological character ; while the por-
phyrite, 80 far as known, may all be referred to one widely
extended sheet. U [

ate of

Melaphyy.

The melaphyrs throughout the Nantasket area differ from
the diabases in being of a finer grain, in that their feldspars
are less decomposed, and in never, so far as observed, contain-
ing a recognizable unaltered trace of either augite or olivine ;
also in being, as a rule, more or less amygdaloidal. They
also carry a considerable proportion of amorphous base. These
characteristics are alone sufficient in every ease examined to
distinguish the so-called melaphyrs from the diabases (or the
surface flows from the later dike rocks). The melaphyrs also
carry a larger proportion of secondary epidote, and less of the
viridite, these two substances occupying the space of the de-
composed feldspars and augites and filling the amygdaloidal
cavities. The feldspars have the appearance described by
Denton! ; and, although still showing twinning striw, are more
or less kaolinized or show traces of the viriditie and epidotic
alteration so common in the dike rocks. The black material
of the ground-mass is in part magnetite, as shown by its bluish
reflections and by its being strongly attracted by the magnet ;
but it is often accompanied by blood red scales of hematite,

The amygdules in these rocks are in part true steam-holes
or vesicles, as shown by their regular oval form, sharply de-
fined walls, and the arrangement of the enclosed and adjacent
minerals. In other cases, however, they are merely cavities
left by the decomposition of some constituent of the rock, and
are irregular in outline, poorly defined and show frequently the
remains of some less easily decomposed mineral, as a feldspar,
projecting from the cavity wall inward. These pseudo-amyg-
dules, like the true amygdules, ате filled with a variety of
secondary minerals, as feldspar, quartz, epidote and chlorite,
often without definite order, or again showing a quite regular
concentric arrangement. I have not, however, been able to
find the marked regularity in the arrangement of these minerals

in the pseudo-amvgdules that I was led to expect from a peru-

1 Proc, B. 8, N. H., Vol, xx., page 416.

—

33

sal of Mr. Benton's paper. The chlorite of the amygdules is
quite different from the so-called viridite of the diabases. With
a low power and by ordinary light it appears like a continuous
sheet of a light green micaceous mineral viewed perpendicularly
to its cleavage; but with a power of 400 or 500 diameters, it
is seen to be a compact mass of minute scales which is almost
black between erossed nicols, but which polarizes faintly as the
stage is revolved and shows at the same time that the individual
foliae have a fan-shaped arrangement. It very likely represents
the final stage of the viriditie alteration, and is presumably the
same as the dark green compact chlorite which. occurs in masses
of some size in the amygdules of the melaphyr at Drighton.
Numerous other minerals of secondary origin are found in
the cavities and crevices of the melaphyr; but as they occur
only in minute quantities in the material at hand and usually
without well defined crystallographic outlines, their correct de-
termination is a matter of great difficulty, and in some cases
would be impossible. For this reason and because they do not
appear to have any important bearing upon the problems under

consideration I have omitted mention of them in my descriptions.

Fourth Melaphyr, Coastal Area. — This is a fine grained
greenish rock, with few macroscopic constituents. ‘True amyg-
dules are rarely abundant in the melaphyr of this area, but
it is very commonly distinctly brecciated, considerable masses
having a highly fragmental aspect. Under the microscope it
is found to consist of the usual lath-shaped plagioclases with
step-like ends imbedded in a grayish amorphous ground-mass
and showing a marked fluidal arrangement. Much secondary
epidote is present, occurring in scattered grains, in strings or
veins and in aggregates filling the numerous small cavities re-
sulting from the decomposition of some of the original constitu-
ents of the rocks. There is also present a considerable amount
of viridite, which results from the alteration of the augites,
and is also derived in part from the ground-mass. This at
times occupies the entire interspaces of the feldspars, and, act-

9

OCCAS. PAPERS B. S. N. H. IV. 3

ing but faintly on polarized light, gives them the appearance of
being imbedded in a light greenish glass.

In the limited bed on the north side of Atlantic and Centre
Hills, bomb-like masses of melaphyr are highly amygdaloidal,
the amygdules varying from 1 to 5 mm. in diameter. These
I regard as true amygdules. They are filled mainly with
quartz and chlorite, though often a little epidote is present.
In such cases the quartz occupies the outer zone with the
chlorite and epidote interiorly. The ground-mass of this rock is
an extremely dense, dark brownish gray, amorphous material
bearing minute feldspar microlites.

'The pseudo-amyedules of this melaphyr are commonly much
elongated, and contain epidote and chlorite with more or less
quartz. The epidote occurs as minute crystals, a single row of
which often forms a border around the wall of the cavity ; while
the entire central portion is occupied by the very compact, light
green chlorite noted above. Outside of the amygdules or seg-
regations of both kinds, scattered through the entire mass of the
rock, are the common decomposition products, ferrite, opacite,
viridite and kaolin.

A typical sample of melaphyr from this area, but bearing
some secondary quartz, which could not be avoided, yielded
50.47 per cent. of silica.

Second and Third Melaphyrs, Central Area.— The mel-
aphyrs of this area are usually of a deep greenish color and often
highly amygdaloidal; but they vary greatly in both color and
texture. A common type is a. dark purplish gray rock, which
is also often profusely amygdaloidal. Under the mieroscope
its color is seen to be due to the large amount of opacite and
magnetite in the ground-mass. Secondary epidote is abundant
in comparatively large grains, which are often surrounded by a
dense black border of opacite. The amygdules are filled with
quartz, epidote and feldspar. The quartz and epidote prevail
and occur in the form of an extremely fine grained and compact

aggregate having a specific gravity of about 9.9. The iron

99

oxides, which are much decomposed, are in part titaniferous, as

shown by their peculiar club-shaped forms so characteristic of

menaccanite. Moreover, the pulverized rock gives a distinct re-
action for titanium, after prolonged digestion with hydrochloric
acid. The green melaphyr differs mainly in carrying a larger
proportion of epidote and in the feldspars having undergone the
viriditic alteration. The amygdules of the rock show first a
zone of epidote, and interiorly quartz, feldspar and chlorite.
A typical sample of the melaphyr from this area, non-amygda-
loidal and carrying no free quartz, yielded on analysis 41.89 per

cent. of silica.

First Melaphyr, Western Area.—The structural features
of this rock are greatly obscured by decomposition, but after
the examination of several sections I am disposed to place it with
the melaphyrs. The microscope shows it to consist of a large
number of greatly decomposed porphyritic plagioclases imbed-
ded in a groundmass so filled with secondary decomposition
products that its original character is wholly obscured. The dull
green macroscopic areas are epidote resulting from the feldspathic
decomposition. The original iron-magnesian constituents have
quite disappeared, even their outlines being no longer recogniz-
able. I assign the rock to the melaphyrs with consider-
able hesitation. Chemical analysis shows its basic character,
three samples from different localities affording the following
percentages of silica: 47.29, 47.97 and 51.05; the latter, at

least, probably including some free secondary quartz.

Porphyrite.

The rock which I here have called porphyrite differs from
the melaphyrs in showing under the microscope a larger pro-
Portion of irresolvable ground-mass, in being of a more pro-
nounced porphyritic structure and in never so far as observed

being truly amygdaloidal. They are also more compaet and

fresh appearing and show in none of the sections examined

36

traces of augite, olivine or other original iron-magnesian sili-

cates.

Western Area. The porphyrite of this area varies from
light greenish to dull red and purplish in color and is well de-
scribed by the term /e/sz/ze. So far ав observed none of its
mineral constituents are of such dimensions as to be recog-
nizable by the unaided eye, nor is it at all amygdaloidal, a
feature so pronounced in the melaphyrs. The purple variety,
in the thin section, shows a densely microlitic base, with much
opacite, carrying numerous porphyritie feldspars, which are
mostly, if not all, triclinic. A few of the feldspars show no
twinning strie; but I eannot obtain such measurements as
would prove these to be certainly monoclinic.

One of the sections examined shows a single corroded bleb
of original quartz, carrying fluidal cavities and moving bubbles.
Large dusky apatites are not rare. In the greenish variety
there is also present alittle viridite from the feldspars, and a
few granules of secondary epidote. The porphyrite presents,
usually, a well-marked fluidal structure. Subjected to chemical
analysis, the green variety yielded 58.02 per cent. of silica,
and had a specific gravity of 2.73. The red variety is more
felsitic and also more decomposed. Two samples, light red,
and dark red in color, yielded silica as follows : light, 56.66 per

cent. ; dark, 56.25 per cent.

Black Rock.—Some of the black rock porphyrite more re-
sembles the melaphyrs in external appearance than does that
of the western area. Macroscopically it is a very compact and
hard, greenish gray to dark purplish and nearly black rock,
bearing small whitish feldspars, and secondary epidote in crys-
tals and granular aggregates of sufficient size to be distinguish-
able by the naked eye.

Thin sections, under the microscope, show it to consist of a
dense ground-mass of needle-like and short, stout feldspar micro-

lites, interspersed with numerous brilliantly polarizing epidote

}
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f

9T

granules, and with little if any truly amorphous base. Im-
bedded in this feldspathic ground-mass are numerous large, clear
and often greatly corroded plagioclases. A few small apatites
are present. The section shows a well-marked flow-structure.
Chemical analysis yielded 58.25 per cent. of silica ; and an aver-

age of four determinations gave a specifie gravity of 2.78.

THe DIKES OR INTRUSIVE Rocks.

The dikes, as has been noted by Professor Crosby, are all of
normal form — sharply defined, wall-like masses making high
angles with the horizon and invariably cutting across the strat-
ified rocks ; no intrusive beds or sheets having been observed in
the Nantasket district, owing, probably, to the absence of thin
bedded or slaty sediments. As previously stated, the dikes
consist mainly of араке ; but they also include an important
series of porphyrite dikes and at least one large dike of mela-
phyr. The dikes of melaphyr and porphyrite are believed by Pro-
fessor Crosby to be contemporancous with the surface flows
of those rocks ; while he finds that the numerous diabase dikes
are certainly newer in most, and probably in all, cases than the
melaphyr and porphyrite. Hence, following the chronological

order, we begin with the last named rocks.

The Melaphyr Dikes.

The dike of melaphyr (1) on the north side of Centre Hill is
80 evidently identical with the melaphyr which it intersects that
it was not examined microscopically. The large dike (2) in
the western area, which Professor Crosby regards as contem-
poraneous with the first flow of melaphyr, is, like that flow, of
doubtful character. Macroscopically, it bears a marked resem-
blance to this flow, and is contrasted in appearanee with all the
diabase dikes of the district. Under the microscope it is seen
to be a fresher and more crystalline rock than the melaphyr,

showing in the section a ground-mass of feldspar microlites and

38

granules injected with epidote and the usual ferruginous decom-
position products. In this ground-mass are numerous phen-
ocrysts of feldspar and an occasional nearly colorless augite.
There are numerous granular areas of secondary epidote ; but
no trace of olivine or its decomposition products; and I am
inclined to doubt its presence as an original constituent. Rec-
ognizing the uncertainty attending the determination of such
rocks from a single section, I can only say that I am inclined
to believe the rock more nearly related to the augite-porphyrite:
or olivine-free diabases than to the melaphyrs. It is classed
here with the melaphyrs simply because the true nature of the
rock is not yet free from doubt, and that arrangement best
accords with its field relations as worked out by Professor
Crosby. The analysis of a single specimen gave 54.47 per
cent. of silica.

The Porphyrite Dikes.

The porphyrite dikes, so far as observed, are, lithologically,
essentially similar to the great flow of porphyrite on Black

Rock, rendering a detailed description unnecessary.

The Diabase Dikes.

These dikes are, as a rule, exceedingly compact, tough and
hard, of a dark gray, nearly black or more frequently greenish
color, and carry few if any original macroscopic constituents,
although pyrite and secondary epidote are often developed in
granules and aggregates of sufficient size to be detected by the
unaided eye. To this secondary epidote and the abundant viri-
dite is due in all cases the green color of the rocks.

I find nothing whatever in the slides, with possibly a single
exception, to indicate that the different dikes belong to distinct
periods of eruption. Slides from dikes which plainly belong to
the same system often show all the variations in texture, struct-

ure and stages of decomposition to be observed in those which

|
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i

89

from their position belong unmistakably to distinct systems.
In grouping the rocks as below I have, therefore, but followed

the notes of Professor Crosby.
FIRST SERIES.

Dike 3. —Rock finely porphyritic with greenish feldspars ;
macroscopic pyrite; color greenish. This is a very typical
diabase, with irregular and sharply wedge-shaped augites, lath-
shaped plagioclases and numerous grains of iron oxide. The
augites are undergoing a chloritic alteration ; and the feldspars
аге во badly kaolinized that the twinning strie are completely
obscured. The iron oxides are in part magnetite and in part
show the whitish alteration characteristic of menaccanite. Pyrite
occurs in brassy yellow irregular clumps. Small, irregular,
brown and strongly dichroic scales attached to the altered au-
gites are evidently secondary hornblende. The porphyritic
feldspars are so thoroughly kaolinized that nothing whatever can
be learned of their original nature. Scattering grains of epi-
dote, kaolin, opacite and viridite complete the list of recognizable
constituents.

Dike 5.— Very fine grained and compact; macroscopic
Pyrite ; color greenish. This presents no distinctions in any
Way essential. oth augites and feldspars are more decom-
posed ; and there is a corresponding increase in the propor-
tions of epidote, viridite, calcite, ete. A few small apatites

appear in this slide.

Dike 22.— Medium fine in texture but distinctly crystalline,
and color greenish. This differs from the preceding only
in being slightly coarser in texture. The iron oxides, judged
by their irregular forms and whitish decomposition products are
largely titaniferous.

Dike 25.— Very fine-grained and compact; a few greenish

feldspars " :
Ispars porphyritically developed; and color greenish. The

40

augite is in most cases completely altered ; and there is much
viridite, epidote and iron oxide. This rock is undoubtedly a
diabase, although I am unable to find a particle of recognizable
augite or even hornblende, the viriditic alteration being complete
in all the sections examined.

The viridite is very abundant and is considered of undoubted
augitic derivation, from its sharp straight and very angular
outlines, which are so characteristic of the augites in this class
of rocks. Minute epidotes are in some cases very abundant
and impart to the rock a yellowish green stain. The section is
traversed by veins of secondary quartz.

Dike 37.— Very fine grained and compact; macroscopic
pyrite; color, greenish. This is of finer grain than any of
the preceding and so highly altered that none of its original
constituents are now recognizable, excepting by pseudomorphs.
The section shows only a fine, compact, fibrous or scaly ag-
gregate of viridite, opacite, epidote, calcite and kaolin, with
numerous grains of iron oxide. Although so highly altered,
I have no hesitancy in referring this rock to the diabase group.

SECOND SERIES.

Dike 12.— Very fine grained and compact; no macroscopic
constituents; color, dark greenish gray. Under the micro-
scope it is seen to be highly altered, but with portions of augite
still fresh and showing pleochroism. Iron oxide very greatly
altered to a grayish amorphous product. Other alteration pro-

ducts as in preceding: sections.

Dike 11.—Differs from 12 only in being of slightly coarser
texture and in having suffered more from alteration ; augites
completely changed.

Dike 10.— Fine grained and compact; porphyritic with small
greenish feldspars. Plainly a diabase, although the feldspars

41

are so muddied by decomposition products as to be scarcely
recognizable. A few augite particles are still unchanged. No

essential differences.

Dike 9.—Like 10, but a trifle coarser in texture. The hand
specimen shows small black segregations of what is apparently

hornblende ; but these do not appear in the section.

Dike 8.—Very compact, with no macroscopic constituents ;
color, dark gray, nearly black. The sample was taken from the
edge of the dike at the contact with melaphyr. It offers no

characters worthy of note to distinguish it from 9.

Dike 7.—Coarse and distinctly crystalline; color, greenish.
This is one of the coarsest rocks of the series. Under the micro-
scope it shows large plates of reddish brown augites, often en-
closing the lath-shaped plagioclases and producing the typical
ophitie structure. The feldspars here are sufficiently fresh to
show twinning striæ. [топ oxide is very abundant, in large
grains, with much pyrite and some apatite. The section shows to
excellent advantage the various stages of augitic alteration into
viridite. The rock resembles 22 more closely than any of the

preceding.

Dike 26.— Very fine, compact and homogeneous. Color, dark
gray, somewhat greenish. This rock is greatly altered ; being
essentially an aggregate of viridite scales, and fibres and gran-
ules of iron oxide, interspersed with epidote. A few cavities or
pseudo-amygdules occur, the cavity wall being lined with vir-
idite, while the central portion is occupied by calcite. The feld-

spars are very obscure; and the iron oxide in part titaniferous.

Dike 29.— l'exture of medium fineness, with large segregations
of black hornblende. Color, uneven, black, greenish and yellow-
ish green; the last tint being due to epidote veins. The augite

is greatly altered; and there is much viridite and epidote, the

latter in scattered granules and veins, The feldspars are ob-

scure; and quite large grains of iron oxide and pyrite occur.
The segregation patches already alluded to are found, under the
‘microscope, to consist largely of deep brown, strongly dichroic
hornblende, in compact and well-defined crystals; sometimes
perfectly fresh or again somewhat altered into the inevitable
viridite. I am not able from the sections prepared of this rock
to state whether this hornblende is original or a product of para-

morphie alteration from the augite.

Dike 31.—This is of medium fineness, with macroscopic pyrite
and segregations of black hornblende. The color is greenish,
the rock being blotched with epidote. The augite is almost
completely altered to viridite, although a few remnants are. still
recognizable. This rock also contains, as stated, coarser grained
segregations in which black hornblendes are readily distinguished
by the unaided eye. In thin sections these are of a deep brown
color and strongly dichroic. Some of the individuals are so
compact and well defined in crystalline outline as to indicate
that they are original constituents of the rock ; while in other
cases the presence of a hornblende border of varying width about
an augitic core indicates unmistakably a paramorphic origin.
The hornblende has, in its turn, undergone in certain cases the
viriditic alteration. This is the only one of the Nantasket rocks
in which I have been able to find paramorphic hornblende so
distinctly characterized as to leave no doubt of its origin, al-
though this is suggested by sections from dike 29.

A few small, grayish, wedge-shaped crystals of sphene are
present. Feldspars, in most part, are considerably altered ;
but in a few instances are still clear and show the twinning
strio very plainly. There is much viridite, which obscures
everything.

Dike 34.—Differs in no essential particulars from 29 and 31,

though the section does not show the hornblendic segregations.

These two dikes are without doubt iden-

Dikes 40 and 41.

tical with the preceding, though nothing in the present compo-

48

sition goes to show that they were not originally diorites rather
than diabases. So far as examined, not a particle of fresh au-
gite remains ; but an occasional fragment of brownish hornblende
still exists so far unchanged as to be recognizable. The light
green viridite is everywhere present, interspersed with magnetite
grains, particles of epidote and apatite needles. A few of the
feldspars are still fresh, though here also the viriditie alteration
has prevailed. My reasons for calling the rocks diabase is that
the products of decomposition, their form and their arrangement,
are identical with those of the augitie rocks throughout the en-

tire area under examination.

+
THIRD SERIES.

Dike 62.—Very fine grained and compaet; nearly black.
Under the mieroscope this rock shows a marked deviation from
the prevailing types of structure as already described. In the
thin section it shows beautifully fresh and clear porphyritic
plagioclases, in single lath-shaped forms and in cruciform aggre-
gates; together with altered olivines and occasional clear but
corroded augites, imbedded in a dense fine-grained ground-mass
of small plagioclases, augites and iron oxide. The olivine, though
changed completely to a chloritic product, is readily recognized
by its form and irregular fracture-lines. The augites are con-
fined almost wholly to the ground-mass ; but the section shows
: few corroded porphyritic forms, as above noted. All are
fresh and free from enclosures. The iron oxide occurs abun-
dantly in rod-like forms, crossing one another at nearly right
angles, produeing grate-like or barred structures. This dia-
base is the least altered of all that has been examined.

Dike 66.—Of medium coarseness, and to the naked eye ap-
parently granular crystalline and fresh. This rock is quite dif-
ferent in general appearance from any yet described.
Macroscopically it is a well compacted, quite fresh-appearing

rock, of , Í š А
k, of a deep purplish black color, in which hornblende or au-

44

gite is apparently the chief constituent. Under the microscope,
however, the section is found to present, aside from the apatite
a fresh and unaltered

needles, not a single original mineral ir
state ; but the entire mass of the rock is filled with small amor-
phous grains and dust-like opaque particles of a black color, in-
terspersed throughout a dirty grayish groundmass, with only
here and there a small fragment of an augite crystal or colorless

portion of feldspar, in which in a few instances twinning striæ

were still apparent. Scattering grains of epidote and shreds of

a brownish mineral, evidently hornblende, complete the list
of determinable minerals. The black opacite and the gray
amorphous material are evidently derived from the decomposed
augite ; it being not infrequent to find the border of a crystal
irregularly outlined by the larger grains, while interiorly is the
gray material and other black grains. The form and size
of the black grains is such as to render their identification by
the microscope alone impossible. The pulverized rock is, how-
ever, strongly magnetic; and after long digestion with hydro-
chloric acid the solution gives a faint reaction for titanium.
The augitic alteration would in this case seem to be similar
to that of the hornblende in certain New Hampshire diorites
described by Hawes.! That the altered mineral in this case is
augite and not hornblende is proved by a few unchanged parti-
cles still remaining and the form of the outlines still preserved.
Viridite, so abundant in the other rocks described, is here en-

tirely lacking.

DETAILED STRUCTURE OF NANTASKET.

The main purpose of this section is to set forth as fully as

may seem desirable the facts upon which the generalizations of

'Geol. of New Hampshire, Vol. III., part IV., pages 43 and 66.

45

the preceding sections are based; not forgetting that the
highest interest attaches to the relations and especially to the
contact phenomena of the bedded rocks—conglomerates and
lavas. For present convenience, as well as in the interest of
future students of the Nantasket ledges, each principal mass or
area will, as regards the sedimentary rocks and associated lavas,
be described somewhat independently and in topographic order,
reserving the general correlation, the complete elucidation of
the stratigraphy, until the close of this systematic itinerary.
The structural details of the dikes, or the intrusive igneous rocks,

will then be presented, in both chronologie and topographic order.

The Atlantic Shore or Coastal Area.

This area, extending from Nantasket Beach to Black Rock,
includes the most accessible and the most frequently visited of
the Nantasket ledges ; and it is а fortunate cireumstance that,
although the melaphyr largely predominates here, some of the
more characteristic features of Nantasket geology аге well ex-
hibited at this natural starting point. The granite, however,
is Wholly wanting; and its interesting relations to the con-

glomerate cannot be observed in this part of the field.

part of Long Beach Rock is a somewhat isolated, half-tide
ledge of conglomerate. It contains many pebbles, mostly
small, of granite and felsite, and also many larger pebbles of
different. varieties of melaphyr. From the latter we may infer
that this bed is underlain by melaphyr, and that it is probably
not the lowest or basal conglomerate. Eight or ten feet below
the top of the conglomerate is an intercalated layer of fine,

hard, red sandstone, eighteen inches thick; and conformably

overlying the conglomerate, are six to twelve inches of a beau-
tifully banded, greenish slate of flinty hardness. The sandstone
and slate show that the strike is N.65° E.*, and the dip S.E.
15990),

1 P Н : : H . 1 soe
All azimuth directions given in this paper are referred to the true meridian,

yr.

2

Melaph

Melaphyr.

Conglomerate.

SCALE,

The Horizontal dotted line marks the low-tide lez

SECTION ACROSS Lonc BEACH Rock.

I.

FiG.

vel.

46

Melaphyr and Tuff The Long
Beach Rock sediments are overlain
conformably by a compact green
melaphyr. The contact of the mel-
aphyr and slate is exposed for about
one hundred feet along the strike ;
and at several points there are fine
transverse sections of the contact,
which is accessible only at low tide.
Fig. lis a general section of both
the conglomerate and melaphyr.
The irregularities of the actual con-
tact, and especially the bending of
the subjacent lamin of slate, as well
as the intense induration of this rock,
which has been baked to a good semi-
porcelainite, indicate that it was but
imperfectly consolidated, if not en-
tirely plastic, when the melaphyr
flowed over it. Sut, on the other
hand, the exquisitely beautiful brec-
ciation and miniature faulting ex-
hibited on some of the wave-washed
surfaces show that before the dis-
turbance ceased the clay had be-
come brittle enough to break. “This
fascinating exposure is the gem
of Nantasket geology; and it is
hoped that it may long remain un-
mutilated, to delight and instruct
future students. That the eruption
was submarine, or at least essentially
contemporaneous with the deposition
of the sediments, is beyond reasona-
ble doubt; although it is probable
that the thin layer of green slate on
which the melaphyr immediately

Occas. papers Bost Soc. Nat. Hist. IV.
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[Red tracing made directly from the stone. |

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This impalpably fine sediment was deposited quietly and uniformly over
the uneven, pebbly surface of the conglomerate; and consequently where
erosion has cut down nearly to the top of the conglomerate, each pebble
determines the center of a series of concentric lines, the outcropping
edges of the laminae of slate. In the portion of the eroded surface here
represented, the banded slate has been worn away sufficiently to actually
expose only two pebbles of the conglomerate.

In the upper part of this thin bed of tuff, where it experienced more dis-

tinctly the mechanical stresses of the overflowing lava, the layers are,

to some extent, finely brecciated and faulted, as shown in the illustrations

on the next plate.

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Occas. papers Bost. Soc. Nat. Hist. IV.

4T

rests should be regarded as an exceedingly fine volcanic tuff,
the eruption of liquid lava having been preceded by an out-
burst of volcanic ashes. The influence of the lava flow has also
penetrated the conglomerate, the layer of sandstone eight feet
below being almost as thoroughly indurated as the slate.

The breadth of the melaphyr is fully two hundred feet and its
actual thickness probably not far from sixty feet. It is, for the
most part, rather compact and of a dark green color. But
toward the top especially it is full of irregular segregations of
quartz and shows, in a rather large way, a very distinct and
somewhat wavy flow-structure, resembling bedding and parallel
With the stratification of the underlying conglomerate. Irreg—
ular veins and masses of compact and impure epidote, occa-
Sionally of considerable size, also occur in the upper part of the
melaphyr. It is very plain that the entire thickness of mela-
phyr is to be regarded as forming one simple and normal flow
homogeneous and massive below, but superficially brecciated
and scoriaceous.

This great lava-flow is undoubtedly continued eastward in the
outermost part of the ledge exposed at low tide north of Centre
Hill ; and it appears probable that further east the line of strike
changes 80 as to connect Little Black Rock with the same bed.
This islet, whieh is about four hundred feet long and half as
broad, is entirely composed of melaphyr very similar to that on
Long Beach Rock, except that it is in part amygdaloidal.

The prominent ledge at the northwestern base of Atlantic
Hill, and partially isolated by the sands of Nantasket Beach, is

Chiefly a dict; |
hiefly ü distinetly and evenly bedded greenish gray sandstone,
alternatin g

especially in the upper part, with layers of greenish
and т;

ег indistinct, small-pebbled conglomerate. This con-

[ni A: . i MISSA
glomerate is made up of much smaller pebbles, and is in every

WAY Very dict! . F
м Very distinct from that on Long Beach Rock. The peb-
)les ar Yat > ]

es are mainly more or less angular fragments of melaphyr,
аге : х ү "
ne not well assorted, the coarsest and finest material being

©

48

mingled indiscriminately, and, with the exception of a rare
pebble of felsite or granite, the entire ledge, sandstone as well
as conglomerate, appears at first to have been derived from the
Long Beach Rock flow of melaphyr. There is much, however,
in both the composition and texture of this mass to suggest that
it may be a true volcanic tuff, a record of explosive volcanic
action either at the close of the eruption already described or

at the inception of the similar littoral or submarine eruption by

which this bed was covered. It will be observed that this in- `

terpretation is adopted on the map, the lava-flows being thus
supposed to have followed each other so closely over this
marine area as to preclude the deposition of any appreciable
amount of the ordinary or normal sediments between them.

This bed of tuff strikes N. 65? E., and dips S.E. 15°. Its
full breadth appears to be exposed, viz., 120 feet, equal to a
thickness of about 30 feet. It can be traced for about 300 feet
along the base of Atlantie Hill, rising at the highest point some
20 feet above the beach. The line of strike carries it directly
across the shingle beach connecting Long Beach Rock and At-
lantic Hill. I have been unable to positively identify it on the
low-tide ledge north of Centre Hill; but it seems impossible to
doubt that it crosses here, although probably with diminished
thickness. It appears necessary, also, to suppose that the tufl
overlies conformably the melaphyr of Long Beach Rock, al-
though there is no exposure of the contact. If the tuff extends
so far to the eastward, it must pass to the north of Gun Rock
and between the two Black Rock islets.

This old ash-bed is overlain clearly and conformably by a
second bed of mélaphyr, the contact being well exposed at sev-
eral points along the northern base of Atlantic Hill. "The con-
tact is similar to that between the green slate and overlying
melaphyr on. Long Beach Rock — conformable in the general
view and yet with many minor irregularities. Fig. 2 shows
the contact as it appears at the edge of the beach, below the
bath house and near the foot of the Atlantic House steps. It
is clearly such a contact as would naturally result from the

19

flowing of liquid lava over unconsolidated sediments. And the
attention of those who may see in the minor unconformities of
the contact evidence that the melaphyr is really intrusive is
“Шей to the fact that no detached fragments of the tuff or sedi-
ment are observed in the base of the melaphyr.

MELA Fl Ye A 7

Fic. 2.— CoNTACT or TUFF AND MELAPHYR AT THE
NORTHERN BASE OF ATLANTIC HILL.
Wund i INCH 5912 PET.

The thickness of the melaphyr, near the bath house, is about
twenty feet ; and it appears to increase eastward. This is very
clearly a second lava-flow, and not an intrusive bed; and in
lithological character it is very similar to the first, being chiefly
dark green | |

and compaet. It is sometimes, however, more brec-
Clated ;

but exhibits the same ill-defined segregations and veins
Oh (тау. t 5

f quartz and epidote. This flow can be traced eastward along
Ü “Q И d ° 7 / Ç

he base of Atlantic Hill to V alley Beach, between Atlantic and

entre Hills; and it undoubtedly forms the landward end of the

9w-tide ledge north of Centre Hill.

Valley Beach is bounded by two good transverse sections of
the formations.

Atlantic Hill ends here in a small cliff, which
shows the
т ^

following thicknesses from below upwards (Fig
16 second melaphyr, already described, about 30 feet;
Volcanic agglomerate and tuff, 7 feet; melaphyr, 9 feet; ag-
slomerate and tuff, 25 feet; and then melaphyr extending
Southward indefinitely, or beyond the end of the section.

lí Y ` Ú mn . .
The first fragmental bed or tuff in this section measures 7

OCCas, PAPERS. B. 8. N. Н. Iv. 4.

S MELAPHYR

FIG. 3.—SECTION ON THE WEST
SIDE or VALLEY BEACH.
SCALE, 1 INCH = 20 TEET:

a

4
(

C

Fic. 4.—FoRM OF THE CONTACT OF THE MELAPHYR AND OVERLYING TUFF ON THE

FEET.

ә

SCALE, I INCH =
inuation of the one next above it.

EAST SIDE OF ATLANTIC HILL.

Each line is the right-hand cont

msc

feet on the cliff, but grows narrower westward and appears to
die out entirely between 200 and 250 feet from the cliff. It is
probable, however, that the tuff really persists until it passes
the edge of the overlying melaphyr, and is merged with the next
bed of tuff, as shown on the map.

The boundaries of this tapering edge of the fragmental bed
are quite irregular. This is especially true of its lower surface,
where it rests upon the second melaphyr. The form of this
contact for 130 feet west from Valley Beach, or as far as it is
clearly exposed, is shown in Fig. 4; for the sake of convenience,
however, the curves are represented as following each other
more nearly in the same direct line than is actually the case.
The upper surface of the melaphyr presents smoothly rounded
hemispherical protuberances or swellings one to three or four
feet in diameter. The actual boundaries of adjacent protu-
berances may usually be traced below the surface of the mela-
phyr ; and some of these remarkably regular and graceful curves
thus describe two-thirds or three-fourths of a complete circle.
Probably the majority of the curves are really sections of rounded
ridges or rolls, the surface flow-structure of the lava. But it 18
very noticeable that these prominences exhibit semicircular pro-
files to some extent on both the transverse and longitudinal &ec-
tions of the bed, suggesting hemispherical tumefactions or
superficial bubbles formed on the liquid lava.

It seems impossible to regard these features as the product of
erosion ; they must be entirely original ; the actual surface of
the fresh submarine lava-flow, which was covered almost in-
tact either by the ashes accompanying a second flow or by the
débris worn from adjacent masses. The fragments are mainly
angular, ranging in size from dust to three inches in diameter;
and this is clearly a thin, local deposit on the surface of the
sheet of lava.

The well-marked north-south depression separating Atlantic
and Centre Hills, and terminating at the lower end in Valley
Beach, does not seem to be oceupied by adike, for the melaphyr
shows half way across the bottom of it, and there is not a ves-

52

tige of dike rock to be seen % situ; nor does it appear to be
chiefly due to faulting, but rather to rapid erosion along a series
of close parallel joints. There has been a little slipping, how-
ever ; for on tracing the thin bed of tuff across the one-hundred
feet of beach, we find its outcrop shifted about twenty feet to
the south, indicating an upthrow on the east of perhaps five
feet. The melaphyr between the two walls, may, of course,
have dropped down almost any amount, producing two con-
verging and compensating faults. East of the beach the bed of
tuff already described crosses the ledges diagonally to the shore
in about one-hundred feet, passing out of sight under the water
without any sensible change of dip or thickness.

Returning to the west side of Valley Beach, the melaphyr,
nine feet in thickness, overlying this tuff is found to be quite
conformable, the contact showing only the minor irregularities
that would naturally be developed where a stream of lava flows
over unconsolidated sand and gravel. The contact is not
always a sharply defined line, but the lava is enough mixed
with the sand so that the two rocks are blended through a
thickness of several inches. The swelling curves characterizing
the lower surface of the tuff are not observed above it.

The structure of this melaphyr, which is really the third
flow, reckoning from the conglomerate on Long Beach Rock, is
quite peculiar. A compact, greenish matrix encloses irregu-
larly rounded amygdaloidal masses from two inches to two feet
in their longest dimensions. The amygdules are usually ar-
ranged in concentric lines or zones parallel with the exterior ;
and the coarsest amygdules are sometimes towards the periphery
and sometimes in the centre. ‘These masses are quite clearly
distributed in irregular lines parallel. with the bed ; and this fact,
as well as the great number of the masses, is decidedly unfavor-
able to the view that they are true voleanie bombs. But the
best explanation which has occurred to my mind is, perhaps,
not wholly satisfaetory, viz., that, during the flowing of the
lava, vesicular layers and crusts were, by the unequal flowing

and revolving motions, broken up and the fragments rounded

53

Into the forms we now see. East of Valley Beach, along the
north front of Centre Hill, this melaphyr expands rather rap-
idly ; and it is very obvious that while the rounded amygdaloid-
al masses or pseudo-bombs are thickly and pretty uniformly
scattered through the middle and upper parts of the flow, they
are almost entirely wanting in the lower part. "They continue
to form a prominent feature of the upper part of the melaphyr
about half way across Centre Hill, and then die out rather
gradually. Beyond this point the identity of the melaphyr as a
Separate flow is lost, since it is indistinguishable from the over-
lying melaphyr, with which it is here in direct contact. That
the third melaphyr actually extends quite across Centre Hill, if
not across Gun Rock, there can be no reasonable doubt. This
flow is of exceptional interest, not alone on account of the
pseudo-bombs, but also because it affords, perhaps, the only
Instance in the Nantasket area where the tapering edge of a bed
of lava сап be clearly traced. It certainly thins out rapidly
toward the west, and, as stated, can not be followed much
шше than two hundred feet west of Valley Beach, the under-
lying and overlying tuffs appearing to come together at this
point. Tt will not escape attention that the amygdaloidal
Masses are thus found where we should most naturally look for
them — in the marginal and superficial portions of the flow.
The second bed of tuff in the Valley Beach section, or that
overlying the amygdaloidal melaphyr, is lithologically similar
to that below the melaphyr ; but it is three times as thick, and
сап be traced westward the entire length of Atlantic Hill, with
DO sensible change of dip or thickness, passing under the
beach on the west where the County or Beach road first
Teaches the base of the hill. It is well exposed near the Atlan-
tie House steps, just above the bath house; from here it con-
"nues around the hill a little above the middle of its height ;
and then gradually descends to the level of the beach on the
Cast. The continuity of the bed is clear; but its strike must
change from N. 65° E. on the west to at least N. 75° or 809 E.

OH Ае бакы s ç
the east side of the hill. Its upper and lower contaets are

4

e

clearly exposed at several points, and are favorable to the view
that the melaphyrs are contemporaneous. It consists, like the
other beds of tuff, almost wholly of more or less angular and
imperfectly assorted fragments of melaphyr imbedded in a dis-
tinctly stratified cement of the same character. On the west
side of the hill it rests directly upon the second melaphyr ; ?
while in the vicinity of Valley Beach the second tuff and the
third or amygdaloidal melaphyr are interposed, as already ex-
plained. East of Valley Beach it is scantily exposed, and can
not be traced much beyond the Waverly House. But it is
here, seemingly, somewhat interstratified or mingled with the
underlying amygdaloidal melaphyr.

Above the last described or third tuff comes the great body
of melaphyr forming all the remaining portions of Atlantic and
Centre Hills and extending south to Conglomerate Plateau, as
shown on the map. The outcrops are so numerous over this
area that there is absolutely no room to doubt the essential
continuity of the melaphyr. Its breadth, measured from the
third tuff, along Valley Beach Avenue, is nearly 1400 feet, cor-
responding to a probable thickness of more than 300 feet. Ог,
classing the three beds of tuff as fragmental lavas, as the facts
appear to warrant, and thus regarding the volcanic series as
essentially continuous back to the conglomerate on Long Beach
Rock, the breadth of the entire series of three tuffs and four
melaphyrs is, in round numbers, at least 1800 feet, and thick-
ness 450 feet.

The great mass of melaphyr above the third tuff is fairly
uniform lithologically. It is usually more or less brecciated,
with numerous highly irregular and limited segregations of

vitreous quartz and chalcedony, as well as epidote. Dut
sometimes it is much more distinctly brecciated, especially im- `

mediately above the third tuff. On Atlantic and Centre Hills
it is rarely amygdaloidal or quite compact in texture. Near
the southern border of the melaphyr area, on the north side of
Willow Ledge Hill, it encloses about twenty feet in thickness |

of greenish tuff and agglomerate. The green arenaceous tuft is

E

55

)

interstratified, especially in the lower part of the section, with
a bright red, slaty rock, which is somewhat contorted ; and in
the upper part of the section with the agglomerate, which is
chiefly composed of quite small, but mostly angular fragments
of melaphyr. These beds, which are very distinctly stratified
throughout, strike about N. 80° E., dip S. 15°, and are overlain
conformably by the melaphyr ; but the lower contact is not ex-
posed. On the west they are quite certainly cut off by a fault, and
this, as shown on the map, is probably also their fate on the east.
At any rate, the melaphyr clearly crosses their strike in this direc-
tion, and it is impossible to trace them for more than three hun-
dred fect. A red slate or tuff similar to that found here, crops
out just west of the Rockland House, accompanied, apparently,
by agglomerate, although this may be only the ordinary con-
glomerate or puddingstone. The correlation of these red out-
crops would require us to postulate dislocations which are not
indicated on the map and for which there is no independent
evidence. It appears wiser, therefore, to regard them as local
and non-synchronous accumulations of volcanic dust and lapilli.

The melaphyr immediately northeast of the Rockland House
is gimilar to that оп Atlantic Hill, and is undoubtedly a con-
tinuation of it; and the same is true of the melaphyr forming
Gun Rock and its neighboring ledges. The beds of tuff on the
north side of Atlantic Hill cannot be traced west of the abrupt
western slope of the hill, perhaps for want of outcrops ; nor can
they be followed eastward more than half way across Centre
Hill. But in this direction the various flows of melaphyr are,
apparently, united ; and in the Gun Rock district there is not a
trace of any sedimentary rock, but the true conglomerate and
the fragmental lava or tuff are alike wanting. It would be
'asy, of course, to explain the absence of the tuffs by faulting,
but it is, perhaps, more probable that these fragmental lavas
were never spread over this area. The submerged or half-tide
ledge east of Gun Rock (see map) is about three hundred feet
long at low tide, and consists wholly of melaphyr similar to
that of Gun Rock.

There is, apparently, no reason to doubt that all the mela-

56

phyr described up to this point, with the included tuffs, on
Rockland, Atlantie, and Centre Hills, Gun hock, the sub-
merged ledges and Little Black Rock, belongs to one great bed
or sheet having an approximate ly east-west trend and southe ly
dip and overlying the conglomerate forming the north end of
Long Beach Rock. Throughout this great mass, however,
there are many indications that it is re ally composite, consisting
of a succession of flows; which either were not submarine, or
followed each other so r: ipidly ав to preclude the formation of
any sensible thickness of intervening sediments. The included
beds of tuff show this very clearly. The zones of brecciated
melaphyr, the fractures being marked by segrations of silica
and impure epidote, seem to show it. Апа there are often
curving lines, such as have been described at the base of the
second tuff (Fig. 4), which appear to mark the contact of two
successive flows.

On the north side of Centre Hill, near the end of Centre
Hill Avenue, is another proof, in the form of a dike of mela-
phyr (No. 1, on the map), that this great bed is not
throughout of the same age. Its compactness and general
resemblance to the enclosing melaphyr, as well as the irreg-
ular contacts, mark this dike as prob: ibly contemporaneous with
some later flow of this immediate series, although, of course
of later date than the flow which it is seen to intersect.

The general absence of the amygdaloidal texture in this mel-
aphyr, except in the flow separating the second and third tuffs,
is very noticeable; and this negative character, as well as the
prevalent brecciation, contrasts it strongly with the most of the
other masses of melaphyr or basic lava in the Nantasket area.
In fact, as Mr. Merrill's descriptions show, both the structure
and composition of this rock assign it a position between the
ultra basie lava or typical melaphyr, and the more acid lava or
porphyrite of this region.

Porphyrite.—The remaining rocks and islets, east of Gun
Rock, constitute a very typical area of porphyrite, bounded on

57

the north, apparently, by the eastward extension of the great
belt of melaphyr indicated by Little Black Rock, and on the
south by the conglomerate of Green Hill. Although, as Mr.
Merrill states, this rock resembles the melaphyrs more in its
general aspect than does the porphyrite of the western part of
the Nantasket area ; chemical analysis shows that it is as acid
as any porphyrite in this district. It is somewhat variable in
its microscopic features, but usually presents a compact, or
apparently felsitic, dark gray or purplish to nearly black, base,
enclosing numerous porphyritically developed feldspars or
minute aggregates of epidote. It is never amygdaloidal, and
‘arely distinctly breeciated ; but exhibits at several points а
well-developed striping or flow-structure.

Black Rock, which is about seven hundred feet east and west,
and half as broad, is a continuous and almost perfectly bare mass
of porphyrite. Fluidal lines are very plainly marked in a por-
tion of the porphyrite, with usually an approximately east-west
trend and a gentle southerly dip, thus proving that the normal
attitude of the rocks, so clearly exhibited about Atlantie Hill,
is preserved to the very eastern limit of the Nantasket area.
Near the middle of the west half of the island, a considerable
mass of porphyrite is split up by close, parallel east-west and
nearly vertical joints so as to present a very shaly appearance.
The character and especially the attitude, or dip and strike, of
this structure indicates that it is possibly true cleavage rather
than jointing, and analogous to the foliation of the diabase on
Calf Island.! The small island north of Green Hill (Green Hill
Rock) is another mass of gray and purplish porphyrite, with flu-
idal lines ; and the half-tide ledge between this and Black Rock
exhibits a similar constitution. Since the porphyrite of this
area is exposed only in these three insular masses, it is mani-
festly impossible to determine with certainty either its bounda-
ries, its thickness or its relations to the bordering melaphyr and
conglomerate. If, however, we may follow the lithologic indica-
tions and correlate it with the porphyrite of the western area, it
must, as will appear later, be referred to a horizon below this

“POG, B; Sv N. H. xxi. 455,

58

melaphyr and conglomerate and thus owe its exposure to exten-
sivefaulting. The map, it will be observed, has been constructed
in accordance with this view ; and reference to the table of Nan-
tasket strata on page 24 will afford some idea of the magnitude
of the displacements which it involves, —at least 50 feet on the
side toward the conglomerate and 400 feet on the side toward
the melaphyr.

Indications are not wanting that Black Rock is near the point
or centre of emission of the Nantasket porphyrites. One evi-
denee of this is the seemingly great thickness of the porphyrite
in the Black Rock area, and its more crystalline character ав
compared with the porphyrite in the western part of Nantasket.

The most important fact pointing to this conclusion, however,

is the series of porphyrite dikes on the Cohasset Shore east of

Green Hill. The most casual observation shows that the ap-
proximately north-south dikes on the Cohasset Shore сап not
all be referred to the third system of diabase dikes ; but besides
the three well-defined systems of dark-colored, finely crystalline
diabase dikes, there is evidently a fourth system having a gen-
eral north-south trend, consisting of a distinctly greenish rock,
which varies in texture from apparently felsitic and porphyritic
to visibly holoerystalline or nearly so. The dikes of this kind
are not only lithologically but chronologically distinct, for they
are repeatedly cut by both the east-west and north-south dia-
base dikes; and hence, although agreeing approximately in
trend with the newest series of dikes, they must be regarded as
the oldest system exposed on this shore.

My interest in this oldest system of dikes was not aroused so
much by the clear proof of their age as by the marked litholog-

ical resemblance which they bear to the more acid lavas or

porphyrites of Nantasket, and especially to the porphyrite of

the Black Rock area. Except that the porphyrite flows, al-
though often visibly epidotie, are rather rarely distinctly green-
ish in color, while the dikes of this system are always so ; and
that the dikes are more crystalline than the effusive rocks, as
we should naturally expect; while the latter exhibit commonly

—

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59

a striping or flow-structure which is wanting in the dikes ; the
superficial resemblance is certainly very obvious, suggesting at
once that the dikes may be the channels or vents through which
the effusive porphyrite reached the surface, or at least referred
to the same eruptions— the same period of voleanie activity.

Mr. Merrill has found (page 88) that, although somewhat
more crystalline, these dikes are essentially similar in micro-
scopic characters to the Black Rock poryhyrite ; and in order to
further test the validity of this hypothesis a typical example
from one of these dikes was submitted, through the kindness of
Dr. T. M. Drown and Mr. G. F. Eldridge of the Massachusetts
Institute of Technology, to partial chemieal analysis, with the
following result, the mean of four accordant determinations :
silica, 60.84 per cent. ; alumina and ferric oxide, 20.12 per
cent.; with which may be compared the percentage of silica
— 58.25 per cent.—-afforded by the Black Rock porphyrite. As
in the case of the melaphyr (page 38), we find that the dike is
slightly more acid than the effusive portion of the eruption ;
its more erystalline charaeter causing it, apparently, to yield
less completely to the deep-seated alteration, thus reversing the
normal relative proportions of silica observed in recent erup-
tive rocks.

That the classification of these dikes by the superficial char-
acters is unsafe is shown by a second analysis, by Mr. Eldridge,
from another dike of the series. This afforded, as the mean of
four accordant determinations, silica, 48.47 per cent. ; alumina
and ferric oxide 31.53 per cent.; and if not a true diabase, it
should, apparently, be associated with the more basic melaphyrs
of Nantasket. Obviously, then, a systematic microscopic and
chemical examination of these dikes will be required for their
accurate classification; and we can now only assume that, as
appearances indicate, they are chiefly porphyrite.

A more detailed description of these dikes will be presented
later, in the systematic account of the dikes of this region ; and
all that I desire now is simply to call attention to the following
points ; — (1) That they are probably of the same age as the

60

surface sheet of porphyrite and connected with the vents through
which it was extruded. (2) That no dikes of this character
have been discovered west of Green Hill or more than one mile
ast of it. (3) That they all have, without exception, а
northerly trend, č. e., they either run toward the Black Rock
porphyrite or so as to intersect the eastward extension of its
line of strike. And yet (4) none of them have been found
breaking through the porphyrite itself, these dikes being thus
comparable with the great melaphyr dike (No. 2), since in both
cases the complete erosion of a portion of the effusive accumu-
lations was essential to expose the intrusive masses — the roots
of the eruptions — in the underlying granite.

Green Hill Conglomerate.—The prominent Jedge of con-
glomerate which has been laid bare by the wearing away of the
northern slope of Green Hill, is probably, as the fault-lines of
the map indicate, a part of the great bed forming Folsom's
Island and Conglomerate Plateau; but it appears more con-
venient to describe it in its geographic than in its geologic
sequence. The ledge is about four hundred feet long from
northwest to southeast, and shows throughout a southeasterly
dip of 20° to 80°, giving an apparent thickness of perhaps 175
feet. Beginning on the southeast, there are 10 or 12 feet of
medium conglomerate with distinct layers of reddish sandstone,
showing the bedding well. This is underlain by about 60 feet
in thickness of very coarse conglomerate, many pebbles, espec-
ially of granite, ranging from six to eighteen inches in diameter.
The remainder of the section is conglomerate of a more normal
character, varying from medium to rather coarse in texture.
The conglomerate is composed throughout of well rounded
masses of granite (coarse and pinkish), felsite of different vari-
eties, and porphyrite; but, apparently, little true melaphyr.
The porphyrite is mainly of compact, dark gray and brown va-

rieties.

61

The Central Area.

This area embraces all that part of Nantasket east of the rail-
road and south of Atlantic and Centre Hills. The geological
relations of the central area to the preceding or coastal area can
be most easily traced in Willow Ledge Hill, which belongs in
part to each. The summit and main mass of this elevation is a
nearly square block of melaphyr and tuff. These rocks have
been described in the preceding section, and the limits of this
block are sharply defined on the map by the bounding fault-
lines. The similar block of conglomerate on the west lies at a
lower level, and its outerops are separated from the volcanic
block by a small artificial pond.

Both of these blocks are separated from the narrow ridge of
conglomerate between them and Hull Street by the well-marked
east-west depression occupied by Spring Valley Road. This
depression is an evident fault-line, for the beds of conglomerate
are so nearly horizontal that they must abut directly against the
melaphyr; and the contact is undoubtedly oblique to the strike
of the conglomerate. As indicated on the map, this is one of
the great faults of the Nantasket region, traversing the entire
area, from Strait's Pond to Weir River Day. Few of the dis-
locations of this district are more obvious, and none are of
greater structural importance. East of Valley Beach Avenue
it is marked by the well-defined escarpment terminating the
melaphyr of Centre Hill on the south, the conglomerate lying
at а lower level here, under a meadow ; while to the westward
it coincides with the similar escarpment along the north side of
Melaphyr Plateau, and, crossing the bay, defines approximately
the abrupt northern shore of the western area. This line of
fracture is thus cl arly the natural boundary line between the
Coastal and central areas. But when we pass to the considera-
tion of the direction and amount of the displacement along this
line, and the stratigraphic relations of the two areas, a much
more difficult problem confronts us. The escarpments, which

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|

62

face now to the south and then to the north, evidently signify
nothing but unequal erosion, appearing always on the side of
the harder and more resistant rock, which is usually melaphyr.
Faets will, however, be aecumulated in the following pages
pointing to the correlation of the conglomerate of Conglomerate
Plateau with a horizon below that of the conglomerate on the
extremity of Long Beach Rock; and hence to the conclusion
that the downthrow is not only on the north, but that it must
exceed the entire thickness of the melaphyr series of Atlantic
and Centre Hills, — four hundred if not five hundred feet.

Although melaphyr occurs abundantly in the central as well
as the coastal area, it is of a different and distinctly more basic
type. It is very generally and often profusely amygdaloidal,
rarely distinctly brecciated and never, so far as observed, ac-
companied by beds of tuff. The abundance of conglomerate
south of the great fault, as well as the numerous faults, still
further contrasts the two areas. It is, of course, quite possible,
if not probable, that some of the faults of the coastal area have
escaped discovery on account of the uniform character of the
melaphyr in Atlantic and Centre Hills ; and the great apparent
thickness of this melaphyr ought, perhaps, to be regarded as
largely due to repetition by faulting.

'The alternations of the conglomerate and melaphyr in the
central area are so frequent that it will be more convenient to
describe these rocks together, following the order of the ledges
or topographic features.

East shore of Nantasket Bay, Crescent Hill and Marsh
Island. — These localities, forming a narrow belt across the
western end of the central area, between the railroad and Mela-
phyr Plateau, may be conveniently described together, since, as
the map shows, they present, as the result of parallel faulting,
simply repetitions of certain beds of conglomerate and melaphyr.
This tract is bordered by two well-marked north-south faults,
and this primary fault-block is divided by no fewer than twelve
transverse fractures, with the slips alternating in direction, and
indicating the unequal rising and sinking of V-shaped blocks

68

(Fig. 8). Commencing at the north end, the first ledges of
conglomerate and melaphyr rise somewhat abruptly from the
marsh and the bay immediately south of the great east-west
fault, and about five hundred feet, following the railroad, south-
west of the broad, low outcrop of red and green slate or slaty
tuff already described ; and, as the map shows, the section is
almost uninterrupted along the east side of the railroad from
this point to the south side of Marsh Island.

Melaphyr. Conglomerate.

Fic. 5.—SECTION SHOWING THE FAULT-DIKE (25) AND THE CONTACT
BETWEEN THE THIRD CONGLOMERATE AND SECOND MELAPHYR,
SOUTH OF THE FAULT. SCALE, I INCH = 30 FT.

It begins with the typical melaphyr of the central area, a
dark green, evidently basic and highly amygdaloidal variety.
The amygdules are not uniformly distributed ; but they are
especially abundant in a superficial layer of the rock from two
to three feet thick. They are usually somewhat elongated and
have a distinctly linear arrangement parallel with the layer; so
that the flow-structure of the melaphyr as a whole is very marked.
This mass of melaphyr has an exposed breadth of about forty feet
north and south and is divided midway by dike 26 ; while it is
abruptly terminated on the south by dike 25. This dike un-
doubtedly marks an east-west fault; for it is bordered by con-
glomerate on the south (Fig. 5). The fault probably hades
with the dike to the north, and the melaphyr is clearly on the
downthrow side. Not more than eight feet in thickness of the

conglomerate are exposed before it is overlain by melaphyr.

64

The contact is very sharply defined, and most admirably
exposed; and it is conformable to the flow-structure in the
melaphyr, dipping south-southeast 5°-10°. This melaphyr
appears to be identical with that north of the fault, the amyg-
dules and flow-structure being even more pronounced ; and they
may be fairly regarded as parts of the same flow. The displace-
ment here has been sufficient to conceal the conglomerate and
to prevent its reappearance at the northern edge of the mela-
phyr, perhaps fifteen fect as a minimum. It should be stated,
however, that the melaphyr on the north or downthrow side
appears to be about horizontal or to have a slight northerly dip.
Above the conglomerate, on the south side of the fault, the
melaphyr has а breadth aeross the strike before we come to the
overlying conglomerate of nearly one hundred feet and an appar-
ent thickness of less than twenty feet. The true thickness, how-
ever, is próbably not less than twenty-five or thirty feet, the
apparent diminution being due to faults with the downthrow
to the south accompanying dikes 29 and 31. Southwesterly or
parallel with the railroad the sloping edge of the melaphyr flow
is exposed broadly and almost continuously as far as dike 35.
Near the railroad this dike marks the contact of the melaphyr
and overlying conglomerate. The exposure is not very satis-
factory ; but there seems no reason to doubt that the melaphyr
passes under the conglomerate conformably and without appre-
ciable faulting. Following the contact to the northeast, it
sweeps around in a curve concave toward the melaphyr. The
two rocks are seen near together, but not in actual contact, un-
til we come to the large inclined dike (31). Here a slight
fault with the upthrow to the north brings the melaphyr into
sight directly under the conglomerate (Fig. 6).

This conglomerate overlying the melaphyr is similar to that
underlying it. It is, however, coarser and more irregular in
composition, containing many rounded pebbles of granite and
felsite 6 to 12 inches in diameter; and it is especially distin-
guished by holding many large pebbles of melaphyr, some of
which are coarsely amygdaloidal and apparently identical with

65
that on which the conglomerate rests. The conglomerate outcrops
broadly south of dike 35, forming a low hill; and a few streaks
of sandstone show that the bed dips southeast about 10°, so
that the thickness can scarcely exceed 25 feet. It is distinctly
overlain on the southeast corner of the outcrop by a second
flow of amygdaloidal melaphyr. The surface of the ledge
slopes with the dip, and thus a thin layer of melaphyr covers
quite a breadth of the conglomerate. A small triangular patch
of the overlying melaphyr is also seen resting on the conglom-
erate immediately north of dike 35, indicating, apparently, a
slight fault with the downthrow to the north on the line either

of this dike or of the small dike (36) which is, probably, a

branch of the main dike.

3
№
X
yy
n ND e
N Conglomerate. N 5.
Melaphyr. Conglomerate.
Fic. 6.—SECTION SHOWING THE FAULT-DIKE (31) AND THE CONTACT

BETWEEN THE SECOND MELAPHYR AND THE FOURTH CONGLOM-
ERATE, NORTH OF THE FAULT. SCALE, I INCH 3O FEET.
Between dikes 29 and 31 the conglomerate erosses the north-
south fault bounding Melaphyr Plateau and extends eastward
into the melaphyr about 100 feet with a breadth of 75 feet.
This extension, being on the upthrow side of the fault, is ele-
vated above the rest of the bed some 20 feet. At its south-east
Corner the extension is overlain by amygdaloidal melaphyr of the
Same character, in precisely the same manner and with the same
southeasterly dip (10-15) as at the southeast corner of the
lower area. The extension is undoubtedly cut off by а fault
on the south, with the. downthrow in that direction, so that
the contact of the conglomerate and overlying melaphyr in the

OCCAS. PAPERS. B. S. N. H. IV. 5.

66

extension can not be traced far in the direction of the same
contact in the main area. The conglomerate extension is bound-
ed on the north by dike 29, which clearly marks a fault with
the downthrow also, probably, to the south. Hence the mela-
phyr north of the conglomerate extension appears to be that
normally underlying the conglomerate, and therefore distinct
from, and separated by a fault from, that overlying the con-
glomerate.

The small meadow or marsh separating the conglomerate
south of dike 35 from Crescent Hill must cover a fault with the
downthrow to the north, as indicated on the map; for the bed
of conglomerate first described and the underlying and over-
lying melaphyrs are repeated, with a stronger topographic re-
lief and fewer dikes, in the hill. The conglomerate forms the
summit of the northwestern part of the hill and extends en-
tirely around the northern and western slopes, its outcrop thus
having the form of a crescent. It isthe irregular or mixed
coarse and fine conglomerate, containing pebbles of amygda-
loidal melaphyr as well as of granite and felsite. ‘The contact
with the overlying melaphyr is very plainly exposed on the
summit. It is a true effusive contact and has a southeasterly
dip of 10°—20°, being conformable with the bedding-planes of
the conglomerate. This overlying melaphyr shows several dis-
tinct zones of amygdules, as well as other indications of flow-
structure, and altogether exposes a thickness of 25 to 85
feet. The contact is also clearly exposed for fifteen feet at the
base of the hill on the east side. The thickness of the con-
glomerate ean scarcely exceed 30 or 35 feet. The underlying
melaphyr exposes a thiekness of about fifteen feet at the north-
west corner of the hill, the contact descending both east and
south. Near the middle of the west side of the hill the under-
lying melaphyr appears again, being elevated some ten feet
above the railroad by an east-west fault with the upthrow to
the north (Fig. 7). This fault gives the conglomerate and
underlying melaphyr a distinctly synclinal structure in the

67
north-south line, the axis of the syncline dipping east 10° or

more; which prevents the melaphyr from reappearing on the
заве side of the hill.

Conglomerate.

Melaphyr.

Fic. 7.—-SECTION ACROSS THE FAULT ON THE WEST SIDE OF CRESCENT
HILL. SCALE, I INCH = 50 FEET.

Crescent Hill is, in fact, divided by a series of east-west
faults. Commencing on the north, the first fault is 65 feet from
the northwest corner of Crescent Hill and exactly opposite the
northern end of Great Hill. It downthrows to the north about
eight feet, jogging the contact of the conglomerate and lower
melaphyr ; and, since it does not now cut the upper melaphyr,
the effect is to increase the apparent thickness of the conglom-
erate. The second fault is thirty feet farther south. It hades
S. 25° and downthrows in the same direction about eight feet,
being compensating with reference to the first fault. Twenty-
five feet farther south is a small dike (43) and the only one
observed on the hill. Accompanying the dike, but not coin-
ciding in plane with it, is the third fault, which hades and
downthrows to the south, the throw being about eight feet, and
the beds sloping steeply from the fault on the north. The next
or fourth fault, seventy feet farther south, is that already re-
ferred to, on the west side of the hill. This is one of the
plainest and most instructive faults in the Nantasket district.

neral: view of the fault as seen from

`:
Figure 7 represents a ge

the railroad, taking in the whole height of the hill. The
fracture is still a well-marked and somewhat open fissure ; and
the jogging of the contact of the conglomerate and upper mela-
phyr shows that the downthrow is to the south and about

68

eighteen feet. The lower melaphyr very clearly rises to the
south or toward the fault, and again to the north, giving the
syncline already described. The fault trends N. 75° E., being
in line with dike 37 in Great Hill; and it can be readily traced
directly across the entire breadth of the hill, following approxi-
mately the boundary line between the conglomerate and upper
melaphyr, and breaking this contact almost as distinctly on the
eastern as on the western face of the hill. About midway be-
tween this fault and the southern end of the conglomerate is a fifth
fault, which is the counterpart of the last, the downthrow being to
the north and about 15 feet. It plainly jogs the contact between
the conglomerate and upper melaphyr; and trends N. 75? W.,
being in line with dike 44 on Great Hill. Finally, at the ex-
treme southern end of the conglomerate there appears to be à
sixth fault, probably parallel with the last, but with the down-
throw to the south. The amount of the throw, if to the south,
is certainly 10 if not 15 feet, being enough to throw the con-
glomerate entirely out of sight. If the throw were to the north
it would need to be 25, and more probably 50, feet to do this.

'The fact that the melaphyr under Crescent Hill, although
rising to the west, can not be found, not even the slightest trace
of it, on Great Hill, is alone sufficient to prove an important
north-south fault between the two hills, with the downthrow to
the east ; and the further faet that not one of the dikes in Great
Hill ean be traced in Crescent Hill points to the same conclu-
sion. Indeed, the contrast presented by the opposite walls of
this narrow defile is one of the especially surprising and puz-
zling features of Nantasket geology. The very abrupt way in
which the conglomerate of Crescent Hill terminates on the east
proves that the hill is boundéd in this direction also by an im-
portant north-south fault, the downthrow probably being as
before on the side of Crescent Hill, or to the west.

Marsh Island is a large ledge of conglomerate and melaphyr
completely isolated by the eastern marsh. Geologically it is
but a repetition, at a lower level and without important, dis-

placements, of Crescent Hill. The conglomerate, which forms

E

the main mass of the ledge, is of the
coarse, irregular type seen in Crescent
Hill and on the east shore of the bay,
containing many large pebbles of gran-
ite, felsite and melaphyr. The breadth
of the conglomerate outcrop, from
northwest to southeast, is about 140
feet; and the thickness of the bed, the
upper and lower surfaces of which are
clearly exposed, is about 25 or 30 feet.
It is underlain on the northwest, at the
very base of the ledge, by a highly
scoriaceous and amygdaloidal green
melaphyr, of which only a slight thick-

hess is exposed ; and is overlain on the

southeast by a very similar flow of

melaphyr, which at one point reaches
up over the conglomerate nearly to the
top of the ledge. These are undoubt-
edly true effusive contacts, and the up-
per one especially shows the dip of the
conglomerate to be S. E. 10°—20°.
The melaphyr contains abundant seg-
regations nd veins of epidote and red
Jasper. Since there appears to be no
reason whatever to question the strati-
graphic equivalence of Crescent Hill
and Marsh Island, it is necessary to
suppose that the marsh between them
conceals an important fault ; the down-
throw being to the north and fully
equal to the combined thickness of the
conglomerate and upper melaphyr,
about 50 or 60 feet.

The foregoing details, which are summarized

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70

features of this belt of ledges are readily explained by one bed
of conglomerate between two very similar flows of melaphyr,
with only such dislocations as are actually seen ог may be rea-
sonably inferred, until we come to the conglomerate underlying
the lower melaphyr south of dike 25. This is a partial expo-
sure only of a lower and older conglomerate which does not
appear elsewhere in the section.

Although the transverse faults of this block are frequently re-

versed in throw and compensating

85

the northerly throw clearly
prevails, the sum of all the displacements giving 161 feet to
the north and 71 feet to the south. Hence the entire series is
equal to one northerly slip of 90 feet.

Melaphyr Plateau.—This rectangular block of melaphyr
is sharply defined by the bounding fault lines and escarpments,
and may be regarded as a very characteristic feature in Nan-
tasket geology. With the exception of the small patch of con-
glomerate on the western edge, it consists throughout of the
most typical, basic, green variety of melaphyr, with frequent
highly amygdaloidal layers. The flow-strueture is very perfect
at many points, and shows that the sheets or flows are still
nearly horizontal, with slight undulations to north and south and,
perhaps, a very gentle general dip to the southeast. The facts
already stated in connection with the mass of conglomerate pro-

jecting into the western edge of the plateau indicate that the

plateau embraces both of the flows of melaphyr observed in the
Crescent Hill section. The lower flow forms the northern bor-
der of the plateau, or that portion north of the conglomerate ex-
tension ; while all the remaining area, or fully three-fourths of
the whole, must be referred to the upper melaphyr. The con-
glomerate, which normally separates the two flows, has been en-
tirely cut out of the section, so far as the surface development
is concerned, by the oblique strike faults. At the extreme east-
ern end of the plateau there are indications, in the form of thin
outliers of sandstone, that the upper melaphyr was also for-
merly covered by conglomerate. It will appear later that this

71

conglomerate overlying the upper melaphyr should not be cor-
related with that forming the adjacent ledges of Conglomerate
Plateau, but rather with that outeropping on Long Beach
Rock and underlying the melaphyr series of Atlantic Hill.

Conglomerate Plateau.—This broad mass of conglomerate,
the largest in the Nantasket district, is of the normal type, well
rounded pebbles of felsite and granite predominating, although
pebbles of porphyrite and compact melaphyr are also common.
The texture varies from moderately coarse conglomerate to
sandstone ; but the bedding can be made out at only a few points
as on the north side of Spring Valley Road. The dip seems,
however, to be always gentle (5°—10°) to the south-southeast,
diminishing toward the granite. The prominent ledges ex-
hibit some well developed joint-planes ; and the plateau is un-
doubtedly bounded on all sides, and not alone on the north, by
fault-lines. It rises abruptly from the marsh on the west ; Hull
Street and Spring Valley Road traverse it in narrow defiles ;
while the straight and solid wall of conglomerate 15 to 80 feet
high in which it terminates on the south is an exceptionally typ-
ical fault scarp. Facts will be presented later tending to show
that the downthrow in the case of all of these faults is on the side
away from the plateau, and hence that this conglomerate is older
than and normally underlies the surrounding formations ; but, as
already indicated, the displacement is, much greater on the north
side than in any other direction. Of the thickness of the con-
glomerate it is impossible to speak with certainty, since neither
the base nor, probably, the original summit of the bed are ex-
posed. It can not be less than the height of the plateau, about
fifty feet, and may exceed one hundred feet, this being, prob-

ably, the thickest of all the Nantasket beds.

Round Hill.—This hill, which, as the map indicates, is named
for its curvetl outline, is a solid mass of conglomerate rising
very abruptly from the eastern marsh on the north and west to

to a height of fifty feet and sloping gently down to the south-

12

east beneath a bed of melaphyr. The conglomerate of Round
Hill is connected with that of the plateau by essentially contin-
uous outerops along the eastern border of the marsh, and must
be regarded as a part of the same bed. It is of medium texture
above, passing in the lower part of the hill into fine conglom-
erate and a considerable thickness (10 to 20 feet) of reddish sand-

stone ; while in the ledges rising from the marsh on the southwest

we have a coarse and irregular conglomerate with layers of

sandstone, which show that in these outlying ledges as well
as in the hill the dip is mainly to the east or east-southeast and
very gentle —5° to 10°, but rarely exceeding 5°. The exposed
thickness of the conglomerate, including the outlying ledges, is
probably between 75 and 100 feet. The low ledge of melaphyr
protruding from the marsh on the north side of the hill is prob-
ably a part of the bed underlying the marsh ; and the fault sep-
arating this melaphyr from the conglomerate is thus given a
general east-west course. Since both this fault and that bound-
ing the marsh on the east cut across the strike of the conglom-
erate, the breadth of the outerop of that rock, as the map shows,
has been greatly diminished at this southeast angle of the marsh
where the fault-lines meet. According to the view here pre-
sented, the melaphyr forming the floor of the marsh belongs to
the same bed as that overlying the conglomerate, and hence the
conglomerate must also underlie the marsh, but at a greater
depth than the melaphyr. More probably, however, as the iso-
lated ledge of conglomerate in the channel east of Marsh Island
suggests, the entire section of Marsh Island and Crescent Hill is
represented beneath the marsh, the latter being bounded by faults
on all sides and separated from Round Hill by a slip of at least
150 feet.

South of the hill, on the bank of the river ( Lyford's Liking),
the granite is well exposed, with conglomerate and sandstone
resting against and upon it. The sandstone is really a fine, red-
dish and intensely hard quartzite ; and both it and the conglom-
erate appear to fill fissures and depressions in the surface of the

granite, in the same manner as the basal conglomerate of the

13

western area to be described later. If we make this correlation,
then, since it is reasonably certain that other beds of melaphyr
as well as conglomerate underlie the conglomerate of Round
Hill, and we can not connect the latter with the basal conglom-
erate on the granite; it appears necessary to suppose that an
important fault, with the downthrow to the north, separates the
granite from the hill,—a continuation with a different direction
of the fault extending from Granite Point on Weir River Day
southeasterly across Great Hill.

The melaphyr on the southeast side of Round Hill is nowhere
seen in satisfactory contact with the conglomerate ; but the slope
of the hill on this side is nearly parallel with the bedding of the,
conglomerate, and all the indications are favorable to the view
that the melaphyr overlies the conglomerate conformably. This
is the typical green and amygdaloidal melaphyr of the central
area, and cannot be distinguished from that outcropping in the
marsh. The flow-lines dip gently to the southeast, as do
the bedding lines of the conglomerate. To the north, this bed
of melaphyr approaches very near the southeast corner of the
marsh, as already explained ; and we here reach the turning
point of a marked and rather abrupt change in the strike of both
the conglomerate and melaphyr. The normal dip of the Round
Hill beds appears to be about east-southeast, and of the strata of
Conglomerate Plateau and the district directly south of it, about

south-southeast.

The District between Round Hill and Hull Street.—
This tract consists of rocky fields sloping southward from the
escarpment marking the southern border of Conglomerate
Plateau to the marshes of Lyford’s Liking. The melaphyr
overlying the conglomerate on the southeast slope of Round
Hil bends to the east, as already explained, to follow the
southern border of Conglomerate Plateau. Dikes 40 and 41
are evidently attended by some faulting ; and the narrow band of
conglomerate in the melaphyr may be aecounted for in this way ;
but whether it belongs to the conglomerate below the melaphyr

74

or to that above it is not quite clear. The conglomerate south
and east of the melaphyr undoubtedly overlies it, although there
are no satisfactory exposures of the contact. The limited mass
of melaphyr between the conglomerate and the granite, although
possibly not 2% situ, is probably a reappearance of the melaphyr
separating the two beds of conglomerate. It is topographically |
below the upper conglomerate. An important east-west fault, a
continuation of the great border fault already referred to, must
separate the conglomerate and melaphyr from the adjoining
ledges of granite; and the latter here exhibit no traces of the
basal conglomerate. The rather abrupt northward jog of the

granite border may be explained by a southward extension with a

slight displacement of the fault between Conglomerate Plateau and
the marsh, as shown on the map. Farther east the melaphyr
rises and broadens out, the overlying conglomerate having been
carried above the present plane of erosion ; and the underlying
conglomerate fails to outcrop along the southern base of Con-
glomerate Plateau. In fact indications are not wanting that the |
melaphyr here covers the entire area between the plateau and h
the marsh. This would be readily explained without increasing
the thickness of the melaphyr by simply regarding the beds as
horizontal here; and the assumption is fully justified, even
though the contact of the melaphyr and conglomerate can not
be seen, by the fact that the flow-structure is well-marked in

this melaphyr and quite horizontal.

The district between Hull Street and Strait's Pond.—
This area is separated from the last by the fault, probably of no
great magnitude, terminating Conglomerate Plateau on the east. |
It is topographically similar, and composed, apparently, of the
same conglomerate and melaphyr, although extending. north to
the great east-west fault and the melaphyr of Centre Hill. \
The chief point of difference is found in the frequent alternations
of conglomerate and melaphyr. These alternations are best ex-
plained by repeated east-west faults, dividing the district into

seyen or eight narrow blocks, as shown on the map. No satis-

75

factory contacts have been observed ; but the ledges are so re-
lated as to indicate that the contacts must mark faults in most cases
atleast. The melaphyr is similar to that west of Hull Street ; and
the conglomerate is probably that underlying the melaphyr,— the
conglomerate of Round Hill and Conglomerate Plateau ; although
it would be difficult to prove that the overlying conglomerate 18
not also represented here. Dike 31, the only one observed in
this district, separates conglomerate and melaphyr and is un-
questionably a fault-plane. On the south, near the Hull Street

bridge, the granite outcrops boldly, with no traces of conglom-

` erate or melaphyr upon it. The melaphyr outcrops very near the

granite on the north ; but, although the contact is not exposed, it
seems impossible to doubt that a fault divides them here, as at
points farther west. Folsom's Island probably lies on the far-
ther side of a moderate north-south fault, which terminates these
narrow fault-blocks and defines the abrupt eastern face of Cen-
tre Hill. There seems to be no reason to doubt that this is
the conglomerate underlying the melaphyr ; and since a careful
search has failed to discover any melaphyr here, we may fairly
suppose that it is unbroken by east-west faults, that the bedding
planes are essentially horizontal, and that it has been elevated
sufficiently by the north-south fault just referred to to carry the
melaphyr wholly above the present plane of erosion. Crossing
Strait’s Pond at this narrowest point, we find what appears to
be the same conglomerate outcropping abundantly between the
pond and Jerusalem Road. It extends quite up to the street,
and immediately opposite are the ledges of granite. The beds
strike about northwest and dip northeast BOP wo AO ДИЗ
plain that the conglomerate runs diagonally against the wall of
granite, from whieh it must be separated by a fault. The con-
glomerate probably forms the whole of this rectangular area north
of Jerusalem Road and it actually outcrops again at the extreme
eastern end, where the small promontory projects eastward into
the pond. In fact, when we consider in this connection the
straightness of the granite border and the important development

of conglomerate at the base of Green Hill, it appears at least,

| x 76

probable that almost the entire basin of Strait's Pond is under-

lain by this conglomerate. It is this view of the geological

the fault-lines on the map. Of course, if the conglomerate is
| carried down to any considerable depth by the rather high dip
| which it shows near Jerusalem Road, it is very probably covered

| in the central part of the basin by the overlying melaphyr.

The Western Area.

This area comprises all that part of Nantasket proper west of
the railroad. It is bounded on the south and west by Weir
| River Bay, and on the north by Nantasket Harbor. We have
| already met the fundamental granite along the southern margins
of the central and coastal areas; but it is in the western area
| that we first encounter what appear to be the oldest beds of
| conglomerate and melaphyr in the Nantasket district. The
| geological structure is quite as varied and complex as in the
central area, and as before, the descriptions may most conven-
iently follow the topographic order.

|| Granite Plateau.— This is the name given to the largest
| mass of granite in the Nantasket peninsula. It forms the large

li southwestern angle of the western area, and is a compact eroup
| © ©

of ledges having, for the most part, an elevation of from twenty
to forty feet. The granite is of the normal type, and, exhibiting
but slight variations, requires no special description ; but the
outliers of conglomerate scattered over the plateau are a feature
| of particular interest. The more important of these are indicated
on the map; and the best and clearest exposures are, naturally,
to be found along the shore. The conglomerate is most abun-
dant around the northeast. corner of the plateau, where it shows
east and southeast dips of 5°—10°, and a maximum thickness of
|| perhaps 15 feet. Elsewhere it is hardly thick enough to conceal
the granite; and everywhere it is largely composed of angular
ranite. This is

and unassorted débris of precisely the same g

| stucture that is intended to be expressed in the arrangement of

Occas. papers Bost. Soc. Nat. Hist. IV. Plate 6.

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SECTION OF AN IRREGULAR FISSURE IN THE SURFACE OF GRANITE
PLATEAU FILLED WITH CONGLOMERATE. SCALE,

ONE FIFTH NATURAL SIZE.

PLAN OF FISSURES IN THE SURFACE OF GRANITE PLATEAU
FILLED WITH SANDSTONE AND FINE CONGLOMERATE.
SCALE, ONE FIFTH NATURAL SIZE.

=
í í

alone sufficient to prove that the conglomerate is newer than,
and was deposited upon the granite ; although at first glance,
or in a general view, the opposite relation might be regarded as
well sustained, the conglomerate appearing as isolated masses
enclosed in the granite. But an examination of the contact
between the two rocks removes all doubt as to the true history
of Granite Plateau. The conglomerate clearly rests upon the
granite and fills the irregularities of its surface. Instead of the
granite being intrusive in the conglomerate, the appearances are
just the reverse, narrow cracks in the granite being filled, dike-
like, with conglomerate and sandstone. These fragmental dikes,
the roots of a once continuous bed of conglomerate, are exposed
repeatedly along the shore, as well as over the surface of the
plateau; and they seem to prove conclusively that erosion has
eut just to the base of the conglomerate, clearly exposing the
original granite floor, but leaving it essentially intact. It is very
evident that the conglomerate was deposited over such a broken
and fissured surface as the granite presents along our shore to-
day.

Besides the granitic débris, the conglomerate includes numer-
ous pebbles of felsite and a large proportion of melaphyr, show-
ing that voleanic action had been instituted in this region before
the formation of this bed of conglomerate and hence that, al-
though resting directly upon the granite, it is not the true basal
conglomerate. Probably, as a rule, each successive bed reached
farther inland, the overlapping edges, only, being in immediate
contact with the granite. The conglomerate in the more south-
ern outlier on the eastern edge of Granite Plateau is curiously
involved with irregular masses of a compact eruptive rock, which
can not be referred directly to either an underlying or overlying
mass of lava but which proves on examination to be identical,
lithologieally, with the first (ew to be described in other parts
of the western area. The significance of its occurrence at this
point, will be explained after we have studied the flow to which
it is related.

The granitic area south and southeast of the west marsh,

"9

id

embracing the isolated ledges of granite in the marsh and the
southern slope of Great Hill, is essentially similar in every re-
spect, including the outliers of conglomerate ; but the interven-

ing marsh is probably underlain wholly by conglomerate, which

Great Hill, where it is seen almost in contact with but not di-
rectly upon the granite, dipping away from the latter or south-
east 5° to 10°. The outlying mass near the bank of the river
is almost certainly 7» situ, and indicates that a considerable
body of conglomerate is concealed here. Traces of conglomer-
ate have been observed on the outlying ledge of granite in Weir
River Bay ; and the preceding description might be repeated for
Granite Point, although it is separated from the plateau by the
boundary fault, which, with the downthrow to the north, marks
the northern limit of the granite at most points.

Granite Point is a knob of granite capped with conglomerate
dipping slightly to the east and northeast, the beginning of an
important bed extending into Cliff Plateau. The relations of
the granite and conglomerate are especially clear on the south
side of the point (Fig 9). A large and irregular hollow or
raviné in the granite is filled with conglomerate largely composed
of granite débris and including several large bowlders of gran-
ite, each of which is precisely similar in character to the nearest
part of the enclosing granite. This conglomerate is from one-
half to two-thirds melaphyr, often in large and angular frag-
ments. Although this is so clearly not the basal conglomerate
of the Boston Basin, it will appear in the sequel that it is prob-
ably the lowest bed now exposed in any of the Nantasket sec-
tions ; and it will, for convenience, be designated hereafter the
first or basal conglomerate. It is interesting mineralogically
on account of the numerous segregations of bright red jasper
which occur in it. “These are exceedingly irregular in outline,
apparently replacing the paste of the conglomerate. The origin
of the jasper is, doubtless, to be found in the alteration of the

melaphyr ; and attention has already been called to the prob-

has been depressed by faulting. The principal exposures of
conglomerate are on the borders of the marsh, directly south of

79

able fact that the intense hardness of the Nantasket conglomer-

ates is due to interstitial silica derived from the same source.

W.

Fic 9.—SECTION OF THE GRANITE AND OVERLYING CONGLOMERATE
ON GRANITE PorNT.
SCALE, I INCH ==10 FEET.

Cliff Plateau. Cliff Plateau comprises the triangular group
of conglomerate and melaphyr ledges between Granite Plateau
and the shore on the north. The average elevation is about
thirty feet, but two points rise above fifty feet. The name re-
fers to the well-marked cliff 25 to 40 feet high which bounds it
on the north, extending from Granite Point to dike 66. Gran-
ite Point, as already stated, is overlain by the basal conglom-
erate, with thin beds of sandstone showing nearly horizontal
dips, the main slope being to the northeast. One bed of sand-
stone two to three feet thick runs the entire length of the cliff
facing West Porphyrite Hill, until it meets dike 33, meeting it
about five feet above the beach and thus sloping approximately
one foot in ten. It is thrown up about three feet by this dike
and then continues with the same inclination, but growing less
distinct, to dike 31, beyond which it cannot be traced, probably
passing under the water. The eastern half of the cliff, rising
abruptly from Nantasket Harbor, between dikes 83 and 66, is
composed entirely of this conglomerate, with the exception of a
narrow slice of the overlying melaphyr which, toward the east-
ern end, has been let down by a fault trending N. 80° E. with
the downthrow to the north. In fact, as the map shows, the

basal conglomerate forms throughout the main body of the

80

plateau, having for the greater part of the area a southeasterly
dip of 5°—10°. It is overlain on the east and southeast by two
considerable patches of melaphyr, detached areas of what must
originally have been a continuous flow. This melaphyr is a
greenish to purplish, compact or finely crystalline, non-amygda-
loidal variety, containing epidote in minute crystallizations and
also larger segregations of both epidote and red jasper. It is
distinctly different from, and undoubtedly older than, any mel-
aphyr in the Central or Coastal areas, although bearing some
resemblance to the more basic porphyrites.

The contact between the basal conglomerate and the south-
eastern patch of melaphyr is very clearly exposed, especially on
the west side of the melaphyr, and the melaphyr unquestionably
overlies the conglomerate. The contact appears on the map as
a regular curve convex to the northwest, indicating that the
melaphyr lies in a shallow trough. This appearance is due
partly, however, to the very plain east-west fault which marks
the northern edge of the melaphyr, with a downthrow to the
south of, perhaps, ten feet. The melaphyr is distinctly and
comformably overlain on the southeast by two small patches of
what must be a second bed of conglomerate, since the melaphyr
separates it from the basal conglomerate. These outlines owe
their preservation chiefly to the two east-west faults shown on
the map and downthrowing to the south ten and fifteen feet,
approximately. The boundary fault separating these rocks
from Granite Plateau is an absolute necessity at this point ; for
their nearly level beds are vis-a-vis with the abrupt ledges of
granite. "Phe displacement here cannot be less than twenty-
five nor, probably, more than fifty feet; for the basal conglom-
erate, which eaps the granite on the south side of the fault, is
at least forty and possibly fifty feet thick, and the overlying
melaphyr is twenty-five to thirty feet.

The northern area of melaphyr appears also to occupy a
shallow trough, and the intervening portion of the basal con-
glomerate must be slightly anticlinal in structure ; although the

ridge coinciding with dikes 22 and 34, and slightly capped with

81

melaphyr at one point, rises so abruptly from beneath the mel-
aphyr on the north as to suggest an intervening fault, which is
indicated on the map, with the downthrow to the north. Layers
of sandstone in the eastern end of this ridge show the usual
easterly dip. The contact between the basal conglomerate and
this northern patch of melaphyr is clearly exposed along the
brink of the cliff and in the gorge formed by dikes 66, where the
dip is southeast perhaps 10°; and it can be traced quite around
the west end of the melaphyr, which everywhere plainly over-
lies the conglomerate. The northern patch of melaphyr, like
the southern, is overlain on the southeast by a thin eap of the
second conglomerate, this upper contact being parallel with the
lower. The normal succession of all the rocks in Cliff Plateau

is clearly exhibited in the accompanying section.

W. Clif Plateau. Dikes бб. E. Porphyrite Hill. E

ERES: ] ы
TRARA j SENS KI GANZ FOU
Granite, Conglomerate, Мару". ` Porphyrite.

Fic, 10.— SECTION FROM GRANITE POINT ACROSS CLIFF PLATEAU AND East
'
PonPHvRITE HILL. SCALE, I INCH = 400 FEET.

East Porphyrite Hill. — This hill, which forms the promi-
nent northeast angle of the western area, is separated from Cliff
Plateau by the group of north-south dikes (66) and the depres-
sion or gorge due to their erosion. It is surprising to find that
these repeated fractures are accompanied by no appreciable dis-
placement ; but the basal conglomerate, the melaphyr, and the
second or overlying conglomerate can all be traced across the
dikes without the least slip (Fig. 10). The basal: conglomerate
just barely shows at the base of the cliff on the east side of the
dikes and does not reappear beyond, the easterly slope of all the
beds being very noticeable. The melaphyr can be followed

OCCAS. PAPERS В. S. N. H. IV. 6.

82

along the shore as far as dike 27, when it also disappears, and
the second conglomerate becomes the prevailing rock on the
west side of the hill. This conglomerate, which is largely a
fine-grained grit and sandstone, and hence plainly stratified, is
characterized by segregations of red jasper, but in a less degree
than the basal conglomerate. The dips are rather high, E.
about 20? on the west side of the hill and S. E. 27? near the
overlying porphyrite at the north base of the hill. The con-
glomerate is conformably overlain by the great mass of porphy-
rite which forms the summit and eastern and southern slopes of
the hill, the actual contaet being particularly well-exposed on
the north side. As a whole this porphyrite is similar to that of
the Coastal area, but it is rather more felsitic in texture, redder
in color, non-epidotic ; and segregations of red jasper occur in
it to a limited extent. The base of the flow is mainly of a
greenish gray color changing upward to a dull reddish tint ; and
the upper part is largely of a deep dull red, often resembling at
a little distance a dark red sandstone. Near the contaet with
the underlying conglomerate it shows a distinct flow-structure,
parallel with the contact, as well as a coarse and fine breceia-
tion, the latter resembling or passing into a genuine **toad-
stone" or spherulitic structure. Dike 27, which crosses the hill
approximately at right angles to the strike, is accompanied by
an important fault. It downthrows to the north and shifts the
contact of the conglomerate and porphyrite about 175 feet, as
shown on the map, equivalent to a vertical slip of fully 50 feet.
This hill is a complex of dikes, and since all, apparently, are
attended by some displacement, the continuity of the contact
between the conglomerate and porphyrite is lost. This confu-
sion makes it difficult, also, to determine the exact thickness of
both the porphyrite and conglomerate. It is quite certain,
however, that the entire thickness of the porphyrite, as well as
of the conglomerate, is exposed here; for it is undoubtedly
overlain conformably by the small patches of conglomerate on

the southeast corner of the hill (Fig. 10).

88

These patches must belong to a third bed of conglomerate, and
since they are parallel with the second or underlying conglom-

rate, the separation of the two beds measures, approximately,
ils thickness of the porphyrite, which cannot well be less than
fifty feet and may be more. I have, however, observed no dis-
tinet, indications that it is a composite flow ; but it is from bot-
tom to top one solid and essentially homogeneous mass. The
underlying conglomerate must be nearly as thick as the por-

phyrite.

West Porphyrite Hill.—This approximately isolated and
well-defined rock-mass is, in both form and structure, an al-
most exact repetition of East Porphyrite Hill, except that the
structure is but slightly complicated by faults and dikes. In
short it is a solid bed of conglomerate underlain by melaphyr
and overlain by porphyrite ; and none of the Nantasket ledges
present a clearer or more instructive section (Fig. E): The
underlying melaphyr is very compact for the most part, green-
ish and, purplish, non-amygdaloidal, and not conspicuously
breceiated or ве 'oriaceous, except near the original surface of the
flow, where it is in contact with the conglomerate. It shows
many minute crystallizations and some whitish veinlets of
epidote, and numerous small, irregular segregations of bright
red jasper. In other words, it is essentially identical with the
melaphyr which we have found overlying the basal conglomer-
ate on Cliff Plateau and passing beneath the second conglom-
erate оп East Porphyrite Hill. The outcrop begins on the
north at the end of the beach leading to Melaphyr Peninsula
and skirts the base of the hill to the middle of the west end.
The contact with the overlying conglomerate is exposed con-
tinuously for several hundred feet, but the base of the mel: aphyr
is not seen, the thickness exposed in the hill not exceeding 15
feet. A greater thickness is proved, however, by the outlying
half-tide le lees, which extend nearly half-way across Weir River
Day. These were carefully examined; and all that are marked

84

as melaphyr on the map are of precisely the same kind as that
just described, and are plainly parts of that flow. The tops of
the ledges sometimes show traces of the overlying conglomerate ;
and these prove a very perceptible westerly dip, as if the bay

occupied here a faintly marked syncline.

Melaphyr W. Porphyrite Granite ;

N. Peninsula. Hn. Cliff Plateau. Plateau. ©.
mg

OC A

SS OUR Ма

TEM ER
AN AREAS

Granite. Conglomerale. Melaphyr. Porphyrite.

Fic. 11.— SECTION FROM NANTASKET HARBOR ACROSS MELAPHYR PENINSULA,
Wrst PoRPHYRITE HILL, AND CLIFF PLATEAU TO GRANITE PLATEAU.
SCALE, I INCH — 400 FEET. THE DIKES ARE, FROM NORTH TO SOUTH,

NOS. 28, 31, 34, AND 22. DIKE NO. 29 IS OMITTED.

The conglomerate is mainly of medium texture, and becomes
finer upward ; but at the base it is ina large part very coarse
and irregular. The contact with the melaphyr, although so
perfectly exposed, is somewhat obscure and indefinite, because
the conglomerate fills the numerous cracks and marked inequal-
ities which naturally characterize the upper surface of the mela-
phyr flow ; and also because at its base the conglomerate is
largely, in some cases almost wholly, composed of melaphyr
debris. Very few of the fragments and pebbles of melaphyr
are amygdaloidal ; and they can nearly all be certainly referred
to the subjacent bed. Near the base of the conglomerate, es-
pecially, many of the component fragments are exactly like the
melaphyr which it rests upon. It would, perhaps, be difficult
to find a clearer example of a contemporaneous lava covered by
newer sediments. The relations of the two rocks are simply
inexplicable on the theory that the melaphyr is intrusive.
Along the contact, especially, granite is very abundant in the
conglomerate ; and several masses were noticed from one to
three feet in diameter, the largest ones resting directly upon

the melaphyr, or seeming to be almost imbedded in ite. Tho

85

granite is chiefly rounded, but some blocks, like many of the
melaphyr fragments, are rather angular. The segregations of
red jasper occur in all parts of the conglomerate, but most abun-
dantly in the lower part. In none of the Nantasket rocks is it
more plentiful or more favorably exposed for observation. In-
terealated layers of sandstone show that the conglomerate is
nearly horizontal, but undulating, along the north side of the
hill; while around the western end the contact with the mela-
phyr, and the strings of sandstone, dip gently (5°-10") to the
east. The true thickness of the conglomerate is probably not
over 25 feet and almost certainly less than the same bed on
East Porphyrite Hill. The outlying ledge of conglomerate
west of Granite Point, instead of belonging to the basal con-
glomerate, as one might have judged from its position, is pre-
cisely like the conglomerate of West Porphyrite Hill, being
coarse and largely made up of.the underlying melaphyr, and
containing numerous large fragments of granite, ete. The
channel between this conglomerate and the ledges of melaphyr
north of it probably marks a slight fault (10 to 20 feet) with
the downthrow to the south.

The overlying porphyrite, of which about 25 feet in thick-
ness still remain, is very similar to the lower half of that on
East Porphyrite Hill. It is compact (felsitic), not distinctly
crystalline, porphyritic or amygdaloidal, and quite homogeneous
except for the scattering and mostly small segregations of red
Jasper. The colors are chiefly pale green and red tints, weath-
ering usually light pink. The absence of the dark red variety
which is so prominent on East Porphyrite Hill is due to erosion,
the entire upper half of the porphyrite having been removed
from the western hill, while the eastern hill still shows a com-
plete section of the flow. The porphyrite is seen near the con-
glomerate at several points on the north and west sides ; but the
only clear and satisfactory exposure of the actual contact is
afforded by the rather precipitous eastern face of the hill.
The contact here shows an exceptionally steep southerly dip

86

of about 25°; and this is still farther augmented by a fault with
a downthrow to the south of perhaps ten feet, which skirts the
northern edge of the porphyrite and crosses the level platform
of conglomerate to the western base of the hill, as shown on the
map, thus explaining the abrupt termination of the melaphyr
midway of the breadth of the hill. Along the contact with the
conglomerate the porphyrite, as on the eastern hill, is some-
what brecciated, and shows a distinct flow-structure parallel with
the contact, as well as a coarse sort of spherulitic or concretionary
structure. On the southwest corner of the hill the conglomer-
ate is seen to pass beneath the porphyrite, but the contact is
not clearly exposed. The southern face of the porphyrite is
precipitous, matching the opposing face of Cliff Plateau ; and
there can be no reasonable doubt that this narrow pass marks
an important fault with the downthrow to the north ; for the
porphyrite is vis à vis with the basal conglomerate, and the
melaphyr exposed at the northwestern base of West Porphyrite
Hill is the equivalent of that forming the southeastern summit
of Cliff Plateau. The displacement must, be equal to the com-
bined thickness of the porphyrite and the underlying conglom-
erate and melaphyr, 2.e., all the beds in West Porphyrite Hill, or
75 feet as а minimum. This fault probably passes southwest
between the outlying ledges of conglomerate on the north and
granite on the south; and the boundary fault between Granite
Plateau and Cliff Plateau, which certainly diminishes rapidly
in this direction, appears to terminate on this line, as repre-
sented on the map.

We now have above the granite, in regular sequence, the
following beds: (1) the first or basal conglomerate, on Granite
Plateau, Granite Point, and Cliff Plateau; (2) the first
melaphyr, on Cliff Plateau and East and West Porphyrite
Hills; (3) the second conglomerate, on Cliff Plateau and

East and West Porphyrite Hills; (4) the porphyrite, on the '

Porphyrite Hills ; and (5) the third conglomerate, on the south-
east corner of East Porphyrite Hill. From Granite Point to

87

the bay east of East Porphyrite Hill the section is continuous
and complete, all five of the beds being represented (Fig. 10) ;
but the evidence along this line is greatly strengthened at the
weakest point by the very clear and simple section afforded by
West Porphyrite Hill (Fig. 11). The lithological comparisons
are almost perfect and certainly safe for so small an area ; and
the succession may be said to rest at every step on the approved
canons of stratigraphy.

The general structural relations of the rocks in those portions
of the western area already deseribed are clearly exhibited in
the two accompanying sections: one (Fig. 10) east and
west, and the other (Fig. 11) north and south. For the sake

of greater clearness the east-west dikes are not shown in the

first section.

Conglomerate Hill.—This completely isolated mass of con-
glomerate is, perhaps, the simplest of all the structural blocks
composing the Nantasket area. It is all conglomerate, with
the exception of occasional thin layers of hard red sandstone,
which show a dip of not more than from 5° to 10° to the east or
alittle south of east. Some parts of the conglomerate are very
coarse, holding pebbles from six to twelve inches in diameter ;
while other parts are very fine and arenaceous. The red
jaspery porphyrite of East Porphyrite Hill is a very conspicuous
constituent ; but the red jasper is also, to a very limited extent,
an indigenous constituent, forming irregular segregations in the
cement of the conglomerate. The hill appears to be entirely
unbroken by either faults or dikes; and the most diligent
search has failed to reveal a vestige of eruptive rock of any
kind, except in the form of pebbles. "There are no overlying
rocks; and the base of the conglomerate is not exposed.
Hence the total thickness of this bed is a matter of conjecture ;
but since the eastern slope of the hill is approximately parallel
with the dip, the exposed thickness must be at least fifty feet.
The hill is surrounded on all sides by grass land and marsh ;

88

and about the only clue to its stratigraphic position is that
afforded by the enclosed pebbles of jaspery porphyrite. There
can be little or no doubt that these represent the flow on East
Porphyrite Hill; and the inference appears to be justified that
this is the third conglomerate, a small outlier of which has
already been observed resting upon the porphyrite. This view
of its position means that Conglomerate and East Porphyrite
Hills are separated by an east-west fault with a downthrow on
the south of fully fifty feet. Following this fault west to the
valley between these hills and Cliff Plateau, we find that it
must terminate in a north-south fault, for it cannot be traced
across the plateau; and this transverse fault must downthrow
to the east, for otherwise there would be insufficient room
beneath Conglomerate Hill for the porphyrite and the second
conglomerate. Conglomerate Hill thus appears as a rectangu-
lar block which has dropped down, the downthrow being
greatest at the junction of the two faults, at the northwest cor-
ner, which explains the fact that the dip is less than in East
Porphyrite Hill and more easterly. The north-south fault
probably coincides with dikes 66 and extends with greatly
diminished throw nearly or quite to the north shore, terminating
all the minor faults erossing Cliff Plateau. East Porphyrite
Hill and Cliff Plateau are thus two broken fault blocks which
have risen relatively to Conglomerate and West Porphyrite
Hills ; while all these blocks must have dropped with reference
to Granite Plateau.

Great Hill.—This is the highest point in the western area,
rising very abruptly 74 feet above the western marsh, and it is
the boldest and most impressive topographic feature in all
Nantasket. With the exception of the granite which supports
the hill on the southwest, it is another isolated mass of conglom-
erate; and we are obliged to depend wholly upon internal
evidence in determining its stratigraphic relations. The entire

lack of correspondence between Great and Crescent Hills has

е

|
|

89

already been cited as proving a displacement between them ;
and the same reasoning applies with equal foree when we pass
from Great Hill to Conglomerate Hill. The Great Hill con-
glomerate is extremely irregular, varying abruptly and re-
peatedly from fine sandstone to a mass of granite and melaphyr
bowlders one to three feet in diameter. Granite pebbles and
bowlders are numerous, but usually rounded. Felsite and
melaphyr are also. abundant, but the latter is never amygda-
loidal. The conglomerate is jaspery at most points—red jasper,
but so highly ferruginous as to be perceptibly less bright and
hard than that in the more western hills. It occurs, however,
in the same way, and lends support to the view that this con-
glomerate should be correlated with one of the three lower or
basal beds. The conglomerate is thoroughly indurated through-
out, although this is most noticeable in the sandstone layers,
which are as hard as quartzite, but always red, felsite debris
largely predominating. The dip is flat as a whole, but vari-
able,—horizontal at some points and inclined 15°-20° at others,
especially near the granite. The normal dip appears to be east
to southeast and not above 5°; and the exposed thickness of the
conglomerate must be nearly the height of the hill or at least
sixty feet.

There is no overlying and there appeared at first to be no
underlying melaphyr or porphyrite ; and the exact correlation of
this conglomerate seemed a hopeless task, until I came to study
in detail the phenomena presented near the contact with the
granite, on the southwest corner of the hill. It is evident at
a glance that the main body of conglomerate does not rest
upon the granite, but is faulted down against it, the boundary
fault crossing the hill at this point, and with essentially the
same result as on the west side of the marsh. The granite 1s,

however, capped by several feet of conglomerate at this point ;

and the fault, which is very clearly exposed, hades irregularly
to the north. Following it down carefully we come finally,

some six feet below the top of the granite and about fifteen

90

feet above the marsh, to the base of the conglomerate. It
is seen to rest upon a compaet greenish and purplish eruptive
rock, which proves to be identical with the first melaphyr of
Cliff Plateau and West Porphyrite Hill. It can be traced north-

ward about one hundred feet, the contact gradually sloping

—

down to the level of the marsh. Its contact with the granite
is clearly a fault with the hade and throw to the north, and
considerable fragments of the melaphyr appear to lie along the
fault-fraeture above the base of the conglomerate, testifying
further to the direction of the slip. In short, we have exposed
at this point, on the south side of the fault, the granite capped
by conglomerate ; and several feet below this contact, on the
north side, the first melaphyr overlain by the main body of
conglomerate forming Great Hill. The conglomerate on the
south is probably the first or basal bed, as indicated by its re-
lations to the granite; and that on the north must be the
second bed, as proved by its relations to the melaphyr. This $5
makes the displacement equal to the combined thickness of the 4
basal conglomerate and the first melaphyr, perhaps 75 feet in
all.

It is a fact, however, that the fault-fracture cannot be traced
upward through the conglomerate ; but the conglomerate over-
lying the granite is essentially continuous with that north of
the fault-line ; and it would, perhaps, be best to refer all of the
Great Hill conglomerate to the first or basal bed, regarding the
underlying melaphyr as a lower flow than has been seen else-
where in the western area. This view, which would dispense
with the fault between the granite and conglomerate, derives
further support from the fact that the basal conglomerate
on Granite Plateau and elsewhere is erowded with the debris of
similar melaphyr ; but it is not followed in the construction of |
the map, simply to avoid multiplying lava-flows where the
facts do not absolutely require it. Still another interpretation
is consistent with all the observed facts; viz., that the con-

glomerate belongs wholly to the second bed and that the

sup-

91

posed fault is really an overlap, the conglomerate having
covered a boss of granite whieh rose like an island through the
first or underlying melaphyr.

Following the fault-contact across to the southeast corner
of the hill, we again find the conglomerate on the north under-
lain by the compact melaphyr; and the granite outcrops at a
little distance on the south without any capping of conglomer-
ate, erosion having cut deeper here. It is evident now that
Great Hill must be on the upthrow side of the fault bounding
it on the northwest; and that the displacement is more than
equal to the combined thickness of the porphyrite and third con-

glomerate, or more than 100 feet.

Correlation of the First Melaphyr.— Passing down
the long southwestern slope of Great Hill, we come at last,
in the ledges of granite isolated by the western marsh, {о
the large dike of melaphyr, No. 2 of the map and the deseriptive
list of dikes. This dike, which is strongly contrasted in
character with all the diabase dikes of this district and certainly
is not, as it appears to be at first glance, a southward continu-
ation of the composite dike (66) separating Cliff Plateau and
East Porphyrite Hill, demands our attention at this time on ac-
count of its important and interesting relations to the first mel-
aphyr, the only melaphyr of the western area. The microscopic
resemblance is certainly very marked, and in his brief descrip-
tion of the microscopic characters (page 37), Mr. Merrill
has cited no fact inconsistent with the view that the dike is a
more crystalline and less altered form of the same original rock
as the flow. Attention was first attracted to this dike, as re-
gards its relations to the effusive melaphyr, in seeking an ex-
planation of the melaphyr which is mingled irregularly with the
first or basal conglomerate on the eastern edge of Granite
Plateau. This ledge of conglomerate and melaphyr is only a
few feet west of the direct course of the dike ; and the question

naturally arose, Why may not these irregular intrusions be

92

regarded as branches from this dike? То actually trace the
connection is impossible, on account of the marsh; but I have
endeavored to show its probability by demonstrating more fully
the identity of this melaphyr with the dike and also with the
surface flow. To this end, characteristic specimens of mela-
phyr from—I. the flow at the base of West Porphyrite Hill,
II. the flow at the base of Great Hill, III. the irregular intru-
sions in the basal conglomerate, and IV. the great dike, have
been submitted to chemical analysis, through the kindness of
Dr. T. M. Drown of the Massachusetts Institute of Technology.
Four closely accordant determinations of the silica and of the
iron oxide (Fe,O,) and alumina were made for each sample by
the chemists of the class of 1892, with the following average
results :—

I II II IV
SiO, 47.97 51.05 47.29 54.47
Al,O,+ FeO; 30.50 30.16 30.20 25.25

Although these results, especially for the silica, do not agree
so well as could be desired, the differences should not excite
surprise, when we consider that, since these rocks are all highly
altered, and filled with quartz, epidote, and other secondary min-
erals, it is well-nigh impossible to select average or normal
specimens. Probably four specimens from the same mass
(dike or flow) would show similar differences. Regarding,
then, these different outcrops of melaphyr, of whatever form,
as essentially identical in macroscopic, microscopic, and chemi-
cal characters, we may reasonably suppose that they are con-
temporaneous, and that the great dike is one of the vents at
least, if not the only vent, through which the sheet of melaphyr
overlying the basal conglomerate reached the surface. The
fissure is straight and regular in the granite; but on passing
into the then unconsolidated gravel of the basal conglomerate
it naturally became very irregular, the melaphyr and gravel be-

coming inextricably mingled, as observed on the eastern edge

d

g

98

of Granite Plateau. The faet that we cannot trace this dike
above the basal conglomerate is thus readily explained ; for this
conglomerate was then the surface deposit, the newest sediment
upon the ocean floor. "Taking all these features into consider-
ation, it is doubtful if there can be found elsewhere in the Bos-
ton Dasin во clear an exhibition of a complete eruption—com-

plete in both its intrusive and extrusive phases.

Correlation of the Central and Western .Areas.—
Up to this point the stratigraphy of the western area presents
no serious or insuperable difficulty, each group of ledges pre-
senting certain recognizable features common to some of the
others. But on attempting to extend this correlation to the
central area the case is very different; for the break between
Great and Crescent Hills, the only point where the two areas are
in contact, is so profound that at first glance the two sections
seem to have nothing in common. The peculiar first melaphyr
and the porphyrite, which are the principal keys to the western
area, are certainly wholly wanting in the central area. I have
already stated that the heavy bed of conglomerate forming Con-
glomerate Plateau, Round Hill, and, probably, the ledges to
the eastward as far as Conglomerate Island and Green Hill,
should, apparently, be correlated with the conglomerate under-
lying the lower amygdaloidal melaphyr at the northern end of
the section along the railroad. If this is done, then, while the
western area consists, exclusive of the fundamental granite and
Melaphyr Peninsula, of three beds of conglomerate and two in-
tercalated flows of lava —one basic and one acid ; the central area
is made up of two beds of conglomerate, each of which is over-
lain by a flow of amygdaloidal melaphyr. The base of the lower
and prineipal conglomerate is nowhere exposed ; but I see no
reason to doubt that it rests upon the porphyrite, and is the
equivalent of the third conglomerate in the western area. This
interpretation of the facts has the merit of simplicity, and it

leaves no observed facts unexplained. The summit of the

04

western section becomes the base of the central section, the two
together ineluding four beds of conglomerate and four beds of
lava.

Strictly speaking, the basal conglomerate is not wanting in
the central area, since it is undoubtedly represented by the
patches of clastic material on the granite south of Round Hill;
and this hill thus admits of comparison with Great Hill. The
relations to the granite and the boundary fault are the
same ; but the Round Hill conglomerate is the third bed and
the Great Hill conglomerate is the second bed.

The satisfactory correlation of the central and coastal areas
presents still greater difficulties, and appeared practically im-
possible, until, fortunately, a partial clue was afforded by the
ledges on Rocky Neck. The completion of the Nantasket
column must, therefore, await the detailed description of the
Rocky Neck section; and it will suffice here to state that all
the melaphyr of the coastal area and the underlying conglomer-
ate of Long Beach Rock belong above the highest bed, the

third melaphyr, in the central area.

Melaphyr Peninsula.—This most northerly and most nearly
isolated part of the western area is the only part that does
not admit of ready correlation without appealing to external
evidence. For this reason, and because it is probably the new-
est part of the area, its description has been reserved to the
last. Melaphyr Peninsula is quite certainly made up of two
prineipal flows of highly basie and amygdaloidal melaphyr.
The northern and older flow shows, at the west end, three fairly
distinct bands of coarse amygdules, averaging about. three
feet thick. The lowest is near the water, and the
second is separated from it by three feet and from the third
by two feet of compact melaphyr. The exposed thickness
of this flow is about 15 feet, and it is clear that we
see only the superficial, vesicular part of it. Immediately

ast of the dike (675, the second and third bands of amygdules

95

are united; but farther east they diverge again and for a
hundred feet or more are separated by from one to four feet of
compact melaphyr. Beyond that they seem to unite ; but other
amygdaloidal bands appear below them, and toward the east
end there are from three to five indistinct bands. These bands
may, perhaps, be regarded as suecessive waves of lava, and
essentially parts of one flow. At many points the amygdules
are arranged in distinct sheets or lines parallel with the main
bands, giving rise to а very beautiful flow-strueture, and in
none of the Nantasket ledges is the flow-structure more per-
fectly developed than here. Both the lines and bands dip S.
5°—10°, the strike being N. 80° E.

The amygdules range from pin-heads to one inch in diameter,
and average fully one fourth inch. They are chiefly composed
in order of importance of: (1) whitish, compact epidote (mixed
feldspar and epidote) ; (2) quartz ; (3) feldspar, which is often
reddish; (4) chlorite; and (5) bright green epidote. The
compact, whitish epidote also forms many irregular veinlets,
which conform with the flow-structure, or cut it at all angles,
and pass insensibly into the large amygdules. In all these
segregations, the quartz was the last mineral to be deposited.
Quartz has not been observed in the same amygdules with
chlorite, but feldspar, epidote, and quartz were always depos-
ited in that order. The sequence of the minerals in the amyg-
dules is quite varied and the following, stated in the order of
abundance and beginning on the outside in every case, have
been observed: (1) mixed feldspar and epidote, and quartz;
(2) feldspar and quartz; (3) feldspar, epidote, and quartz ;
(4) epidote and quartz; and (5) mixed epidote and feldspar,
and chlorite. Occasionally bright red jasper occurs with the
quartz. The amygdules were carefully examined to see if
their elongation and orientation throw any light upon the direc-
tion of the flow. The smaller amygdules are mainly spherical ;
and the larger ones, although elongated, are usually very irreg-
ular. So far ав the evidence goes, it favors the view that the

lava eame from the east.

96

The second flow of melaphyr begins with about fifteen feet of
compact, green melaphyr overlain by five to ten feet, in one to
several bands, of amygdaloidal melaphyr. It is similar to the
first flow, except that the amygdules are not го crowded and
the bands are not so well defined. These flows are strongly
contrasted with, and undoubtedly much newer than, the flow
forming the base of West Porphyrite Hill, one hundred feet
away. The isolation of this outcrop makes an absolutely safe
correlation impossible. It is reasonably certain, however, that
it should be referred to either the upper or lower amygdaloidal
melaphyr of the central area. It looks like a block detached
from Melaphyr Plateau: in fact the lithological resemblance
is almost perfect. And this view is strengthened by the low-
tide ledges of precisely the same melaphyr which stretch away
to the eastward, in the direction of Melaphyr Plateau, and
virtually extend Melaphyr Peninsula as far as the base of Kast
Porphyrite Hill. It is a foregone conclusion, then, that the
short beach connecting Melaphyr Peninsula and West Porphy-
rite Hill conceals an important fault with the downthrow to the
north, an extension, apparently, of the line of displacement
dividing the coastal and central areas. The slip can searcely be

less than 150 feet, and may easily be 100 feet more.
Rocky Neck.

The part of Weir River Bay separating Rocky Neck from
Nantasket is not a geological boundary; for the western shore
repeats, with slight variations, the geological conditions with
which we have become familiar on the eastern shore. These
variations are, however, of some moment, since they supply miss-
ing links in the chain of evidence essential to the correlation of the
Nantasket strata. But although Rocky Neck is simply a de-
tached part of Nantasket, we can go no farther west in the study
of Nantasket geology ; for not only 1з a natural boundary pro-

vided in the drumlins of World's End, Planter's Hill, and Pine

97

Hill, but beyond these we encounter, in Hingham Harbor and
northern Hingham, structural features. of a radically distinct
type.

The eastern base of Planter’s Hill is the typical, coarse, and
somewhat reddish granite, which extends eastward in numerous
ledges to Weir River Bay. Advancing north along the west
shore of Rocky Neck, we find the granite, near dikes 22 and
23, overlain by the basal conglomerate. It is largely com-
posed, as on the Nantasket side, of the more or less angular de-
bris of granite and felsite, including, however, few large frag-
ments, and it has, in consequence, a prevailing reddish tint.
Erosion has evidently again cut just to the base of the con-
glomerate over a considerable area; and, as on Granite Plateau,
several thin patehes have been isolated by unequal erosion.
These outliers show that the conglomerate conforms in the
usual manner to the fissures and other inequalities of the origi-
nal granite floor. The dip is S. E. 0°-10°. The granite rises
from beneath the conglomerate on the northwest, forming
Granite Head ; while on the south the conglomerate is certainly
cut off by an oblique fault with a decided upthrow on the
south. Hence the conglomerate is naturally thickest toward the
east, where the ledges overlook the little valley running north-
west to the bay north of Granite Head. In this natural section
the conglomerate encloses considerable layers of sandstone and
even slate, and the bedding is very distinct. The fault just
referred to divides the basal conglomerate into two principal
areas; and in going castward the conglomerate again appears,
first in small outliers capping the granite ledges, and then, ар-
parently, in a continuous bed reaching to the shore. This is a
drift slope, however ; and the outerops are, perhaps, insufficient
to make this inference entirely safe.

These masses of granite and overlying conglomerate must
be bounded on the north by a fault transverse to the strike
of the conglomerate and reaehing from Granite Head to the

o

”
7

OCCAS. PAPERS B. S. N. H. IV.

98

eastern shore, the continuation of the boundary fault on the
Nantasket side. The downthrow is, of course, to the north,
and evidently greatest toward the eastern shore; for north of
this fault, on the western shore, we again find the granite and
basal conglomerate. The granite rises somewhat: abruptly to a
height of from twenty to thirty feet on the west end of the hill,
and is immediately overlain by the conglomerate, matching Gran-
ite Point on the Nantasket side. The hill is precipitous at this
end on both the north and south sides, being bounded by fault-
scarps ; and these natural sections show that the granite is cut
off in its eastward extension beneath the conglomerate by a
north-south fault having an easterly throw of perhaps 10 or 20
feet (Fig. 12). This fault appears to end in the east-west
faults, and accounts for the form of the conglomerate outcrop, as
shown on the map. The basal conglomerate on this hill is
similar to that south of the boundary fault, except that the
pebbles include a large proportion of compact forms of mela-
phyr, and the red jasper is more conspicuous. The dip is S. E.
5 10% equivalent to a thickness of thirty or forty feet.

W. E,

Granite. Conglomerate. Melaphyr.

Fic. 12.— AN BAST-WEST SECTION ACROSS ROCKY
NECK, NORTH OF THE BOUNDARY FAULT.
SCALE, I INCH = 400 FEET.

The conglomerate is certainly separated from the melaphyr
on the southeast by a fault, which is probably an extension of
the southwest- northeast fault in the granite, already referred to,
but with the throw reversed (Fig. 12). This melaphyr is the
compaet, green and purplish, jaspery variety—the first melaphyr
— which normally overlies the basal conglomerate; and it is

99

indistiaguishable from that flow on the east side of the bay.
It is never amygdaloidal, but often somewhat breeciated. The
fault just referred to must, therefore, downthrow to the south-
east, but probably not more than 20 or 30 feet, possibly less.
Doth this displacement and the preceding one, parallel with it,
are evidently added to the boundary fault, so that this now
becomes sufficient to conceal entirely the basal conglomerate on
the downthrow side. The first melaphyr is overlain in regular
order by the second conglomerate (Fig. 12), which is thus,
in consequence of the increased displacement, brought vis à vis
with the strongly contrasting basal conglomerate (Fig. 13).
The second eonglomerate contains so much melaphyr debris
derived from the underlying flow that it is not always easily
distinguished from the brecciated portions of the melaphyr. It
is also the richest in red jasper of all the Rocky Neck strata,
agreeing perfectly in this respect with the same bed in the
porphyrite hills of Nantasket. This highly jaspery conglom-
erate is well exposed on the shore for nearly two hundred feet,
before it gives way to the underlying melaphyr, which extends
150 feet farther, but bears several distinct outliers of the con-

glomerate, as mapped. These outliers are seen to rest upon

а very uneven surface of the melaphyr; but the normal dip

is plainly S. E. 10° or more. At the northern edge of the
nielaphyr it seems to end quite abruptly ; but the narrow beach

between it and the puddingstone on the north is partly occupied

by the firm, jaspery, second conglomerate,— another outlier
let down by faulting.

We have now reached the north end of the northeast-south-
west fault which has already been seen to bound the first mela-
phyr on the northwest. It appears to be a double fault at
this end (Fig. 13) ; and following 1t southwest, we find that it
encloses not only the depressed outlier of the conglomerate,
but also an isolated ledge of the melaphyr with traces of the
conglomerate upon it.

The hill or ridge just described, composed of the first and

100

second conglomerates and the first melaphyr, and terminated
by the granite on the west shore, is bounded on the precip-
itous northern side, as previously stated, by a fault, which, as
the map shows, coincides with a dike (30). The dike can only
be traced about half-way to the N. E. — S. W. fault, passing
under the grass; but both the dike and its displacement un-
doubtedly continue until the faults meet, as mapped. This
dike-fault and the part of the N. E. — 5. W. fault northeast
of the junction may be regarded as one continuous fault,
with the downthrow on the north. That it is an important
displacement is evident from the fact that the rocks north of
this line are wholly different from those south of it. There is
first a broad area of non-jaspery conglomerate — а normal
puddingstone — sloping gently to the southeast and dipping
in the same direction 5°-15°. It forms the eastern shore for
about 100 feet; but the true thickness of the bed probably does
not exceed 25 feet. This conglomerate is underlain con-
formably and very clearly by typical, basic, green, amyg-
daloidal melaphyr, which is identical with that on Melaphyr
Peninsula and Melaphyr Plateau (Fig. 13). . The melaphyr
outerops broadly around the conglomerate and the exposures
of the contact are all that could be desired. The contempo-
‘ancous origin of the melaphyr is sufficiently demonstrated by
the fact that near the base of the conglomerate, especially, it
encloses many water-worn fragments, from three to eighteen
inches in diameter, of exactly the same kind of melaphyr. The
melaphyr plainly consists of two flows, each of which is amyg-
daloidal above and compact below, with an aggregate thickness
of forty or possibly fifty feet; and this fact seems to identify
it with Melaphyr Peninsula, and with the upper flow in the
Crescent Hill section — the third melaphyr. — If this correlation
be made, the displacement along the dike-fault can scarcely
fall below 200 or 250 feet.

The east-west fault crossing the summit of the. hill, with a

southerly throw of ten feet, is very clearly exposed on the west

4

101

Granite. Conglomerate. Melaphyr.

FIG. 13.—A NORTH-SOUTH SECTION ACROSS Rocky NECK.
SCALE, I INCH — 400 FEET.

side, where it breaks the contact of the conglomerate and mela-
phyr (Fig. 13). It is nearly parallel in direetion, but opposed
in throw, to the fault bounding this hill on the north. The mela-
phyr and conglomerate rise here in a sloping cliff of 20 to 40
feet, extending the entire length of the hill and overlooking
the small plain which forms the extremity of the Neck.

The melaphyr north of this fault-scarp is similar to that south
of it; but itis underlain on the west by a limited bed of con-
glomerate, which, dipping S. E. 2*—5*, must abut against the cliff
of melaphyr, although the contact is not exposed. This proves
the fault, for the northern conglomerate, rising toward the west,
could not otherwise fail to appear in the western half of the
cliff. This conglomerate is of normal character, with an average
thickness not exceeding ten feet. It is plainly cut on the north
shore by a north-south fault, sinking the conformable contact
with the overlying melaphyr about eight feet on the east. The
conglomerate is, in its turn, clearly underlain on the west by a
non-amygdaloidal melaphyr, of which about twenty feet are ex-
posed at low tide. Superficially, at least, this melaphyr is highly
brecciated, so that it is not quickly distinguished from the con-
elomerate. It encloses numerous masses of compact melaphyr
similar to the matrix, more or less rounded, and of all sizes up
to eighteen inches in diameter. It is probably not a conglom-
erate or tuff, but a very coarse original brecciation, something
similar to the melaphyr holding the pseudo-bombs on the north
side of Atlantic and. Centre Hills. The entire rock is similar
in color and texture to the melaphyr of Long Beach Rock and

102

the eoastal area generally ; and it seems impossible to correlate
it with anything else in the Nantasket region. But in that case
the conglomerate and the highly amygdaloidal melaphyr over-
lying it must be referred to the very summit of the Nantasket
seetion ; and this amygdaloidal melaphyr north of the fault-scarp
їз thus widely separated in time from the very similar melaphyr
south of the fault. This view also implies the recurrence at the
top of the Nantasket section of the basic flows characterizing
the middle portion of it. |

According to the foregoing interpretations, the Rocky Neck
section, as exposed to observation, is evidently far from com-
plete or continuous; but the following list of the Nantasket
strata will show more clearly than either the map or sections
just what the deficiencies are :—

Granite.

l'irst conglomerate, basal.

First melaphyr, compact and jaspery.

Second conglomerate, highly jaspery.

Porphyrite, wanting.

Third conglomerate, wanting.

Second melaphyr, wanting.

Fourth conglomerate, wanting.

Third melaphyr, amygdaloidal.

Fifth conglomerate, normal puddingstone.

Fourth melaphyr, compact and brecciated.

Sixth conglomerate, ten feet.

Fifth melaphyr, amygdaloidal.

On referring to the map and sections (Figs. 12 and 13), it be-
comes obvious that this great break in the middle of the series
is due wholly to the east-west dike-fault, which, as previously
explained, divides Rocky Neck into two areas having nothing
in common so far as the stratigraphy is concerned. This im-
portant displacement is quite clearly a continuation of that sep-
arating Melaphyr Peninsula from West Porphyrite Hill and
dividing the central and coastal areas,

Sh tr

— —

103

The porphyrite is, possibly, not wholly wanting on Rocky
Neck ; for the small point jutting into Weir River Day south of
the basal conglomerate and mapped as granite, consiste of angu-
lar masses, probably overlying a ledge, of a semi-erystalline, red
rock, apparently quartzless, and resembling the Nantasket por-
phyrite. Its rather crystalline texture would indicate a dike of
porphyrite breaking through the granite ; but if it is effusive, a
surface flow, important bounding faults must be assumed to ac-
count for its topographic position with reference to the granite.

The Nantasket Peninsula North of Atlantic Hill.

As previously stated, the only exposure of the underlying
rocks north of Atlantic Hill is the ledge of slate on the shore,
south of Thornbush Hill; and with the exception of thé ledge
of green and gray contorted slate on the railroad northwest of
Rockland Hill, which has been doubtfully referred to the prin-
cipal bed of tuff in the Atlantic Hill section, this is the only
considerable outcrop of slate on the Nantasket Peninsula. It
is exposed along the shore for about 900 feet and with an ex-
treme breadth across the strike of perhaps 400 feet. It is a
‘ather coarse or arenaceous, purplish slate, distinctly and finely
banded with gray, and occasionally with greenish, layers. The
strike is N. 65°-70° E. ; and the dip is variable, but always low.
In the western ledges it is S. 10°-15° or 20° at the most; but
in the middle ledges the beds are approximately horizontal,
broad contortions or undulations showing gentle dips in various
directions ; while the eastern ledges show a northerly dip of
10°-20°. This is evidently an oblique section of a flat anti-
cline. It includes no pebbly or distinctly arenaceous beds;
and the exposed thickness of the slate, although difficult to es-
timate, probably does not exceed 40 or 50 feet. The slate is
characterized throughout by a distinct cleavage-structure, which
strikes N. 80° E., and hades, usually to the north, occasionally
to the south, 0°-5°, being always transverse to the bedding

104

planes. Somewhat prominent joint-planes hade S. about 45°.
Dy taking advantage of the lowest stage of the tide, it is pos-
sible to add a little to the meagre knowledge of the geology of
northern Hull afforded by the ledges just described. The
Toddy Rocks, one fourth mile north of Telegraph Hill, were
visited with a boat at low tide and found to be ledges of a
bright green slate, with very perfect cleavage, which strikes
№. 75° E. and hades to the north 0°—5°. The true bedding
lines dip north at low angles, 10°-20°. Some thin and very
hard compact layers of a gray color are probably intrusive
(trap). This outcrop is evidently essentially similar to that
south of Thornbush Hill; and slate of the same general char-
acter is the most abundant rock in the drumlins and on the
stony beaches of northern Hull. Bowlders of conglomerate
and sandstone are so rare as to indicate that they have prob-
ably been transported by ice; and we hazard little in reg: rding
the part of Hull between Point Allerton and Windmill Point
as having a continuous foundation or axis of slate. One and
one half miles east of Point Allerton, and fully a mile south of
the line of strike of the slate at the foot of Thornbush Hill, is
Harding's Ledge. This is partly bare at low tide, exposing a
hard grayish and greenish sandstone varying to a fine conglom-
erate. It is an undoubted ledge ; and these rocks are probably on
the line of the anticline of Hough’s Neck, which we may sup-
pose to extend by the north side of Grape Island, Bumpkin
Island, Strawberry Hill, and Strawberry Ledge to Harding’s
Ledge. It is manifestly impossible, without going beyond the
limits of Hull, to reach any definite conclusion concerning the
relations of the slate beds to the conglomerate and melaphyr
series. It is probably safe to assume, however, that here, as
elsewhere in the Boston Basin, these soft and arenaceous slates
normally overlie the conglomerate, the latter appearing, as a
rule, only along denuded antielines. According to this view,
not only northern Hull, but the entire peninsula north of
Atlantic Hill, is underlain chiefly by slate; and to account for

105

the broad exposure of the conglomerate series at the southern
end of the peninsula we have only to add one more to the east-
west displacements with the downthrow on the north which we
have observed in the Nantasket ledges, the fault crossing the
beach near the base of Atlantic Hill.

| THE DIKES OF NANTASKET.

The dikes of this district, as previously stated, consist almost

wholly of diabase and are more recent than the bedded rocks.

The existence of dikes of melaphyr and porphyrite, at least in

the granite and the older strata, is, however, probable, с priori;

and two dikes of melaphyr have aetually been recognized in the

Nantasket ledges; while in the granite ledges along the Co-

hasset shore there is a fine series of porphyrite dikes. The

dikes of melaphyr and porphyrite clearly antedate all the dia-

1 base dikes, dating, probably, from the effusive eruptions to

| which they correspond in composition ; and they will, therefore,

| be deseribed first, in each topographic division. Since the

special map shows so clearly the location and approximate

trend of each dike, and the descriptive lists give in addition the

exaet trend and the hade, width, and other features, it is only

necessary to cite here such additional facts as may be of particular
interest.

The Melaphyr Dikes.

No. Trend. Hade. in Remarks.

| | E-W.
2 | 8, 09-809. E.

Vertica 3-7 | Irregular in form and hade.
ADM 32 Regular in form and uniform in

| Width
|
| texture.

The two dikes belonging to this class have been referred to
in connection with the associated flows of melaphyr. Тһе first
one (No. 1) breaks through the breceiated and compact green-
ish gray melaphyr on the north side of Centre ПШ. Except in

106

being more compact and homogeneous, it is very similar, litho- |
logically, to the enclosing melaphyr ; and hence is quite incon-
spicuous and not easily found. The outcrop begins on the
north side of the road immediately east of the Waverly House
and ean be traced due east several hundred feet to the shore.
Ft is somewhat irregular in form, and varies in width from
three feet or less to seven. feet, narrowing eastward. “The hade
is also variable, but averages about vertical.

Its compactness, similarity to the enclosing melaphyr, and
irregular outline are the characters that especially mark this as
an approximately contemporaneous dike. The melaphyr of the
dike is firmly welded to that of the walls, and the contacts are
clearly neither joint-planes nor fault-fractures. Although this
is the only undoubted dike of melaphyr that I have observed in
the Nantasket flows, the eye is frequently attracted by sharply
defined, vertical, dike-like contacts between the ordinary brec-
ciated, amygdaloidal or scoriaceous melaphyr and more com- |
paet forms; but as a rule these cannot be traced far, ог only
one wall can be found, and it is quite clear, in most cases at
least, that they are not dikes in the ordinary sense.

'The second dike (2) of the list is much more impressive and
interesting than the first, being decidedly the largest of all the
Nantasket dikes. It cuts the granite south of the west marsh
with a nearly due north-south trend and a width of 32 feet. Its
interesting relations to the effusive melaphyr of the western
area, and its marked change in form on passing from the gran-
ite to the basal conglomerate on the eastern margin of Granite
Plateau, have been fully described (page 77) ; and in these
respects, that is, as a clearly exposed volcanic vent the outflows
from which can be fully identified, it is, perhaps, unique among
the dikes of the Boston Basin. It is exposed, as the map -
shows, on two islands in the marsh, forming the western edge
of one and the eastern edge of the other. It commences on the
north with a due north-south trend, but on passing to the
southern ledge it changes to S. 10? E., and then to S. 80° E.

—

107

It can be traced almost to the northern shore of the bay; but no
trace of it could be found in the almost continuous ledges of
granite on the south shore. Its last observed course, however,
would сату it directly up the mouth of Weir River; and
the absence of additional outerops is not, therefore, evidence of
an important displacement. That this great dike should not,
on account of its trend, be referred to the third or newest
system of diabase dikes is obvious not only from its lithological
character, but also and especially from the fact that it is clearly
eut by, and hence is older than, one of the east-west diabase
dikes (39), the outcrop of the latter continuing directly
through the melaphyr dike.

The Diabase Dikes.

The occurrence of the diabase dikes in three systems — two
older systems having approximately east-west trends and a
newer north-south system — has been noted; and also the fact
that the east-west systems form three belts, corresponding in a
general way with the principal dislocations of the district. All
of the dikes agree in direction with well-determined faults,
although often not actually coinciding with a fault of sensible
displacement. The two east-west systems are certainly much
more closely related to each other than to the north-south
system, and since, in the absence of clearly exposed intersec-
tions, they are not always readily distinguished, it appears best
not to attempt to. describe them separately ; but we will,
instead, follow the classification by belts, commencing with the
northern or coastal belt.

Although Mr. Merrill has not observed any marked or
constant difference in mieroscopie characters between the older
or east-west dikes and the newer or north-south dikes, they are
usually somewhat contrasted in external aspect. The east-west
dikes are commonly of a dark greenish gray color, due to the
abundance of secondary chloritic minerals and epidote. They
are, relatively, chemically stable, resisting atmospheric influ-

108

ences and preserving glacial striae almost as well as the mel- |
aphyr and granite ; and they rarely weather brownish, or show
in any marked degree peroxidation of the ferruginous
compounds. Finally, they are, as a rule, irregularly jointed,
exhibiting only exceptionally a regular transverse or prismatic
joint-strueture. The north-south dikes, on the other hand, are
usually black on a fresh fracture and not conspicuously chloritic 4
or epidotie ; they weather readily to brownish masses of rounded

outline and later to a brown earth; and a well-developed cross

or columnar jointing is often a conspicuous feature and never

wholly wanting.

Hast West Dikes of the Northern or Coastal Belt.

Width
No. Trend. Hade. in Remarks.
feet.
3| N. 759-809 E.] Vert.-N.109| 2.5-8.5 | Finely porphyritic.
4 N. 75 B. 8. 159-209 3 Slightly porphyritic.
0 N. 805 BN 8. 5° 3-5 Double dike; cut by 11. \
618.1807 B Vertical 8—4 Jogged or faulted in crossing
beach.
УЫ SOUP EN Vertical 15-20 Walls irregular and faulted.
8 | 8, 829 E N. 25° 2.75-8 | Cross-jointed.
9. | 8. S09 . Vertical 10 Probably divided or composite /
toward the east.
10 | S. 759-809 E. | Vert.-N. 150 4-5 Irregular in width and hade.
iT | 8. 7692809 B. | N. 107 2.5 Encloses fragments of granite.
12 8.75" E. Vertical 8.5-4.5 | Encloses fragments of granite

апа syenite.

18 S. 759-809 E. | Vert.-N.10° Very irregular at east end.
14 | S. 759-809 E. | N. 109-209 Very irregular and broken.
10 ЙАА Ba N. 59-109 —47 No satisfactory exposures.
16 | S. 759-809 E. | Vertical 4 Quite irregular in trend and
hade.
17.79. S07 0. №. 109-15? | 4-5 Regular; probably a branch of
18.
18 | S. 759-80? Е. | N. 45° 8 Regular; unusually strong
hade.
19 S. 769-80? E. | N. 80? 8-9 Very irregular апа branching.
)
D

The dikes of the coastal belt present many interesting and in-
struetive features. Ав the map plainly shows, they form a
well-defined belt coinciding with the immediate shore-line ; and

109

it is very certain that their non-occurrence over the broad area
of melaphyr forming Atlantic and Centre Hills cannot be
attributed to the absence of exposures. Only three dikes in this
belt (3—5) belong to the first or oldest system ; while no fewer
than fourteen (6-19) find a place in the second system. The
older dikes will be described first and, so far as possible, in the
topographie order of the outerops.

On the northeast corner of Atlantic Hill, the melaphyr south
of the great dike (7) is, for a breadth of nearly twenty feet,
very shaly, being divided into thin vertieal layers running N.
80° E. Careful examination discovers in the middle of this
shaly zone a dike (3) having the same trend. It must meet
the great dike near the east side of Valley Beach ; and is prob-
ably eut and slipped by it, although the intersection is not
exposed; while to the westward it passes under the shingle
beach. It is finely porphyritic, and reappears with the same
lithological character, width, and hade on Gun Rock and the
half-tide ledge to the eastward.

From 25 to 75 feet south of the melaphyr dike (1) on
Centre Hill, and extending from near the end of Centre Hill
Avenue across the ledges to the shore, is the narrow chasm
holding the very obscure outcrops of the next dike (4), the atti-
tude and dimensions of which are taken from the fissure rather
than from the dike itself. It trends toward the middle of Gun
Rock, but no traces of it could be found on that promontory.
Nearly 150 feet south of this dike is a broader chasm or de-
pression, in which a small dike occurs, or possibly in the lower
end of the chasm two small parallel dikes with several feet of
melaphyr between them. Looking eastward across the bay we
see in line with this depression a more sharply defined chasm in
the first ledge south of Gun Rock. Here the double dike is
well developed in a fissure five feet wide, the south dike meas-
uring 2.5 feet and the north dike 1 foot. Keeping this course,
we find the same double dike cutting the ledge east of Gun

Rock. The two members here measure 3 and 1.5 feet respec-

110

tively, with 3 feet of separating melaphyr. The dike is thus
widening eastward, and it undoubtedly begins оп Centre Hill
near where it is first observed. In the third or eastern outcrop
it is very plainly cut by a dike of the second system (11), this
intersection (Fig. 14) being the one chiefly relied upon to
determine the relative ages of the two systems

We pass now to the dikes of the second system, commencing
with the three typical examples crossing Long Beach Rock.
The most northerly of these (10) is about 50 feet from the
north end of the ledge at low tide. It crosses the strike of the
conglomerate, but the intersection is under water. About 60
feet of the melaphyr separate this from the large dike (9),
which crosses the eastern end of the conglomerate, and is there
seen to be attended by slight faulting, the contact between the
conglomerate and melaphyr being displaced in such a way as
to show a downthrow to the north of about 5 feet. The third
dike (8) is 120 feet south of this and crosses the conglomerate
contact without apparent faulting. It is, perhaps, the most
distinctly cross-jointed of all the east-west dikes. These three
dikes appear to cross the low-tide ledge north of Valley Beach ;
and 10 appears again on the south side of Gun Rock, but with
a northerly hade of 15°. In the place of 9 we find on the shore
south of Gun Rock a series of six small parallel dikes, with an
aggregate thickness of 9 feet, which appear to be the equiva-
lent and continuation of it. But 8, if it extends this far,
must pass under the beach.

The shore along the northern base of Atlantic and Centre
Hills appears to be determined immediately by the great dike
(7), which must also pass under the beach south of Gun
Rock. It shows several small faults or lateral shifts of from 3
to 8 feet at the base of Atlantic Hill, and suffers a displace-
ment of perhaps 10 feet in crossing Valley Beach, passing
beneath the foot-bridge in front of the Waverly House. The
smaller dike (6) parallel with this is well exposed in front of
the Pacific House, but cannot be traced more than half-way to

111

the Waverly House, although there are plenty of ledges on its
course. It is quite possible, however, that it passes under the
street, in front of the Waverly House, as far as Valley Deach.
It is perhaps possible, also, that this dike cuts 4 and 5, but
more probable that they end before reaching it. On crossing
the beach east of the Pacific House, this dike is seen cutting
through the first ledge, near the road. Its characters are
unchanged ; but in erossing the beach it is evidently jogged or
shifted to the south about 25 feet, as if a transverse fault inter-
vened (see map).

Gun Rock, with its broad surface of bare melaphyr, is an
uncommonly favorable point for the study of dikes. As
already pointed out, it embraces two dikes (3 and 5) of the
first system and five dikes (9—18) of the second system.
Beginning on the west shore we have first the double dike (5)
in a fissure about five feet wide. About 23 feet north of this
begins the series of small dikes supposed to be the extension of
the large dike (9) on Long Beach Rock. The following suc-

cession is readily made out :—

(1) Diabase, 0.5 feet. Separating melaphyr, 2 feet.
(2) "M XO A 10 n tara
(3) ч 80 ү gius
(4) М O e Wi MEL
(5) " TOREM "T М uu
(6) bs ко"

9 feet. 11 feet.

Some 75 feet farther north, at the base of the steep slope, is
the extension of 10; and about 100 feet of melaphyr separates
this from 11, which hades N. 10° and, passing under the house
at the top of the cliff, reappears оп the east side of the ledge,
and again in the same direct line on the ledge east of Gun
Rock, where it cuts the double dike of the first system (Fig.
14). The intersection is clear and well exposed; and, so far

as one intersection can, it establishes the relations of these two

112

/

systems. It is a curious fact, however, that of the ten possible
intersections of the dikes of these two systems on or near Gun
Rock, only this one is actually exposed above the water.

—

—

— e 7

FIG. 14.— PLAN SHOWING THE INTERSECTION OF DIKES ON
THE SHORE, EAST OF GuN Rock, AND THE ENCLOSED
FRAGMENTS IN DIKES II AND 12. SCALE, I INCH ==25 FEET.

The next dike in regular order is the continuation of 3 of the
first system, and the exposure is perfect from one side of the
promontory to the other. About 40 feet north of this dike,
and parallel with it in strike and hade, is an interesting quartz
vein one inch or a little more in width. . It is perfectly straight
and continuous, its outcrop dividing the entire mass of Gun
Rock by one sharply-defined white line. Dike 12 of the second
system is about 120 feet north of 11; and nearly 20 feet
farther north is 13, a parallel and much smaller dike. Dike 13
is closely parallel to 11, appearing just to clear the extremity of
the ledge east of Gun Rock and to be continued in a similar dike
(13) on Green Hill Ledge. But dike 12 takes a more diagonal
course, being, on the eastern ledge, 100 feet from 13 and only

42 feet from 11. The large dike (12) is, perhaps, the most

Ney

118

interesting of the series. It crosses Gun Rock and all the
ledges to the east, increasing in width from 3 to 4.5 feet. On
the summit of Gun Rock it makes a clear intersection of the
quartz vein just described, without faulting it. Throughout
its entire extent this dike has the appearance of being very
coarsely porphyritic along a zone from six inches to a foot wide,
between the middle of the dike and the south wall. Careful
examination shows, however, that it is not truly porphyritic,
but what appear to be indigenous feldspars are really angular
fragments of a coarsely crystalline feldspathic rock which is,
apparently, partly granite but mainly syenite. The fragments
range from the smallest size up to six inches or more in length ;
and their true character is most obvious on the eastern ledge
(Fig. 14). Dike 11 presents the same feature for a part of its
sourse. The enclosed fragments in this dike are chiefly coarse
granite ; and they begin very abruptly at a point about 20 feet
from the intersection of the double dike and the same distance
from the shore. They are of all sizes up to six inches and, as
in 12, are crowded in a layer from 6 to 12 inches wide between the
middle and south wall of the dike. Although no granitic rocks
are developed on the surface nearer to Gun Rock than Jerusa-
lem Road, the inference is irresistible that they actually under-
lie Gun Rock; and the restricted distribution of the frag-
ments in the dikes is a very plain indication that the upward
movement of the melted rock in these fissures had nearly
ceased before the underlying granitic rocks were sufficiently
disintegrated to yield this debris.

The intersection of the quartz vein on Gun Rock by a dike
of the second system, and its perfect parallelism with the dikes
of the first system, show that it probably dates from the form-
ation of the first system, or else from some period between
the two systems. South of this vein is another, smaller and
parallel with it, but not observed to meet any of the dikes.

Under the Park House on the summit of Centre Hill are two

OCCAS, PAPERS B. 8, N. II. IV. 8.

114

conspicuous quartz veins from 2 to 3 inches wide, parallel, and
75 feet apart. Curiously enough, however, they conform in

direction with the second system of dikes. This is only one

of several indications that the dikes of the two systems are not

widely separated in time.

The outermost dike on Gun Rock (13) is undoubtedly con-
tinued in the Green Hill conglomerate, as mapped. In the
eastern half of the ledge, both this dike and 14 are especially
characterized by extreme irregularity and lack of continuity,
the conglomerate, apparently, having been repeatedly faulted
after they were formed. One result of this disturbance is that
they converge eastward, so that, although 50 feet apart along
the north-south dike (62) they are almost in contact at the east
end of the ledge. About 50 feet from the extreme northern
edge of the conglomerate and 22 feet north of 14 on 62, but
diverging from it eastward, is dike 15. It is not well exposed,
but it is clearly a dike of the second system. The chief interest
at this point centers in the clear intersections of all these dikes
(18-15) by the single small representative (62), in all the
coastal area, of the third system of dikes.

Dike 16, on the north side of Green Hill Rock, is quite
irregular in trend, but doubtless belongs to the second system.
If so, it is probably parallel with, and passes to the south of,
all the dikes (17-19) on Black Rock. Dikes 17 and 18
converge eastward, their outerops being 50 feet apart at the
west end of the island and almost in contact at the east end.
Dike 19 on the northeast corner of the island ends abruptly on
the west and is much given to irregular branching, one of the
easterly branches splitting a prominent vein of quartz.

sy far the most prominent set of joints in the rocks of the
coastal area trends due north and south. They form or bound
the valley between Atlantic and Centre Hills and are very
marked along the shore of Centre Hill and Gun Rock, and, in
faet, along the entire Nantasket coast, producing the many

fissures and chasms that indent the shore. It is certainly,

acid

Ji

С)

115

therefore, a very interesting fact that the only dike of the

coastal area conforming in direction with these joints is the one

small example of the third system.

East-west joints are also

well developed in the coastal melaphyr ; but they have, with few

exceptions, a decided southerly hade of 20° to 80° ; while the

east-west dikes, almost without exception, are vertical or have

a northerly hade, showing a tendency to be, as a rule, approxi

mately normal to the bedding planes of the melaphyr, tuff, and

conglomerate.

East- West Dikes of the Central Belt.

No. Trend. | Hade.

20! Мег ДА Vert.-N.15'

2L Nd d N. 159-20"

49 NOTA" M Vert.-N. 8

28 | N: 7109-809 E.] Vertical

24 IN TO UN Se

25 М. 809 E N. 20°-25

n N. 5

e N. 15

98 | B, 80° R Vertical

29 | S. 80° E Vert.-N.10'
M, Т0” 2 N. 10
S. 809 Е. N. 200-30
S. 80° E N. 159-20
m Td H N. 169-20*
S. 759-809 E. | N. 409-50
S. 75°-77° Е N. 109-15*

86 | E.-W. N. 59-10

The central belt extends

Width
| їп
feet.

| 6-8.5

Remarks,

North end of Rocky Neck.

Probably a branch of 20.

Probably faulted six times.

Parallel with 22 on Rocky Neck

Porphyritic. Parallel with 22.

Porphyritie; a fault-dike.

Cuts 25 on Melaphyr Plateau.

Porphyritic; a fault-dike.

Regular; and no apparent
faults.

Possibly not one continuous
dike.

An important fault-dike.

Possibly a continuation of 84.

Branch of 81.

Probably a branch of 81.

Double dike.

Unrecognized west of the rail-
rond.

Branch of 35.

from the shore of Strait’s Pond, near

Folsom's Island, across Conglomerate and Melaphyr Plateaus,

and thence along the south shore of Nantasket Harbor to Rocky
g À

Neck.

Doth of the east-west systems are represented in this

belt: but, as before, the second system largely predominates.

Although the exposures are, as a. rule, less satisfactory than in

the coastal belt, individual dikes can, in several instances, be

116

traced much farther, the extreme distance being опе and а half
miles. It is a singular fact that the number of observed dikes
is exactly the same as in the coastal belt.

The most northerly dikes of the first system (20 and 21) on
the northern extremity of Rocky Neck are of similar character,
and, converging eastward, may be regarded as, possibly,
branches of the same dike. The next dike of this system (22)
first appears on the western shore of Rocky Neck with a width of
from 8 to 10 feet ; and only 20 feet south of, and parallel with, it
is the very similar companion dike (28), enclosing a large
wedge of granite. These two dikes can be traced in the same
direct line only 400 or 500 feet at the farthest ; but it is quite
certain that they are shifted to the eastward by the fault between
the
This is clearly the case with the northern one, which may be

granite and conglomerate and continue to the east shore.
safely correlated with the dike of the same number in the
western area, although it is 100 feet too far north to be just in
line with it. Crossing the bay, we find this dike, with a width
of 12 feet, dividing the half-tide ledge of melaphyr off Granite
Point; and striking the shore on the north side of the point,
with a trend N. 80? E., it takes a perfectly straight course to
the southwest corner of East Porphyrite Hill, where it is jogged
to the south its own width and, skirting the edge of the con-
glomerate, is lost beneath the grass. It is probably either
broken here by a fault not marked on the map, or dies out and
is resumed along a parallel line; for what appears to be essen-
tially the same dike begins a little to the northward and, though
cut and shifted by three dikes of the younger system (27, 29,
31), can be traced across the hill to the shore, where it is well
exposed with a breadth of 9 feet. Closely parallel with this
dike, on East Porphyrite Hill, and sharing the same accidents,
but with the intersections less clearly exposed, is 24 ; and this
dike, which is distinctly porphyritic, might safely be correlated
with the dike of the same general character (25) east of the

bay and the railroad, but for the fact that it lies so far to the

2

117

north as to involve a displacement of improbable magnitude.
The latter (25) is a good example of a fault-dike (Fig. 5) and
its northerly hade agrees well with the throw. It is eut on the
northern edge of Melaphyr Plateau by the most northerly dike
of the second system (26) ; and near the railroad both of these
dikes are dislocated by the same transverse fraeture, the hori-
zontal displacement being 7 feet for dike 26 and, on account of
its greater hade, 15 feet for dike 25.

'The fault-dike (27) on East Porphyrite Hill is one of the
most important in the distriet, marking a vertical slip of about
50 feet. It is porphyritic and strikingly similar to 25, east of
the bay ; but since they belong, one to the first and the other to
the second system, it seems best not to connect them. This is,
however, one of many indications that the two systems are not
very distinct, chronologically. The next dike (28), though
presenting precisely the same characters on Rocky Neck and
Melaphyr Peninsula, cannot be identified on Kast Porphyrite
Hill; for it seems best to connect the next dike to the south on
the hill (29) with the dike south of 28 on Rocky Neck. The
last-mentioned dike (29), like 27, breaks the dikes of the first
system on Hast Porphyrite Hill; and it is certainly continued
on the east side of the bay. In fact it can be identified with
reasonable certainty, as shown on the map, from east of Hull
Street to the western shore of Rocky Neck. The outcrop on
the north side of West Porphyrite Hill, however, is quite un-
certain; and it may be that what is regarded here as one
continuous dike may include two or three independent dikes.
In contrast with this dike of exceptional length, the next dike
(30) has been recognized only on the west side of Rocky Neck.
It is interesting chiefly on account of the important displace-
ment which accompanies it, being the only clear fault-dike on
Rocky Neck.

The next dike (31) is especially characterized by its strong
northerly hade. It probably ends westward against the fault
between Cliff Plateau and West Porphyrite Hill. At any rate,

118

it begins abruptly and with its full width on the northern
escarpment of the plateau, not a trace of it or of 33, which is
probably a branch of 31, appearing on the bare ledges of the
hill. It crosses the composite N.-S. dike (66) without per-
ceptible displacement, slips the dikes of the first system оп East
Porphyrite Hill and is itself thrown abruptly to the south about
25 feet at the eastern base of the hill. It crosses the bay with-
out further important displacement and is readily recognized on
both sides of the railroad by its width and hade. It can be
followed by frequent outcrops across Melaphyr Plateau, gradu-
ally approaching and appearing to unite with 29. But they
are widely separated in the conglomerate ledges beyond, 31, on
account of its greater hade, suffering a greater lateral displace-
ment in crossing the fault between Melaphyr and Conglomerate
Plateaus. In the ledges west of Hull Street some interesting

inclusions of conglomerate are clearly exposed. The last ap-

FIG. 15.— NATURAL SECTION OF THE INCLINED DOUBLE
DIKE (34) ON THE WESTERN SHORE OF Rocky NECK.
SCALE, T INCH =O) FEET,

119

pearance of this dike to the e: stward is on the shore of Straits
Pond, more than a mile from its starting point. On Melaphyr
Plateau it appears to give off a branch (32), which ean be
traced for a considerable distance parallel with the main dike.

Granite Head on Rocky Neck is crossed near the northern
edge by a double dike (34) having a northerly hade of 50° and
the following section from south to north: main dike, 7 to 8
feet; granite parting, 6 to 18 inches; small dike, 15 inches
(Fig. 15). It can be traced entirely aeross the head, and
presents the same section and hade in each escarpment. This
dike crosses the valley occupied by the boundary fault without
sensible displacement and reappears on the opposite slope at the
point where it erosses the great N. E.-S. W. fault, which here
downthrows to the southeast. There are indications, however,

that the dike is subsequent to the fault and has simply been

jogged in erossing it; for it is shifted or jogged to the north

some 50 feet in crossing the fault. If the fault were subse-
quent to the dike, the displacement should be in just the oppo-
site direction. The character of the dike is unchanged, except
that it is simple instead of composite, no trace of the smaller
member being observed. On account of the hade, the outerop
curves in passing down the hill to the shore ; but it is not clear
enough to determine with certainty its relations to 22. Crossing
Weir River Day in the same direct line, and apparently without
displacement, it reappears as a double dike with a strong
northerly hade (40?—50^) on the southwest corner of West Por-
phyrite Hill The section here from south to north is: diabase,
3 feet ; conglomerate, 8 feet ; and diabase, 7 feet. The hade is
most marked in the southern member, which is possibly a
branch of the other, but more probably an independent dike ;
for оп Cliff Plateau they are reversed in position, the small
dike, which is now on the north, retaining its stronger hade.
Near the eastern edge of the plateau it suddenly disappears,
apparently joining 22 ; possibly, however, it is continued along

a more northerly line by the other inclined dike (31). The

last of the large dikes of this belt (95) first appears immedi-
ately east of the railroad, at the junetion of the conglomerate
and underlying melaphyr, and сап be traced thence directly
across Melaphyr Plateau, where, passing beneath the private
road, it reappears in the eastern part of Conglomerate Plateau.
The small dike (86) south of 35 near the railroad is, doubtless,
a branch of the latter.

Bast— West Dikes of the Southern Belt.

Width
No. Trend. Hade. in Remarks.
feet.
Dd Na . Nie ШОР 5? Cuts 44 and 45?
08 |N. 70°. B. Vert.-N. 59| 6-7 Porphyritic and branched.
89 | N. 80° E. Nr 6-10? Cuts the great melaphyr dike.
40 | N. 88? E. Vert.-N.10?| 3.5-4.5 | Irregular.
41 | N. 86° E. ee 4.5 Possibly unites with 40.
42 N. 759-809 E.] N. 10° 6-7 In granite; regular.
to | S, 80° E. N. 105 .25-.5 Very regular and continuous.
44 S. 78° E. N. 10° 2.5-4 Ends abruptly toward the east.
45 | S. 759-809 E. | N. ? 2? Imperfectly exposed.
46 | S. 80° E. N. 50-100 10—12 Regular and continuous.
Viena SO B ОЁ 6 )ranch of 44.
48 | S. 85° E. [CUN LOS T M ide SA
e | N. 109 5-1 „„
50 | S. 720 E. N. 159-20? | .5-.75 «t 1
DI S SOT INST if Porphyritic.
52 | S. 759-809? E. | N. 59-109 6-8 ү

On account of the marshes, the dikes of the southern belt
appear much less continuous than those of the central belt;
and they present, on the whole, fewer points of interest.
Although six dikes in this belt аге referred in the table to the
first system, it is doubtful in several cases whether this refer-
ence should stand. "Thus 37, on Great Hill, has the normal
trend of the first system, but it appears to cut two small dikes
(44 and 45) of the second system. The two systems are thus
no longer distinct in both age and trend, as in the coastal area.
Dike 38 has the extreme trend of the first system (N. 70° E.).
It appears first on the west shore of Weir River Bay and can
be traced about half-way across Granite Plateau, or until it

— wy

у

—h

121

meets the great dike of the second system (46). It is some-
what branching as well as porphyritie, and crosses the channel
without sensible displacement, although it is somewhat jogged
in the plateau. The E.-W. dike (39) in the granite south of
the west marsh is imperfectly exposed; and its chief point of
interest is that it cuts the great melaphyr dike (2).

The dikes beyond the east marsh (40 and 41) have rather
noncommittal trends, not belonging distinctly to either system.
These are, possibly, extensions of the Great Hill dikes ; but it is
impossible to prove it. They converge eastward and upward.
The dike in the granite southeast of Round Hill (42), on the
other hand, is, so far as can be seen, a very typical example
of the first system.

The most northerly dike of the second system in this belt is the
smallest one, the solitary dike on Crescent Hill (48). It can
be traced continuously across the conglomerate and melaphyr
from the western to the eastern face of the hill. If it were con-
tinued across the railroad in the same direct line it would strike
Great Hill about 40 feet from the northern end ; but no trace of
it could be found in the bare ledges of conglomerate and sand-
stone. Advancing south across Great Hill, we come first to
dikes 44 and 45, which, as already stated, appear to be cut by
37 of the first system.. These two dikes are somewhat con-
verging eastward, and 44 ends very abruptly in the conglom-
erate before reaching the face of the hill. The principal dike
of this entire belt is 46, which begins on the western shore of
Granite Plateau with a breadth of about 12 feet. It is jogged
to the north 8 or 10 feet in ascending the slope, and cannot be
clearly traced more than half-way across the plateau, passing
under the grass; but there is a natural path for it under the
narrow meadow along the north side of the high, precipitous
ledge of granite and conglomerate to the western marsh.
Here it is directly in line with what is clearly the same dike in
Great Hill. On the precipitous western face of Great Hill,

just north of the junction of the conglomerate and granite, it

122

measures 11.5 feet ; and about half-way across the hill it appears
to divide into two nearly equal dikes, which are separately
numbered (47 and 48). Near the point of bifureation, which is,
unfortunately, concealed, a much smaller branch (49) starts
from the north side and can be traced across to the western cliff.
The similar small dike (50) just south of 46, on the western
shore of Granite Plateau is probably also a branch of this main,
parent dike. Parallel with 46 on the south is 51, which can
be traced for about the same distance across Great Hill, crossing
the boundary fault between the conglomerate and the granite
without sensible displacement. The most southerly dike on
Granite Plateau (52) is, perhaps, a continuation of 51. It is
first seen about half-way across the plateau and ean be followed
to the western shore of Weir River Bay, crossing and probably

cutting 38 under the water.

Hast- West Dikes South of Weir River.

| Width

No. Trend, Hade. | in Remarks.

| | feet.

58 N. 809 E. N. 59-100 | 1 In granite west of railroad.
54 N. 809 E. N. 592405 8 N incl b: t
55 INNO E “№. 109-159 | 8.5 M " w n “
OO | N. 80“ E. Ny 109 4? б, iH Ae Fk fe
N 80° E. | " " east A
58 N. 80° E N. 300-400 10-15? | “ n V NE n
59 | 8.809 E N. 350 4 T " west '' "T
00 | 8, 85° E N 20У T " y. mre "
61 | E.-W. S, 15 2.5 ti (dy east <“ p

No special or systematic search has been made for dikes
among the granite ledges south of Weir River Bay; and the
few that were casually observed have been noted chiefly to
render the special map more uniform and complete, and not
on account of their intrinsic interest. It should be understood,
however, that these outlying non-sedimentary arcas of the map
have not been uniformly treated in this respect, and un-

doubtedly many dikes have been overlooked, especially west of

— —

the bay and along Rockland Street and Jerusalem Road. The
dikes of this section may be regarded as forming a fourth east-
west belt, and it is noteworthy that, for the first time, the
older system predominates. The outcrops are so imperfect

that no attempt will be made to deseribe these dikes in detail.

North-South Dikes.

Width
No. 'Trend. Hade. in Remarks.
feet.
62 | N.-S. W. 389-5 75-1.75] Cuts 18, 14, and 15.
63 | N.-S. Imperfect exposure.
64 | N.-S. W. 10' 1.5 Irregular and. interrupted.
65 | N.-S. Imperfect exposure.
66 | N.--S. Vertical t0 Composite, six paralel dikes.
67 | N.-8. Vertical 8 Probably ends against fault on
the south.
08 | N.-8. E. 8° 2.—2.25
69 | N.-S. Vertical 1-1.5
TORN UA W. 5°-105 2.5 In granite south of Weir River
Bay.

The dikes of the third or newest system, with the normal
trend N.—S., have their best development in the central and
western areas, only one having been observed in the coastal area
and none on Rocky Neck. Unfortunately, the only clear. and
satisfactory intersections with the east-west dikes are those
afforded by the small dike in the Green Hill ledge; but that
the dikes of this system are all of about the same age, and newer
than the east-west dikes, there can, I think, be no reasonable
doubt. They are, as previously explained, darker colored
(less chloritic and epidotie ) than the older dikes, more prone
to weather brownish and to disintegrate on exposure, and
more generally characterized by transverse or columnar jointing.

The single small example of this system in the coastal area
(62) breaks through the Green Hill conglomerate and cuts all
the east-west dikes (18, 14, 15) in this ledge, without sensibly
displacing them. But it is itself slipped or jogged to the west

4 feet near its intersection with 14; and north of 15 it is

double, giving off a braneh parallel with itself. It runs

directly toward the middle of Green Hill Rock, but no trace of

it can be found in this bare mass of porphyrite.

The fault bounding Melaphyr Plateau on the west is
accompanied by several small dikes, one of whieh (64) сап be
traced, with some interruptions and dislocations, the entire
breadth of the plateau, crossing and probably cutting all the
east-west dikes. The most important and decidedly the most
interesting of all the dikes of this system is the parallel series
(66) separating Kast Porphyrite Hill and Cliff Plateau. Near
the shore these afford the following section from east to west :—

(1) Diabase, 9.75 feet. Separating melaphyr, 8. feet.
(2) s vd» Au " conglomerate, 5.75.
(3) K 6 n i е
(4) “ 9% “ “ 29
(5) A ИЕЛ, n Lo 8. ۳
(6) » ‚100%

89.75 feet. 74.75 feet.

This composite dike or series of dikes is thus equivalent to
about 40 feet of diabase and 75 feet of separating conglomerate
and melaphyr, or 115 feet for the entire fractured zone. The
series is actually exposed for only a few rods; but there is a
clear path for it between Conglomerate Hill and Cliff Plateau
to the northern end of the western marsh. Almost in the
same direct line at the southern end of the marsh is the largest
of all the Nantasket dikes (2), which might be regarded as
due to the union and continuation of the composite dike. This
view is precluded, however, by the lithological contrast, the
southern dike being a true melaphyr and undoubtedly contem-
poraneous with the first flow of that rock, while the northern
or composite dike, representing the latest period of igneous
activity at Nantasket, is very much newer. Probably both the
melaphyr dike and the composite dike are intercepted by the
boundary fault under the western marsh.

125

The three-foot dike (67) so clearly exposed on the north
shore of Melaphyr Peninsula and traceable across West
Porphyrite Hill certainly does not extend so far in this line as
Granite Plateau, probably ending under the water against the
N. E.-S. W. fault. Nearly 200 feet west of this line, on the
north shore of the plateau, is 68, which сап be traced about
150 feet back from the water; and about 500 feet farther west,
following the shore, is 69, of which only about 50 feet in

length are exposed.

DIKES ON TIIE COHASSET AND SCITUATE SHORE.

Dikes are abundant in. the granitic rocks south of Nantasket
and the boundary faults ; in faet, almost every large ledge or
considerable exposure of the granite and diorite shows upon
careful examination one or more dikes of diabase. As a rule,
however, except along the shore, where the almost continuous
ledges are clean and bare, the outcrops of the dikes are obscure
and сап be correlated only to a limited extent; 7. e., the same
dike can rarely be traced with certainty from one outerop to
another. And when we further consider that the dikes
naturally tend, through their more rapid erosion, to follow the
depressions and the drift-covered portions of the surface ; and
also that the drift-deposits and marshes are continuous over
large areas, especially toward the south and south-east, the
futility of attempting to trace out or map the dikes of Cohasset
and Scituate becomes apparent. In the more limited Nantasket
area the conditions are comparatively favorable for such
thorough work ; but it is probable that even here many dikes are
wholly concealed, and the map claims to represent with only
approximate accuracy and completeness the dikes which are
actually exposed. Particular attention was given to the Nan-
tasket dikes, because it was seen that the outcrops are sufficient
to permit of their correlation with the faults of the district and

in systems of different ages; and about all that it has seemed

wise to attempt in the study of the dikes of Cohasset and

Seituate is to determine to what extent they сап be referred to
the same systems as the Nantasket dikes. Systematie observa-
tions for this purpose were confined to the immediate vicinity of
the shore ; and the following paragraphs and aecompanying lists
give the results of a complete canvass of the littoral dikes from
Green Hill to the shore beyond the Glades. No attempt has
been made to trace any of the dikes inland or beyond the clean
exposures afforded by the waves ; but such casual observations
as have been made away from the shore indicate that the littoral
zone is, in the number and trends of the dikes, representative of
the entire area. No dikes have been represented on the general
map, partly on account of the inconveniently small scale, but
chiefly to avoid giving the impression of great inequality in the
distribution of the dikes,

It is readily apparent that the dikes of Cohasset and Scituate
exhibit a general agreement in character and trend with those
of Nantasket. No dikes of melaphyr have been certainly
identified ; but in their place, as previously explained (page
105), there is a well-defined series of porphyrite ‘dikes. These
are clearly older than all of the diabase dikes and probably date
from the Nantasket flows of porphyrite. Each of the three
systems of diabase dikes is clearly represented, and the ex-
posed intersections show that their relative ages are essentially
unchanged. But the oldest system, having the normal trend
N. 75° to 80° E., largely predominates, though, as at Nantas-
ket, it cannot always be clearly distinguished from the.second
system, with the normal trend S. 75° to 80° E. Only a few
dikes ean be referred to the second system. The dikes of the
newest or N.—S. system also are relatively less numerous than
in the Nantasket area; and it is especially noteworthy that they
become less numerous eastward, not a single clear example
having been observed beyond the Cohasset Rocks or more than
three fourths of a mile east of Green Hill Beach. This
apparent limitation of the newer dikes to the Nantasket area

and its immediate vicinity is an interesting and suggestive fact,

—

127

but а more extended study would be required to discover its
full significance. An occasional dike only, such as 92, 109,
114, 136, ete., is distinetly aberrant or ambiguous in trend ;
and some of these afford intersections determining the relative
age and hence the system. Although faults are, perhaps, ав
numerous in Cohasset as in Nantasket, none have been
definitely located; and we can only conjecture that their
relations to the dikes are unchanged. The dikes are often
observed to coincide with prominent joint-planes in the granite ;
but as at Nantasket, the joint-structure is evidently mainly of

more recent origin than the dikes.

The Porphyrite Dikes.

Width
No. | Trend. Hade. in Remarks,
| feet.
|
71 | N; 6229, E. 159-209 | 4 In granite on Green Till Beach.
72 | N. 809 E. Ij, 109 8 750 E. of 71; irregular.
TE NS ADS Е. Vertical 18 15“ E. of 72; imperfectly ex-
| posed.
74 | N. 89 F. Vertical 2 250! E. of 73; near Black Rock
| House.
75 | N. 18" Шу E. 8° 8 Near Black Rock House; cut
| һу 88.
10 N. p> I Vertical 13—14 Behind Black Rock House;
cut by 88 and 89.
T ING 40° D. N.W.10°-159 4—5 Branch of 76; cut by 88 and 89.
78 N.-S. Vertical 1-6 About 450! E. of 77; cut by 89.
79 | N.-S. Vertical 10—15
80 | N. 20? E. Vertical 8--10 About 2550! E. of Forest Ave. ;
cut by 150.
81 | N.-S. Vertical 8-10 About 8000! Е. of Forest Ave. ;
cut by 90 and 91.
82 N. 20° Е. | Vertical 1-1.5 About 450! R. of 81.
88 | N. RO" E Vertical 2-8 6' east of 82; probably con-
nected with 81.
84 | N. 15° E. Vertical 6 Near 88; cut by 92 and 93.
8 N. lO K. Vertical 25-30 About 100! E. of 84: cut by 94.
SO UNS LOS B Vertical 3-4 About 200! E. of 85; also cut
by 94.
Sii UN VADO. ç Ми ДО” 8-8.5 West end of Pleasant Beach;
cut by 97 and 98.

The dikes of this series have not been observed along the
entire shore, but only for about one mile, between Green Hill
Beach and Pleasant Beach or Walnut Angle. This is the

comparatively straight and eminently rock-bound part of the

128

shore known as Cohasset Rocks. Although it is convenient. to
class all the dikes of the above list, provisionally, as porphy
rite, it is well to bear in mind that a few at least are more
basie and that the list quite possibly includes some melaphyr
dikes. "The chief points of interest. presented by the porphy-
rite dikes have been noted in connection with the Black Rock
flow ; while most of the details of any importance, including
especially those required for the identification of the individual
dikes, are presented in the table. We learn from the table
that nine of the seventeen dikes are visibly cut by the oldest
system of diabase dikes. On account of the approximate
agreement in trend, intersections by the north-south diabase
dikes were scarcely to be expected ; but, fortunately, one such
is presented ; and it is, perhaps, the prettiest and most unique
phenomenon to be observed among all the dikes of this district.
The relations of the two dikes are shown in Fig. 16. The
diabase dike (150) advances obliquely from the south until it
strikes the east wall of the porphyrite dike (80), follows this
wall for 25 feet, amputating a branch of the porphyrite dike,
and then passes in a graceful double curve diagonally through

the latter and follows the west wall as far as either can be

traced, —24 feet. The intersecting dike is a typical example of

the third system of diabase dikes—black, brownish-weathering,
and beautifully eross-jointed. This porphyrite dike (80) is the
one affording the analysis given on page 59. On the north

LEELA TES AAKE O k EA E E A уке,
X X » XXX is Wi x n
x y y X 00. E: ah

e

Fic. 16.— PLAN SHOWING THE INTERSECTION OF A PORPHY-
RITE DIKE (80) BY A DrABASH DIKE (150)
ON THE COHASSET Rocks. SCALE,
L INCH = 25, FEET,

129

side of Beach Island, near Sandy Deach, is а small exposure of
a porphyrite (?) dike—N. 5? E., 4 feet, vertical—which is
cut off by a fault.
The Diabase Dikes.
No attempt is made in the list of E.—W. dikes to distinguish
or separate the first and second systems; but in both this and
the N.—S. list the dikes are described in topographic order,

g in each case at the western end of the Cohasset Rocks,

°

beginnin
near Green Hill Beach and Forest Avenue. The data of the
lists are sufficient for the identification of the individual dikes,
if taken in order; but in the brief notes which follow the lists,

some additional landmarks and bearings аге given.

Hast- West Dikes.

Width
No. Trend. Hade. in Remarks.
feet.

88 N. 80? E. "or 3.5 Coarsely porphyritic; cuts 75,
76, and 77.

89 N. 80° Е. Vertical 6.5-7 Exposed about 750'; cuts 76,
Tt, and 78.

VO INIT 7 Ha NV. 0 25-30 Branches toward east end: cuts
81; cut by 152-8-4.

90a! N. 85° E. N, 97 2-3 A northern branch of 90.

DT. М 809 E. Nd 1.5-2 A south. branch of 90; cuts 8I.

92 E.-W. Vertical 2-3 Exposed over 300“; probably
unites with 93.

VB N. SOS Ii O 2-8 8' S. of 92; 92 and 98 cut 84.

94 | N. 80? E. Vertical 9 Exposed about 300'; cuts 85
апа 86; magnetic.

95 N80 B. SV 8-8.5 60/ S. of 94; cut by 156.

96 N. 80° E. Vert.-S. 59 | 8 75'S. of 95; parallel with flow-
structure of granite.

9T N. 607.20. N. 159-209 | 4-5 125’ S. of 96; cuts 87.

98 N. 80° E. Vertical 1-2 50! S. of 97; irregular; cuts 87.

99 | N. 659-70? E. | N. 20° 2-4 E. end Pleasant Beach; cut by
100; irregular.

100 | N. 80? E. Vertical b E. end Pleasant Beach; ex-
posed long distance.

LOL UNS TOT B, iN 1 150! S. of 100.

102 N. 80? Е. S. 59-109 8—4 About 250! S. of 101.

108 | N. 85° E 26! S. of 102; W. end of Sandy
Beach ; irregular.

104 | N. 759-80? E. | Vertical 3-4 N. side of Beach I.; probably
cut by 105 and 107.

105 | N. 85° E. Vertical 10 N. side of Beach I.; exposed
long distance.

OCCAS. PAPERS В. 8. N. H. IV. 9.

106
107

| 108
109
110
111
112
118

114
115
116
| 117
| 118
| 119

120

121
129
128
124
125
126
127
128
129
130
| 181
| 132
188
134
135
136

| 137
| 138
189
140
141

142
148

144

N. 70°-

N. 859
N. 80°
H.-W.
N. 80°

H.-W.

N. 82°—

80? E.

E.

E.

E.

909 E.

IS; (or E

E.-W.
N. 85°
N. 80°
N. 80°
N. 80°

E.
E.
E.
E.

N. 80? E.

N. 80°

N. 759

E.

E.

8, 75° E.

N. 80°
N. 70°

E

E.

Irregular.

N, 707
Nono
N. 709
INS

N. 405

N. 80°
INK 80%
N. 80°
N. 82°
N. 80°
E.-W.

N. 707
N; 75
N. 846
N. 80?
N. 809

E.-W.
N, 157

N. 70°

E.
E.
E.

E.
809 E.
E.
E.
E.
E.
E.
E.
E.
E.
E.
E.
E.

E.

Vertical
Vertical

9. 5°

N. 159-20?
N. UV
Vertical

Vertical

E
E

2 2 E 4 0

N. 50

Vertical
Vertical
Vertical
N. 15"
Vertical
Vertical
Vertical
N 159
IN. 10V

Vert.—N. 109|

N. 5?

N, 6°
Vertical
Vert.-N. 8°
NI, LO
Irregular

N. 0"
М, 105
INA LO
N. 50

Vertical

Vertical
Vertical

Vertical

1

70! S. of 105; very irregular.

50! S. of 106 and 75! N. of arti-
ficial harbor; faulted.

S. side of artificial harbor:
branching.

E. side of Beach I.; compos-
ite and branching.

Cuts 111 obliquely, both longi-
tudinally and vertically.

80! N. of 112; interesting in-
tersection by 110.

First dike N. of outlet of
Little Harbor; branching.

First dike E. of outlet of Little
Harbor.

40! N. of 113.

18! N. of 114.

150! N. of 115.

5! N. of 116.

057 М. of 11T.

60! N. of 118; 150! from point;
exposed 800/.

Point nearest Quamino Rock ;
exposed 200/.

N. W. shore of Sandy Cove.

Extreme end of Hominy Point.

South side of White Head.

| In coarse gran-

Dikes 113 to 118
are all on shore
E. of the outlet
of Little ITarbor.

180! S. of 124.
75 S. of 125. tof Deacon
115^ S. of 126. | Bourne’s Is-
12! S, of 127. | land.
On Hog Rock in
Harbor
On shore W. and E. of Glades
Hotel; coarsely porphyritic.
6’ N. of 130 ; near Glades Hotel.
About 200’ N. of 131; in diorite.
About 90' N. of 132; in diorite.
30! N. of 133 ; in coarse granite.
20' N. of 184; in coarse granite.
16! S. of 130; E. of Glades

ite on E. side

Cohasset

Hotel.

80! S. of 181; E. of Glades
Hotel.

70! S. of 135; on Strawberry
Point.

About 100! S. of 138.
About 25/ S. of 189.
About 175' S. of 140; irregular,
branching.
About 75! S. of 141; branching.
Headland E. of Strawberry
Point.
About 800! 8.
quarry.

of 143; in old

wl

181

146 | N. 70? E. Vertical 1-1.25 | About 100/ S. of 144, in quarry ;
porphyritic.

N. 55° Е. N. 309 8 150' S. of 147; coarsely por-
| phyritic and branching.

146 | N: 70% W INN OT 1-1.5 Separated by short beach from
145.

147 | N. 759 E N. 5° 2-3 Near 146.

148 5

As a rule, the east-west dikes cut the shore ledges very
obliquely ; and in going eastward we cross the first system
from north to south, and the second system from south to
north. The outcrops begin in the rear of the Black Rock
House, where 88 is readily recognized by ite coarsely and
distinctly porphyritic character. About 50 feet south of 88 is
89, a splendid dike, which can be traced almost continuously
for fully 750 feet. About midway of the outcrop it is some-
what irregular and split up; and the well-preserved glacial
striae upon it, and especially the freshness of an inscription bear-
ing the date 1852, testify to the stable and resistant character
of those diabase dikes which have, by deep-seated alteration,
been largely changed to such almost indestructible species as
epidote, chlorite, magnetite, and quartz. A little more than
half a mile (2860 feet) from Forest Avenue brings us to
90, the largest of all the east-west dikes; and its outcrop is
quite a center of eruption (Fig. 17). It cuts one of the
porphyrite dikes (81) and is eut by no less than four of the
north-south diabase dikes. The small dikes (90% and 91) are
probably branches of 90. The magnetic dike (94) is so
highly charged with magnetite as to make the compass observa-
tions quite unreliable. It not only cuts 85 and 86, but just
before entering the sea is cut by 156 of the third system, which
also cuts 95. “The next three dikes of this series (96, 97, 98)
follow in regular order and bring us to Walnut Angle or the
'astern end of Cohasset Rocks. No dikes have been observed
in the ledges on Pleasant Beach; but near its eastern end we
strike 100, which ean be traced the entire length of the head-

land, trending just north of the extreme north end of Brush

182

Island. In like manner, 108, on the east side of this headland,
is exactly in line with a large dike near the southern end of

Brush Island.

jili

A
T^
2 v2

T
8
s

=
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D

De
BAN

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ШИШИШИ
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š
S
z
š
š
š
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:

Fic. 17.—PLAN SHOWING A GROUP OF INTERSECTING DIKES
ON THE CoHAssET Rocks. SCALE, I INCH = 50 FEET.

Crossing Sandy Deach, we come at once, on the north side
of Beach Island, to 104 and 105, which can be traced for long
distances, the latter enclosing, near the eastern shore, a long
narrow mass of granite. The second dike beyond (107) is
slipped about twice its width. The large dike south of the
artificial harbor divides eastward into two unequal branches
separated by from 8 to 10 feet of granite; and 15 feet north of
it is a one-foot branch. ‘The composite dike (109) on the east
side of Beach Island is near the contact of the coarse typical
granite and the mixed granite and diorite. The intersection of
111 by 110 (Fig. 18) is a clear case of the confusion of the
first and second systems. The intersection is very oblique,
extending over 20 feet along 111. Immediately north of 110
is an unenumerated six-inch dike. The next dike (112) is a
fine and instructive example of a branching fissure, and shows
flow-structure parallel with the walls.

The dike on Hog Rock (129) is exposed again in the
re-entrant angle of the shore south of Strawberry Point. The
next dike (130) is beautifully porphyritie, with large clustered

188

i st LM CET

ріне Uh.

Fic. 18.— PLAN SHOWING THE INTERSECTION OF TWO
DIKES ON THE EAST SIDE OF BEACH ISLAND.
SCALE, 1 INCH — 8 FEET.

crystals. It outcrops west of the hotel, east of the hotel, and
then, after some faulting and branching, on the east side of
Strawberry Point. The remaining dikes on Strawberry Point
(131—142) are easily found and identified by taking the ledges
in order from west to east, one of them (135) cropping on three
distinct ledges.

North-South Dikes.

Width
No. Trend. Hade. in Remarks.
feet.
149 | Nos WA Vertical 3-4 On Cohasset Rocks, 400! E. of
| Black Rock ITouse.
In the valley immediately E.
149% N.-8. Vertical | of the Black Rock House;
imperfectly exposed.
150 N. 50-10 E. | Vertical 2-3 On Cohasset Rocks; cuts 80
| obliquely.
151 | N.-S. Vertical 8-4 Interrupted and faulted; cuts
90.
152 | N.-S Vertical 8-6 Cuts 90; irregular.
158 | N.-S Vertical 6-7 Cuts 90; unites with 152.
154 | N.-S. Vertical 7.5 Cuts 90; branching northward.
105 CN: 8 Үү, Vertical 8-8.5 About 200! E. of 154, on Cohas-
set Rocks.
156 N. 59-109 E. | Vertical 2 On Cohasset Rocks; cuts 94
and 95.
157 | N. 159-459 E. |S. E. 800-45 2-8 On Hominy Point, near White
Head.

On the short, stony beach immediately east of the Black Rock
House thereare traces of several — at least three and possibly
more—dikes (149a) of this system, recalling the composite dike
of the Nantasket area (66). The last of the north-south dikes

184

(157) is the only really doubtful one. This is the dike shown
in Fig. 19. In tracing it across the ledge to the mainland, its
sasterly trend, hade, and thickness are all greatly. increased;

and it would, perhaps, be best to class it as a highly aberrant
member of the first system.

FIG. 19.— DIKEZ157 CUTTING A LEDGE OF GRANITE, NEAR WHITE
HEAD, ом Hominy POINT, Conasser.

SEQUENCE OF EVENTS RECORDED IN THE NANTASKET
LEDGES.

It is unnecessary to review or summarize farther the evidence
supporting the conclusions that have been reached in the preced-
ing pages concerning the succession of the beds of conglomerate
and the flows of lava; but we may fitly close this section with
a general statement of the geological history of Nantasket so
far as it is recorded in the hard rocks. The granite, with the
associated diorite and felsite, is the fundamental rock in this
part of the Boston Basin; and it had, apparently, been
exposed to erosion for a long time before the basal conglom-

erate was deposited over it, The character of this conglomerate

135

is, however, a sufficient indication that outflows of lava and,
very probably, the deposition of conglomerate had already
begun in the deeper parts of the basin; and we may fairly
suppose that both conglomerates and lavas older than any
exposed in southern Nantasket underlie at a great depth the
northern part of the peninsula. Eventually the slow subsidence
of the land then in progress carried the shore-line over and
beyond the Nantasket area, and the so-called basal conglom-
erate was formed upon the uneven and fissured surface ofthe
granite. This conglomerate had attained a maximum thickness
of perhaps 50 feet, when the first flow of melaphyr was spread
over it. This was probably originally an ordinary compact,
black lava; but through subsequent alterations it has become
greenish and purplish, compact and jaspery. The lava-flow
being submarine, the sedimentary process was uninterrupted ;
and the uneven and scoriaceous upper surface of the melaphyr
was slowly covered by the second conglomerate, which is
largely composed of debris derived from the melaphyr, and like
this melaphyr is especially characterized by the segregations of
red jasper. When the second conglomerate had attained
approximately the same thickness as the basal conglomerate,
a flood of more acid lava (porphyrite) was spread over the sea-
bottom to a depth of from 50 to possibly more than 100 feet ; and
all the porphyrite in the Nantasket area probably belongs to
what was once one continuous flow, increasing in thickness
astward, or toward the vent from which it issued. Over
the porphyrite was gradually accumulated the third conglom-
erate to a thickness of from 50 to 100 feet or more, but termi-
nated at last by a comparatively thin flow of highly basie and
vesicular basaltic lava, which is recognized now as a typical
green and amygdaloidal melaphyr. The fourth conglomerate,
with a thickness of from 20 to 30 feet, separates this second
melaphyr from the very similar third melaphyr. The latter
consists, however, of two flows, having an aggregate thickness of
from 40 to 50 feet. Once more the beach conditions prevailed,

186

and the fifth conglomerate was formed. When the volcanic
activity was again renewed over this area, the eruptions were,
for the first time, partly of an explosive character, forming, as
the Atlantic Hill section shows, beds of tuff alternating with
beds or flows of compact and brecciated melaphyr. This marks
the culmination of the volcanic energy, flow sueceeding flow,
until they attained an aggregate thickness of several hundred,
possibly five hundred, feet. This series of eruptions completes
the conglomerate and melaphyr series as now developed in the
Nantasket ledges ; but on the extremity of Rocky Neck we find
evidence that the great melaphyr was finally covered by a
sixth bed of conglomerate, and that by a flow of green, amyg-
daloidal melaphyr resembling the second and third sheets of
that rock.

The volcanic energy finally died out ; and these alternating
sheets of conglomerate, melaphyr, and porphyrite were probably
covered by a great thickness of conglomerate and sandstone
without interbedded lavas. But of this upper conglomerate
series there are now no visible traces in the Nantasket Penin-
sula, although Harding’s Ledge, as already explained, affords
some evidence that it underlies the middle part of the beach, in
the vicinity of Strawberry Hill. As the subsidence progressed
and the water became deeper and the shore more remote, the
deposition of the coarser fragmental rocks over this area
gradually ceased, the conglomerate changing through sandstone
to slate, which, we may fairly suppose, underlies almost the
entire peninsula north of Atlantic Hill, and has a thickness of
several hundred, possibly a thousand, feet, The deposition of
the slate probably occupied a much longer time than that of
the conglomerate series; but it was finally terminated by the
period of disturbance during which the sediments of the Boston
Dasin were strongly folded, faulted, and elevated to form dry
land. It is to this geological revolution that we owe nearly all
those structural complexities which make the stratigraphy of
the Boston Basin such a difficult problem. This was also

187

probably a period of intense igneous activity, the numerous
dikes of diabase traversing the ledges of Nantasket and other
parts of the Boston Basin appearing to date from this time.
The dikes were probably, in many cases, the feeders or channels
of supply of effusive eruptions ; but these surface lavas of every
form, not being protected by later sediments, have, at least in
in the Nantasket district, been long since completely swept
away by the agents of erosion, which during all subsequent
time have worked unceasingly and so efficiently that not only
have these ancient volcanoes been destroyed but the very
foundations on which they stood. In southern Nantasket the
entire thickness of the slate series and probably, on the average,
half of the conglomerate series have been removed; and over
broad areas erosion has exposed the original granite floor or cut

deeply into it.
AGE OF THE NANTASKET ROCKS.

Up to the present time no fossils have been found in the Nan-
tasket ledges. It is, of course, improbable that any ever will be
found in the conglomerate and volcanic series; but we may
reasonably entertain the hope that the slate ledges will yet afford
us some clue to their geological age. Fragments of а some-
what calcareous slate have been observed in the drift of Straw-
berry Hill, and suggest the possibility that beds of impure
limestone (of organic origin, if not actually fossiliferous) underlie
the northern end of the peninsula. It seems impossible to feel
quite sure that the hard massive slate on the railroad, northwest
of Rockland Hill, whieh has been referred provisionally to the
bed of tuff outcropping at the base of Atlantic Hill, is not really
an isolated exposure of the Cambrian slate of Braintree and
Weymouth. The Weymouth beds are now referred by
Walcott, provisionally at least, to the Lower Cambrian ; and
the Braintree beds to the Middle Cambrian!. The granite and
diorite are undoubtedly here, as elsewhere in the Boston

! Tenth Ann, Report U.S. Geol. Survey, p. 567.

Basin, younger than these Cambrian strata; and it is perfectly
clear that the conglomerate and melaphyr series is newer than
the granite, and therefore distinctly more recent than the
Middle Cambrian. They are quite certainly Paleozoic, but
whether as late as the Carboniferous strata in the Narragansett
Basin is at least doubtful. The conglomerate and melaphyr
and newer slates of the Nantasket area may be safely correlated
with the same rocks in other parts of the Boston Basin; and it
is hoped that, before the revision of the geology of the basin
is completed, evidence will be forthcoming which will finally
and definitely settle the question as to the age of these strata ;
but for the present the problem must be regarded as unsolved.

In January of this year (1892), Mr. Т. A. Watson found
on Pleasant Beach, Cohasset, a smoothly rounded and evi-
dently water-worn bowlder, between five and six inches in
diameter, of a highly fossiliferous, compact, red (ferruginous)
limestone. The fossils appear to belong wholly to two species,
which have been identified by Mr. C. D. Walcott" as the two
Lower Cambrian types Straparollina remota Billings and
Hyolithes communis Billings. Although Mr. Watson was
unable, after the most thorough search, to find a second speci-
men of this limestone on the Cohasset shore, its Cambrian age
made it seem highly probable that it had been derived from
some point within the Boston Basin, and possibly within the
Nantasket and Cohasset district, thus encouraging the hope
that it would yet afford us the desired clue to the geological age
of the Nantasket strata. But this hope has been dispelled ; for
Mr. Watson has recently found several water-worn fragments
of precisely the same kind of limestone, holding the same fossils,
but not quite so abundantly, on the beach at Bass Point,
Nahant. We can no longer doubt that this rock is a part of
the Cambrian limestone of Nahant, although representing a
more ferruginous and more conspicuously fossiliferous bed than
any now exposed on that peninsula ; and the Cohasset speci-
men must be regarded as a solitary glacial erratic.

1 Proc. Biological Society of Washington, VII., 155.

THE SURFACE GEOLOGY OF NANTASKET AND
COHASSET.

As a starting point for the surface geology of this district,
including both its glacial and post-glacial history, we must
aceept the preglacial peneplain, the evidence for which has
been presented in sufficient detail in the general description of
the topography (pages 5 to 7). The Mesozoic and Cenozoic
eras are, apparently, unrecorded here, except in the erosion of
the hard rocks. But, while it is to these long geological
cycles that we must refer the removal of a great thickness of
stratified rocks from this area, and the wearing down to the
base-level of the hard granitic rocks, lavas, and conglomerates,
the rugged contours presented today by the preglacial pene-
plain undoubtedly date chiefly from the marked clevation of
the land which, it is probable, ushered in the great ice-age.
On account of the intense hardness of the rocks, these deeply
incised lines were only partially effaced by the powerful abra-
sive action of the ice-sheet ; but, as we have already seen, they
are to a large extent obscured or concealed by the non-lithified
deposits of glacial and post-glacial origin.

The principal events recorded in the surface geology, since
the development of the preglacial peneplain, are: (1) A con-
siderable elevation of the land at the beginning of the glacial
epoch. The elevation was, probably, a principal cause of the
subsequent glaciation, and is proved by the deeply eroded
character of the peneplain. (2) Long-continued glaciation,
during which the divided peneplain was strongly eroded and
the ground-moraine or bowlder-clay accumulated irregularly
upon it, chiefly, at the last, in the form of drumlins. (8)
A marked depression of the land, accompanied by the final
melting and retreat of the ice-sheet, and the accumulation,

either in the sea or in temporary lakes and streams, of the ex-

140

tensive deposits of modified drift. (4) A slight re-elevation
of the land immediately after the disappearance of the ice
and, probably, during the formation of the modified drift.
The elevation is proved їп other parts of the Boston Basin by

the occurrence of clay beds above sea-level, and generally, it is

believed, by cut terraces on the drumlins and marine deltas of

sand and gravel. (5) The development of the modern shore,
including the growth of the beaches and marshes, accompanied
by a slow movement of subsidence.

There remains now, in order to complete this paper, simply
the task of setting forth more fully the tangible and material
faets upon which the foregoing statements rest, taking up the

topies, so far as possible, in the chronological order.

DRUMLINS, GLACIAL STRIAE, AND BOWLDERS.

The unmodified drift or till of this district, so far as it now
exists above the sea and is not covered by the modified drift,
occurs almost wholly in more or less typieal drumlins. The
positions, outlines, heights, and names of these are indicated on
the general map, so far as they have been determined.

Rockland Hill, however, is only an incipient drumlin, and
might be more properly classed as a lenticular slope of till.
Telegraph Hill, World's End, Planter's and Pine Hills, Tur-
key Hill, Bear Hill, and others are examples of composite
drumlins; what were originally separate accumulations of till
becoming more or less perfectly united by their continued
growth.

As the map shows, the longer diameters of the drumlins
have without exception a strong easterly trend, varying
between south-southeast in the southern and east.southeast in
the northern part of the district, and agreeing closely at all
points with the glacial striae on the ledges.

The following observations on the directions of the striae em-

brace the extreme range and are sufficiently numerous to show

141

the normal courses. It is very obvious that in crossing what
is now the Nantasket Peninsula the ice was strongly influenced
by the eastward trend of the main valley of the Boston Basin,
being held to this course partly, perhaps, by the eastward slope
of the ground and the proximity of the sea in that direction,
and partly by the continuous barrier of granite 200 feet or
more in height presented by the south shore of the harbor.
But when the ice had escaped from this influence by flowing
out of the harbor into the bay, or by scaling the ledges to the
peneplain of Cohasset, it resumed more nearly its normal
south-southeast trend, as shown by the drumlins and the glacial
striae along the Cohasset shore.

Directions of Glacial Striae.

Slate ledge south of Thornbush Hill.

(a Ue
Northeast base of Atlantic Hill. 48° E.
Double dike on Little Gun Rock. 87°-47° E
Dike 13 in Green Hill Ledge. 80°R

Slate on the railroad south of N: untasket Station.

East side of the bay, near the railroad.

North base of East Porphyrite Hill.

West ond of Granite Plateau.

East side of Rocky Neck.

West side of Rocky Neck.

South side of Weir River Bay. ... . .

Jerusalem Road, south of Straits' Pond.
* “ pear Green Hill Beach.

35*- enia
85°.

41° E.
40° R.
80" E.
EUR THUS

= z ол ол m ол c cn Ф ол р uu ОО zn tn р tp Ол ОО р р

Cedar Street, Cohasset. 809-56" E.
West side of Little Harbor. М 229-259? E.
Cohasset Rocks, east of GreenHill Bei ITO h. TOS B.
M o one fourth mile east of beach. 469 E.
ly i near Pleasant Beach. 400 E
Beach Island, near the artificial harbor. 329-359 Е.
ka “ near Pleasant Beach. 259-309 E.
Strawberry Point, Scituate shore. 959R
Scituate shore, south of Strawberry P oint. 25?E

Doubtless the most interesting feature presented by the
composition of the till is the occurrence of fossil shells in some of
the drumlins. Mr. Warren Upham has noted! the finding of
fragments of shells in the sea-cliffs at several points about
oston Harbor, including the northern member of Tele-

de, B. 9, N; Ho АУТ, 1972181.

142

graph Hill and Sagamore Head inthe Nantasket area. He
has also called attention to the fact that, so long ago as the
Revolutionary War, a fort was built on the top of Telegraph
Hill, and a well was dug inside the fort of which the commander,
Gen. Benjamin Lincoln, wrote as follows“: „There is a large fort
on the E. Hill, in which there is a well sunk 90 feet, which
commonly contains 80 odd feet of water. In digging the well,
the workmen found many shells, smooth stones and different
stratas of sand and clay similar to those on the beach adjoining
to the hill. These shells and appearances were discovered
from near the top of the ground to the bottom of the well."
Of more than thirty species of shells collected by different
observers from the till of the Boston Basin, Mr. Upham has,
in a hasty examination, found but three — Venus mercenaria
(the round clam or quahaug), Cyclocardia borealis, and Cliona
sulphurea (a boring sponge)—in the section at the north end of
Telegraph Hill; and traces of the round clam, which is by far
the most abundant species in all the sections, in Sagamore
Head. More recently, by careful searching, with the assistance
of Mr. Н. D. Card, I have found the shells in the eastern
searps of Point Allerton Great Hill, and Strawberry Hill; and
have extended the list of fossils known to occur in the till of the
Nantasket Peninsula to eleven species, as follows :—
List of Species in the Nantasket Drumlins.

Sales; Telegraph | Point yita,
Hill. Allerton, Hill.

Balanus (sp. ?). be ж

Tritia trivittata, Adams. 10

Ilyamassa obsoleta, Stimp. т

Crucibulum striatum, Say. i

Buccinum undatum, Linné. "

Mya arenaria, Linné. 10

Venus mercenaria, Linné. * i "
Cyclocardia borealis, Conrad. * 0 л
Astarte undata, Gould. * ^ *
Scapharca transversa, Say. *
Cliona sulphurea, Verrill. " " ^

p.56. (Only a small part of this work was published.)

I have failed to find even the slightest trace of shells in any
of the drumlins south of Nantasket Beach. This is an instance,
however, where negative evidence is of very little value, the
apparent presence or absence of shells depending very largely
upon the character of the exposure. A deep section is usually
essential ; for, as Mr. Upham has explained, the shells are very
likely to have been dissolved out of the upper, oxidized or
weathered layer of the till. It is probably for this reason that
they are not found in the shallow sections afforded by Point
Allerton Little Hill, Little Hog Island, White Head, World's
End, and Planter’s Hill, as well as all the inland drumlins.
We may reasonably hope, however, that shells will yet be found
in Green Hill and on Bumkin Island. That shells actually
occur in the drumlins of Cohasset we have satisfactory evidence
in the fact, to which my attention has been called by Mr. H.
W. Nichols, that fragments of the round clam were found by
Mr. Titus Burbank some years ago in digging a well near the
summit of James Hill, northeast of Scituate Pond. The well
is 45 feet deep; and the shells were observed only near the
bottom. They cannot, therefore, be referred to any post-
glacial source.

Although shells are now so generally wanting in the buff or
oxidized till, evidence of their former existence is afforded by
the calcareous material which may, in certain cases, be detected
in the till. This evidence is especially clear in the buff till of
Telegraph Hill. The carbonate of lime has, in part at least,
been dissolved and segregated, locally but firmly cementing
the till, and forming in the finer parts regularly rounded concre-
tions, from a fraction of an inch to several inches in diameter,
which are sometimes attached to the flat surfaces of the large
stones or bowlders. These nodules effervesce freely with acid,
and no other source of the carbonate than the shells is apparent.

Mr. Upham has correctly explained the shells in the till,
which are always in a fragmentary form, as having inhabited
the bottom of Boston Harbor before the coming of the ice-sheet,

144

by which they were gathered up and incorporated in the till;
and he has shown that some of the species, at least, like the
round clam, are now found only in more southern waters,
indicating that the sea in this vicinity just before the glacial
epoch was warmer than at the present time. The shells are
also of special interest as proving not only that Boston Harbor
was in existence in preglacial times, for otherwise we could not
know but that this basin was above sea-level before as well as
during its occupation by the ice-sheet ; but they also show that
the harbor has now approximately its preglacial outlines and
hence that the sea has regained, on this coast, very nearly its
preglacial level.

The peneplain, as already stated, and especially when we
make an allowance for the glacial erosion upon its surface,
proves that this coast is now one hundred feet or more above
its preglacial level. jut this greater elevation is off-set, so
far as the harbor is concerned, by the excessive erosion which,
being a valley in comparatively soft rocks, it suffered in glacial
and especially in immediately preglacial times.

Although the fragments of shells are unquestionably the most
interesting feature of the till орвегуе( іп Nantasket and Cohasset,
other components of more normal character—the ordinary
stones and bowlders—demand brief mention. No particularly
striking instances of glacial transportation have been noted.
The till of the Nantasket Peninsula, as may be most readily
seen in the marine sections of the drumlins, consists very
largely of fragments of slate, which is undoubtedly the under-
lying rock not only of this peninsula but of the main part of
the harbor to the northward. Bowlders of conglomerate are
not wholly wanting north of Atlantic Hill, but they are so few
and small as to prove that it сап occur only very sparingly, if
at all, in the harbor area or beneath the extensive drift-deposits
north of the harbor. In fact, pretty much all of the conglom-
erate in the till north of the Nantasket ledges might be referred
with considerable probability to the known exposures of that

4|

145

rock in Medford, on the extreme northern margin of the Boston
Basin, or at least to the probable eastward extension of those
beds beneath the drift; although the strong easterly trend of
the glacial movement in the harbor allows us to regard the
prominent conglomerate ledges in the central part of the basin
as a possible source of some of the conglomerate erraties in the
Nantasket Peninsula and Cohasset. In the Nantasket drumlins,
granitic rocks, felsites, quartzite, etc., from the high land
north of the Boston Basin, form a small but very obvious or
noticeable fraction of the till, and in some cases exhibit lith-
ological peculiarities enabling us to refer them to the parent
ledges, as in the case of the bright red felsite from Saugus
Centre. А portion of what might be mistaken for conglom-
erate is brecciated felsite, and another portion is brecciated or
pebbly slate. Over the granitic area of Cohasset and Scituate,
as a matter of course, the crystalline rocks predominate in the
till, and the slate, which does not bear glacial transportation
well, is subordinate, except, perhaps, in the finest part of the
H

till. There is, however, among the larger masses or fragments
in the till, a quite liberal sprinkling of both melaphyr and con-
glomerate from the Nantasket ledges. These are especially
noticeable along the shore, the number and average size, with
oecasional exceptions, diminishing rapidly as the distance
increases.

The surface distribution of the larger bowlders indicates that
they are not equally abundant in all parts of the till; for
while the majority of the drumlins are remarkably smooth, pre-
senting on the surface but few bowlders larger than those to be
seen in the stone walls, others, like Booth Hill in Scituate, are,
at least on certain slopes, thickly strewn with masses from
three to eight feet or more in diameter. The bowlders are, in
some cases at least, clearly most abundant along those lines
where the till has been eroded by standing or running water and

which may thus be regarded as in some sense ancient shores or

OCCAS PAPERS B. 8. N. Н. IV. 10.

146

channels, resembling the modern shore in this respect; or over
areas where there is at least evidence that the till has been sub-
jected to the sorting action of water and the finer materials
removed. It is reasonable to suppose that the bowlders are
most abundant in the lower parts of the drumlins, and in gen-
eral where the till is thinnest, as may be observed at many
points over the rocky peneplain of Cohasset ; and it is certainly
not improbable that, as suggested to me by Mr. Upham, this
is owing, not wholly to the fact that the source of the bowlders
is near at hand, but partly to an actual combing of bowlders
out of the till or ground moraine as it was swept past the
ledges by the movement of the ice, the ledges thus serving as
gathering points for bowlders.

An oceasional bowlder merits special attention on account of
its size; and some of these are rather striking also in their sit-
uations. As might be expected, the largest erratics consist
almost exclusively of granite, the most massive and resistant
rock of the region. Опе of the most impressive examples
observed in the Nantasket-Scituate area is the block of granite
on Booth Hill, in Scituate, known as Hatch’s Rock (Fig. 23).
It is very regular in form, approximately twenty feet square
with ап average height above the ground of at least ten feet.
It stands on. the summit of the hill and some fifteen feet above
a broad level terrace or platform. Near the eastern base of
Booth Hill, on the grounds of Mr. Silas Peirce, is Toad Rock,
a block of diorite about twenty feet long resting on a local
deposit of modified drift.

A granite bowlder about twenty feet long, twelve feet wide,
and ten feet high lies in the woods close on the south side of
Beechwood Street, about one mile northwest of Beechwood
Village, with several others nearly as large near it; and east
of the Village a block of granite which is approximately a ten-
foot cube lies on the west side of Bound Brook, a few rods south
of the street ; while in the rocky woods south of Scituate Hill
may be found a striking example of a large bowlder which has

been disrupted by the action of the frost.

147

In a walk along the shores of Nantasket, Cohasset, and Scit-
uate, one observes occasional large bowlders, sometimes local
and sometimes traveled or truly erratic, but the only one that
need be specially mentioned is an angular block of coarse,
massive, pinkish granite, one among a large number of smaller
masses, on the eastern shore of the Glades, in Scituate, and
nearly opposite the Osher Rocks. It is about twenty-five feet
long ; but being of precisely the same lithological character as
the neighboring ledges, it would, perhaps, eseape partieular
notice, except for its position, being supported by smaller
bowlders and the ledge in such a manner that when the tide is
out one can pass under it from one side to the other. The fact
that it maintains this insecure position, although exposed to
the full force of the breakers, testifies to its great weight.

The best example, however, of a perched block, and one of
the largest bowlders in Cohasset, is the mass of granite on the
estate of Mr. Edward Wheelwright west of Little Harbor,
which has long been known as Tittling Rock (Fig. 20).
Going north from Cohasset Village on Jerusalem Road, we
come in about half a mile to the residence of Mr. Wheel-
wright, whence a private road runs nearly due west for a
considerable distance through beautiful rocky woodland,
bringing us, when about two thirds of a mile from the highway,
to Tittling Bock. The bowlder, which lies close to the road on
the south, is a rude and somewhat oblique parallelopipedon
measuring approximately 20 feet in extreme length (north-
south), 12 feet in breadth, and 12 feet in greatest height. Its
location is almost the highest point in this part of the peneplain,
probably 100 feet above the sea. It rests upon a low,
glaciated ledge of granite sloping gently to the north, seeming
to lie in a shallow depression or glacial trough ; and the actual
base or supporting surface is not more than six feet long and
from two to three feet wide. But although seemingly so
nicely poised, considerable force would probably be required to

disturb it; and it is not properly a rocking stone.

Fic. 20.—TITTLING Rock, COHASSET.

On the northwest slope of Telegraph Hill, just above the
junetion of the main hill with the lower, flat-topped part, is a
block of coarse granite about fifteen feet in diameter and from six
to seven feet high above the ground. А very similar bloek rests
in a similar position on the north side of Strawberry Hill. It
is about fifteen feet square and eight if not ten feet thick. Two
bowlders of granite, essentially similar to these, are similarly
situated on the northern slope of Otis Hill, in Hingham. The
occurrence of these isolated blocks on the northern aspects of
the hills, and at approximately the height of one of the principal
sand plains, is certainly suggestive of their transportation by
floating ice rather than the ice-sheet ; and it appears necessary
to regard them as having been derived from the granite ledges
north of the Boston Basin, the nearest possible source of granite
of this character, in the direction of glacial movement, being

twelve to fifteen miles distant.

GLACIAL POTHOLES.

Four years ago, my attention was first called by Mr. T. T.
Bouvé to the existence of a fine series of glacial potholes
on Cooper’s Island in Little Harbor, Cohasset, and I had the

149

pleasure a little later of examining the locality in company
with him and Mr. Warren Upham. In the following year Mr.
Bouvé published a full account of the potholes," from which
the greater part of the following deseription is taken, together

with the illustrations ( Figs. 21 and 22).
о

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Fic. 21.— Ротногк, CooPER's ISLAND.

Cooper’s Island, so-called, is the peninsula consisting of
granite ledges and salt-marsh which extends into Little Harbor
from the west side. The eastern end, which reaches nearly to
the middle of the harbor, is an approximately north-south
ridge of granite one fourth of a mile long and from 15 to 25 feet
high. This ridge is divided transversely near the southern

extremity by a short stretch of grass land ; and it is near the

1 Proc. B. S. N. H., XXIV, 219-228.

150

southern end of the main ridge, on the east side and quite close
to the water that the potholes occur. The most southerly and
most perfect holes are on the north side of a slight indentation
of the rocky shore and may be readily found when the tide is
out (Fig. 21). Of the lowest and best preserved of these
(No. 1) the bottom is still intact and perfect, holding water to
a depth of 21 inches and having a well-defined rim just at the
surface of the water. The diameter at the rim is 254 inches;
below the rim, 30 inches. Above this rim the whole southern
side of the hole is wanting; but on the northern side the rock
is smoothly concave and the characteristics of the pothole are
plainly discernible for a height of four feet, with a breadth of from
three to four feet, making the total depth about six feet. The
whole has the appearance of a large, imperfect *or one-sided
pothole with a smaller and perfect pothole in the bottom of it.
Exterior to this pothole, the tide sinks below the level of its
bottom, but at high tide all is covered.

The second pothole has its bottom three and one half feet
above that of No. 1; and their centers are three feet apart hori-
zontally. The whole southern side of this hole (No. 2) is
wanting and water can now stand in it only to the depth of
about two inches. Above the bottom, the granite is smoothly
concaved for a breadth of three feet and a height of five feet;
and this hole, when entire, was evidently about as large as the
first. "The slope of what remains of the walls of these holes
shows, apparently, that the flow of water over the rocks was
from the west and northwest. Of the third pothole there is
but little to be said, except that it is small and shallow. It is
about five feet above the bottom of No. 2, still in a northwest
direction ; and there may be traced from it in the same general
direction а narrow water-worn channel about six feet in length.
The high-tide mark is about midway between the second and
third potholes.

It is quite obvious that these potholes were once entire.

Their sharply-defined vettical edges are seen to coincide with

181

one well-marked joint-plane sloping steeply to the south; and
the southern sides of all three of the holes have, apparently,
been carried away, by the removal of a single large joint-block
of the granite. Since this block appears to have been torn

away bodily, leaving an angular, unglaciated surface, and no
trace of the block itself can be seen in the vicinity, we may
fairly regard its removal as the last work accomplished by the

ice-sheet on the lee side of this ledge.

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Fic. 22—PorHoLE, CooPER's ISLAND.

The fourth pothole is or was the largest of all, and hence
has been commonly called the Well." Passing over the rocky

n

elevation in a northerly direction, it may be found about a

152

=

hundred feet distant from the others; and it is a little farther
than the others from the shore, its bottom being about four
feet above high tide (Fig. 22). "The bottom of this hole,
which is still entire, so as to hold water to a depth of about one
foot, is cut in the solid granite at the base of a small scarp.
It is somewhat oval in form, the horizontal diameters varying
from two feet ten inches to four feet. The ledge rises abruptly
nine feet from the margin of the Well and ten feet from its
bottom ; and the Well itself was probably as deep at least as
ten feet, the curvature and wearing of the rock clearly showing
this. The beautifully rounded and water-worn form of the
granite on the southeast side of the Well, above the water, is a
plain indication, however, that it has not been entire on this
side since the water ceased to swirl through it. On the north
side the concave wall is vertical or slightly overhanging; but
on the west side the wall is widely flaring, receding upwards in
such a way as to indicate that the water entered at this point,
the Well agreeing in this respect with the group of potholes
(1 to 3). It seems probable that the pothole was entire in its
early stages ; but as it increased in size the outer or lee wall
was either worn through or carried: away bodily by the ice,
after which the rush of water continued long enough to develop
the graceful contours of its natural exit, as shown jn the
figure.

Besides these four potholes, there are other depressions
which are evidently incipient or embryo potholes. A linear
group of these may be observed about twenty feet north of,
and parallel with, the southern series (1 to 3). The upper
one is shallow, like the bowl of a spoon, about a foot across,
showing, extending from it, a water-worn channel sloping
easterly about ten feet to the brink of the ledge, connecting the
first depression in its course, with two others of similar form ;
while on a lower surface there is a larger depression just where
the water from the first might descend.

Considering the shallowness of the entire portions of all the

158

potholes, and the probability of their visitation by generations
of both the Indian and the white man, it is not surprising that
nothing of their contents is left in or about them. There is,
however, one rounded stone in the possession of Mr. Charles
S. Bates, the owner of Cooper's Island, which tradition states
to have been taken from the deepest pothole (No. 1). It is a
smoothly rounded and nearly spherical ball of granite about
four inches in diameter,—a typical pothole bowlder ; and there
seems to be no reason to question the truth of the tradition.

On one of the higher ledges south of Little Harbor and
Beach Street, and near the village, there is a smooth hollow in
the granite, about a foot across and nine inches deep, which,
from its form, has been called the ** Devil's Armchair.”
Although not well-defined, it is clearly a small pothole ; and in
this instance, as in others, an apparently water-worn channel
extends cast-southeast on the rather abrupt slope of the ledge
for several feet, ending in another and smaller hole. During
the past year I have discovered still another pothole. This is
on Cohasset Rocks, immediately behind the Black Rock
House, on a bare surface of granite sloping down into the sea,
and only two or three feet above the high-tide level. It is a
smoothly-worn and sharply-defined basin, somewhat pear-
shaped in outline, and measuring 41 by 33 inches in maximum
length and breadth, the major axis trending about S. 15 W.
It holds about six inches of water, but the depth below the
well-defined rim varies from 6 to 18 inches, being shallowest
on the side toward the sea.

The potholes of Cooper's Island, although on the shore, do
not directly face the sea, and the Well, especially, is quite shut
in by the granite ledge on the seaward side, as the eut shows.
They are also partly above the high-tide level; and the lower
ones are bathed only by the quiet waters of Little Harbor. If
the sea were high enough to sweep over Beach Island and the
other barriers between Little Harbor and the Atlantic, the pot-
holes would be completely submerged and so still beyond the

154
reach of the surf. In short, both the situation and the forms
of the potholes make it impossible to regard them as in any
sense the product of marine erosion. It is equally clear
that no ordinary or surface river ever flowed over these ledges.
We are forced, therefore, to ascribe their origin to the action
of glacial streams. But here the question arises as to whether
they have probably been formed by subglacial rivers rushing
along over the ledges, or by streams which, flowing over
the surface of the ice-sheet, plunge through crevasses to
the solid rocks below. Such a waterfall in the ice is called a
moulin or glacial mill; and since the crevasses must often be
of great depth—hundreds or even thousands of feet—these
glacial mills are generally recognized as the most efficient of all
agencies in the formation of potholes. Subglacial streams,
it is believed, must usually, like ordinary rivers, follow the
depressions of the rocky surface; while moulins—the chief
source of the subglacial stream—may strike the ledges with
resistless force at almost any point, and especially on the crests
and southeastern or lee slopes of prominent ledges and ridges,
where the conditions are most favorable for the formation of
profound crevasses. А strong presumption is thus created in
favor of the glacial mills as a cause of the potholes of Cohasset.
One difficulty, however, still remains. As the ice-sheet moves
continuously forward, carrying the crevasses and moulins with
it, how is it that the potholes escape elongation in the direction of
the movement? Of course a moulin cannot move forward
indefinitely, but only until a new crevasse is formed behind
the one through which the water first fell, a few feet or yards at
the most ; and the moulin then returns to its starting point. But

why do the potholes not show even this small amount of elonga-

o
tion ?

Mr. Bouvé has discussed this point in the following words! :
** [t has, indeed, been thought strange that, as the ice moved

continuously on, the holes were not found generally elongated

1 Proc. B. S. N. H., XXIV, 224.

in the direction of the movement of the glacier rather than
circular. Such thought, however, is only consistent with the
presumption that the holes were made just where the water
first fell upon the rock surface below. Far more reasonable is
it to suppose that the holes were formed somewhat distant from
this place, where the masses of rocks borne by the waters
found a lodging in some depression and there by rotation
worked out the potholes. The ice might move on and the
waters descend through the moulin far from where they first
fell, yet continue their flow in the same direction as at first and
go on with the work of rotating the contents of the hole through
a whole season. In such саве there could be, of course, no
reason to expect the elongation." In explanation of potholes
in close proximity to others and yet seemingly independent, as
in the case of the Well, Mr. Bouvé says: ** Observation upon
Alpine glaciers shows that as a crevasse is carried forward by
the general movement of the ice it closes. Subsequently а new
one is formed just where in relation to the land at the margin
of the glacier the former one existed : and the waters again
descend upon the rock surface near where they before fell, but
not often, probably, in exaetly the same place; and thus pot-
holes аге formed contiguous to each other and yet far enough
distant to make it evident that they were not produced by the
same flow of water." This is undoubtedly a true explanation
of many glacial potholes : and its weakest point, as applied to
most of the potholes of Cohasset is that it requires the water
to flow up over instead of around the prominent ledges.

Mr. Upham has suggested a different explanation. He
says': ‘ The time of the excavation of these glacial potholes
was probably the early part of the epoch of glaciation, when the
ice-sheet was being formed upon the land by snow-fall. | Upon
any hilly country the ice must have attained an average depth
somewhat exceeding the altitude of the hills above the adjoin-

ing lowlands before any general motion of the ice-sheet could

Proc. В, 8. N. H., XXIV, 226-228.

begin. During this process of slow accumulation of the ice-
sheet, the summer melting upon its surface would produce
multitudes of rills, rivulets, and brooks, which might unite
into a large stream, and this, pouring through a crevasse and
melting out a cylindric moulin, might fall a considerable depth
to the bed rock, perhaps one or two. hundred feet or more upon
an area so moderately uneven as Cohasset, while yet the ice-
motion, though sufficient to permit the formation of the crevasse,
might not have gained a definite current to carry the crevasse,
moulin, and water-fall away from the spot where they were
first formed. We may thus explain the continuation of a glacial
water-fall in one place while it was excavating one of these
** Giant's Kettles " or potholes. After the ice-sheet acquired a
current because of the greater thickness and pressure of its mass,
such deep cylindrie excavations in the bed-rock could not be
made; and during the recession and final dissolution of the ice-
sheet, it seems probable that its receding border had steeper
gradients and consequently even more rapid motion than in the
culmination of the glacial epoch.”

In criticism of this view it may be pointed out, first, that
while the movement of the ice-sheet. was still practically nil, it
would probably have accommodated itself to the irregularities of
its rocky floor without cracking; second, that the face of the
country, before it had been swept by the long-continued advance
of the ice-sheet, was probably buried beneath a considerable
thickness of soil and half-decomposed rock, the product of
chemical and mechanical decay during long preglacial ages ;
and third, that it makes no adequate allowance for glacial erosion
of the hard rocks during the long periods of the maximum
development and waning of the ice-sheet, which would inevitably
have obliterated the potholes. In short, Mr. Upham’s view
'arries as its logical consequence the reduction of glacial erosion
not only to aminimum, but almost to nothing.

While seeking for additional light upon this problem it has
oceurred to me, first, that a moulin may remain approximately

— me

=

— 1

stationary, while the ice moves on, through the backward ero-
sion and melting of its up-stream side; and, second, that when
a pothole is formed at the bottom of a moulin it is not the
direct impact of the water upon the face of the ledge that does
the work, nor do the stones carried down by the water wear
the ledges appreciably by their direct fall, but the pothole is due
to their subsequent movement and especially their rotation by
the water. This rotation implies an antecedent depression or
hollow to hold the stones, and thus the conditions are seen to
be essentially the same as for ordinary river potholes. Since
the rotation of stones in a pre-existing hollow is an essential
condition of the glacial as of other potholes, and the moulin
simply supplies the power, it would seem to make little or no
difference whether the water plunges into the up-stream side,
the middle, or the down-stream side of the hollow. The pot-
hole will be made where and only where the hollow is; and
during the progress of a moulin across the hollow there
would not, apparently, be any marked tendency to elongate it.
In the case of a linear group of potholes on the lee slope of a
ledge, it 1s reasonable to suppose that the upper one, which, on
Cooper's Island, is always the smallest and most indefinite, marks
the shifting position of the moulin, and that the others were
formed by the subglacial flow of water from the bottom of the
moulin.

Reverting once more to the question as to what phase of the
continental elaciation——the beginning, the maximum, or the
waning—was most favorable for the formation and preservation
of glacial potholes, it may be noted: First, that during the de-
velopment of the ice-sheet precipitation was mainly in the form

g; the true

of snow, and greatly exceeded the waste by melting
glacier ice was covered by a great thickness of névé and snow ;
the water escaped largely by seepage ; and the superficial streams
were small. Second, that while the growing sheet of ice, névé,
and snow must have been virtually stationary for a long time,

it must also have attained quite a high velocity (for an ice-sheet)

158

before crevasses extending up through the thick layers of

yielding névé and snow to the surface could be formed ; and the
latter condition must, of course, be realized before glacial
moulins are a possibility. Third, that it is in the highest degree
improbable that during the period of maximum glaciation such
comparatively slight inequalities as the Cohasset ledges could
have produced crevasses traversing the entire thickness of the ice-
sheet ; just as in the case of inequalities in the bed of a river,
the surface disturbance diminishes with the depth of the water.
Fourth, that sinee the decay and final disappearance of the
ice-sheet were due to a general amelioration of the climate,
resulting in part it is probable from a depression of the land,
it was probably aecomplished by such a general ablation of its
upper surface as would have caused only a slight increase in
the gradient, an increase that may have been neutralized, so
far as its effect upon the rate of movement of the ice was con-
cerned, by the greater depression of the land toward the north,
and the steadily diminishing thickness, weight, and power of
the ice. It certainly seems more reasonable to suppose that
the rate of movement of the ice-sheet diminished as its weight
and power declined, than that it continued to increase until the
final disappearance of the ice. Fifth, that when the decay of
the ice-sheet over any area was well advanced, it must have
presented, at least during a considerable part of the year, a sur-
face of hard, brittle ісе ; the conditions being thus more favor-
able than at any previous period for the existence of superglacial
streams and of crevasses traversing the entire thickness of the
lee; and it is obvious that such crevasses are now consistent
with a very slow movement of the ice over very slight inequal-
ities of the ground, for the ice has lost, in its loss of weight,
not only the power tó flow rapidly forward, but also the power
to flow around the ledges and mold itself accurately against
them without rupture of its mass. Sixth, that with the dimin-
ishing thickness of the ice-sheet there must have come a time

when it could no longer overcome the friction of so rough a

159

surface as the Boston Basin presents, and it became essentially
stationary, or was subject to slight local movements only ; and
presenting still a fairly continuous sheet of solid iee, with
surface streams and crevasses, the conditions were especially
favorable for the formation of normal glacial potholes by
stationary moulins. Seventh, the contours and distribution of
the modified drift in this region indicate that the ice-sheet
became, later, very ragged, being frayed out at the margin
and divided into numerous detached masses, so that important
surface streams and moulins ceased to exist. In conclusion,
attention may be called once more to the fact that the glacial
potholes prove by their very existence that there has been no
appreciable glaciation of the rocks in which they occur since
they were formed. In other words, they are a record of a
time when the ice-sheet still covered the land, but had become
nearly or quite stationary.

It may be remarked in this connection, also, that while the
till in this region is found to lie almost universally upon strongly
glaciated rock surfaces, the deposits of modified drift repose in
part upon glaciated, but very largely upon unglaciated and
distinctly water-worn surfaces. It is a common circumstance
also to find the protruding and stoss portions of ledges from
which the sand and gravel have been removed more or less
distinetly glaciated, while all the depressed and lee surfaces are
beautifully sculptured by water, showing many incipient pot-
holes connected with winding channels and separated by
smoothly hummocky and undulating surfaces. The glacial
striae often end abruptly at the margins of these depressed areas,
showing very distinctly the co-operation or simultaneous action
of water and ice at a time when the ice had but little erosive
power, and proving very clearly, also, that the water was

subglacial.
MODIFIED DRIFT AND TERRACES.

It is generally conceded that the recession and final dis-

appearance of the ice-sheet was attended by, and probably

160

consequent upon, a marked depression of the land. This
depression was, apparently, more than sufficient to restore the
preglacial relations of the land and sea; or at least evidence is
not wanting that the shore-line was, for a brief period, somewhat
above its present level. The re-elevation of the land here
referred to amounted to 520 feet in the vicinity of Montreal,
225 feet on the coast of Maine, but only to 20 or possibly 30
feet in the Boston Basin; and the evidences for both the eleva-
tion and the slow subsidenee which has since been in progress
are much less perfectly developed in the Nantasket area than in
other parts of the Dasin.

Partly by subglacial and superglacial streams, but mainly
by the great torrents and the temporary lakes resulting from the
final melting of the ice-sheet, the till or bowlder clay was very
largely modified, — that is, washed, assorted, and stratified in
the sand plains or deltas, gravel ridges or eskers and kames, and
clay beds. The explanation of modified drift in general requires
a constant blending or intermingling of glacial, fluvial, and
lacustrine conditions ; and probably, also, in the Boston Basin,
for the clay beds if not the lower deposits of sand, we may pos-
tulate marine conditions.

Among the various forms of modified drift the eskers or
gravel ridges appear usually to be the oldest; and being the
product, chiefly, of superglacial and subglacial streams, they
evidently record the actual presence of the ice-sheet as a fairly
continuous body of ice and not as the detached masses of ice
indieated by the kettles and other features of the sand plains.
If any important or esker-forming glacial streams crossed the
Nantasket and Cohasset area, their accumulations have been
effaced by subsequent erosion or deposition ; for no distinct
eskers have been observed. The sand plains, also, are rather
scantily and imperfectly developed.

Sand plains, in general, we must regard in part as repre-
senting the flood-plains of great rivers flowing from the melting

ice-sheet and in part as deltas formed by these rivers in temporary

—

Г

———

161

lakes or, possibly, in the case of the lowest plains, in the sea.
Most of these lakes were undoubtedly truly glacial, —that is,
enclosed partially, sometimes wholly, by solid walls of ice.
The kettles, dimpling the sand plains, it is well understood, are
due to the subsequent melting of masses of ice which were
buried in the stratified deposits; and the steep and sharply-
defined marginal slopes which the sand plains often present are
doubtless in part the natural, free, growing edges or fronts of
the deltas, and in part due to the subsequent melting of walls
of ice against which either flood-plain or delta deposits had
been accumulated, the sand and gravel, as it was thus grad-
ually let down, naturally assuming the maximum declivity.

The Nantasket Peninsula, being then surrounded by deeper
water than at present and even more remote than now from the
main land and the mouths of rivers, was almost exempt from
these delta and flood-plain deposits. The higher plains are
entirely wanting here, and the lowest, which varies usually
from fifteen to twenty-five or thirty feet in height, is developed
in only a very scanty and fragmentary manner, as on the south
side of Point Allerton. In Cohasset and Scituate the plain,
having usually a height of from forty to sixty feet, is well devel-
oped at intervals. It forms the comparatively level land along
the railroad between Hingham and Cohasset Village ; and in the
northern part of the village, and especially between Little
Harbor and the railroad, it is a very typical plain, this part
of the village being designated as the Plain“ on the published
maps of the town. It rises quite abruptly 45 feet from the
marshes of Little Harbor and, deeply dimpled with beautiful
kettles, slopes gently down on the south to the marshes and
James River. One of the kettles, on the north side of Beach Street
near the Plain, has long been known as the **Punch Bowl";
and several of them hold permanent ponds, the pond near the
south end of the Common being an example. When this plain
was formed, the ice must have completely filled the basin of
Little Harbor ; and the stream supplying the detritus probably

OCCAS. PAPERS, B. S. N. Н, IV. 1l.

162

came from the west or northwest. It is interesting to observe
that this deposit of sand is probably the only barrier between
Little Harbor and Cohasset Harbor at this point.

The considerable deposits of modified drift in the Beechwood
district, south and southeast of Scituate Pond, may be regarded
as a fragmentary development of this plain. A much lower
plain, which, quite probably, is of marine origin, has a very
perfect development in the district south of the Glades. It
terminates on the east in Mitchell’s Beach and presents a very
flat, unbroken surface, from 10 to 15 feet aboye the sea, over an

area, nearly half a mile square. It is also scantily developed

along the northern edge of the peneplain, in the vicinity of

Rockland Street, and at other points; and we may reasonably
suppose that farther inland it underlies the broad, level, and
swampy valley of Bound Brook and its tributaries.

Beds of clay, or the finer portion of the modified drift, appear
to be wholly wanting above sea-level in the Nantasket and Co-
hasset area. There can be but little doubt, however, that they
are extensively developed under the salt marshes and in the
drowned or marine portions of the valleys generally. The slow
development of the salt marshes during post-glacial time
probably commenced in every important instance with a broad,
level expanse of the finest glacial silt or clay covered by a mod-
erate depth of water and supporting а luxurious growth of eel
grass. In the meshes of this dense jungle or mat of vegetation
the detritus brought bv the tides is entrapped and slowly sinks
to the bottom. The upper surface of the deposit is thus slowly
'aised to the low-tide level, when the conditions become unfa-
vorable for the growth of eel grass and the area enters upon the
mud-flat stage. Although the main sweep of the flowing and
ebbing tides is now through more or less definite channels or
creeks, but little additional silt lodges on the unprotected flat
until it has been overrun from the shore outward by higher
forms of vegetation, when the silting-up process proceeds un-

checked until the high-tide level is reached. A dense, firm

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mat of peat-forming vegetation then covers the surface, and the
marsh is complete. In other parts of the Boston Basin, how-
ever, and generally wherever excavations have been made in
the salt marshes, the evidence is conelusive that a slow sub-
sidence has permitted the formation of several successive beds of
peat, separated by beds of clay.

Although, as the beaches, marshes, and marine cliffs so
nt

level for a very long time, the rocks of our coast are so intensely

plainly indicate, the sea has undoübtedly maintained its prese

hard and resistant that there are very few points where they
show any appreciable amount of marine erosion. All along
the Nantasket and Cohasset shore it is perfectly obvious that,
save where the rocks are very finely jointed, or a dike has
yielded to the ceaseless pounding of the waves, the ledges are
still essentially intact, showing still the roche moutonnée forms
impressed upon them by the ice-sheet, even the glacial striae
being, in some cases, well preserved for several feet below the
high-tide level.

Above the present level of the shore evidences of marine
erosion on the hard rocks are, so far as I have observed, wholly
wanting. If the sea has stood at higher levels in post-glacial

times, the evidence must be sought in the erosion, not of the

hard rocks — granite, felsite, melaphyr, etc., but of the
non-lithified or drift deposits. Fortunately, the drumlins,
which are such a prominent feature of Boston Harbor, present
in their firm but yielding material and regular outlines condi-
tions exceedingly favorable for making and preserving a record
of even a very brief occupation of a higher level by the sea.
Any one who notes the extensive erosion of the drumlins by
the sea at its present level and the comparative stability of the
erosion scarps, cannot doubt that if similar features — terraces
and scarps — had ever been developed on the drumlins at higher
levels, some indications of them would still be traceable. A
general study of the drumlins of the Boston Basin has satisfied
me that undoubted horizontal erosion-marks are a common

feature up to a height of 100 feet or more above the sea. They
exist as strongly-marked and approximately horizontal and
gy р] $
longitudinal scarps and terraces, frequently bearing bowlder
g | l À g
pavements and showing a general correspondence in height with
the sand plains of the region. They have been observed in all
] 8 M
parts of the Boston Basin; but are, perhaps, most favorably
exposed for study on several of the drumlins northeast of
Boston, including Breed's Hill, Pleasant Hill, Mt. Revere, Mt.

Washington, and Powder-Horn Hill. They are also a promi-

nent feature of several drumlins on the South Shore, including

Baker's and Otis Hills, in Hingham, and have been observed
on some of the drumlins of the Nantasket-Cohasset area. These
may be briefly deseribed, commencing on the north.

On the southwest side of Telegraph Hill, in Hull, a sloping,
bowlder-strewn terrace, from 20 to 30 feet above the sea and
backed by a steep declivity, extends directly across from shore to
shore. The only terraces on Point Allerton Great Hill are such as
have probably been formed by the gradual accumulation of soil
by rain-wash against stone walls at the lower edges of culti-
vated fields. Three such artificial terraces may be readily
traced along the southern slope at heights of from 55 to 75 feet.
The 20- to 30-foot terrace is well developed on the south and
southwest sides of Sagamore Hill, opposite the steamboat
wharf, and it may be traced along the southwest side of the
northern drumlin of World's End. A strongly marked terrace
extends for a short distance on the northeast side of Planter's
Hill, at a height of 55 feet. Occasional indications of terraces
have been noted on the southwestern slopes especially of other
drumlins in Nantasket and Cohasset, including Strawberry
Hill, Bumkin Island, Turkey Hill, Scituate Hill, ete. But the
only others requiring particular deseription are the series on the
northeast side of Booth Hill, in Scituate.

This great drumlin extends southeast from Bound Brook,
near North Scituate, for about one mile. Its northern base is

closely skirted by the railroad, and on this side it immediately

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SCITUATE.

IN

23.— НАтСН'5 Rock, ox BoorH HILL,

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165

overlooks the salt marshes tribu-
tary to the Gulf. Going up from
the marsh we come, at a height
of 25 feet, to a distinct and level
terrace, which is several hundred
feet wide midway of the length of
the hill, where the church stands
upon it, but narrows toward either
end. The road running from the
church over the hill rises, at the
cemetery, 15 or 20 feet to a well-
defined terrace and bowlder pave-
ment from 100 to 200 feet wide.
In the vicinity of the church the
main highway between North
Scituate and Scituate Harbor runs
on the lower terrace. But farther
west, near the school house, it
rises to the second terrace. From
the school house a private road
ascends the hill and brings us at
a height of about 30 feet (70
feet above the marsh) to a third
terrace, which is at least 300 feet
wide and thickly strewn with
bowlders. This terrace cannot
be clearly traced so far east as
the highway across the hill; and
like both the others, it dies out
near the west end of the hill.
Following the private road about
30 feet higher (100 feet above
the marsh) we come to a broad,
level plain, which seems at first

to form the summit of the hill; but on tracing it eastward

166

across the highway the hill is found to rise abruptly some
20 feet above the plain to the true summit, on which Hateh's
Rock (Fig. 23) stands. The plain is here at least 300 feet
wide; and at all points, save where it has been cleared for
cultivation, it is strewn with bowlders. From this terrace the
view seaward is broad and unobstructed ; and if the sea has
ever stood at this level since the drumlin was formed, its
storm waves must have broken with such resistless force against
the banks of till as to require only a short time to carve this
broad platform.

Several of the lower drumlins of this region, which show no
terraces—Green, Hoop-Pole, and Mann Hills—are so flat-topped
as almost to suggest that their original summits have been
worn away by the sea or some agent of horizontal erosion.

The lowest terraces on the drumlins consist chiefly and the
upper ones to a very limited extent of modified drift, being,
properly, narrow, fringing sand plains. But that the drumlins
of this region also exhibit many true terraces of erosion, and
that these are in many cases of such magnitude as to profoundly
modify the normal contours of the hills, there can be no doubt
or question. Concerning the origin and real significance of the
cut terraces, however, there is still room for the widest differ-
ence of opinion. As stated in the general description of the
topography (page 7), I at first regarded these terraces as true
shore-lines, and for the most part as marine shores, finding in
them evidence of an important postglacial elevation of this
coast. But a more extended and critical study of these features
since the printing of this work began has convinced me that,
as already pointed out, they lack the essential characteristics of
true shores. They are not only deficient in continuity and
uniformity of level; but it is further impossible to regard them
as marine shores, because, with rare exceptions, like Booth
Hill, they are on the southwest sides of the drumlins and do
not face the sea; or as the shores of temporary lakes, because
they are often, as in the Village of Hull and on the World’s

D

А

*

167

End, in very narrow valleys between the drumlins, that is, in
the most sheltered spots, where we would least expect erosion
by waves. Still another difficulty in the way of regarding them
as ordinary shore-marks is the fact that they occur only on the
sides of the drumlins, the scarps never, so far as observed,
changing their directions so as to cut across the ends of the
hills. hat is, they notch or break the transverse, but never
the longitudinal, profile of the drumlins. We may safely
conclude, then, that they have not been formed in or by &tanding
water.

It has been suggested to me that the terraces may, perhaps,
be regarded as original features of the drumlins, owing their
formation to the molding action of the moving ice, the drumlins
having been grooved or fluted by the ice on alarge scale. This
explanation would account for their uniform parallelism with
the major axes of the drumlins; and I am inclined to believe
that some of the less distinct terraces are really gigantic glacial
grooves, formed, of course, while the ice still covered the
drumlins, and not by the edge of the ice, since it is probable
that when the ice had melted sufficiently to uncover the tops of
the drumlins its motion had practically ceased. Starting with
the normal type of drumlin, we recognize departures from it in
several directions. Thus some are long and narrow or ridge-
like, while in others the longitudinal and transverse axes are
nearly equal, and still others, though presenting the normal
proportions of length and breadth, are unusually flat-topped,
like Booth Hill and many of the lower drumlins, including
Hoop Pole and Green Hills. "These variations are probably
original and due in part to the contours of the underlying
ledges and the character of the till and in part to the varying
thickness and motion of the ice ; and no reason is apparent why
the latter cause, at least, may not give us departures from the
regular curvature of the transverse profiles.

Asa rule, however, the terraces are too deeply incised and
much too sharply defined to be explained in this way. It is

168

highly improbable that in the case of a terrace due to olacial
ghiy g

molding we should pass abruptly, over a well-defined edge,
from the steep erosion slope or scarp behind the terrace to the
normal slope of the drumlin above the scarp; for it is incon-
ceivable that the erosive action of a continuous sheet of ice
could be so sharply differentiated. The scarps are not simply
a little steeper than the normal slopes, but they are in most
cases as steep as the material will maintain, as steep and
sharply defined, in fact, as the scarps of till on the modern
shore which have been protected for a few years from the action
of the sea.

We are thus, apparently, forced to refer the terraces to the
erosive action of running water. It is improbable that strictly
subglacial streams would cross the drumlins at all; and if they
did, the natural result would be to produce channels rather than
terraces. The same would be true of superglacial streams,
wherever they cut through the ice into the underlying drumlins ;
and such channels, notching their summits, have been observed

on some of the drumlins of eastern Massachusetts. The only
alternative view that now seems worthy of consideration is that
which refers the terraces to the erosive action of lateral streams.
When the ice had retreated from the summits of the higher
drumlins and, presumably, had ceased to flow, the heat reflected
from the ground would naturally melt away the edge of the ice
sufficiently to afford pathways for the superglacial streams, and
drainage channels for glacial waters impounded north of or
behind the drumlins. These lateral streams, having banks of
iee on one side and till on the other, would necessarily erode
the latter in the manner indicated by the existing terraces.
This explanation accounts for the longitudinal direction of the
terraces ; for the observed irregularities in height and level; for
the accumulations of modified drift sometimes observed at the
astern ends of the terraces ; and especially for the occurrence
of the terraces chiefly оп the southern slopes of the drumlins,

since it is on these slopes that the heat of the sun would be

]

—

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De

=

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169

most effective in melting away the ice, the surface of which
would naturally slope toward the drumlins. The terraces are
thus a joint produet of glacial and fluvial conditions, contempo-
raneous with the modified drift ; and like that record the rapid
waning of the ice-sheet when it had ceased to flow but yet lin-
gered in the valleys. Some gullies or vertical erosion channels
appear to have been formed on the drumlins at this time; and
probably the many other depressions and hollows observed on
their slopes, which are often ill-defined, but commonly some-
what saucer-shaped, are the result, chiefly, of local landslips
dating from this period when the till was first laid bare and was

yet unprotected by vegetation.

THE MODERN SHORE.

When the sea finally retired from its highest postglacial
level, it appears to have subsided quite rapidly to a level some-
what below the modern beach. Evidence that the land has re-
cently stood higher than now is found at other points in this
region chiefly in the form of submerged peat beds and forests,
and the former, at least, would probably be found if ехсауа-
tions were made in the Nantasket marshes. Although the
depressions occupied by the marshes and the winding channels
of Weir River Bay, Strait’s Pond, ete., are clearly submerged
land valleys, as previously explained, their origin — the erosion
of the hard rocks — undoubtedly dates from the strong eleva-
tion of the land at the beginning of the ice-age. In fact, it is
not in land erosion, but in the absenee of marine erosion аё
certain points where it might otherwise be expected to exist,
that the evidence sought for is to be found. It is a fact familiar
to all that the drumlins of the harbor exhibit marine erosion
wherever freely exposed to the sea. This erosion scarp exists,
in general, not only on the seaward, but also, in less degree, on
the landward, sides of the drumlin islands. Being the work,
almost exclusively, of the waves, it varies somewhat with the

170

breadth and depth of the water, and is most, marked on the sides
of the prevailing winds. It is, however, almost entirely
wanting on the eastern ends of Thornbush and Telegraph Hills,
Little Hog Island, Bumkin. Island, the western part of White
Head and Hampton Hill, as well as the eastern shores of
World's End and Planters’ Hill. At present, of course, these
points are bordered by shallow water and, what is still more
important, they are in the lee of Nantasket Beach, and are thus
protected from the ceaseless beating of the ocean swell. It is
perfectly clear, however, that this broad beach has been formed
since the existing relations of the land and sea were finally
established ; and if the postglacial elevation of the land had
ceased when the present level was gained, the sea, sweeping
freely through the broad passes between Strawberry Hill and
Point Allerton on the north and White Head on the south, and
the narrower gaps still further south, would inevitably have
worn away the eastern ends of the masses of till which now
enjoy the protection of the beach. Considerable and, at some
points, ineffaceable results would undoubtedly have been accom-
plished before the sea sueceeded in throwing up the barrier
beach that now checks its own depredations. The absence of
sensible erosion at these points is, however, readily explained,
if we admit that the comparatively rapid elevation of the land
probably continued until it stood somewhat higher than at
present ; and that a slow and gradual subsidence has since been
in progress. Whenever, during this subsidence, the depres-
sions between the drumlins of what is now the Nantasket
Peninsula were brought within reach of the waves, barrier
beaches were naturally formed across them ; and these barriers
have kept pace in their upward growth with the subsidence, so
that the land behind Nantasket Beach has been continuously
protected from the action of the surf. This view relieves us of
the necessity of imagining a cordon of drumlins outside of the
present beach which have been completely washed away,
although. it is not improbable that Harding's Ledge and. the

Oo

= —

171

Black Rock Islets are the foundations of such vanished
drumlins.

It is obvious to the most casual observation not only that
the present shore is strongly marked in all its various features,
but also that its level has been unchanged for a long time.
'l'his is seen especially in the essentially finished state of the
salt marshes, which, over broad and continuous areas, have
clearly attained their maximum elevation —extreme high tide ;
and in the great breadth of Nantasket Beach. The front of the
beach, when it was first established, probably formed a series
of rather strongly concave curves connecting the drumlins, all
of which were then fully exposed to the surf. The over-wash
of the beach at this time formed what is now its rather low and
marshy inner margin ; while through the additions made by
the erosion of the drumlins the line of the beach has been
gradually straightened and advanced seaward until Strawberry
Hill and the drumlins south of it, although their 'sea-cliffs are
still sharply defined, are separated and protected from the
breakers by a wide belt of sand and shingle. The Point
Allerton drumlins are still within reach of the waves. The
little hill would probably have been completely swept away
by this time but for the massive sea-wall which has been built
around it. Its present area is less than two acres, but it
probably extended once as far as the beacon and had an esti-
mated area of forty-five acres—as large as Boston Common.
Between 1847 and 1860 its cliff receded 65 feet, equal to the
loss of half an aere. The great hill has lost an estimated area
of fifteen acres, but the outward growth of the beach has now
nearly stopped the erosion. The finer material washed from
the north side of Telegraph Hill has been carried to the west-
ward to build Windmill Point, which is a miniature Cape Cod.
In like manner the waste of Green Hill, and possibly of other
drumlins which once existed in that vicinity, has formed the
barrier beaches between Strait’s Pond and the Atlantic; and

even the widest of these, Crescent Beach, is still so narrow that

172

it grows chiefly by over-wash during great storms. The same
is true of the barrier beaches separating Little Harbor from
the open sea, and those on the Scituate shore. The supply of
fresh material being now virtually eut off, the outward growth of
the beaches must substantially cease or the additions consist of
much finer material, such as might be brought from more
distant sources. The increasing fineness of the sand on Nan-
tasket Beach is apparent from the fact that distinet sand dunes
may now be seen along the front of the beach north and south of
Strawberry Hill, having a height in some cases of ten feet or
more, while they are entirely wanting on the back side of the
beach. At the south end of the beach the sand is also very fine,
and abundant traces of it may be observed at heights of from 20
to 30 feet on the north side of Atlantic Hill ; while in the valley
between Atlantic and Willow Ledge Hills a considerable bed,
not of modified drift, but of the same fine white beach sand
may be traced as far south as Atlantic Avenue. The accumu-
lation of beach sand in this sheltered spot does not appear to be
in progress now. But, although it could be used as an argu-
ment for the recent elevation of the coast, I am disposed to
ascribe it to strong northerly winds in past years.

178

SUPPLEMENT.

During the time that has elapsed since the printing of this
work began, some additional observations have been made
which, although not involving any material changes in my
views concerning the geological structure of the region, it is
thought best to indicate briefly, in order to make the exposition
of facts as complete as possible.

Besides the two small granite quarries on the Cohasset and
Scituate shores, referred to on page 17, there is a third quarry
on the south side of Scituate Hill which has afforded some
good stone for general use.

Undoubtedly the most interesting flow of melaphyr in the
Nantasket district is the second melaphyr in the Valley Beach
section described on page 52. This flow is of particular
interest, not alone because it can be traced in one direction to a
distinct edge, but also on account of the numerous, irregularly
rounded, amygdaloidal masses which it incloses. I have referred
to these masses as pseudo-bombs ; but, although well satisfied, as
stated, that they are not true volcanic bombs, or projected
masses of lava, it is very gratifying to find this view confirmed
by the observations of Prof, J. D. Dana on the lavas of the
Hawaiian Islands; and I am grateful to Mr. Geo. P. Merrill
for calling my attention to the following statements by Pro-
fessor Dana.!

The aa streams are remarkable also for the presence of lava-
balls of concentric structure that have been wrongly called
bombs. These lava-balls are smoothish exteriorly, more or less
rounded and bowlder-like, and vary in size from an inch or less
to ten feet and more.

Some of these lava-balls have, outside, a crust of hard lava,
and, inside, fragments of scoria; others consist of concentric

shells, hard and scoriaceous shells alternating with one another.

! Amer, journ. sci., 1887, ser. 3, vol. 84, p. 364.

174

One on Hawaii, near Punaluu, was found to have a nucleus of

scoria eighteen inches in diameter, and around this successively a
stony shell of three inches, a scoriaceous layer of fromone to two
inches, a stony shell of from four to five inches, and then outside
а rough lava shell six inches thick. One of large size, broken
open on one side, had had its inside filling of scoria worked out by
the natives, and so made into a small cave. A common size on
Hawaii is from three to five feet in diameter ; but one enormous
lava-ball, in the aa field west of Punaluu, measured 24 by 12 by
9 feet in its extreme dimensions, and contained at least a thou-
sand cubic feet. Enough of its hard outer shell was peeled off
to ascertain that the second layer was quite vesicular ог scoria-
ceous, and the next layer inside hard basalt again. These
Hawaiian lava-balls lie in the midst of the other blocks of the
aa stream, proving that all had a common origin, and that they
are not projected bombs, and hence properly not bombs at all.
Professor Dana! also refers in the following terms to the
paper on **Fragmentary ejectamenta of volcanoes” by Dr. H. J.
Johnston-Lavis?, who has studied with much care the Vesuvian
lavas and eruptions. He (Johnston-Lavis) shows that the
‘¢yoleanic bombs" of writers on European volcanoes are not
bombs any more than those of Mt. Loa; that they were
not projected into the air; that they occur sc: ittered. over
lava streams in great numbers when the adjoining country
is free from them, and occur within lava streams; that
they vary in size from a walnut to some cubic yards, and yet
have often a thin shell and friable nucleus; that they occur most
commonly by far on lava streams whose surface is rough and
scoriaceous instead of corded. He regards them as formed of
lapilli that fell upon the flowing lava, and, in consequence of its
forward motion, became incorporated with it, and may undergo
partial fusion, but usually congeal around themselves a coating
of the lava in which they are involved. Dr. Johnston-Lavis

! Amer, journ. sci., 1888, ser, 3, vol. 36, p. 103.

2 Proc. geologists’ assoc., London, vol. 9, no. 6.

175

also points out that the ejected blocks of solid lava are wholly
different in origin, and readily distinguished from the so-called
voleanie bombs.

The red slate referred to on page 55 as outcropping on the
edge of the marsh, west of the Rockland House, is probably not
a tuff; but it appears better to regard itas a slaty layer in the
conglomerate. A similar layer occurs in the conglomerate
south of Willow Ledge Pond; and, without wishing to suggest
the exact correlation of these very limited deposits of finer sedi-
ment, it has appeared best, on the map, to indicate the con-
glomerate in the vieinity of the pond as extending westward far
enough to embrace the second outerop of slate.

There appears now little reason to doubt that the large out-
crop of hard, slaty roek on the railroad, west of the Rockland
House, is a true tuff; and it should probably, in spite of its
finer texture, be correlated with the main bed of tuff at the base
of Atlantie Hill.

In the description, on page 67, of the faults crossing Cres-
cent Hill, it should have been noted in connection with the third
fault, which accompanies the small dike, that the dike itself
marks a fault plane of later date than the other and compensating
with reference to it. The two faults hade in opposite directions,
and the only displacement of the strata actually observable is
the elevation of the triangular mass of melaphyr between the
two faults. It now appears probable, also, that there is one
more fault on the southern slope of Crescent Hill than has been
described in the text; and it has been so represented on the map.

There appears now no sufficient reason to extend the fault
which is supposed to cross the western end of Conglomerate
Plateau southward to the granite and the boundary fault, as
suggested on page 74; and it has not been so drawn.

The position of the conglomerate overlying the melaphyr
south of Conglomerate Plateau and east of Round Hill appears
to be slightly synclinal, probably rising both to the east and west.

It now appears improbable that all of the contacts of conglom-

176

erate and melaphyr in the area between Hull Street and Straits
Pond are due to faults, as stated on page 75. The more prob-
able view is that expressed on the map, viz., that the alternate
contacts are faults. The conglomerate of Round Hill and Con-
glomerate Plateau, with a gentle southerly dip, is overlain by
the second melaphyr; and the outerops of these two beds are
repeated several times by east-west displacements downthrowing
to the north.

Recent observations have thrown additional light upon the
relations of the dikes of the central belt in the vicinity of Hull
Street ; and nearly all the dikes in the section on the railroad сап
now be traced across Conglomerate Plateau. Nos. 25 and 27 are
clearly the same dike; and this can be traced now, as shown on
the map, to the intersection of Hull Street and Atlantic Avenue.
Just west of Hull Street it appears to cross the course of
29; and what appears to be a branch from the latter quite clearly
cuts 25 on the north side of Hull Street, and then runs parallel
with it. No. 81 also branches near Hull Street; but there
seems to be no way of definitely deciding whether the dike
which ultimately reaches the shore of Strait’s pond is 29 or 31.
No. 24, on East Porphyrite Hill, is cl 'arly а continuation of 38,
on Granite Plateau ; and there is a clear path for it across Cliff
Plateau, although it does not appear to be actually exposed.

l'or the elevations of some of the drumlins in Cohasset I am
indebted to Mr. H. W. Nichols, a student in the Geological
Department of the Massachusetts Institute of Technology ; and
for many of the names on the maps I am under obligations to
Col. E. T. Bouvé. He has prepared a very interesting chap-
ter on the ancient land-marks of Hingham and Cohasset for the
forthcoming history of Hingham, and has kindly permitted
me to use the proof-sheets.

I also gladly embrace the opportunity which this supplement
affords to acknowledge the important aid in the prosecution of
this work which I have received during the past year from Mr.
Thomas A. Watson. He has not only made possible a great

€

—

economy of my time while in the field ; but he has proved him-
self an unusually acute and accurate observer, with a ready and
clear comprehension of the bearings of nearly every new fact ;
and my work has been at some points materially strengthened
by the data and conclusions which he has freely placed at my

disposal.

Occas, Papers. Boston Soc Nat. Hist.. IV. |
— t
11955" | B. umm EE P - wise i
і = at ^ |
* Р |
U H
$
Hardin gs Ledge
T *. E
Z/Z 7
@ Ay * i
* » E Í |
a Peddocks Id. |
Z / 2 |
4 |
. 2 Б he P ree. 3 D lille Black Rock —
: „„ . E Minots Ledge Lights
i ER Кы.
Lo. o [io:; AMA C /Aselnahd 1 VM | TE :
$ V Serait’s nd s i
v G . 7
Мыр у < Black Ledge |
Granite
Plateau zi 7 o T E
Z š. ^i Kimballs Pt. e. |
ae E А Brush: ld,
7 ADT
^" Stoddards Neck MUN
Baker: 5
141 |
r а. j;
7 yr us Danoda a torri as brai لد لونک‎ a
rd i
TY
Hockley“ *
LJ
|

GEOLOGY OF THE BOSTON BASIN, BY W. O. CROSBY. GENER:AL MAP, SHEET 1.

| j Scale sana or linch =2400 feet. |

| 600 1200 2400 4800
n 4 1

One Mile.

иур | , DU 777 S 5 [us
„ jones ш в ш O = p S

Quterops of Іт portant Bowlders Granitic Rocks, Conglomerate en, ^r Slate. Melaphyr Porphyrite, Outlines of Aumes — Salt Marsh. Fresh Marsh. Kettle-holes. Fau (ts.
Diorite. and Ledges. (Granite, Diorite, Series, and Tuff. Dramilins.
and Felsite.) | ; |

II

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