Rocks and rock minerals; a manual of the elements of petrology without the use of the microscope, for the geologist, engineer, miner, architect, etc., and for instruction in colleges and schools online

and from the shales, though many exceptions must occur.

It is therefore easy to understand that many mica-schists contain
more or less of feldspar grains among those of quartz, which are diffi-
cult to detect without the aid of microscopic investigation. These may
increase in amount until the rock passes over into a gneiss, and no
hard and fast line can be drawn between them, as previously stated.
The decision as to whether a given rock should be classed as a gneiss
or mica-schist is often a very difficult thing to make on purely mega-
Bcopic grounds; in general if the amount of mica is large, and little or
no feldspar can be detected with the lens, it is best to classify it as a



366 ROCKS AND ROCK MINERALS

mica-schist ; if the amount of mica is small and feldspar can be seen,
to define it as a gneiss.

On the other hand, in proportion as the original sandstones were
more and more purely composed of quartz grains there would be less
and less of mica made, and in this way formal transitions into quartz
schist and quartzite are produced in their metamorphic representa-
tives. We thus see that gneiss, mica-schist, and quartzite form a
graded series whose divisional lines must be purely arbitrary.

Again, as the rocks become finer and finer in grain and in texture,
the mica-schists pass into micaceous slates and so on into slates, and
this becomes more marked if the amount of carbonaceous matter
increases, as it tends to mask the mica. The divisional line thus
becomes an arbitrary one in this case also.

QUARTZITE.

Quartzite is a firm, compact rock, composed of grains
of quartz-sand united by a cement consisting of the same
material, that is, of deposited quartz. They are in general
metamorphosed sandstones, and while no hard and fast
line can be drawn between the two rocks, since all degrees
of transition can be found between them, the quartzites
are much harder and firmer than the sandstones; the
latter have a more or less sugar-granular feeling and
appearance; the individual grains are distinctly visible to
the eye or lens, while in the quartzites the fractured sur-
face is uneven, splintery or conchoidal; the luster vitreous
or greasy, like that of quartz, and the grains are imper-
ceptible or nearly so. This difference arises chiefly from
the fact that in breaking the sandstone the fracture takes
place in the cement, leaving the grains unaltered and
outstanding, while in quartzite the grains are so firmly
cemented, that there is nearly a homogeneous substance
formed and the fracture takes place through cement and
grains alike. This difference will serve as a practical dis-
tinction between the two rocks.

Minerals and General Properties. While some quartz-
ites are very pure in mineral composition, others carry in
greater or less abundance other minerals, which may be in
part remains of original mineral grains, such as feldspar



367



mixed with those of quartz, or new ones which have
resulted from the metamorphism of the clay or lime
cement, which formerly filled the interstices between the
grains of the sandstone. Such are muscovite, chlorite,
cyanite, epidote, etc. Iron hydroxides may be con-
verted into magnetite or hematite, and carbonaceous
substance into graphite. These resultant minerals are
usually of microscopic size, and may give the rock a dis-
tinct color green, blue, purple, black, etc.; sometimes
they are large enough to be clearly seen with the lens.
The most important of them is muscovite, which, as it
increases in amount, gives the rock a more schistose char-
acter, through which it attains a capacity for cleavage
along the planes of the mica. Eventually this produces a
transition into mica-schist, as previously explained under
that rock. The normal color of quartzite is white, light-
gray or yellowish into brown, but these are often modified
by included material acting as a pigment, as explained
above. The jointing of quartzite is usually platy, but
sometimes very massive, and such rocks are in some
places quarried and furnish good material for structural
purposes.

The chemical composition of a pure quartzite is nearly
that of silica alone, but as more or less clay or calcareous
material was mixed with the sand, small amounts of
alumina, iron, lime, and alkalies appear. This is illus-
trated in the contrast of the two analyses quoted below.





SiO 2


A1 2 O 3


Fe 2 3


FeO


MgO


CaO


NasO


K 2 O


H 2 O


Total


I...
II..


97.71
74.22


1.39
10.61


1.25
7.45


0.85


0.13
1.48


0.18
0.56


2.12


1.08


1.79


100.66
100.16



I, Pure quartzite, Chickies Station, Pa.; II, Impure quartzite,
Pigeon Point, Minn. Contains small quantities of feldspar, mica,
chlorite and magnetite.



368 ROCKS AND ROCK MINERALS

Varieties. The different varieties of quartzite are chiefly those
which are occasioned by the presence of some included substance.
Thus we have epidotic quartzite, graphitic quartzite, sillimanite-
quartzite, and many others. Micaceous quartzite is also called
quartz-schist. In very strongly folded and compressed mountain
regions even pure quartzite may suffer such shearing as to break
and crush the original grains and impose a more or less schistose
structure. Such rocks are called stretched quartzites. In some
places these rocks contain pebbles, of varying sizes, which retain
their original shape and are sometimes by pressure and shearing
reduced to lenticular, ovoid, or cylindrical bodies. These are called
conglomerate-quartzite and were formed from gravels, like conglom-
erate-gneisses and conglomerate-mica-schists.

Oolitic quartzite is a variety consisting of rounded grains, com-
posed of chalcedony, a slightly hydrated form of silica, deposited
around fragments of quartz which serve as nuclei. It resembles
the roe of a fish, and if the globules are sufficiently large, their con-
centric structure can be plainly seen with a lens on the broken or
polished surface of the rock. Such quartzites have been found at
State College, Pa., and in Sumatra. Buhrstone is a name given
to a variety of quartzite which is full of long, drawn out hollows
or pores. Notwithstanding the porosity, it is quite firm and its
hardness and toughness have caused its use as a millstone. It is
thought to have been originally more or less of a limestone, filled
with fossils, which, by the action of solutions containing silica, has
been converted into a quartzite, consisting mostly of chalcedony,
whose cavities represent the leached out fossils. It occurs in
western Massachusetts, Georgia, South Carolina and in the Paris
Basin in France. It is chiefly of Tertiary age.

Occurrences. Quartzite is a widely distributed rock,
mostly among the older metamorphosed strata. Thus
it is common in eastern North America, in the Rocky
Mountains Cordillera and in various localities in Europe
and other parts of the world. The occurrence of some
special varieties has been already mentioned.

Alteration. On account of the insoluble, unyielding
nature of its constituent grains and their cement, quartzite
resists erosion and the atmospheric agencies well, and,
where it is prominent in mountain regions and areas
undergoing denudation, it forms prominent features of the
landscape, bold ledges, cliffs, castellated crags, spires, etc.



DESCRIPTION OF METAMORPHIC ROCKS 369

Eventually the rock breaks down into sandy soil of poor
quality.

Distinction from Other Bocks. Quartzites, which are
very homogeneous appearing rocks, may be confused, in the
outcrop or hand specimen, with some limestones or felsites
of a similar color and texture. From the former they are
easily told by a test of the hardness, or by lack of effer-
vescence with acid; from the latter, in the field by the
different mode of geological occurrence, by the cleavage
of the feldspar if visible under the lens, or by blowpipe
test. It should be remembered that the chief minerals
composing these three rocks are quartz, calcite, and feld-
spar respectively, and they should be tested accordingly.

SLATE OR ARGILLITE.

Slates are dense, homogeneous rocks, of such fine tex-
ture that the individual mineral particles composing them
cannot be distinguished by the eye or lens, and character-
ized by a remarkable cleavage, by means of which they
split readily into broad, thin sheets, which, as is well
known, may be used for a variety of purposes.

The slates represent in metamorphic form the finest
material of the land waste by erosion, which, among the
unmetamorphosed stratified rocks, appears as clay,
shales of various kinds, etc., as previously described.
With such material more or less volcanic dust and debris,
or tuffs, may be mingled. The cause of the slaty cleavage
is discussed in a following paragraph. The difference
between slate and shale has been discussed in the de-
scription of the latter rock.

Mineral Composition and Other Properties. The mineral
particles are so fine in slate that the composition from
the megascopic standpoint is not a matter of importance.
It may be mentioned, however, that since the clays, silts,
etc., from which they are formed come from a great variety
of sources, so the microscope detects in them many and
varied minerals, the chief of which are quartz, mica,



370



ROCKS AND ROCK MINERALS



chlorite, carbonaceous substance, etc. The kaolin and
feldspar particles, which one might naturally expect, are
rare and appear to have been converted into other min-
erals. They not infrequently contain crystals of pyrite,
readily seen with the eye or lens, which may attain large
size, sometimes as distinct crystals, sometimes as concre-
tions, or replacing fossils. Veins, lumps, and lenses of
deposited quartz are also common in them, those of cal-
cite more rare. The color is chiefly gray, to dark gray, to
black, according to the amount of carbonaceous substance,
but they are often green from chlorite, or red, purple,
yellow, or brown, from the oxides of iron. The surface of
the slaty cleavage is apt to have more or less of a silky
luster, sometimes scarcely perceptible; the cross fracture
has a dull surface. While the rock is firm and never
friable, it is also rather soft, so that it may be quite readily
cut, a feature of great value for technical purposes. The
specific gravity of an average slate is about 2.8. The
chemical composition is shown in the following analyses
of typical examples, made in the laboratory of the U. S.
Geological Survey.





SiO 2


A1 2 8


Fe 2 3


FeO


MgO


CaO


Na 2 O


K 2 O


H 2 O


C


XyO


Total


I


59.7


17.0


0.5


4.9


3.2


1.3


1.4


3.8


4.1


0.5


3.8


100.2


II


67.6


13.2


5.4


1.2


3.2


0.1


0.7


4.5


3.3


_


0.7


99.9


III


59.8


15.0


1.2


4.7


3.4


2.2


1.1


4.5


3.8


_


4.3


100.0


IV


56.4


15.3


1.7


3.2


2.8


4.2


1.3


3.5


4.8


0.6


6.5


100.3


V


60.5


19.7





7.8


2.2


1.1


2.2


3.2


3.3








100.0



I, Black roofing slate, Benson, Vermont. XyO = TiO 2 , P,O 5 , CO,,
FeS 2 , etc.; II, Red roofing slate, Washington Co., New York
State; III, Green roofing slate, Pawlet, Vermont, CO 2 , 3.0; IV,
Black roofing slate, Slatington, Pennsylvania, CO 2 , 3.7; FeS 2 , 1.7; V,
Roofing slate, Wales.

The general predominance of magnesia over lime in the
analyses, as well as the small amount of the latter, shows



DESCRIPTION OF METAMORPHIC ROCKS 371

that the soluble lime silicates have been mostly dissolved
out of the silt in the process of erosion and laying down
of the sediments. The presence of carbon in the black
varieties, and of ferric iron in the red, is to be noted.

Varieties. Roofing slates are compact, very fissile varieties which
split with a smooth, even cleavage. All the different colors are used,
but the most common is a dark gray. Some slates fade when taken
from the quarry, on continued exposure, through incipient altera-
tion and the possibility of this can only be determined by practical
trial. The presence of pyrite in any notable quantity is very
prejudical, as this substance on exposure quickly alters and
gives rise to rusty stains. The slates used for blackboards and
ciphering are the blackest and most compact kinds. Calcareous
slates are those which contain a good deal of intermingled calcite,
or chalky material, which may rise to 30 per cent of the whole;
they represent slates which have been formed from original
marls.

Cleavage of Slates and its Origin. The cause of slaty cleavage
has occasioned much speculation and has been the subject of investi-
gation, both experimental and mathematical, as well as geological,
by a number of scientists. From this work it has become clear that
it is the result of great pressure upon the material and that the
planes of cleavage are at right angles to the direction of pressure.
When the fine-grained sediments are subjected to intense pressure,
unevenly shaped particles tend to rotate, so that their longer axes
are perpendicular to the direction of pressure ; they also tend to
become flattened perpendicularly to it. This tends to give the rock
a grain, an arrangement of particles, by which it tends to split more
readily along such a direction than in any other. Moreover the rock
minerals, which naturally tend to be flattened or elongate in the
shape of their particles, such as the micas, kaolin, hornblende,
chlorite, etc., possess an excellent cleavage parallel to the elongate or
flattened directions, and this is a great help in promoting the capacity
of the rock cleavage. Slaty cleavage is thus partly molecular, or
mineral cleavage, where it passes through a single mineral particle,
and partly mechanical where it passes between arranged, unlike
mineral particles. Not necessarily all of the minerals whose cleav-
age and arrangement induce the slaty cleavage are original; some
of them, micas for example, may have been formed by the meta-
morphism accompanying the pressure.

The planes of cleavage do not necessarily bear any definite rela-
tion to those of original bedding. The beds were laid down horizon-




372 ROCKS AND ROCK MINERALS

tally and the direction of pressure is also usually horizontal; the
cleavage planes are at right angles to this, and may therefore cut the
bedding at right, or highly inclined,
angles. But, as the beds may be folded
before the pressures become intense,
the cleavage planes may pass through
the bedding at various angles, although
they themselves are strictly parallel, as
seen in the diagram, Fig. 74.

Fig. 74- Slaty Cleavage in Slates, in addition to their cleav-
Folded Beds. ' . , , , , . . ,

age, are intersected by cross joints

which are frequently so numerous as to divide them into small
blocks and prevent their technical use. They generally form sys-
tems intersecting at definite angles. In the older mountain ranges
the slates are frequently crumpled by repeated movement and show
this upon their cleavage surfaces.

Occurrence. Slates are common rocks in metamorphic
regions and range geologically from the Algonkian up to
recent periods. In eastern North America they are chiefly
Paleozoic and have an extensive developement in Maine,
in Vermont, in Pennsylvania, and in Georgia. They are
also extensively distributed in the Lake Superior region
and in the older ranges of the Rocky Mountains Cordillera.
They are found in southern England, in Wales and in
many other parts of Europe.

Phyllite. Closely connected with slate by intermediate
types are a group of rocks to which the name of phyllite
has been given. The name means " leaf stone " and is
used on account of the remarkable cleavage of the rocks,
by means of which they split into exceedingly thin sheets,
in typical examples. The surface is sometimes flat,
sometimes curved, folded, or crumpled by crustal move-
ments. It differs from ordinary slate in containing a
larger amount of mica, or at all events the mica is in
larger flakes, and is more evident, giving the surface of
cleavage a shimmering or micaceous appearance, and thus
furnishing a transition form between slate and mica-
schist. The mica is a fine, scaly, silky variety of mus-
covite to which the name of sericite has been given.



DESCRIPTION OF METAMORPHIC ROCKS 373

Quartz is the other chief mineral and may sometimes be
seen on the cross fracture. Rocks, which in this country
have been called " hydromica-schists," are in large part
such phyllites. Their color is sometimes pure white,
more often tinged with reddish, yellowish, or greenish
tones, and sometimes dark colored, or black, from pig-
ments, like those of slate. They are apt to have a soft
talcy or greasy feel, and to be more brittle than slate, and
to lack its toughness and firmness. Sometimes they con-
tain visible crystals of pyrite, garnet, and other minerals.

The origin of phyllites, as shown by the researches which have been
made upon them, is a varied one; in some cases they represent sedi-
mentary material which has been metamorphosed, like the slates,
but has attained a more complete degree of recrystallization than
they have. On the other hand a considerable part of the phyllites
represent original felsites igneous rocks which have been
subjected to the energetic operation of rnetamorphism through
dynamic forces, to pressure and great shearing, aided probably by
liquids and heat. Their feldspars have been largely, if not entirely,
converted into mica, and a thin schistose or slaty cleavage has been
imposed upon them. In some extreme cases the rock appears
as if wholly composed of this silky mica. The chemical analyses of
these rocks show them to have compositions similar to that of many
felsites or felsite tuffs.

Porphyroid Sheared Felsites. In many places where phyllites
occur, they may be traced into types which are firmer, with less pro-
nounced but yet distinct cleavage, and which contain visible pheno-
crysts of quartz and feldspar, similar to those in felsite-porphyries
(embedded in the phyllitic ground mass). Such rocks have been
termed porphyroid. These again may be further traced into un-
doubted felsites which still retain the phenocrysts, flow structures,
spherulites, etc., characteristic of lavas, or the broken, angular, frag-
mental features of tuffs and breccias, in spite of the slaty cleavage,
which to a greater or lesser degree, has been imposed upon them
by the dynamic movements and shearing to which they have been
subjected. These again may be followed into undoubted, unsheared
felsites. Rocks with these characters, in their varied types as
described above, occur in various places among the older metamor-
phosed Paleozoic areas of eastern North America, in Maine, at South
Mountain, Pa., in Virginia and North Carolina, in Wisconsin, the
Lake Superior region, etc. They have been found of various ages
in Great Britain, Germany, the Alps and other places in Europe.



374 ROCKS AND ROCK MINERALS

In Sweden such ancient felsites and felsite tuffs, hardened and
more or less metamorphosed, have been termed hdlleflinta.

It is only in comparatively recent years that such altered igneous
rocks, with more or less schistose appearance and cleavage, have been
recognized and their significance appreciated. The older geologists,
confused by their cleavage, regarded and mapped them as slates and
considered them as of sedimentary origin. They are of interest,
because, as stated in the introduction to metamorphic rocks, these
latter comprise material both of igneous and sedimentary origin.
Of the feldspathic, igneous rocks, the coarser-grained ones, like
granite, as we have seen, yield gneisses ; the compact felsites and their
tuffs under the metamorphic agencies of pressure, shearing, etc., are
turned into phyllites, porphyroids, and compact slaty rocks, according
to the degree to which these agencies have acted. The igneous
ferromagnesian rocks we shall see later among the amphibolites
and other schists.

TALC-SCHIST.

Talc-schist is a rock of pronounced schistose cleavage and
character, in which talc is the predominant mineral. The
talc is present in fine scales to coarse foliated aggregates.
Other minerals also occur in different varieties of the rock,
such as quartz in grains, lenses, and veins ; or magnetite and
chromite in black specks and grains; hornblende, usually
in white or green prisms, or crystals of enstatite; chlorite
mingled with the talc, etc. The color is usually light,
white to pale green, or yellowish, or gray; sometimes dark
gray or greenish. The rock is soft and the talc gives it
a greasy feeling, and often a pearly or tallowy appearance
on the cleavage surface. In addition to its micaceous
appearance and soft greasy feel, the talc is easily told by
its infusibility before the blowpipe, and its insolubility in
acids. The rock cleavage is sometimes thinly fissile,
sometimes thicker, and sometimes cleavage is nearly
wanting, the rock is more nearly massive, is compact, and
has a lard-like or wax-like aspect, and approaches soap-
stone in character. Chemically, these rocks consist mostly
of silica and magnesia with small amounts of water and
other oxides.



DESCRIPTION OF METAMORPHIC ROCKS



375





SiO 2


A1 2 O 3


Fe 2 O 3


FeO


MgO


CaO


Na 2 O+K 2 O


H 2 O


Total


I..


58.7


9.3


4.4




22.8


0.9




4.1


100.2


II..


53.3


4.4


5.8


1.0


29.9


1.5


1.5


2.6


100.0



I, Talc-schist, Falun, Sweden; II, Talc-schist, Zobtau, Moravia,
Austria.

The composition is quite similar in its general features
to that of the peridotites among igneous rocks, as may be
seen by reference to their analyses.

The talc-schists undoubtedly represent material which was some-
times of igneous origin, peridotite, pyroxenite, or dunite, and some-
times of sedimentary origin, dolomitic, ferrugineous marls, etc. It
may not be possible from field work and an inspection of specimens
alone, unless aided by chemical analyses and microscopical study,
to decide in any given case which origin the material had, and some-
times not even then. The presence of chromium, either in the form
of chromite or of secondary minerals derived from it, such as
kammererite or fuchsite (a variety of muscovite green from chrom-
ium), is indicative of igneous origin, while that of much quartz and
dolomite mingled with the talc, which produces the variety of talc-
schist called listwanite, would be on the other hand more indicative
of a sedimentary one.

Talc-schists do not form important formations like
gneiss, mica-schist, and slates, but are limited in occurrence,
being found as interbedded layers or inclusions, chiefly as
lenticular masses, in other metamorphic rocks, and are
really not very common. They show transitions in places
into other rocks, such as chlorite-schist, crystalline dolo-
mite, quartzite, etc. Such transitions, or the lack of
them, may furnish useful hints in regard to their origin in
particular cases. In eastern North America talc-schists
occur, associated with other metamorphic rocks, in
Canada, in the New England states, in northern New
York, and south to Georgia. They are also found in the
Rocky Mountains region and in the Pacific states, Cali-



376



ROCKS AND ROCK MINERALS



fornia, Oregon, etc. In Europe they occur in the Alps,
Germany, and various other places, in Sweden, Finland,
etc. They also occur in Brazil and other parts of the world.
Their occurrence, though not generally of wide geologic
interest, is important because they furnish a source of
supply for talc, which is used for a variety of purposes.

CHLORITE-SCHIST.

These rocks are schists which have the mineral chlorite
as their chief determinant mineral. It occurs as fine,
scaly aggregates, sometimes too fine for the individual
scales to be seen by the eye; more rarely in foliated to
coarse foliated aggregates. It is sometimes thinly, some-
times thickly, schistose, and in some cases almost massive;
and although the rock is very soft and may be readily cut,
it is very tough in the more massive varieties. The color
varies through different shades of green, yellow-green, to
dark green. Different minerals are apt to accompany the
chlorite, some of which may be in megascopic sizes; of
these may be mentioned magnetite, often in fine crystals;
hornblende in slender needles or prisms; corundum and
cyanite in some cases; quartz, which is generally in veins
and lenses; epidote in grains and crystals; in some instances
graphite, calcite, dolomite, etc. The chemical composi-
tion of these rocks is very variable, so far as is known,
for not many have been investigated; it indicates that