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

states and on the Pacific coast there are also large deposits of Creta-
ceous and Tertiary age. Elsewhere, in England, Belgium, Germany,
France and Russia, in South Africa, Australia, India and China, this
coal occurs and is mined in quantities. It is the chief coal of the
world, and the enormous increase in production in these later years
(in the United States from 137,000,000 tons in 1896 to 463,000,000
tons in 1922) points in no uncertain way to its exhaustion in a not
distant future.

Anthracite. Hard Coal. This is a compact, dense
rock, iron-black to velvet-black in color. It is brittle;
has a strong vitreous to sub-metallic luster, a more or less
pronounced conchoidal fracture, and a hardness of 2-2.5.
Specific gravity 1.4-1.8. Anthracites vary in the amount
of carbon they contain from 80-95 per cent; Pennsylvania
varieties from 85-93 and of Wales 88-95. The amount of
fixed carbon varies from 80-90 per cent; the volatile hydro-
carbons generally do not much exceed 5 per cent and the
remainder consists of moisture and ash. An analysis of
a Welsh anthracite is given as:

Carbon Hydrogen Oxygen Nitrogen Sulphur Ash Total
90.4 3.3 3.0 0.8 0.9 1.6 = 100.0

Anthracite requires a strong heat for ignition and with abundant
access of air burns with a pale blue flame, giving great heat without
smoke or odor. These qualities, with its relative cleanliness, par-



320 ROCKS AND ROCK MINERALS

ticularly adapt it to household purposes. Some anthracites exhibit
on broken surfaces a strong play of spectrum colors produced by
iridescent films and are called " peacock " coal.

Anthracite occurs not only in regions of folded strata as previously
stated, but also, though usually in no great quantity, where beds
containing ordinary bituminous coal or lignite have been invaded
by intrusive masses of igneous rock, as in New Mexico, Colorado,
Montana and Scotland. The largest and most important deposits of
anthracite are those of eastern Pennsylvania, a considerable part
of which has been already mined. It occurs also in Wales, Belgium,
France, Russia, and in the province of Shansi in China, as well as in
other places.

In addition to the carbonaceous rocks described above,
other carbonaceous and sometimes combustible substances
occur, such as graphite, ozokerite or mineral wax, asphalt
and various modifications of it, petroleum, etc., but not in
such a manner or relation that they may be properly in-
cluded in a work treating solely of rocks. Also between
the coals and the sandstones and shales, intermediate
types exist in great variety, but these are best treated
under the description of the latter rocks.

Sedimentary Deposits of Mechanical Origin.

These include the products of land waste by various
erosive agencies, which have been laid down in stratified
form, by moving currents of water in seas, lakes, and on
the flood-plains of rivers, and afterwards consolidated
into rocks, as described in the foregoing chapter. Accord-
ing to the size- of the particles they are divided into the
gravel rocks or conglomerates and breccias, into the sand
rocks or sandstones, and into the mud or clay rocks, or
shales, as previously mentioned.

CONGLOMERATES AND BRECCIAS.

Conglomerates. These consist of pebbles of various
sizes, intermingled with a finer material which acts as a
cement. The pebbles may vary from the size of a pea up
to large boulders. They are rounded by the action of



PLATE 32




A. CONGLOMERATE, OF SEDIMENTARY ORIGIN.




B. BRECCIA, OF SEDIMENTARY ORIGIN.



DESCRIPTION OF STRATIFIED ROCKS 321

water. They may consist of any kind of rock, though
generally of the harder and more resistant varieties, or
they may be of a simple mineral, usually quartz or feldspar.
The pebbles may be all of one kind or of a mixture of
several kinds of rocks or minerals. The cementing mate-
rial may also vary greatly; it may be composed chiefly of
consolidated sand, either purely siliceous or mixed sub-
stances; it may be calcareous in nature, or chiefly com-
posed of clay, or of these substances largely mingled with
iron oxide. There may be a sharp distinction between
the relatively large pebbles, and the very fine matrix in
which they are enclosed, and if this contrast is pronounced
and the matrix present in considerable amount, such
conglomerates are often called pudding stone. On the
other hand there may be gradations in size from the peb-
bles down into the matrix. There is of course great vari-
ation in the color of these rocks ; in some cases the pebbles
are sharply defined by their colors from the matrix; in
other cases the rock may have one general hue, alike for
pebbles and matrix this is more apt to be the case
where the rock has been somewhat changed or altered
from its original character.

Breccias. In a breccia the fragments which correspond
to the pebbles of a conglomerate, instead of being rounded,
are sharp and angular in character. (See Plate 32.)
This indicates, if the material has been laid down in
water, that they have suffered very little transport and
are close to their place of origin. In other respects what
has been said in regard to conglomerates will also apply
to breccias.



Conglomerates and breccias, which are composed of a single type
of rock, are generally called by its name and we thus have quartz-
ite conglomerates and breccias, limestone conglomerates and
breccias, etc. Volcanic breccias, produced by the fragmental
accumulations of eruptive activity, are really igneous rocks and
have been already described (see page 269). The material may,
however, fall into water and be rounded, assorted, and stratified,



322 ROCKS AND ROCK MINERALS

giving rise to volcanic conglomerates; such rocks are very difficult, and
sometimes impossible, to distinguish from conglomerates formed
by the erosion of such land areas as are formed for the most part of
surface extrusions of lavas.

Breccias are sometimes produced as the result of the breakage
and grinding of the rock masses along some fault plane upon which
powerful movement is occurring. The fragments thus formed may
be afterwards cemented together into firm rock by deposits from
solutions circulating in the zone of crushed and broken rock. Such
types are called friction breccias and they naturally show no evi-
dence of stratification.

Conglomerates are normally formed from deposits laid down by
swiftly moving currents of water which tend to carry away the
lighter and finer material in suspension. Hence they represent the
deposits of rapid rivers and estuarine currents. Also, when a sink-
ing land surface passes under the sea and the edge of the latter
advances, a beach formation sweeps over the land as the initial
stage to later deposits. The waves throw the coarser material,
the gravel or shingle, toward the upper part of the beach and as the
latter sweeps inland a conglomerate is thus the first deposit laid
down on the new sea bottom. Thus it is general to find a conglom-
erate or coarse sandstone as the first member of a new series of
stratified rocks, resting upon an unconformable lower series, and in
thus marking divisions of geologic time they may be of great im-
portance. They are quite common rocks and are everywhere dis-
tributed in the sedimentary formations.

In the older, and especially in strongly folded mountain regions
where the strata have suffered great pressures and shearing, the
pebbles of conglomerates are generally distorted and flattened into
lenses, or drawn out into spindle-shaped forms. The process is
generally accompanied by mineralogical changes which may be
especially noticeable in the cement. This is the first stage in the
conversion of these rocks into gneisses and schists through meta-
morphism, as described in the following chapter. On account of
their coarse and irregular appearance and unhomogeneous char-
acter conglomerates have been little used for structural purposes,
except in the roughest stone work, as in foundations, piers, etc. In
some cases breccias, which are compact and capable of a good
polish, have been cut as ornamental stones, as a reddish conglomerate
breccia from South Dakota and a vari-colored limestone breccia
from Japan. Since the discovery of the wonderful gold deposits
in conglomerates in the Rand district, South Africa, these rocks
have received much attention, as representing possible fossil placers
in which, if the gold has been concentrated by natural processes,
available sources of the precious metal might be expected.



DESCRIPTION OF STRATIFIED ROCKS 323



SANDSTONE AND RELATED ROCKS.

Sandstone. Typical sandstones are composed of grains
of quartz held together by some substance acting as a
cement. The size of grain may vary from that of peas
down to that of fine seeds; as they become finer the rocks
pass into shales, just as on the other hand they graduate
upward into conglomerates, and thus no sharp line can
be drawn between the three kinds. While some sand-
stones are very pure, consisting of quartz grains alone,
others contain intermingled particles of feldspar, garnet,
iron ore, tourmaline, flakes of mica and fragments of other
minerals. It can generally be observed with a lens that
the grains tend to be spheroidal, and that the larger they
are, the more perfect the rounding is apt to be. The
general appearance of many sandstones, with respect to
their granular texture, is much like that of loaf sugar.
As described under quartzite, to which reference should be
made, the fracture, in breaking sandstone, takes place
chiefly in the cement, leaving the grains outstanding, and
this gives the rock its sugary appearance and feeling.

Sandstones differ very much in regard to the cementing
material which holds the grains together, and thus different
varieties are produced. Sometimes it is deposited silica,
sometimes a carbonate commonly calcite, but on occa-
sion dolomite or siderite, sometimes extremely fine
argillaceous material or clay, and at other times deposited
oxides of iron, either reddish (hematite, turgite), or yellow-
ish (limonite).

The colors are very variable, white to gray, buff to
dark yellow, and brick-red to reddish bro.wn and brown, are
common; green, purple and black are rare. These colors
depend largely on the nature of the cement; in the yellow,
red and brown sandstones oxides of iron predominate, with
the other, lighter colors, it is apt to be calcareous or argilla-
ceous. In addition, the calcareous sandstones are readily
detected by their effervescing when touched with acid,



324



ROCKS AND ROCK MINERALS



while the argillaceous ones give the characteristic odor of
clay, when breathed upon. The green color is due to
glauconite, or in some cases admixed chlorite. Some
varieties appear to be almost devoid of a cement.

Sandstones are usually very porous rocks, and this
appears to depend to a large extent upon the amount and
character of the interstitial cement. Thus the ratio of
the volume of pore space to that of the rock has been
found to vary from 5 to almost 30 per cent, the latter
being about the greatest amount theoretically possible in
deposited sand grains.

The same characters also condition to a large degree
other physical properties and also explain their variations :
thus the weight per cubic foot varies from 125-150 pounds,
the crushing strength from 1500 to 15,000 pounds per
square inch. The specific gravity is about 2.6 (2.5-2.7),
with the rock pores filled with water, when weighed in it.

The chemical composition of sandstone varies con-
siderably; the chief element is silica, but the proportions
of the other elements depend on the nature of the asso-
ciated minerals and cement. Some analyses of promi-
nent sandstones used for building purposes are as follows:





SiO 2


A1 2 O 3


Fe 2 O 3


FeO


MgO


CaO


K 2 O


Na^O


H 2 O


CO 2 ,
etc.


Total


I....


99.4


0.3












0.2




99.9


II...


86.6


8.4


1.6








trace


2.4


0.7


_


_


99.7


III..


92.9


3.8


trace


0.9


trace


0.3


0.6


0.3


1.2


_


100.0


IV ..


69.9


13.6


2.5


0.7


trace


3.1


3.3


5.4


1.0


_


99.5


V ...


87.1


3.9


1.3


_


1.1


2.7


1.3


0.8


0.5


1.4


100.1


VI ..


90.7


4.6


0.4


0.1


0.1


0.1


0.5


2.8


0.4


0.3


100.0



I, White, very pure Potsdam Sandstone, Ablemans, Sauk Co.,
Wisconsin; II, Lake Superior Brownstone, Houghton, Bayfield Co.,
Wisconsin; III, Sandstone, light gray, Berea, Ohio; IV, Brownstone,
Triassic, Portland, Conn.; V, Sandstone, Triassic, near Liverpool,
England; VI, Bunter sandstone, Heidelberg, Germany.



PLATE 33.




A. SANDSTONE, OF FINE GRAIN.




B. LAMINATED SANDSTONE, WITH SLIGHT FAULTS.



DESCRIPTION OF STRATIFIED ROCKS



325



The presence of the alkalies points to that of feldspar
(or mica) in the rock; in IV the amount of feldspar must
be large, and such a rock is to be classed as an arkose
rather than a sandstone.

The structure of sandstones is essentially that of the stratified
rocks. They are sometimes thinly laminated and fissile, and some-
times very thick bedded and within the individual bed may show
a very even texture and be practically free from any evidence of
stratification. Sandstones such as the latter are valuable for
structural purposes on account of their homogeneous character
and capability for cleaving or working equally in all directions;
they are often called freestones.

These rocks are frequently distinguished according to the
character of the cement or admixed material as described above;
thus there are calcareous sandstones, argillaceous sandstones,
jerrugineous sandstones and siliceous sandstones. Micaceous sand-
stones contain considerable muscovite; the tabular flakes are par-
allel to the bedding and induce a more or less ready cleavage in the
rock, giving it a fissile character; the cleavage faces are apt to be
somewhat silvery in appearance from the mica films coating them.
Grit is a term applied to coarse-grained sandstones whose particles
are in general more or less sharply angular, and whose cementing
material is, as a rule, quite siliceous. They have been considerably
used for grindstones and millstones, hence the term " millstone grit."
In siliceous sandstones it may happen that the deposited silica is
precipitated upon the rounded or angular quartz grains in crys-
talline position, thus reconverting them outwardly into crystals;
examination with the lens shows the crystal forms and faces of the
little regenerated quartzes; these are known as crystal sandstones.

Green sandstone is a variety full of grains of glauconite which
impart a general greenish color to the rock. Sometimes these
rocks are very friable, indeed scarcely coherent, as in the Cretaceous
formations of the Atlantic border, especially in New Jersey. They
are then called greensand or, inappropriately, greensand-marl. They
are apt to contain, in addition to the sand and glauconite, iron oxides
and fossil shells, either whole or fragmentary. These deposits have
been considerably used as fertilizers. Analyses of typical green-
sands from New Jersey are as follows:



SiO 2


P 2 5


SO 3


A1 2 3


Fe 2 O 3 FeO


MgO


CaO


K 2 O


HjO


Total


34.5
51.2


1.2
0.2


1.3
0.4


6.0
8.2


31.5
23.1


2.2
2.0


2.5
0.5


1.5

7.1


18.8
6.7


99.5
99.4



326 ROCKS AND ROCK MINERALS

Arkose. This is a special variety of sandstone in which a notable
quantity of feldspar grains is mingled with those of quartz. Often
there is considerable mica present and, if the material is firmly
cemented, the rock to a casual glance may bear no small resemblance
to a granite. The particles are generally sharply angular, and the
feldspar is apt to be soft and more or less changed to kaolin. Under
a lens the irregular, clastic, angular shape of the particles readily dis-
tinguishes it from a granite. The mineral composition and the shape
of the grains show that the material has been derived from quickly
disintegrating granite and has suffered but a very short transport
before being deposited. Arkoses often grade into conglomerates
and breccias by increasing size of some of the particles. They
occur in all of the different geological formations. The red-brown
Triassic sandstones of New England are in large part arkose and
conglomerate or breccia.

Graywacke. These are sandstone-like rocks of a prevailing gray
color, sometimes brown to blackish, which, in addition to the quartz
and feldspar of an arkose, contain rounded or angular bits of other
rocks, such as fragments of shale, slate, quartzite, granite, felsite,
basalt, etc., or of varied minerals, hornblende, garnet, tourmaline,
etc. They are in reality fine-grained conglomerates and readily
pass into them by increase in size of some of the component particles.
The amount of cement, as in sandstones, is usually small and it is
generally argillaceous, but sometimes siliceous or calcareous. Such
rocks, when fine grained and compact and largely composed of feld-
spathic material, may be difficult in the hand specimen to distinguish
from some felsites, but close examination with a good lens will
generally show their nonhomogeneous character. The name has
been rather loosely used and has never had the vogue in America
that it has in Europe.

Uses of Sandstone. As is well known, sandstone is everywhere
used for constructional purposes. The ease with which it is worked,
and the large size of the blocks which may be quarried, make it
particularly valuable for this purpose. Thus in the United States
a very considerable portion of the buildings of the eastern cities are
wholly or in part of the red-brown sandstone, generally called
" brownstone," of the Triassic areas of the Atlantic border, while
for instance in Great Britain the city of Edinburgh is largely built
of the Carboniferous sandstones of that region. On account of the
insoluble nature of the iron oxide forming their cement, the red and
brown sandstones in moist climates retain much better the details of
fine cutting and carving for architectural effects, than do the
lighter colored gray or buff stones. The latter are liable to have a
calcareous cement, which dissolves under the action of atmospheric
agencies and water, allowing the stone to crumble, and thus in the



DESCRIPTION OF STRATIFIED ROCKS 327

course of years the fine details of carving are spoiled. Many ex-
amples of this may be seen in the older cities where expensive and
beautiful buildings have been much injured. If possible, in building,
a sandstone should always be laid upon the quarry bed as it is then
much less liable to flake or spall.

Sandstones are of such wide and general distribution
in all parts of the world where stratified rocks are found,
that it is unnecessary to give any detailed account of their
occurrence.

SHALE AND BELATED ROCKS.

Shale is the name given to compacted muds and clays
which possess a more or less thinly laminated, or fissile
structure. The parting is parallel to the bedding, and is
the result of natural stratification. When such rocks
have been subjected to folding and pressure, they assume
a slaty cleavage which has nothing to do with stratifica-
tion; they are then slates or phyllites and are described
among the metamorphic rocks. This distinction, that
rocks showing slaty cleavage are not shales, should be
clearly noted, as the two are often confused.

Shales are, in general, too fine grained for the component
particles to be determined with the eye, or even with the
lens. By microscopical and chemical analysis they are
known to be formed mostly of kaolin and related sub-
stances, with which may be associated much white mica,
but these are often accompanied by tiny fragments of
quartz and other minerals. As the amount of quartz
increases, and also the size of grain, the shales pass over
into sandstones, and such intermediate rocks represent
deposited silts. There are also all transitions between
clays and shales, depending on the relative firmness and
fissility of the mass.

Clay when dry is a fine, earthy, lusterless mass, giving a char-
acteristic odor when breathed upon. It clings to the tongue, and
when strongly rubbed to a powder between the fingers, it finally
produces a soft, greasy, lubricated feeling, usually thus differing
from loess, adobe, and similar appearing deposits. It absorbs water



328



ROCKS AND ROCK MINERALS



eagerly and becomes plastic. When pure it is white, but it is
generally colored red or yellow by iron oxides, forming the red and
yellow ochers, or gray, blue or black by organic substance. The
colors are sometimes evenly distributed, and sometimes irregularly
blotched, through the mass.

Shales are apt to be soft, cut more or less readily with
the knife, and are brittle and crumbly, so that taken in
connection with the fissility, it is often difficult to prepare
hand specimens of them. Like clays they exhibit a great
variety of colors, white to buff or yellow, red to brown,
purple, greenish and gray to black, and from the same
causes. Different shades of gray are perhaps the most
common. They often contain various accessory mineral
substances, such as carbonates, gypsum, rock-salt, pyrite,
etc. Some of these are frequently seen in the form of
concretions, which may attain large size, up to several feet
in diameter.

The chemical composition is somewhat variable, depend-
ing on the relative proportions of clay and other minerals.
The following analyses will serve to show the general
chemical character.



SiO 2


A1 2 O 3


Fe 2 O 3


FeO


MgO


CaO


NaaO


K 2 O


H 2 O


XyO


Total


55.0


21.0


5.0


1.5


2.3


1.6


0.8


3.2


8.1


1.9


100.4


60.6


16.4


4.9


_


1.4


1.6


0.9


3.0


9.7


1.5


100.0


61.2


15.6


1.4


3.0


4.2


3.4


0.4


6.7


2.7


1.1


99.7


53.6


17.6


4.1


3.7


5.2


2.3


2.5


2.2


8.5


0.2


99.9



I, Cambrian Shale, Coosa Valley, Cherokee Co., Alabama;
TI, Cretaceous Shale, near Pueblo, Colorado; III, Devonian Shale,
Morenci district, Arizona; IV, Cretaceous Shale, Mount Diablo.
California.

XyO = Carbonaceous matter, CO 2 , and small amounts of other
substances.

There are many varieties of shales, depending chiefly on the
presence of accessory materials. Thus there may be a large amount



DESCRIPTION OF STRATIFIED ROCKS 329

of organic matter, mostly carbon, present,, and such are called car-
bonaceous sfiales. They are black in color, and by increase of car-
bon, grade into coaly shales, shaly coals and so on into coal. They
are a very common type, and are found associated with coal and also
independently of it, sometimes covering wide areas and of great
thickness. From the nature of the organic matter they are some-
times called bituminous shales. It is probable that the total amount
of carbon in the shales far exceeds that existing in coal beds.

In other varieties of shales large amounts of carbonates, especially
carbonate of lime, are present, and these are known as calcareous
shales. By increase of this substance they pass into shaly lime-
stones. They are apt to be associated with limestones and these
calcareous varieties are detected by their ready effervescence with
acids. Or the carbonate present may be chiefly carbonate of iron
and thus produce transition forms between shales and clay iron-
stone previously described. The connection between clays, shales
and marls has been mentioned on a previous page. Alum shale is
a variety full of pyrite, or of sulphates resulting from its alteration;
it has been used for the manufacture of alum.

Uses of Clay and Shale. The use of clay in the making
of bricks, tiles, pottery, etc., is too well known to need
further comment. Shale has no value for 'structural pur-
poses, but in recent years, along with clay, it has become
of value and is used in many places as a material for the
manufacture of Portland cement, when mixed with the
proper proportion of limestone and burned. A pure,
clean shale or clay of the general composition shown in
analysis No. II, given above, is one best adapted for this
purpose, when combined with a non-magnesian limestone.

Clays and shales are such common rocks in all parts of
the world, where the unmetamorphosed stratified forma-
tions are found, that their occurrence needs no special
description.

Surficial Deposits.

This small group of geologic materials is of somewhat
diverse origin, and they are here included under this head-
ing largely as a matter of convenience. They would