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 23 of 35)
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by flows of lava, and the cone is consequently of com-
posite character.


Classification. The particles of magma driven into the
atmosphere and solidified and the pieces of rock are of all
dimensions from the finest dust, which may float for years,
to huge masses weighing several hundred pounds. By
general usage, for the sake of convenience, the following
sizes are roughly distinguished: pieces the size of an apple
or larger are called bombs; those the size of a nut are
termed lapilli; those the size of small peas or shot, ashes;
the finest is known as volcanic dust. Sometimes the
bombs, lapilli, etc., are sharply angular and sometimes
smoothly rounded off a form caused by the grinding
and attrition of the pieces upon one another in the upward
rush from the volcanic throat. They should be dis-
tinguished from bombs which have been afterwards
rounded by the action of running water. The larger
bombs sometimes present a sub-angular appearance, are
porous and their surface is penetrated by cracks as shown
in Plate 23. Such have been called bread-crust bombs.
The ashes, and lapilli which usually make up the greater
part of the material are frequently spoken of as volcanic
cinders and cones composed of them are called cinder cones.

In a sense, of course, this loose material may be estimated
as a rock formation; so far as the individual pieces are
concerned they are to be considered merely as fragments
of the various kinds of rocks treated in the foregoing
pages, to be named and described as there set forth.

But in process of time great accumulations of such
material may be spread over wide tracts of country,
covering up existing rock formations. The heavier and
coarser particles fall first, then the finer, giving a grada-
tion from top to bottom and, as successive outbursts occur,
there is produced in this way a rough bedding. By its
own weight as it accumulates, aided by the action of
percolating water which may carry and deposit substances
in solution, it gradually becomes compacted into a more
or less firm mass, having a certain individuality as a kind
of rock and deserving of special treatment. When the


rock is composed entirely of the finer particles, dust and
ash, it is called volcanic tuff; when this is mixed with the
coarser bombs and lapilli it is termed volcanic conglomer-
ate, or better, volcanic breccia, with reference to the broken
angular character of the embedded fragments.

Volcanic Tuff. This is generally a fine-grained rock,
light in weight, and often of a chalky consistency, some-
times dense, compact, and breaking into small chips. The
color is usually light, white, pink, pale-brown, gray or
yellow, sometimes passing into darker shades. The more
compact varieties may be easily mistaken for f elsite lavas ;
it is possible, indeed, in some cases, that they cannot be
distinguished from them megascopically, but generally
attentive examination with a good lens will reveal angular
particles of quartz, feldspar, and often other minerals in
them, and possibly small fragments of other rocks. When
breathed upon, they usually exhale a strong argillaceous
odor, probably owing to partial or complete alteration
of feldspathic particles to clay. When not too compact
they have a rough feel and yield a gritty dust, when
strongly rubbed between the fingers, unlike the smooth-
ness of pure clay or chalk, owing to the hard, angular
character of the dust particles. Sometimes such tuffs con-
tain fossil remains of vegetation, when they have fallen
upon land surfaces covered with it, and carbonaceous
remains of stems, twigs, or leaf imprints may be found
in them. If the material has fallen into water the tuff
may be rich in various kinds of fossils, such as marine
organisms, of possibly great perfection of form, and for
the same reason it may be well stratified. All of these
varied characters, including the mode of occurrence and
relation to other rocks, must be taken into account in
judging the nature of the deposit.

Volcanic Breccia. This has a base or cement of tuff,
more or less completely filled with lapilli of angular
shapes, and these are often mingled with larger bombs
and masses which are apt to be rounded. Interspersed



with these are apt to be fragments of other rocks, pieces
of the basement through which the conduit has drilled,
of limestones, shales, sandstones, and massive crystalline
rocks, granite, gneiss, schist, etc. They have therefore a
strongly conglomeratic aspect, like the specimens seen in
Plate 32. Even when these rocks have been greatly
indurated by contact metamorphism, or other agencies,
they still reveal, by differences of color and texture on a
freshly broken face, the angular shapes of the fragments and
their composite character. When not too indurated they
are apt to erode very unevenly; the finer cement being
less resistant washes away first, leaving the contained
masses projecting, and in this way along the edges of
cliffs strange and weirdly shaped figures of erosion are
produced, called " hoodoos " in the Rocky Mountains'
region. The colors of these breccias is variable, browns,
reds and chocolate being common, along with lighter tones,
depending partly on the state of oxidation of the iron-
bearing compounds they contain, and partly on the nature
of the magma, whether felsitic, which tends to lighter
colors, or basaltic which produces darker ones.

Occurrence of Tuffs and Breccias. These rocks are of
wide distribution, being found in all regions where volcanic
activity has taken place; their presence indeed is the best
confirmation in many regions of such activity in the past.
In places where vulcanism is still active, or has only
recently ceased, they are represented by the still uncom-
pacted material, but no definite line can be drawn be-
tween the different conditions of consolidation.

In the eastern United States, tuffs and breccias have
been found in several localities in Maine; near Boston; and
at South Mountain, Penn. They occur also, to a limited
extent, with the Triassic eruptives of the Connecticut
Valley. Further research will probably reveal other
localities, but they are neither common nor conspicuous
rocks, being limited in volume and so greatly changed in
character by various agencies that in many places their


true character is difficultly recognizable. They probably
had once a much greater extension, but erosion has mostly
carried them away, during the vast period of time which
has elapsed since volcanic activity was displayed.

In western America, however, the case is very different;
in the various ranges of the Rocky Mountains; in the
Coast and Cascade Ranges, and in fact over most of western
North America, these are common rocks and in many
places in Colorado, Wyoming and Montana, they occur
in immense deposits, forming often an important factor in
building up the bulk of the mountain masses. They are
especially well displayed in western Wyoming, in the region
of the Yellowstone Park, where the serried peaks of the
Absaroka Range are mostly carved out of tuffs and
breccias aggregating thousands of feet in depth, thus
testifying to the enormous volcanic energies which this
region formerly displayed. A section cut into them
by erosion, exhibiting their rough bedding, is shown in
Plate 24. In this region they are frequently interbedded
with flows of lava.

In Europe tuffs and breccias have a wide extension.
They occur in many places in the British Islands, as in the
Lake district in northern England, and associated with
the volcanic rocks of the old red sandstone and Car-
boniferous of Scotland. They are often interbedded with
sedimentary rocks and are frequently so changed by
metamorphic processes as to be recognized only by
careful petrographic research, having been changed into
slates, etc. Such altered tuffs form a part of the so-called
" halleflintas " in Sweden or the " porphyroids " of
Continental geologists. Tuffs and breccias occur in many
places in Germany, France, Italy, etc. The mention of
these localities is sufficient to show their wide extension
and importance. In this connection the reader is referred
to what is said of adobe.


THE stratified rocks consist of material which has
already formed a part of pre-existent ones, and which has
been deposited from some fluid by which it has been
moved from its former position. The shifted material
may have been moved and deposited by the action of
water, the atmosphere, or glacial ice. The first case is
by far the most prominent and important, especially with
respect to the volume of the masses involved, and the
frequency of their occurrence, and thus when stratified
rocks are mentioned such water-formed rocks are always
understood, unless it is otherwise stated. In contra-
distinction to them, the material which has been moved
and deposited by the action of the atmosphere, forms the
class known as Aeolian rocks, one of far less importance.
From what has been said, it is clear that the stratified
rocks are secondary ones in the respect that their material
in some form or other has been derived from already exis-
tent ones. An exception to this would be found in beds
of coal, which are truly stratified rocks derived from plant
life, or in beds of volcanic ashes which have been deposited
from the atmosphere, and which have been described by
preference under the igneous rocks. But in general the
statement, that the material of the stratified rocks is sec-
ondary, holds true, and it has been derived from former
rocks of all classes igneous, metamorphic and strati-
fied and in the case of the earliest sediments from the
earth's original crust, if such ever existed.

The rocks which have been formed in water may be
divided into two main groups, according to the manner



in which the material has been deposited; they consist
either of substances mechanically held in suspension, and
then directly dropped, or of that which has been in solu-
tion, and through chemical agencies, either of organic life
or otherwise, has been rendered insoluble, and has been
therefore deposited. The first we may call mechanical,
the second chemical sediments. Yet even between these,
as we shall see later, it is difficult to draw a definite line.
We have then the following classes to deal with :

Sedimentary rocks ; water-formed ( mechanical.

i chemical.
Aeolian rocks; wind formed . . mechanical.

Decay of Rocks ; Formation of Soil. When firm and even
dense rocks are exposed to the action of the atmosphere,
they gradually decay and are turned into soil. This is
brought about by a variety of agencies. All rock masses
are penetrated in various directions by cracks and fissures
called joints; these are both great and small, and in addi-
tion the individual mineral grains contain cleavage and
other cracks. Thus water is able to thoroughly permeate
the rock masses, and in cold regions where alternate
thawing and freezing goes on, the expansion of the water
in turning to ice keeps on splitting and crumbling the
rocks until on the surface they are reduced to a mass of
debris. The expansion and contraction of rocks in hot
countries and in arid regions, under great daily and yearly
changes of temperature, accomplishes the same thing more
slowly. The expansion of the growing roots of trees and
plants tends to the same end. By such processes there is a
constant tendency for the rock masses to be broken up,
mechanically, into smaller and smaller fragments. In the
meantime the substances dissolved in the water, such as
air, acids from decaying vegetation, and especially carbonic
acid gas, are acting chemically upon the rock minerals, con-
verting the silicates, oxides, and sulphides into other
forms, into carbonates, hydrated silicates, hydroxides,


^^^E^^K I^^^Vi^M





sulphates, etc. Much material goes into solution, is leached
out, and by running water is carried into lakes and the
ocean, where it concentrates, and where we must again
consider it under the formation of the chemically precipi-
tated sediments. Some minerals, such as quartz, are not
attacked to any appreciable extent, or but very slowly,
under ordinary circumstances, and these remain to form
the chief part of the rock debris. It is for this reason
that silicates, and especially quartz, play the chief min-
eral r61e in the sedimentary rocks formed by mechani-
cal processes. This debris of broken, crumbled and
altered rock, which constitutes a detritus, has been
called by various names, and the finer upper portion
in which vegetation grows is the soil. Under this latter
name for convenience we may consider all of it. The
gradual change from rock below to soil above is illus-
trated in Plate 17.

Movement of Soil. The surface of the land in general
is covered by a mantle of soil resting on the rocky crust of
the earth. The latter, which is popularly known as the
" country rock," here and there in ledges, precipices, and
the craggy tops of hills and mountains projects through
this covering. By the action of running water, aided by
gravity, this crumbled rock and soil mantle, which is appar-
ently at rest, is, geologically considered, actually in motion,
and is continually being urged downward into the sea, its
ultimate goal, Plate 25. On steep slopes it goes more
rapidly, in valleys more slowly; in level plains, like water
in a lake, it is temporarily impounded. Its rate of motion
varies continually from time to time and from place to
place. Its movement in mass is of course very slow;
when suspended in running water, that of the water which
carries it; when resting on the stream bottom it varies
according to circumstances. Thus the land waste is being
ever carried away and ever renewed by the destruction of
the rocks. The greater part is carried into the sea, but a
considerable part is deposited in inland lakes and seas,



and on the lower plains and deltas of great rivers, which
from time to time are heavily flooded. It is this material
which forms the sedimentary rocks of mechanical

Gradation of Material. The detritus of the land con-
sists of material of very variable sizes, and in northern
countries over which, glaciers have passed this is particu-
larly apt to be the case, as rock masses showing great
extremes in dimensions are moved and mingled by them.
When such material is moved by running water it becomes
sorted and graded, according to the strength of the cur-
rent, into masses consisting approximately of equal sized
particles. When they are larger than peas the material
is called gravel and the individual pieces are termed
pebbles; large, loose pieces of rock from the size of a small
melon up are spoken of as boulders. Pieces smaller than
peas, which form a non-cohering mass when wet with
water, are termed sand, while the finest particles which
are readily lifted and transported by movements of the
atmosphere are known as dust, and these when wet and
then dried generally cohere into solid material. All these
grade into one another. The following table shows a
more accurate division according to size.

Name of Material.

Diameter in Millimeters

Fine Gravel .
Coarse Sand .
Medium Sand
Fine Sand . .
Very fine Sand

0.05-0 01

Fine Silt

0.01-0 005



Thus, roughly speaking, the material may be classified
into, (a) gravel, (6) sand, (c) mud, clay or silt. Since
this division is made the basis of classification of the


mechanically formed sedimentary rocks, each of them may
be examined somewhat more in detail.

Gravel. The pebbles which compose a gravel are
pieces of individual rocks and like them are generally
made up of grains of different kinds of minerals. In
some cases they are composed of only one mineral, and of
these, quartz is by far the most common. Such quartz
pebbles may be fragments derived from quartzite strata,
from a quartz vein, or from large quartz crystals from
some granite-pegmatite dike. Such coarse granites or
pegmatites may furnish pebbles consisting of other single
minerals, especially feldspar.

The form and appearance of pebbles depends on the conditions
to which they have been exposed. Those which have suffered
considerable transport in the bed of streams, or have been rolled
on the shores of lakes and of the sea, are, as is well known, rounded
and become ovoid to spherical. They are apt to have a very smooth
surface with a characteristic faintly dimpled, slightly dented, or
inverted shagreen appearance, caused by their repeated collisions
under movement. This is best seen on a pebble of a hard homo-
geneous substance, as in one of quartz. If composite in nature
they are often pitted by the decay and removal of softer or more
easily altered particles.

The degree of rounding shown by pebbles depends on the distance
and length of time they have been transported and on the hardness of
the material. Sedimentary rocks, as will be shown, are sometimes
composed of pebble-sized fragments, which have suffered very little
movement, and which still retain their original rough, angular

Pebbles and boulders which have been transported by glaciers
are sometimes seen in sedimentary rocks. These have character-
istic sub-angular forms, with faces ground upon them, which are
polished and scratched by parallel and crossing grooves or
scratches. Pebbles, partly buried in the sand of the seashore
and of deserts, are also often subangular and facetted, the faces
being ground by the sand drifting past them, but these lack the

Pebbles buried in the soil often show fern or moss-like markings
or dendrites upon them, or are sometimes covered with a shiny skin
of dark color. This comes from a deposit from water, of manganese
or iron oxides.


Sand. Strictly speaking, sand means particles of a
certain size, as mentioned above, and has no reference to
their composition: thus we have quartz sand, coral sand,
volcanic sand, etc. It happens, however, that by far
the greater part of the sands are composed of particles
of quartz, and some are exclusively made up of it. For
this reason when sand is spoken of briefly, quartz sand is
always understood.

The composition of ordinary sand is quite variable, depending
on the locality. In addition to the quartz grains, those of many
other minerals are present, depending on the rocks of the region.
Feldspar, garnet and iron ore are very common. Various silicates
such as hornblende, pyroxene, tourmaline, etc., are apt to occur.
Some grains may be made of pieces of very fine-grained rocks of
composite character. Twenty-three different kinds of minerals
were found in the dune sands of Holland by Retgers.

Like pebbles the sand grains are more or less rounded, depending
on the amount of transport. In some rather coarse sea sands they
are almost all spherical. Below a certain degree of fineness the
grains do not become more rounded by attrition in water among
themselves; this is due to the fact that the capillary film of water
covering them acts as a buffer and prevents them from coming in
contact when they collide; in the larger grains it is not able to do

Mud, Silt and Clay. This consists of the finest ma-
terial of the land waste. As -sedimentary deposits they
are characteristically found off shore, or in sheltered bays
and sounds, where the slow movement of the water does
not permit the transport of the heavier sand and gravel,
and as the material forming the lower flood plains and
deltas of great rivers. On account of their minute size
the particles are little apt to be rounded, but under the
microscope show angular forms. Like the sands they
may be composed of a great variety of minerals, kaolin,
mica, quartz, feldspar, etc., but just as quartz is the char-
acteristic mineral of the sands, so is kaolin that of muds
and clays. As shown elsewhere the decay of the feldspars
of the rocks produces kaolin or clay, while the quartz


grains are unaltered; the clay particles are excessively
fine and light, while the quartz ones are mostly larger and
heavier. From this there tends to be a separation of the
two by moving water; as the current slackens the quartz
is deposited first, forming sand, while the lighter clays are
carried beyond and settle in still water. Fine flakes of
white mica are apt to accompany them.

In fresh water a portion of most clays, consisting of the very
finest and lightest particles, will remain in suspension almost indefi-
nitely. Turbid water of this kind acts much as if it were a solu-
tion of clay in water ; if salts be added to it, or if it be mixed with sea
water, the clay then curdles into lumps or flocculates and is quickly
deposited, leaving the liquid clear. This behavior is analogous to
that of salts in solution, and it has an important bearing on the
deposition of material carried into the sea, and on the formation of
certain kinds of rocks.

Muds or clays are characterized according to the pre-
dominance of certain constituents; thus some are cal-
careous, containing more or less carbonate of lime and
are often called marls; some contain a good deal of fine
quartz and are spoken of as siliceous, others are rich in
deposited iron oxides and are ferrugineous clays or
ochers, while in many these constituents are present in
minimum amount, or are wanting, and these are plain
clays or argillaceous deposits. Such mixed forms are
transitional to the chemically deposited rocks described

Dissolved Material. The waste of the land includes not
only the material mechanically transported by water, but
also that which is taken into solution and ultimately
carried into the sea. A rough estimate of this for the
continents places it at 5,000,000,000 tons per annum.
It is an important fraction of the whole amount removed,
compared with the mechanical sediments. It varies
greatly in different rivers, depending on the composition
of the rocks forming their basins. It is inferred that
through the concentration of this material in solution


during past ages the salts now in the ocean have been
produced. From these salts, those sedimentary rocks,
whose material through chemical agencies, either of
organic life or otherwise, has been redeposited from solu-
tion, have been formed. It includes the important class
of carbonates, limestones, dolomites, etc., and the less
important sulphates and chlorides, such as gypsum and

It is probable that the carbonates of lime, magnesia, and
alkalies were all originally derived from silicates of these
oxides. Water containing carbon dioxide has converted
them into carbonates, as illustrated under the decom-
position of feldspar, and has then dissolved and carried
them into the sea. The sulphates have been formed by
the oxidation of sulphides in the original rocks and the
union of the sulphuric acid with the stronger, more
soluble, alkaline bases. The chlorides have in part been
derived from minerals of the original rocks and, perhaps,
made in part by volcanic emissions from deeply seated
magmas within the earth.

Structure of the Sedimentary Bocks. The sedimentary
rocks, as geological masses, differ greatly from the igneous
ones in that they form widely extended, relatively thin
bodies, making part of a coating or mantle upon the earth's
outer surface; they never prolong themselves by extension
into the depths, as the latter always do. It is thus their
horizontal, as contrasted with their vertical extension,
which gives them importance as geological masses. The
most characteristic feature about their structure is that
they are stratified. This means that they consist of layers,
varying in material, texture and color, and in thickness,
which, if undisturbed by geological events more recent
than their formation, are in general horizontally disposed
one upon another. This is illustrated on Plate 26.
It is due to the fact that the mechanical sediments have
been deposited by moving currents of water in lakes and
seas and on the flood plains of rivers, and these currents,



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 23 of 35)