Louis V. (Louis Valentine) Pirsson.

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

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Online LibraryLouis V. (Louis Valentine) PirssonRocks 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 text (page 24 of 35)
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acting with diverse material and continually varying in
strength, have assorted the sediments and deposited them
in strata. For further consideration of this subject
information should be sought in any of the numerous
manuals of Geology, but it may be stated as a general
law that sedimentary deposits are always stratified and
that, conversely, perfect stratification, resulting from an
assortment of particles, is regarded as a proof of deposit
in water.

Volcanic ash deposits are commonly rudely and sometimes well
stratified; the heavier of the particles projected upward falling first
to be succeeded by smaller lighter ones; repetitions of the process
make individualized beds and thus a rude stratification (see Plate
24 ,) while the lighter dust may be deposited by moving air currents
much as if in water.

For stratification variation, both of conditions and in the size of
the particles, is necessary; during a period in which uniformity pre-
vails in either it will be wanting. Thus aeolian deposits, which
consist of the finest sands and dust driven by the wind, are often so
uniform with respect to the size of the particles in any one area that
no stratification, or but very little, is produced. In the same way
deposits of carbonate of lime, as limestone and chalk, in the open
ocean may take place under such uniform conditions and size of
particles that beds of these rocks, perhaps a hundred feet in thick-
ness or more, are quite structureless and devoid of apparent strati-
fication throughout that extent.

Mere parallelism of layers in a rock is not in itself a mark of
stratification and therefore a proof that the rock exhibiting it is of
sedimentary origin. Its mineral composition, texture, and relation
to the accompanying rock masses, and to the general geology of the
region, must also be taken into consideration. For the spreading
action of flowing lava may draw out portions of ' unlike character
within it into thin superimposed sheets as illustrated in Plate 22.
Such lavas when fresh are easily recognizable, but buried in the
midst of sedimentary deposits and changed in appearance by geologic
ages of exposure to various agencies, they may be confused with the
accompanying stratified rocks. And again very perfect parallelism
of layers and structure may be induced in all kinds of rocks by the
shearing and metamorphism accompanying movements of the
earth's crust and mountain building. These may superficially
simulate stratification quite perfectly, but consideration of the points
mentioned above is generally sufficient to show the difference


between them. Many serious errors in understanding the real
origin of the rocks of different places and their geology have occurred
in the past through failure to properly appreciate these facts.

The individual layers of stratified rocks, which are uni-
form in texture, color and composition, may vary from the
thinness of paper to a hundred feet or more. Usually,
it will be observed that a certain layer, which has a general
similarity of character and composition that serve to
distinguish it clearly from others above and below, is
made up of much smaller subdivisions whose differences
from one another are not very marked. The larger division
is usually known as a layer or bed, the smaller ones are
termed lamince. The main differences between laminae
are generally in coloration; as shown in Plate 33, between
beds in texture and composition. As explained above,
under uniform conditions lamince may be wanting in a
particular bed. The general homogeneity of a bed is
shown by its particular hardness and appearance, its
individual method of cracking or jointing, and the way
in which it is affected by erosion, which differs from the
beds above and below it. A collection of beds lying con-
cordantly one above another, and deposited during a given
geological period of time, is called a formation.

Texture of Sedimentary Rocks. This depends upon the
relative size of the particles, which determine the fineness
or coarseness of grain, upon their shape, and upon the
amount and character of the cement, which determines the
firmness or friability of the rock. The size of grain varies
within wide bounds, but as explained previously under
gradation of material, this in itself determines largely the
kind of rock. Thus conglomerates are of necessity
coarse-grained rocks; sandstones, medium-grained ones,
and shales, fine-grained or compact. Still within the
limits of each class there is variation in this respect and
we are accustomed to speak of fine, medium and coarse-
grained sandstones; a medium grain in this rock is about
that of ordinary loaf sugar.


The shape of the component grains, when these are
megascopically visible, depends on the amount of trans-
port which they have suffered, as explained under gravel.
Usually they are more or less rounded or ovoid, but
sometimes quite angular. The latter is more apt to be
the case as the size of grain increases. Sometimes this
broken, angular character of the particles can be dis-
tinctly seen in medium-grained sandstones and arkoses
by close observation with a good lens. It shows the rock
to have a distinctly clastic nature. In the case of coarser
rocks and in conglomerates it becomes very striking, and
such rocks are called breccias and are said to have a
brecciated structure. This is illustrated in Plate 32.
Such breccias are not to be confused, however, with
volcanic breccias, as described on page 272.

The cement is that which binds the particles of sedi-
mentary rocks together and converts them from loose
material into firm rock. Various substances act in this
capacity, according to circumstances; sometimes it . is
carried into the rock from outside sources in solution
and deposited in its pores, sometimes part of the sedi-
ment itself goes into solution and is redeposited, and some-
times it consists of fine material mechanically enclosed
with the sediment. In the first and second cases silica
and carbonate of lime are common binding materials, in
the third, clay or clay-like substances perform this func-
tion. Iron oxide, probably according to the second case,
is also a not uncommon cement in the form of hematite,
or gothite, or limonite. The fine deposits of mud and
clay appear to be able to consolidate into firm rocks,
under the pressure of superincumbent masses, without
the presence of a perceptible cement, though it is some-
times present.

The firmness of the rock depends then, in part on the
amount of cement and its quality, and in part on the pres-
sure. As a result all degrees of this character are shown
by sedimentary rocks; some are very hard, firm and


compact, breaking like igneous rocks under the hammer
and susceptible of a polish, as in the case of some
limestones and sandstones, while others are so loose,
incoherent and friable that they may be readily rubbed to
powder under the fingers, as with chalks and some sand-
stones. And all gradations may be found between these

Color of Sedimentary Rocks. This depends partly on
the color of the constituent mineral grains or particles, and
partly on included substances which act as a pigment.
The most common minerals which form the sedimentary
rocks are quartz, kaolin, feldspar, calcite and dolomite;
these are white or colorless substances naturally, though
they sometimes display exotic coloration, and rocks com-
posed purely of them, without included pigment, are white,
as illustrated by certain sandstones, clays and chalk.
Generally more or less pigment is present, and the common
ones are the oxides of iron and carbonaceous matter.
The iron occurs in the form of ferric oxides, or hydrated
oxides, as hematite, or probably hydrohematite (turgite),
which gives red to red-brown colors, or as limonite, or
gothite, which produce yellow to yellowish brown tones.
Carbonaceous matter or finely divided carbon is black,
and this is the color of the rock, if it contains an excess of
it; as the amount lessens dark grays are formed, and so
on into pale grays. If both organic carbonaceous matter
and iron oxides are present in the rock, the former exerts
a controlling power over the coloring capacity of the latter
in this way; in the presence of organic matter, especially
when it is decaying, iron is reduced from the ferric to the
ferrous condition, it changes from ferric oxide to ferrous
carbonate, and as ferrous compounds are colorless or light-
colored the rock has the tones of color produced by the
carbonaceous pigment. If such rocks are exposed to
weathering and the carbonaceous material destroyed, the
iron is reoxidized and the red and yellow colors show
themselves. This is illustrated in the outcrops and on the


joint faces of many black slates which weather red or
yellow. On the other hand if the rocks are devoid of iron,
when the organic pigment bleaches out, they become white
or very light in color. And again, if solutions containing
organic matter leach through the rocks, the iron oxide
is not only changed into the ferrous condition, but when
reduced to this state, or in it originally, goes into solution
also and is carried out, the rocks thus becoming light or

The most common colors then for the sedimentary
rocks are white to light gray, to dark gray and black, or
from white to pink into red, to dark red and red-brown, or
from pale yellow to buff, to yellow-brown. The reds and
yellows are often seen commingled in the same rock mass
or layer, according to the varying iron hydroxides. In
the case of conglomerates and coarse arkose sandstones,
these colors may be modified by those of the frag-
ments of the unchanged original rocks which they may

Chemically formed Rocks. These rocks are formed in
those cases where material, which has been in solution, has
become insoluble by reason of some agency, and is pre-
cipitated. The chief agencies involved are concentration
of the solutions and organic life. In the latter case ani-
mals living in water, chiefly in the sea, secrete inorganic
material in the production of their hard parts, either skele-
tons to stiffen them, or shells as defensive armor for their
soft organisms. As the animals die these collect as depos-
its. The chief substances secreted are carbonate of lime,
CaCO 3 , and silica, Si0 2 , the former being much the more
abundant and important. Examples are seen in the for-
mation of reefs and islands by corals, and in the shell-
banks made by mollusks. Vegetable organisms also,
under certain conditions, secrete silica, and give rise to
deposits of that substance.

The deposits produced by concentration occur when
bodies of sea-water are isolated from the ocean by geol-


ogic processes and become so concentrated by evapora-
tion that they are no longer able to retain the salts
in solution. These are then deposited in the order of
their solubility. Gypsum and anhydrite, sulphates of
lime, and common salt, sodium chloride, are the most
important substances deposited in this way. The same
result occurs in lakes and inland seas in arid regions,
which have no outlet and where there is a steady concen-
tration of material in solution, brought into them by in-
flowing streams. Carbonates, sulphates and chlorides
are the main salts deposited. In a somewhat similar
manner, when water passing through the rocks becomes
mineralized by taking substances into solution and then
attains the outer air, as in springs, these substances are
deposited. Such deposits are, with respect to the masses
involved, geologically speaking, of minor importance, and
are illustrated by the deposits of carbonate of lime around
springs, and in caves, and of silica around geysers and hot-
springs in volcanic regions. A more important case is
where water, in the presence of organic matter, has leached
iron oxide from the rocks and soils and carrying it in
solution into swamps and shallow waters, has there depos-
ited it, either in the form of ferrous carbonate (siderite),
if there is excess of organic matter present, or in the reoxi-
dized form of ferric hydroxide (limonite) if it is wanting.
By this means widely extended beds of iron ore have
been formed, which are of great technical value and

Circulation of Material. Geological science is not yet
in a position to state definitely concerning the origin of
the material of the earliest formed sediments upon the
earth. We have only the fact that, wherever upon the
continents the deepest amounts of erosion have occurred
and the basement upon which the visibly earliest sedi-
ments have been deposited is exposed, this basement is of
igneous rock or of apparently igneous rock which has been
metamorphosed, and the sediments such as could have


been derived from its erosion and weathering. What-
ever the nature of the original sediments was, it is evident
that when they had been elevated to form land, since
erosive processes continued, any new sediments would be
derived from the old ones plus any material that would be
added by the continued erosion of such areas of the
original surface as the first sediments had not covered and
which still remained land, and of any fresh igneous rocks
which had come up to occupy a place in either. This con-
dition of affairs has continued to the present time; sedi-
ments have been laid down, and then elevated to form
land, sometimes being greatly metamorphosed in the
process and sometimes not, and these by their erosion
have in turn yielded fresh sediments, and so on. Thus
there has been a circulatory round of material, with
changes of conditions to affect the minerals at each stage,
and only the most resistant, such as quartz, have been
able to undergo it without change. One is a downward
course from land to sea; the return journey is the ascen-
sion of the land from the sea. The silicate minerals, which
chiefly form the mechanical sediments, have performed the
downward journey in suspension, the carbonate minerals,
on the other hand, have made it mainly in solution. This
means that sandstone, for example, on erosion is mostly
carried away mechanically, while limestone, which consists
mainly or entirely of carbonate of lime, ultimately disap-
pears mostly by going into solution, although at the begin-
ning of erosive work upon it, it may be largely mechanical
processes, which break down the rock. Some cases of
mechanical sediments consisting of carbonate of lime occur,
though not relatively of great importance, and these are
described under limestone, along with some deposits of
lime formed on land by evaporation, which may be
regarded as temporary stoppages of the material in solu-
tion on its way to the sea. This latter case is illus-
trated in the formation of travertine around springs and
in caves.


Minerals of the Sedimentary Rocks. From what has
been said in the foregoing pages, it is evident that the
minerals of the sedimentary rocks consist of those which
compose the igneous ones and which have been able to
endure without change the various conditions to which
they have been subjected, as well as the. new ones formed
by weathering and erosion. The finer the material and
the longer the time of its transport has been, the more
thoroughly it will be changed into new mineral combina-
tions. Hence quartz and feldspar are important in the
coarser-grained rocks, quartz, kaolin and mica in the finer-
grained ones; while calcite, dolomite, siderite, limonite
and gypsum represent minerals of the chemical deposits.
In the fine-grained and dense sedimentary rocks, formed
of silts, muds and clays, the particles are so fine, that from
the megascopic point of view the mineralogical composi-
tion is an element of little value in determining and classi-
fying the rock, compared with its color, texture, structure,
hardness and other qualities.

Chemical Relationships. The chemical and mineral-
ogical composition of sedimentary rocks is not dependent
on definite laws, as that of the igneous rocks evidently is.
There are no rules governing the associations of minerals,
since these have been brought together by chance, depend-
ing mostly on specific gravity, and on size of grain in the
assortment. The chemical composition has not in conse-
quence the same significance that it has in igneous rocks.
Analyses of a few of the more important types are given
in the following descriptive portion, since these may be
useful in several ways.

Classification of Sedimentary Rocks. Two modes are
used to classify the stratified rocks; one, without reference
to composition and character, is based upon the period
of their formation in the geological time scale; the other
is founded on composition and physical characters.
According to the first, strata are classified as Cambrian,
Devonian, Jurassic, Tertiary, etc.; according to the


second, as sandstones, limestones, etc. The first has its
bearing in historical geology, the second is the petrological
method, and is the one that concerns us here. In this
work the following classification is adopted.

Classification of Stratified Rocks.

1. Material of chemical origin, from solutions.

a. Deposits from concentration.

Sulphates; GYPSUM and ANHYDRITE.
Chlorides; ROCK-SALT.
Silica; GEYSERITE and related rocks.
Carbonates; TRAVERTINE and related rocks.
IRON ORES of several kinds.

6. Deposits through organic life.*

Silica; FLINT and related rocks.
Phosphate rock.
COAL, asphalt, etc.

2. Material of Mechanical Origin.

a. Water-laid deposits.




6. Wind-formed deposits.

Volcanic ashes.

c. Surface accumulations.

Laterite and various soils.

In the nature of things a classification of stratified rocks
cannot always draw exact lines between different kinds of
rocks. For shales may pass into limestones on the one
* Geyserite, Travertine, and Iron Ore may be also partly organic.


hand, and into sandstones on the other, and no sharply-
defined boundary can be drawn between them. Many
such instances could be cited.* And in cases of many
rocks of mixed materials and origin it would be difficult
to know just where to assign them. The classification
must be considered as based upon clear and unmistakable
types, which serve as center points around which the
rocks group themselves. In the descriptive portion
which follows, the exact order of this classification, in
respect to some minor rocks, for convenience in refer-
ence, may not be always exactly followed.

* Thus geyserite and travertine are in places and at times partly
organic in origin.


Chemical Deposits by Concentration and Organic Agencies.

THE more important of the deposits produced from
aqueous solutions by the material becoming insoluble
through concentration are gypsum, anhydrite, rock-salt
and calcium carbonate. Certain deposits of silica from
hot waters should also be placed here and iron ores as well,
although in the latter case the process of deposition is not
usually one of simple concentration. The connection
between gypsum, anhydrite and rock-salt, in respect to
their origin and occurrence, is very close. They are
formed in bodies of sea-water that have been separated
from the ocean by the raising of coast-lines, or by accumu-
lations of deposits, and under such climatic conditions that
the isolated water concentrates by evaporation to such a
degree that its salts must crystallize out of solution and
deposit. Or in a similar way they may be formed in
inland lakes, which have no outlet because they are in
arid regions, where the evaporation equals or exceeds
the amount of inflow. All natural flowing waters contain
more or less of various salts in solution, and in such
a lake they must indefinitely increase until the deposit-
ing point of concentration is reached.


As a rock, gypsum is fine-grained to compact; some-
times a foliated aggregate showing the excellent cleavage
of the mineral; sometimes it has a fine fibrous structure;
these forms are less common than the first one. The foli-



ated is sometimes cavernous with crystal ends projecting
into the cavities, and this may be from recrystallization
of the more compact varieties. The fibrous variety is
more apt to occur when gypsum forms thin layers or
lenses in shales and sandstones. The usual color is white,
but it is sometimes yellow or red from iron oxides, or
gray to dark gray from mingled clay or organic matter.
It is soft and easily scratched with the finger nail. For
other properties reference may be had to the description
of gypsum as a mineral.

Gypsum is likely to be accompanied by a great variety
of minerals depending on the local occurrence. The most
common and intimately related of these are rock-salt and
anhydrite, the three having a common origin as previously
stated. Clay, marl and bitumen are common impurities.
Dolomite, calcite, quartz, sulphur, iron pyrites, are not
uncommon accessory constituents. Varieties containing
bituminous substances generally yield a disagreeable odor
when broken. Gypsum is used in the manufacture of
plaster of Paris, and in the raw state as fertilizer. The
very compact white or tinted varieties are sometimes
called alabaster, and cut into ornamental forms, vases, etc.

Occurrence. Gypsum is widely distributed in the strat-
ified rocks, in the form of extensive beds, often of great
thickness, and is especially associated with limestones and
snales. It is very commonly found accompanying beds
of rock-salt; in such cases it is likely to underlie the salt.
It is also found in sedimentary formations, especially in
clays and shales, in lenticular masses or scattered through
them in isolated crystals, sometimes of great size, as in the
Cretaceous beds of the western United States.

It also occurs in volcanic regions, around fumaroles,
where sulphurous vapors are escaping, and especially
where limestones have been subjected to such action. In
some places where it is found in rocks it may be due to the
oxidation of iron pyrites and a chemical reaction of the
product with carbonate of lime.



As a rock, anhydrite is a compact to fine granular sub-
stance; sometimes coarse and showing the apparently
cubic cleavages of the individual grains. It may be
somewhat translucent, and usually has a somewhat
splintery fracture with a shimmering or pearly luster.
Its color is generally white, though, like gypsum, it is often
tinted reddish, yellowish, bluish, gray or dark by oxides
of iron, or commingled clay, or organic matter. It is
harder than gypsum but easily cut with a knife. For
the other properties see description of it as a min-
eral. The most commonly associated minerals are rock-
salt and gypsum, but locally it may contain many
others, as those stated under gypsum. In the anhydrite
beds in the strongly folded regions of the Alps, the
clay impurity has been converted into cyanite, sillimanite,
mica, etc.

The occurrence of anhydrite is similar to that of gypsum,
which it frequently accompanies. It is changed on ex-
posure to the air into that substance. The beds do not
usually show any distinct stratification. In America,
extensive deposits occur in Nova Scotia.


This is an aggregate of grains of common salt, halite
or sodium chloride. It is sometimes fine, sometimes
medium, and sometimes coarse grained. The color is
white but it is often red or yellowish from oxides of iron,
gray from intermingled clay or organic matter, and the
latter may at times produce bluish or greenish tints.
The properties of halite are mentioned in the chapter on
rock minerals.

Associated minerals sometimes found in the salt are
quartz, anhydrite and sometimes, though rarely, carbonates
or pyrite.

Rock-salt occurs in geological formations of the sedi-


mentary rocks of all ages and in many parts of the world,
The beds vary greatly in thickness, from one foot to 4000 or
more. Such enormous thicknesses cannot be explained by
the simple concentration of an isolated body of sea-water
along an arid coast-line. There must have been sub-
sidence gradually going on; at first the less soluble gyp-

Online LibraryLouis V. (Louis Valentine) PirssonRocks 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 text (page 24 of 35)