light. There are also numerous irregular fragments of aegirine and a few
small, triangular patches of nepheline with a ground-mass of quartz as a
cement. Several interesting dikes in this vicinity cut the granite and
syenite rocks. A solvsbergite dike at Andrews' point cuts hornblende
granite, and a third of a mile east from Squam lighthouse are several wide
dikes of tinguaite and quartz syenite porphyry. Within two hundred
yards of the light, there is a biotite tinguaite dike.
Dikes and masses of Labradorite gabbro occupy the greater part of
Davis' neck, Bay View. It is a very conspicuous rock with crystals of
Labradorite, some of which are two inches wide, and from three to six
inches in length.
The Dry Salvages or Tri-Salvages, and the Little Salvages, islands
east from Sandy Bay, are outcrops of hornblende syenite having little
or no quartz. The larger number of the outcrops on the north side of
Gap Head and Straitsmouth island are syenite, with some areas of micro-
graphic granite. In a southwesterly course from Gap head to the Rock-
port pumping station, extending on both sides of Cape pond, all the out-
crops are augite syenite.
From Bass Rocks, Gloucester, to Cobblestone beach, the outcropping
bed-rock is quartz syenite, porphyry, and aplite. From Brace's cove,
westerly across Eastern point to East Main street, all the outcrops are
augite syenite. Thatcher's island has about eighty acres of surface, and
is a massive outcrop of augite syenite cut by several basic dikes. The
augite syenite outcrops at Emerson's point on the mainland opposite the
' Zeitschrift fur Krystallographie, Vol. XVI, p. 261.
> - *Â»<iÂ«er''
^^^^P'^ ''r 1
^Bi ^ ^
Fig. 99. â€” PULASKITE SYENITE FORMED INTO BOULDERS W SITU BY DISINTEGRATION AND EROSION.
Fig. 100.- PULASKITE SYENITE VEINS CUTTING DECAYED HORNBLENDE GABBRO.
island, and also on the north shore of Loblolly cove. Probably this akerite
bed-rock is continuous under the sea from the mainland to Thatcher's
Pulaskite. â€” This formation varies from the augite syenite or akerite
in that it contains little or no nepheline. (See Figs. 99, 100.) In some
places, noticeably in outcrops on the Beverly shore at Curtis' point,
it becomes distinctly a hornblende zircon syenite. Here the feldspars are
microperthitic intergrowths of albite and plagioclase, with a large pro-
portion of magnetite. Still farther to the eastward, along the coast, at
Gale's point on the Manchester shore, occur veins of syenite rock, from a
few inches to two feet in width, which the author has described as aegirine
syenite, for these veins are completely filled with acicular segirine crystals,
some of which are two inches long and one-sixteenth of an inch wide.
The feldspar in this rock has the optical character of anorthoclase. North
of Fort Lee, on Salem Neck, the pulaskite becomes slightly massive in a
ledge which was opened during the construction of the fort. West from
this opening are two exposures of pulaskite, having little or no hornblende.
On the Beverly shore from Mackerel cove to Woodbury's point, the syenite
rock is largely pulaskite and a coarse akerite. Beyond the point, the out-
crops are pulaskite, and extend to the western end of West's beach.
Nordmarkite. â€” This is a mica hornblende quartz syenite rock, and
was so named by Professor Henry S. Washington of New Jersey. Its micro-
scopical structure shows augite syenite minerals, microcline-microperthite,
and the soda-microcline, which are characteristic minerals described by
Professor Brogger as occurring in the augite syenite rocks of Norway.
There are outcrops of this rock on both sides of Lobster cove, Annis-
quam, extending for a third of a mile. It is also massive in West Gloucester,
with many large outcrops. (See Figs. loi, 102.) The trend of the out-
crops from Essex avenue, West Gloucester, to Lobster cove, Annisquam,
is northeast to southwest, which is parallel to the prevailing strike of the
sedimentary beds. A narrow vein of nordmarkite extends southerly near
the creek by Essex avenue, beyond the railroad track, and an outcrop
may be seen near the cellar of the Russia Cement works. The south-
eastern extension of this rock is seen in large outcrops on both sides of
the railroad near the Anchor Forge and Iron Works, and on Rocky Neck,
Thin sections prepared from specimens collected on Pierce's island in Squam
river (see Pig. 103), have the following mineral composition : Nos. i, 2, 3, contain
numerous patches of red biotite, hornblende, and augite, in perfect crystal form.
microcline, orthoclase, microcline-microperthite, and numerous inclusions of zircons,
apatite, and magnetite, the whole cemented in a ground-mass of quartz. Thin sec-
tions prepared from specimens collected in an old and deserted quarry on the north-
east side of this island are much more porphyritic. The larger crystals are always
microcline-microperthite (sp. gr. 2.60 to 2.64). One of the sections has fine
crystals of titanite, and the quartz is in thinner films as a ground-mass or cement,
otherwise the minerals are of a similar character to Nos. i, 2, 3. Sections from an
outcrop by the side of the road leading to Coffin's beach, near a deserted quarry in
West Gloucester, are of a fine-grained rock, slightly porphyritic, with an abundance
of biotite, perfect well-twinned crystals of albite, much microcline in large, irregular
patches, microcline-microperthite, hornblende, augite, and titanite, some of the
orthoclase feldspars having areas of micropegmatite. From the great abundance
of biotite in this rock -mass it may be locally called a biotite nordmarkite (sp. gr. of
feldspars in this rock, 2.57 to 2.62). Thin sections from the augite syenite outcrop
at Wheeler's point, Gloucester, and extending to Goose cove, Annisquam, and Bay
View, give the microscopic structure as follows: Nos. i, 2, Wheeler's point, numer-
ous large porphyritic crystals of microcline-microperthite, albite and orthoclase,
good crystals of augite, hornblende, numerous crystals of titanite, some biotite,
magnetite, a little quartz, some crystals of apatite and zircons. No. 3, section from
Goose cove, is the same as the last. Nos. 4, 5, 6, sections from Bay View quarries,
contain augite and segirine. In one section. No. 5, there is a complete felting of
these ffigirine crystals which sink to the finest dust as inclusions in the microcline-
microperthite, giving the rock a deep green color.
Solvsbergite. â€” The Bostonite porphyry (Rosenbusch) on Coney Island,
is known now as solvsbergite, the name having been first applied by
Professor Henry S. Washington of New Jersey. Naugus head, on the
Marblehead shore, is largely composed of metamorphosed Cambrian sedi-
mentary rocks cut by veins and large dikes of pulaskite syenite and solvs-
bergite, which extend to Peach's point and Orne's island.
Coney island and the Coney island ledges are largely composed of a coarse
pegmatitic mass of microcline-microperthite in which the albite equals the
microcline in volume. There are also coarse plates of biotite and small
zircons, the latter held as inclusions in both the soda-microcline and the
biotite. Pulaskite syenite and hornblende gabbro, with numerous dikes,
are also present. A large dike of vesicular basalt, the vesicles being filled
with crystals of epidote, cuts across the island from northwest to south-
east. Other interesting dikes, not yet determined, cut the pulaskite and
Great Haste rock and the Haste ledges are nepheline syenites contain-
ing nepheline, sodalite, zircons, and much titaniferous magnetite.
A dike of solvsbergite, cutting the hornblende granite at Andrews'
. ^ik - jK^ ^-tAB^
^^^' ^ ' - Â«
t^- 231-1, J
Fig. 101, â€” NORDMARKITE LEDGE BROKEN DOWN BY ACTION OF FROST.
Fig. 102. â€” QUARRY OPENED IN A LEDGE OF NORDMARKITE, SHOWING IRREGULAR JOINTING
OF THE ROCK. WEST GLOUCESTER.
BIOTITE TINGUAITE 205
point, the northeastern extremity of Cape Ann, has been analyzed by
Professor H. S. Washington as follows :
CaO â– â– â– 0.85
Na O 7.28
HjO ig 0.20
Specific gravity, 2.703 at 22Â° Cent.
Biotite Tinguaite. â€” At Manchester, on Gale's rocks, two hundred yards
south of Gale's point, near low-water mark, there is a dike of a very pe-
culiar color. (See Fig. 95.) It is six inches wide, and is exposed for
twenty feet. It cuts the augite syenite in a nearly horizontal position
six feet below the surface of the syenite mass. This outcrop is only ex-
posed to view at low water. On the surface the color is a grayish-green,
mottled with bluish-black spots. A freshly broken surface is of an olive-
green color, and the spots are black. Its occurrence in the immediate
region of the segirine tinguaite dike at Pickard's point and the ffigirine
syenite at Gale's point, attaches to this rock a special interest. A micro-
scopical examination gave the following minerals in its composition:
aggirine, nepheline, sodalite, biotite, a triclinic feldspar, microperthite, and
some larger feldspars that gave optical characters suggesting anorthoclase,
and having nearly the same structure as the anorthoclase phenocrysts in
the keratophyre rock from Marblehead harbor. The black spots in the
rock were magnetic iron, a decomposition product of an original biotite.
Dr. A. S. Eakle of Harvard University made the following microscopical
and chemical analysis of the rock:
The rock is composed mainly of feldspathic laths and plates with much
nepheline and less amounts of segirine, magnetite, and biotite. A little
sodalite, apatite, and zircon are also present. The feldspars have a fib-
rous appearance caused by lamellar intergrowths of the soda and potash
feldspars, microcline and albite, forming microcline-microperthite. Carls-
bad twinning of the laths is common. Nepheline occupies the position of
206 BIOTITE TINGUAITE
a filling matter in the interspaces formed by the feldspars. The nepheline
has altered, and is present as grayish, muddy, granulated sections, which
are apparently mixtures of nepheline with kaolin and very fine grains of
quartz, ^girine is disseminated in the rock in fragments and small
crystals, in sufficient amount to give it its greenish cast, shading from
deep grass-green to an almost colorless appearance. Magnetite is promi-
nent, and marks the remains of rather large plates of a former dark silicate.
Most of the original silicate has completely disappeared, leaving only the
patches of black oxide of iron; but in an occasional section, a greenish-
brown silicate still remains between the black borders of magnetite, which
from its absorption, parallel extinction, and characteristic shimmer, is evi-
dently biotite. Sodalite is present, and also a few small crystals of apatite
and zircon as inclusions in the feldspars.
The tinguaite dike at Pickard's point, Manchester,^ contains much
analcite, and is classified as analcite tinguaite. Very little isotropic min-
eral occurs in the dike here described, and from its appearance and the
presence of chlorine, what is present is judged to be sodalite, so the dike
can hardly be classed with the one at Pickard's point. The structure of
the rock also differs in that the component minerals do not occur in needle
forms, but in much stouter lath-shapes, showing a greater degree of crys-
tallization of the individual minerals, and producing a much less dense phase
of tinguaite. The presence of many plates of feldspar tabular to M indi-
cates an approach to a solvsbergite, and the rock might perhaps with
equal right be considered a phase of a nepheline solvsbergite. It seems in
structure and composition to lie intermediate between a nepheline tinguaite
and a nepheline tegirine solvsbergite. The analysis of the rock yields :
TiOs and ZrOj o.ii
HjO at no 0.15
HjO ig 1.26
' Bulletin of Essex Institute, Vol. XXV, p. 4; and American Journal Science, Vol.
VI, p. 176.
Fig. 103. â€” PHOTOMICROGRAPH OF NORDMARKITE, GLOUCESTER.
Fi^. 104. â€” PHOTOMICROGRAPH OF /EGIRINE TINGUAITE FROM PICKARD'S POINT, MANCHESTER.
^GIRINE TINGUAITE OR ANALCITE TINGXJAITE 209
The specific gravity is 2.708. The dike is difficult to reach, and the
specimens examined come from near the surface and have altered enough
to make it difficult to estimate the mineral contents with any degree of
.^girine Tinguaite or Analcite Tinguaite. â€” Thin sections of this phono-
lite dike rock, when studied under the microscope in polarized light (see
Figs. 104, 105, 106), show that it is composed of some crystals of sodalite,
hexagonal in outline, and numerous long, irregular feldspar phenocrysts,
which sometimes are in Carlsbad twins with a quite fine multiple-twinning
and in one section there is double-twinning of the microcline structure.
Several of the -feldspar crystals have a perfectly square cross-section which
is very noticeable, and suggests a resemblance to anorthoclase phenocrysts.
Micro-chemical tests of this feldspar in hydro-fluosilicic acid give, upon
evaporation of the acid, equal numbers of crystals of sodium (NajO) and
potassiiun (K2O), but with no calcium (CaO) ; specific gravity 2.572 to
2.58. The analysis of the anorthoclase feldspars in the keratophyre rock
which was made at the laboratory of the United States Geological Survey
at Washington, by Dr. Thomas Chatard, gave Kfi, 6.98 ; NajO, 6.56. This
micro-chemical test, therefore, shows that the feldspar in this phonolite
rock is very near if not chemically equal to anorthoclase. The hexagonal
outlines of the sodalite phenocrysts are isotropic, and the mineral gelat-
inizes readily with acid, which upon evaporation gives an abundance
of common salt crystals. There are also some crystals of green augite
and brown hornblende, one of the outline hornblende crystals being filled
with minute crystals of gegirine. The holo-crystalline ground-mass is
composed of feldspars and feebly polarizing nepheline in a nearly com-
plete felting of segirine crystals and grains, some of which sink to the
finest dust. These cegirine grains are so abundant in the feldspars of the
ground-mass that the specific gravity of the feldspar in the rock-powder,
even after passing through the 100 mesh sieve, could not be clearly made
out, but with the inclusions of segirine it was as low as 2.59. This rock-
powder, gelatinized readily with acid, and, upon evaporation, an abundance
of gypsum crystals appeared, thus characterizing some of the minerals in
the ground-mass as belonging to the haiiyne group.
A chemical analysis by Dr. Henry S. Washington gave the following
' Bulletin of Essex Institute, Vol. XXIX, p. 58.
210 UMPTEKITE GABBRO
MgO 0.1 1
NazO 1 1. 45
H2O 110Â° 0.04
H2O 110Â° + 3.18
Specific gravity, 2.474 at 22Â° Cent.
Umptekite Gabbro. â€” On Salem Neck, the Beverly shore, and on
Misery island, are masses and dikes of a hornblende gabbro which varies
greatly in structure (see Figs. 107, 108) In the same dike, or general
mass, three different types have been found with various intermediate
grades. First, a compact, tough, bluish-white, feldspathic mass containing
a few grains of hornblende. Second, a crystalline rock composed of nearly
equal amounts of feldspar, hornblende, and titaniferous magnetite. Third,
a rock in which the feldspar becomes subordinate, serving merely as a
matrix to hold large porphyritic crystals of hornblende, some of which are
six inches long and three inches wide. From an analysis made by F. E.
Wright, under the direction of Professor Rosenbusch of the University of
Heidelberg, this hornblende mineral was shown to be umptekite. The
chemical analysis was as follows :
MgO . 1.30
H2O- ' Â°-''
Specific gravity, 2.732.
Fig. 105. â€” PHOTOMICROGRAPH OF /EGIRINE SYENITE FROM GALE'S POINT, MANCHESTER.
SHOWING THE /EGIRINE CRYSTALS ARRANGED IN A PLANE WITH ORTHOCLASE.
Fig. 106. â€” PHOTOMICROGRAPH OF /EGIRINE SYENITE FROM GALE'S POINT, MANCHESTER.
UMPTEKITE GABBRO 213
The rock-mass in which this umptekite form of hornblende occurs may
therefore be called umptekite gabbro.
On Salem Neck, near Collins' cove, there is an outcrop of umptekite gab-
bro, varying from exceedingly coarse- to very fine-grained forms, greatly
differing in portions of the same mass and making three distinct types.
Microscopic structure: No. i. Orthoclase with fine zonal structure, some
plagioclase with very coarse twinning, a little hornblende with inclusions of augite,
much biotite, with zircons that show pleochroic halos, much magnetite, and a
few apatite crystals scattered through the orthoclase.
No. 2. Large masses of brown hornblende, some augite, much biotite and mag-
netite, some plagioclase, a little orthoclase and apatite and zircons as inclusions in
No. 3. Orthoclase somewhat kaolinized, a little plagioclase, hornblende, augite,
and biotite. The augite is very fresh, and numerous good basal sections are seen
in the field. There is also much magnetite, some micro-zircons, garnets, and apa-
tite inclusions in the biotite.
Umptekite gabbro from Salem Neck and vicinity in the nepheline
syenite belt has the following microscopic structure when studied in thin
sections in polarized light. (See Figs. 109, no.)
No. I. Hornblende or umptekite gabbro: Numerous well-twinned plagioclase
crystals, some orthoclase, green hornblende, an abundance of perfectly fresh biotite,
crystals of olivin, some irregular patches of quartz, and some glassy plagioclase
as inclusions in the biotite and hornblende. Some of the olivin is inclosed in these
hornblende masses, and is much altered, forming magnetite. Nvmierous lime-iron
garnets and cubical iron pyrites are also seen as inclusions in the plagioclase. Crys-
tals of apatite and micro-zircons are abundant in all parts of the section. The
specific gravity of the plagioclase is 2.69.
No. 2. Salem Neck. Hornblende or umptekite gabbro: Much green horn-
blende, good sections of augite, some olivin, large patches of biotite, fine well-
twinned plagioclase, some orthoclase, a little quartz, numerous masses of quite
large apatite crystals and a few zircons. Some of the olivin is partly altered to
magnetite and serpentine.
No. 3. Salem Neck. Hypersthene umptekite gabbro: Much plagioclase, some
orthoclase, hypersthene, augite, olivin, hornblende, biotite, and a little quartz.
Otherwise as in No. 2.
On the northeastern side of Woodbury's point, Beverly, a dike-like
mass cuts the pulaskite syenite and Pre-Cambrian schistose rocks, and is
principally composed of umptekite with some brown hornblende, with
biotite developed on the edge of the hornblende. The umptekite occurs
in rectangular to irregular blocks or large crystal forms which are easily
cleavable. Occasionally a piece is found which is nearly a perfect cube
some two inches square. Usually the feldspars are in large, felty masses
of a bluish-white color, and are much decomposed, but in some places
still showing the multiple-twinning of anorthite. Magnetite occurs in
large, irregular crystal masses surrounding the umptekite/
The same rock occurs on the southwest side of Great Misery island, and
thin sections give the same microscopical character, except that the ortho-
clase and plagioclase are much fresher.
Keratophyre. â€” This formation (see Fig. iii) may be seen at low tide
near the residence of Mrs. Harding on Boden's point, Marblehead Neck.
It appears as the much eroded remains of a surface flow, and extends two
htmdred yards in a northeasterly direction, with a width of sixty feet at
the lowest point of observation. There are smaller masses of this rock three
hundred yards from this point in the same strike (northeast), which are
exposed only at extremely low tides. About five hundred yards south of
Boden's point near Flying point, the eruptive granite cuts the metamor-
phic slate, and near this point also the granite is cut by dikes of quartz-
porphyry (felsite). Near the keratophyre, and dipping under it, is a
banded aporhyolite. Both the granite and the felsite are cut by diabase
dikes. The aporhyolite tends to the northeast and forms the larger por-
tion of the bed-rock of the Neck. The banding of this aporhyolite dips
towards the harbor nearly north, and lying upon it is the keratophyre.
Between the lowest points of observation and the banded aporhyolite, a
conglomerate of varying thickness composed of fine felsitic debris, holding
rounded and angular fragments of the aporhyolite, is found in several
places inclosed in the keratophyre. In some places the keratophyre rests
directly upon the aporhyolite, while in others the conglomerate intervenes
between them. The line of contact between the keratophyre and the
rhyolite debris is well marked, and specimens detached at this point show
a basal surface very rough and pitted where it conforms to the irregulari-
ties of the conglomerate. The rock is much decomposed on the surface,
but the least altered specimens obtained are of a brownish or bluish-gray
color, having a conchoidal fracture and a compact ground-mass, holding,
occasionally, large, glassy crystals of anorthoclase, some of which are one-
fourth of an inch in length, and, rarely, plates of biotite.
Microscopical analysis shows the ground-mass to be filled with lath-
shaped feldspar crystals, which are somewhat decomposed. The base is
an earthy kaolinized mass, with irregular masses of quartz and earthy
' Min. Pet. Mitth., Vol. XIX, p. 368.
Fig. 10 7. â€” SALEMITE OUTCROP (IN THE FOREGROUND) AND LEDGE OF UMPTEKITE GABBRO
(BEYOND THE PATH), SALEM NECK.
Fig. 10 8. â€” UMPTEKITE GABBRO CUT BY VEINS OF PULASKITE SYENITE, SALEM NECK.
limonite. The phenocrysts occur as crystals with a square cross-section,
owing to the presence of the base and brachypinacoid ; in addition to the
two cleavages, there is a rough transverse Assuring. The crystals are
quite glassy when fresh. The different feldspar sections show marked
optical peculiarities ; there is often a very fine single- or double-twinning
(microcline) ; sometimes the whole of one section of the mineral consists of
irregular areas not extinguishing in common, which resemble the phenom-
ena produced by mechanical causes. These areas contain very fine lines
crossing each other at various angles in the different areas; in other cases
there is a very fine zonal structure. Sections prepared parallel to the base
show this fine, irregular double-twinning, and give an extinction iÂ° to 2Â°
oblique to the line of the second cleavage ( 00 P'SS) ; and sections parallel to
the latter cleavage give an extinction about 9Â° oblique to the line of the
first cleavage, with an obtuse positive bisectrix about perpendicular to
the face, the acute bisectrix making an angle of 9Â° with the basal cleavage.
These sections sometimes show a very fine, indistinct microperthite
striation. The angle between the two cleavages was determined in the
reflecting goniometer as approximately 89Â° 42', about that of microcline.
The specific gravity of fragments determined by Westphal balance and
Thoulet solution was between 2.570 and 2.572.
The following analyses of the feldspar (I.) and the rock (II.) were
made in the laboratory of the United States Geological Survey at Wash-
ington by Dr. Thomas Chatard.
HgO at 110Â° C .04
H3O at red heat .37
The Ti02 was not very pure, and its presence is not absolutely certain