218 KERATOPHYRE

III. IV.

Gmelin, No. I. Gmelin, No. 2.

SiOj 65.90 65.19

AljOj 19-46 19. 99

Fefi, 44 .63

CaO 28 .48

MgO

K2O 6.55 7.03

NajO 6.14 7.08

HjO 12 .34

Specific gravity, 2.587.

It is evident from the analyses and optical properties that this is a
triclinia soda-potash feldspar of remarkable purity, with very evenly bal-
anced percentages of Na and K, belonging to the anorthoclase group of
Rosenbusch. For comparison, are appended analyses (III. and IV.) by
Gmelin, of anorthoclase from the augite syenite of Norway (Brogger, " Die
Sil. Etagen 2 und 3," etc., p. 261). In the rock as a whole, the same
even balance between Na and K is noticeable, and the insignificant quan-
tity of lime and magnesia. Allowing for the free quartz, base and de-
composition products as causing a relative increase of silica and iron and
decrease of the alumina and alkalies, it is evident that the feldspars of the
ground-mass are closely allied chemically to the porphyritic crystals, and
are probably also anorthoclase. The rock is therefore a very pure type
of keratophyre.

The microscopical structure of the sections made is as follows :

No, I. Keratophyre with anorthoclase crystal cut obliquely to an optic axis.
Ground-mass made up of minute twinned lathshaped crystals of feldspar, some-
what kaolinized, some quartz, and an earthy fibrous kaolinized base. In the center
of the porphyritic feldspar crystal are numerous microliths and a few ferritic masses,
similar to and probably composed of the base, which penetrates the edges of the
crystal.

No. 2. Keratophyre and an aggregate of the porphyritic crystals. Ground-
mass nearly as in No. i. One of the phenocrysts shows twinning after the Carls-
bad type.

No. 3. Keratophyre with one porphyritically inclosed crystal (see Fig. 113).
The crystal is cut nearly parallel to the second cleavage, and gives an almost per-
fect interference figure of the positive bisectrix. The basal cleavage is well devel-
oped, and the striae, or fine twinning, are well marked in polarized light. The
ground-mass is more generally composed of the minute lath-shaped feldspar crystals,
some of which are clearly twinned anorthoclase of the same form as the larger crys-
tals. There are also small patches of quartz.

No. 4. Keratophyre with one large porphyritic feldspar crystal cut obliquely to




Fig. 10 9.— PHOTOMICROGRAPH OF HORNBLENDE UMPTEKITE GABBRO FROM SALEM NECK.




Fig. I 10. — PHOTOMICROGRAPH OF UMPTEKITE GABBRO FROM EASTERN SIDE OF MISERY ISLAND.



KERATOPHYRE 221

the brachydiagonal (see Fig. 114), which in polarized light shows a microperthitic
intergrowth and a very perfect example of fine and interrupted twinning. Through
the crystal are several fluid cavities and a few microliths of a reddish color. The
ground-mass is more kaolinized, and the minute lath-shaped crystals are less distinct.
Small, irregular masses of quartz and considerable limonite and earthy matter per-
vade this section.

No. 5. Keratophyre section cut across a joint plane which is filled with vein
quartz : numerous irregular patches of quartz are scattered all through the section.
One mass is a basal section of original (?) quartz; it gives the uniaxial cross, and is
shown to be positive by the mica plate. Some scales of biotite and numerous
small grains of magnetite are seen in the ground-mass, which is composed of a fib-
rous, feebly polarizing kaolinized mass of the decomposed minute lath-shaped feld-
spar crystals. One of the inclosed phenocrysts cut nearly parallel to the base
shows numerous microlithic inclusions, and several fluid cavities in which the bubble
movement is seen. The outer edge is deeply penetrated by the ground -mass.

The occurrence of this keratophyre as a surface flow, in close proximity
to the large intrusive masses of nepheline syenite, pulaskite syenite and
Essexite of Salem Neck and the islands in Salem harbor, and the augite
syenite of Marblehead and the Beverly shore, is interesting, as showing
the various forms assumed here by the alkaline magmas under different
geological conditions or at different periods.



CHAPTER VIII

IGNEOUS VOLCANIC ROCKS

These rocks are easily separated into two great groups, the acid and
the basic volcanics. The acid volcanics occur in massive forms. Expo-
sures may be seen on Marblehead Neck, at Swampscott, Lynn, and Saugus,
and from there extending into Middlesex County. (See Figs. 115, 116.)
These rocks also extend easterly into the bay by islands and ledges.
Another area appears at Rowley and Newbury in the form of a long and
comparatively narrow mass not over one mile in width, extending in a
northeasterly direction from Batchelder's brook, at Clay lane, Rowley,
across Rowley and Newbury to the tidal marsh beyond Pine island.

The acid volcanics are tufaceous, fragmental, and massive, and the
exposures are more extended in area than the basic volcanics. The mas-
sive eifusives often possess a compact cryptocrystalline felsitic texture, and
on weathered surfaces are seen as banded structures with light and red
to gray tints which conspicuously reveal curving and crumpled lines of
flow movement. This fluxion banding, if the rock is much weathered, is
accompanied by an easy cleavage into slabs parallel to the fluxion planes.
A good example may be observed at Boden's point, Marblehead Neck-
Below high-water mark, in the harbor off Boden's point, occurs a coarse
breccia or aporhyolite tuff, which is covered by a sheet of lava rock, kera-
tophyre, composed of anorthoclase feldspars. The keratophyre at this
point is only exposed below high-water mark. It also appears on a ledge
east of the Eastern Yacht Club's pier. On Marblehead rock, weathered
surfaces of these banded forms of volcanic rocks are conspicuously seen.
(See Fig. 112.) Amygdaloidal types of the basic volcanics occur in Row-
ley on the northern side of Clay lane, near the Dummer Academy grounds,
and may be traced for nearly two hundred yards on the summit of the
ridge. These amygdules are round, and of a red color merging into lighter
shades. In cross section, they are seen to be composed of a fine radiating
structure with an open center. In some instances the centers are much
lighter in color. A partial analysis by Mrs. E. H. Richards, of the Massa-
chusetts Institute of Technology, shows a composition of about one half




Fig. I I I. — KERATOPHYRE FROM THE HARBOR SIDE OF MARBLEHEAD NECK.
Anorthoclase crystals appear as white spots. Size of block 6x10 inches.







Fig. 112. -FOLIATED APORHYOLITE FROM A BARE LEDGE OFF IVIARBLEHEAD NECK, SHOWING
WEATHERED SURFACE.



IGNEOUS VOLCANIC ROCKS 225

silica. They are forms of spherulites and vary in size from a small pea to
a half inch in diameter.

At Kent's island, Newbury, these rocks are found in great variety.
One large mass occupies the greater part of the center of the island and is
so much decomposed that it is nearly kaolin. An area on the west
side of the island has a deposit which is fine and white as chalk and is
an excellent pottery clay.

Fliixion-banded basic forms are found on Eagle hill, Kent's island.
The rock is vesicular on the bank of Parker river and at the extreme eastern
part of the island, but it is epidotic on the summit of the hill. Previous
to the intrusion of these volcanics, the original bed-rocks of the island were
slates and sandstones cut by a mass of quartz hornblende diorite which
is now cut and brecciated by the aporhyolites, while the sedimentary beds
are calcined and so greatly metamorphosed that in places they are hardly
recognizable. At the contact of the volcanics and the diorite mass, on
the southern part of the island, the rocks are well mineralized with zinc,
copper, iron, and galena. During the mining excitement in 1875, a shaft
was sunk at this point. On the southeastern part of the island, by the
side of the railroad track, there is an exposure of a conglomerate in which
nearly all of the fragments are angular, and not only comprise volcanics
but also fragments of diorite and sandstone. Thin sections of the volcan-
ics from Pine island, Newbury, show trichites in a glassy ground-mass.
South of Parker river, in Rowley, is a spherulite rock commonly called
toadstone. It is a volcanic form in a long tongue or dike extending from
the mass north of the river and here cutting the diorite which is in turn
cut by an olivin basalt dike. This dike has calcined the spherulitic rock
so that the spherulites are of a reddish color merging into black and
giving a distinct form to the rock near its contact with the dike.

At Marblehead Neck and on High rock, Lynn, narrow dikes of quartz
porphyry, a later formation of the volcanic series, cut through all the
other numbers of these rocks, showing them to belong to a later period. A
fine example raay be seen at the first outcrop on approaching Marblehead
Neck, and another exposure, cutting across the Neck to the ocean, appears
by the side of the lane which leads from Tucker's landing to the Main
street on the Neck. South of the lighthouse this rock is extremely hard.
It has the characteristic conchoidal fracture with a very perfect jointing,
and is fotmd in blocks two or three inches wide and equally thick and
suggesting, in appearance, tiles in regular layers. Cat island and Lowell
island are largely agglomerate with several distinct forms, both coarse and



226 IGNEOUS VOLCANIC ROCKS

fine, and green and darker colored. Satan rock is a brick-red spherulitic
aporhyolite with a glassy base. The Gooseberry islands are a conglom-
erate aporhyolite, and Halfway rock is a mass of coarse porphyritic apor-
hyolite containing numerous lithophase forms. These delicate shell-like
structures are identical in form with those taken from the obsidian cliffs in
the Yellowstone and figured in the Seventh Annual Report of the United
States Geological Survey, pages 264-265.

A quartz porphyry volcanic dike at Marblehead which cuts hornblende
diorite, has ntimerous fine examples of quartz phenocrysts surroimded with
spherulites. This rock is a liparite. (See Fig. 117.) East of Boden's
point, on the harbor side, the banded-fltixion rock (see Figs. 118, 119,
120), is succeeded by a massive porphyritic form of the basic series which
contains veins of epidote. This is joined to a coarse breccia, the bed
upon which the flow of keratophyre rests. Northeast of Sparhawk
beach, the high bluff is composed of a purplish feldspar porphyritic apor-
hyolite which assumes a fluid structure near the Point and contains
numerous concretionary formations, varying in size from a small pea to a
diameter of two feet. (See Fig. 121.) Some are flattened as if after having
been ejected into the air they had fallen back into the viscid lava.

The primary constituents which have been preserved in these volcanics
are the alkaH feldspars, quartz, and magnetite. Lime-magnesian and
ferro-magnesian minerals are only found at or near a contact with some
basic dike or in the agglomerate series. On Lowell island, glaucophane
is usually present. Feldspars occur both as scattered phenocrysts and as
components of the grotmd-mass, assuming granular forms in lath-shaped
crystals, and radiating fibers. The lath-shaped feldspars are usually mi-
croperthitic. The extinction indicates that albite, orthoclase, and anor-
thoclase are present. Quartz occurs in the form of phenocrysts and also
as a constituent of the ground-mass. The textures found in these lavas
are the granular trachytic porphyritic fluxion, spherulitic, perlitic, and
amygdaloidal.

By megascopic examination, hand specimens exhibit as great a range of
color as of texture. Light-green to dark grass-green shades occur and gray
with various shades of pink and purple and also a brick-red. The frag-
mental materials, such as the breccias and agglomerates, are readily recog-
nized by the weathered surfaces, which owe their character to the vari-
ously colored fragments contained in a light-green or pink base. Red
hematite is disseminated through the feldspathic mineral as a microscopic
dust and produces the various shades of red and pink. The green colors




-PHOTOMICROGRAPH OF AN ANORTHOCLASE CRYSTAL IN KERATOPHYRE FROM
MARBLEHEAD NECK.




fig. I 14. — PHOTOMICROGRAPH OF AN ANORTHOCLASE CRYSTAL IN KERATOPHYRE FROM
MARBLEHEAD NECK.



IGNEOUS VOLCANIC ROCKS 229

are due to pyrite and epidote, derived from the alteration of the feldspa-
thic ground-mass.

In this area, the age of these volcanics is somewhat problematical.
As veins and masses are erupted into the Cambrian sedimentary rocks at
Saugus and Lynn, and especially at Kent's island, undoubtedly they are
Post-Cambrian. They are also younger than the hornblende granite, for
on Marblehead Neck they cut through and inclose masses of these granites
which are younger than the diorites. Hornblende diorite is cut by a
spherulitic aporhyolite liparite dike at Throgmorton's cove, Marblehead.

The agglomerates on Lowell island and Cat island are rhyolitic tufas,
the microscopical structure showing them to be composed of sharp-edged
fragments of the aporhyolites and volcanic glass embedded in a ground-
mass of ashy materials. Much of the glass has been altered to quartz
and the ash to an earthy chloritic mass. Some magnetite appears and
also spherulites and skeleton-crystals of augite. In places the fluxion
of the micro-felsitic ground-mass shows secondary quartz in radiating
fibrous lines.

North Gooseberry island is a large mass of porphyritic aporhyolite con-
taining considerable amounts of original glass, and a conglomerate apor-
hyolite in which the pebbles are weathered out very conspicuously. (See
Fig. 12 2.) Some of the well-rounded pebbles are from one to three inches
in diameter. The fine-grained flow has a perlitic structure and contains
much original glass, in twisted and curved forms.

At the northeastern end of Thatcher's island there is a wide, dike-like
mass occupying at least a third of the area of the island, which seems to
be a spherulitic quartz porphyry. A microscopical examination of thin
sections of this rock shows it to be composed of quartz phenocrysts, with
phenocrysts of orthoclase perthite, having albite intergrowths across the
crystals. Glaucophane, biotite, and magnetite are present; also a felsitic
ground-mass in which is considerable quartz containing original glass
and numerous fluid cavities. In the ground-mass are rutile crystals with
prisms joined at right angles with each other. Macroscopically this dike
resembles the paisanite at Magnolia,^ and in the cove between Wood-
bury's point and Hospital point, Beverly.

' Journal of Geology, Vol. VII, pp. 111-113.



CHAPTER IX

THE MINERALS OF ESSEX COUNTY

The following list of the minerals of Essex County has been prepared
after a careful examination of the work of the earlier mineralogists, and
diligent search in almost every portion of the County for species not pre-
viously noticed. Doubtless it is not absolutely complete. In studying
the rock formations, more than sixteen htmdred thin sections have been
made for microscopical study. Of the minerals enumerated nearly all
are represented by excellent specimens in the Cotmty collection in the
museum of the Peabody Academy of Science at Salem. In connection
with the minerals will be found collections illustrating the rocks of the
County and the geological formations, including photographs of the more
interesting features. The rocks from which these minerals -were taken
represent twenty-nine distinct rock-formations, and several thousand
outcropping ledges, the greater nimiber of which have never been broken
into except to collect the few specimens required to determine the char-
acter of the rock. As these ledges are worked into and studied they will,
without doubt, furnish many mineral species new to the County, and an
extremely interesting field is open to the mineralogist. The arrange-
ment of the minerals in this list follows the text book of Prof. E. S. Dana,
the tenth revised edition.

Gold. The gray copper, galena, and quartz, from the Chipman silver
mine at Newbury, contain gold, and gold has been reported from various
other mines in the neighborhood, and also from Boxford, Topsfield, Lynn-
field Centre, and Saugus. The analysis of the gray copper from the Chip-
man mine made by Prof. R. H. Richards of the Massachusetts Institute
of Technology,' gives: silver, $1,422.00 per ton; gold, $145.12 per ton and
27 per cent of copper. The galena (30 pounds) from the Chipman mine
analyzed by Professor Richards, yielded 25 lbs. of refined lead, 436.32 grains
of silver and 4.19 grains of gold. An analysis of this galena made by the
author at the Lawrence Scientific School gave silver at the rate of 27
ounces per ton. The gray copper of the quality above indicated is very

' Proceedings, Boston Society of Natural History, Vol. XVII, pp. 200-204.

230




Fig. I I 5. — CASTLE HILL, SAUGUS, A MASSIVE OUTCROP OF APORHYOLITE.
An ancient volcanic rock.




Fig. 116. — CASTLE HILL, SAUGUS.



THE MINERALS OF ESSEX COUNTY 233

rare even in Newbury, and it is doubtful if it is to be found in the County
in sufficient quantities to be mined at a profit.

Graphite. This occurs in minute foliated scales in the granitic rocks
of Peabody and Danvers, and in the slaty, carboniferous shales of Tops-
field, Middleton, and Lynnfield Centre.

Stibnite: Gray Antimony. Found associated with galena at the
Newbury and Newburyport silver mines.

Molybdenite. Found in foliated masses of considerable size at the
Pomeroy granite quarry at Gloucester, in the augite syenite at Salem
Neck, and some good specimens have been found in the diorite at Marblehead.

Silver Ore. At Newbury, Newburyport, Amesbury, Rowley, Box-
ford, and Lynnfield Centre.

Galena: Lead Ore. Found in the places last named.

Bornite : .Variegated Copper Ore. At the Luther Noyes copper mine
and the southern part of Kent's island, Newbury.

Chalcopyrites : Copper Pyrites. Found at the Luther Noyes copper
mine, the Chipman silver mine, and at Kent's island, Newbury, the
Stephen Osgood mine in South Georgetown, and the old Governor Endi-
cott copper mine in Topsfield.

Sphalerite: Zinc Blende. This occurs in considerable masses at all
of the mines in Newbury, Newburyport, and Rowley and also in much
larger quantities in the John Pettingill mine at Amesbury.

Pyrrhotite : Nickel Ore. From the Luther Noyes nickel mine in
Newbury, and in a small vein exposed in the augite syenite at Poorhouse
hill in Beverly.

Pyrite: Iron Pyrites. This occurs in large masses near the Harri-
man mine at Boxford, and in Newbury in connection with the galena
and silver ores. It is also common in small quantities in nearly all of
the bed-rocks of the County.

Marcasite : White Iron Pyrites. Found in large masses at the Luther
Noyes nickel mine, Newbury.

ArsenopyriTe : Mispickle. This occurs in thin sheets or veins at the
John Pettingill mine, Amesbury, and good specimens were found at an
old mine near the Parker river, Rowley.

Tetrahedrite : Gray Copper. Good specimens of this mineral were
found in the dump heaps of the Chipman silver mine, Newbury, and at
the Stephen Osgood mine. South Georgetown.

Halite: Salt. Found as incrustations and in acicular crystals on
rocks and the borders of tide pools at the sea-shore.



234 THE MINERALS OF ESSEX COUNTY

Fluorite: Fluor-spar. In irregular, crystalline masses in the grani-
tite at the quarry of the Rockport Granite Co., Rockport, and also asso-
ciated with galena at Lynnfield Centre.

Hematite : Specular Iron. Found on the surfaces of the slickensides
of diorite at Salem, in amphibolite at Putnamville, and in hornblende
granites at Peabody.

Hematite, var. Micaceous Hematite. Found in the bed-rock of the
Tophet hill lost gold mine, Lynnfield Centre.

Hematite, var. Red Ochre. At Beverly Cove, Danvers, Topsfield,
etc. This is the common anhydrous form.

Menaccanite : Ilmenite : Titanic Iron. Seen in microscopic patches
in nearly all of the eruptive rocks, especially in the augite syenites, dio-
rites, and mica-schists.

Leucoxene. This mineral, a decomposition product of the titanite, is
usually seen surrounding the titanite or entirely replacing it.

Magnetite ; Iron. This occurs in masses in the elceolite zircon syenite
at Great Haste ledge, Salem harbor, and is common in crystals and small
grains in all of the eruptive rocks of the County.

Chromite ; Chromic Iron. In octahedral crystals in the limestone and
serpentine at the Devil's basin, Newbury.

Rutile. Common in microscopic crystals in the metamorphic Cam-
brian rocks in all parts of the County. Larger crystals occur in the granites
at Swampscott, West Wenham, etc.

Turgite: Red Ochre. An earthy form of this mineral occurs in a
hillside, northwest of the old meeting-house, at Beverly Farms.

Limonite: Brown Hematite: Bog Iron Ore. Found in the beds of
brooks and small ponds in nearly all of the towns in Essex County. This
was the ore used at the Saugus Iron Works, in 1643, the first iron-casting
works in America.

Limonite: Brown Ochre. At the mineral paint mine, Georgetown.

Limonite : Yellow Ochre. At Danvers, Topsfield, Newbury, etc.

Limonite : Clay Iron Stone. Good specimens of this mineral are
found in pockets in the granite at the Pomeroy quarry, Gloucester.

Xanthosiderite. Found in segregated masses, stalactitic and bot-
ryoidal in form, in crevices of the granite at the quarry of the Rockport
Granite Co., Rockport.

Brucite. a mineral belonging to the magnesia group, found asso-
ciated with serpentine at the serpentine ledge, Lynnfield Centre.

Wad : Bog Manganese. Found in large masses in a meadow and brook




Fig. I 17.— PHOTOMICROGRAPH OF LIPARITE.
A quartz porphyry aporhyolite with spherulites.




Fig. I 18. — APORHYOLITE OUTCROP ON THE HARBOR SIDE OF MARBLEHEAD NECK, SHOWING THE
FLOW OF THE LAVA.



THE MINERALS OF ESSEX COUNTY 237

at Putnamville, and in the form of rotinded concretions in small ponds
and spring-holes at Peabody and Topsfield.

Quartz. Massive vein quartz occurs at North Beverly, Danvers,
and various other places in the County.

Quartz: Rock Crystal. Found in large masses and crystals in peg-
matite veins at Andover, Nahant, Rockport, etc.,

Quartz : Drusy Quartz. In minute crystals at Danvers, Nahant, and
West Newbury.

Quartz: False Topaz. Light Yellow color, Rockport.

Quartz : Smoky Quartz. The massive vein form is found in the rhyo-
lites of Marblehead and in the granite of Gloucester and Rockport.

Quartz: Cairngorm Stone. Found in nearly black crystals at the
Pomeroy quarry, Gloucester, and at Rockport.

Quartz: Milky Quartz. Massive veins occur at South Georgetown
and Groveland.

Quartz : Ferruginous Quartz. In the carboniferous slates of Topsfield.

Quartz : Rose Quartz. Occasionally found in the glacial drift.

Prase : Actinolitic Quartz. A vein occurs at Bass Point, Nahant.

Chalcedony. Good specimens occur at Prospect Hill, Beverly, and it is
also foiuid filling the amygdules of the amygdaloidal melaphyre at Saugus.

Basanite : Chert. Found in the Cambrian rocks at Peabody, Mid-
dleton, Rowley, and Nahant head, Nahant.

Jaspelite. At Saugus Centre and Nahant. This is the so-called red
jasper as popularly known.

QuARTZiTE. At Saugus, Lynnfield Centre, etc., forming large beds in
the lower Cambrian rock-mass.

Opal, var. Silicious Sinter. Found as segregated, granular, stalactitic
masses at the contact of the augite syenite and granite in Beverly.

Opal, var. Tripolite: Infusorial Earth. Found in beds of brooks and
meadows in Danvers. At "West Boxford, beds occur two feet or more
in thickness.

Hypersthene. In irregular, cleavable, crystalline grains and masses
in the hypersthene gabbro at Misery island and Salem Neck.

Wollastonite : Tabular Spar. A bladed variety of this mineral is
found at the Devil's den, Newbury.

DiALLAGE. Found in large crystalline masses at the Luther Noyes
nickel mine, Newbury.

Pyroxene, var. Augite. In irregular crystals in the augite nepheline
syenite at Salem Neck, Beverly, and Manchester.



238 THE MINERALS OF ESSEX COUNTY

DiOPSiDE, var. Brown Augite. This occurs as irregular, microscopic
crystals in the augite nepheline syenite on the Pickman estate, Beverly
Cove.

AcMiTE. This occurs as small acicular crystals in the augite syenite
at Powder House hill in Essex, and at Lanesville in Gloucester.

.^Egirine. Typical bent crystals, sometimes three inches long, are
foimd in the EEgirine syenite at Gale's point, Manchester. It is also seen
in thin sections of the elaeolite zircon syenite of Salem Neck and Beverly,