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 28 of 35)
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include seolian deposits, or those made by the wind, and
those formed by the disintegration and decay of previously


existent rocks. They are appended here to the stratified
rocks, because they are in general closely connected with
them, and in many cases pass insensibly into them.
Many indeed, which might be classed here wholly or in
part, have already been described elsewhere because of
their close connection with other rocks. Thus volcanic
tuffs and breccias have been described under igneous rocks;
clays under shale, marl under carbonate rocks, sands in
Chapter VIII, etc. Most of these substances, under the
ordinary usage of the word, would not be considered rocks
at all, and their treatment entails matters of geological
interest rather than such as enter into a work of this char-
acter. This applies to such things, for instance, as soils,
talus heapings, morainal deposits from glaciers, etc. Their
description and mode of origin should be sought in the
handbooks on geology, or in special manuals. Only a
few of them, which from their widespread occurrence and
great importance as geological formations are of particulai
interest, are here included.

Loess. This is a deposit of a pale to buff yellow color,
running into brown, of an exceedingly fine grain; friable,
with scarcely the consistency of ordinary chalk when
coherent, and passing into looser forms, and of a rather
harsh feeling when rubbed between the fingers. It is of
a remarkably homogeneous appearance, and commonly
shows no signs of stratification, though this is sometimes
clearly seen. It has been found to consist chiefly of
angular grains of quartz, mixed with considerable amounts
of clay-like substances, tiny specks of other minerals, and
a calcareous cement, the amount of carbonate of lime
rising in some cases to 30 per cent. This latter produces
an effervescence in acid which quickly ends. The analysis
of a loess from Kansas City, Missouri, may be quoted to
show the general chemical composition.

SiO, A1,O 3 Fe 2 O 3 FeO MgO CaO Na 2 O K 2 O H 2 O CO 2 XyO Total
74.5 12.3 3.3 0.1 1.1 1.7 1.4 1.8 2.7 0.5 0.4 = 99.$
XyO - minute quantities of P A. TiO-j, SO 8 MnO and C.


Loess occurs in widespread areas in the valley of the Mississippi,
In the states of Ohio, Indiana, Illinois, Iowa, Kansas, Nebraska,
Arkansas, Missouri, Tennessee, Kentucky, Alabama, Louisiana,
Mississippi, and Oklahoma. It is found also in Europe in various
places, especially in the valleys of the Rhine and its tributaries,
lying in isolated patches on the upper hill and mountain slopes and
in the same way in the Carpathians. It covers an enormous area in
northern central China with thicknesses attaining 1500-2000 feet,
and the yellow color which it imparts to the Hoang-ho (Yellow River),
and eventually to the Yellow Sea, into which the former discharges,
gives to these their names.

It is now generally accepted that the loess is an seolian, that is,
a wind blown deposit of dust which has accumulated through long
periods of time. This is shown by its lack of stratification, the
spread out manner in which it lies upon the surface, filling former
inequalities, the remains of land shells which are found in it, and
by the small, vertical tubes running through it caused by the roots
and stems of former vegetation. In places, however, where it has
been washed down into former lakes, ponds and streams, it becomes
stratified. In America and Europe, the material of the loess is
supposed to represent the finely ground rock powder of the glacial
ice sheet.

A characteristic feature is the common occurrence of concretions
of carbonate of lime and of oxide of iron. They often assume the
odd shapes seen in the flint nodules of chalk. The perpendicular
tubules give to the loess a vertical cleavage, which produces along
river banks bold bluffs.

Adobe. This name is applied to a very fine-grained,
coherent, yet friable material which covers wide areas in
the semi-arid and arid regions of western North America,
especially in the southwestern states and in Mexico. It
resembles loess in many ways, has usually the harsh feeling,
when rubbed between the fingers, and is of a yellowish,
yellow-brown, gray-brown or chocolate-brown color. Its
use in the form of sun-dried brick for building is well-
known. It is the result of the finer detritus of rock decay
on the higher slopes of hills and mountains accumulated
on the lower slopes, plains, valleys and basins, in part by
rain wash, and in part by the action of the wind in moving
it as dust. It forms a valuable soil when irrigated and
brought under cultivation.


Laterite. This is a red soil or deposit found in tropical
regions and is the result of the sub-serial decay of many
rocks, especially of granite. In the process the rocks lose
their alkalies and alkali-earths more or less completely,
and there remains a reddish, cellular mass, consisting of
quartz sand mixed with clay-like substance (chiefly
hydrargillite, A1(OH) 3 ) with iron oxides which give the
color. When dried it may become very hard and rock-
like. It frequently contains concretions of the iron oxides.

Loam. The common arable soils of the greater part of
the world are comprised under this heading. Loam con-
sists of a mixture of sand and clay, colored yellow, brown,
or reddish by iron oxides, or dark to black from organic
matter. The sandy particles are chiefly quartz, often
mingled with fragments of other minerals. On rubbing
between the fingers, it first feels harsh from the gritty sand
particles; if the rubbing is continued and these are allowed
to drop out, the greasy smooth feeling of the clay is finally
perceived. The proportion of organic matter varies very
greatly; the black soils of India and Russia are very rich
in it.



Introductory. The metamorphic rocks are those which,
originally sedimentary or igneous, have been changed
either in mineral composition or in texture, or in both, so
that their primary characters have been altered, or even
entirely effaced. Here constantly, as elsewhere in geology,
gradations exist, and no definite line can be drawn on the
one hand between the sedimentary rocks and their meta-
morpnio products, or between the igneous rocks and the
metamorphic ones formed from them, on the other. Thus
loose chalks pass into limestones, and these into crystalline
marbles, just as dolerites merge into greenstones, and so
on into hornblende schists, without any sharp line of
domarkation. But there comes a point in the change of
each original rock, either of composition or of texture and
usually of both, where its characters and relations to other
rocks have become so individual that, for practical pur-
poses, it is best regarded as a distinct kind of rock. Where
this line shall be drawn must depend upon the experience
and judgment of the observer; in this work only those
cases are treated where the change has been so defi-
nite and pronounced as to produce typical metamorphic

Rocks for the most part are composed of minerals, and
minerals for the most part are definite chemical combina-
tions, which are only, as a rule, permanent under stable
conditions. If the minerals are submitted to new condi-
tions, quite different from those under which they were
formed, with new chemical and physical factors operating
upon them, they will tend to change into other minerals,



that is, to turn into new chemical combinations, which will
be the most stable under the new conditions. A familiar
example is the decay of the feldspar of igneous rocks, and
its change into clay and other substances through the
action of water and carbon dioxide, as treated under
granite. The change in conditions may be so slight that
some rock minerals may be able to resist them indefinitely,
while others less stable may succumb. Thus igneous
rocks, formed by the cooling and crystallization of molten
magmas, may remain in the depths for millions of years,
and on coming to the surface through erosion and denuda-
tion, may be found entirely unchanged, or with only one
or two of the constituent minerals altered. At the surface
they are at once subjected to new conditions, to the com-
bined effects of changes of temperature, to moisture, the
various gases of the atmosphere, the products of organic
life, etc., and they commence to break up and to form into
new compounds. Then their ultimate conversion is only
a question of time. The same is true of the sedimentary
rocks, only in lesser degree. They are formed of mineral
particles, deposited in water and, usually, cemented by
pressure and deposits from solution. While they remain
deeply buried and under fairly stable conditions, they are
unchanged; when they are exposed to the atmosphere
they also tend to change and decay, especially in those
minerals that are susceptible.

All these changes which occur upon the surface are
strictly to be classed as metamorphic ones, and the prod-
ucts, in a geologic sense, are metamorphic rocks. But
for practical purposes all these materials formed by the
action of weathering and by the decay of rocks on or near
the surface, such as the soils, are not here included. They
have been previously mentioned under the foregoing rock
types, so far as seems desirable for the object of this work,
and only those rocks are treated as metamorphic which,
while buried at depth below the surface, have suffered,
through the action of certain agencies to be presently


described, changes, which have practically converted
them into new kinds of rocks.

Metamorphic Agencies. The chief metamorphic agen-
cies are mechanical movements of the earth's crust and
pressure, the chemical action of liquids and gases, and the
effect of heat. We may simplify these into the effects of
movement, water solutions, and heat, and all three of
these are required to produce complete metamorphism
in rocks, though not necessarily all to the same extent,
since sometimes one factor is more predominant, and
sometimes another. Thus in the metamorphism which
has been already described as contact metamorphism,
induced by the intrusion of a body of magma, the effect
of heat is the most important, that of gases and liquids
less so, while the effect of movements of the crust, or
pressure, is negligible. The rocks produced, however, are
actually metamorphic, but for practical reasons they have
been given separate consideration, and are not included
among these under treatment. We will consider the
different agencies separately.

Movement and Pressure. Pure simple downward pres-
sure, to the amount exerted in the upper part of the earth's
outer crust, appears to have little metamorphic effect. It
tends without doubt to consolidate the material of sedi-
ments by bringing the grains closer together, but many
instances may be cited of sediments, buried under great
thicknesses of deposits for geologic ages, which on being
raised and exposed by erosion without disturbance, such
as folding, are found to be practically in unchanged

On the other hand, as commonly supposed, through the
gradual contraction of the earth, the outer crust is under
compression, and this finds relief from time to time by
buckling or wrinkling up of the outer shell into mountain
ranges. This compressive force, thus acting with lateral
thrust, is therefore spoken of as orogenic, i.e., mountain
forming. By it whole masses of strata with possibly


included igneous rocks intrusive, extrusive and frag-
mental volcanic are folded, crushed, and mashed
together in the most involved and intricate manner. Not
only are the rocks then subjected to vast pressure, but
they are also subjected to enormous shearing stresses,
which tend to produce forced differential movements
among the rock particles. It is particularly this latter
effect which is of great potency in producing meta-
morphism Its effects may often be seen megascopic ally
by the manner in which large cryrjtak, included pebbles,
or fossils are flattened and elongated, or broken into frag-
ments which are drawn out into thin, lenticular masses
in the direction of shear. The microscope shows that
even minute crystals are broken, and their optical proper-
ties affected, as the result of the strain. It is possible
indeed, for this agency working alone to produce rocks
having the characteristic outward metamorphic texture,
without any change in their ^riginal mineral composition,
but in combination with heat and waier, it is of the highest
importance in inducing chemica* changes, and the produc-
tion of new minerals. It is indeed a noticeable fact that
so long as the rocks retain their original position, "hey are
unaltered, but as we commence to find them disturbed
by erogenic forces, they begin to show signs of meta-
morphism, and in proportion to the degree to which they
have been folded up, mashed, and sheared, they become
more and more metamorphosed.

Heat. The effect of heat as a metamorphic agent is very
powerful, as is so well shown in local or contact meta-
morphism. It increases very greatly the solvent action
of solutions; it tends in many cases to break up existing
chemical compounds which form minerals, and to promote
new chemical arrangements. The heat needed for meta-
morphism may come from the interior of the earth, which
increases greatly with the depth; it may be supplied in
part by the transformation of energy resulting from the
movements, the folding and crushing of the rock masses.


and in part it may result from intrusions of molten magma,
which are very liable to rise and invade the rock masses
as they are uplifted and folded.

Liquids and Gases. The chief of these is of course
water, which under heat and pressure becomes a powerful
chemical agency. It acts as a solvent, and promotes
recrystallization, and taking part in the chemical compo-
sition of some of the minerals, such for example as micas
and epidote, it is a substance necessary to their formation.
It is, without doubt, aided also in its action by substances
it may carry in solution, such as alkalies, and by volatile
emanations coming from magmatic intrusions, like boric
acid, fluorine, etc., as already explained under contact
metamorphism. It is this which explains the presence
in metamorphic rocks of such minerals as tourmaline,
chondrodite, and vesuvianite, which are characteristic of
pneumatolytic contacts, and of micas, hornblendes and
other minerals which contain fluorine.

Effect of Depth. The outer crust of the earth has been
divided by geologists into different zones, according to
the various geological processes at work. In the outer-
most one, down to the level at which ground water stands,
the rocks are full of fractures, and are exposed to atmos-
pheric agencies moisture, carbon dioxide, oxygen, etc.
In this the rocks tend to decay, to be converted into car-
bonates and hydroxides, and to form soils. It is called
the belt of weathering, and is the one of rock destruction.
Below this lies another, in which the rocks are also full of
fractures and cavities filled with water. Its upper level
is that of ground water; below, it reaches to the point
where the pressure of the superincumbent masses and the
contraction of the crust becomes so great that all fractures
and openings are closed up, since the stress is so much
greater than the strength of the rocks, that they crush
under it, and are to be regarded as being in a relatively
plastic state. In this zone the chemical action of water
is most important, aided by the substances it may carry


in solution. The tendency is to change the minerals to
hydrates, and to a lesser amount to carbonates; thus
oUvine, an anhydrous silicate, becomes converted into
the hydrous silicate, serpentine. Substances are taken
into solution and, reinforced by those leached out from
the belt above and carried down, are deposited in the
pores and fissures of the rocks; hence it is called by Pro-
fessor Van Hise the belt of cementation, because the rock-
grains are thus cemented together.

Below this lies the zone where, as stated above, the
pressure becomes so great that all openings are closed up,
and the rocks may be regarded as in a plastic condition.
Its upper level is variable and depends on geological con-
ditions; in times of quiet it may be as deep as six miles
below the surface; in times of mountain making, it may
rise much higher than this. Of what may be its lower
level, we know nothing. In this, the chief agencies are
the enormous pressure and the increasing heat of the
earth; the r61e played by liquids and volatile substances
is of less importance; the tendency is for them to be gotten
rid of, to be squeezed out. The chief work done in this
zone is molecular rearrangement, in which less stable
mineral compounds are broken up, and new ones of higher
specific gravity and smaller volume, through condensation,
are formed. Carbonates are converted 'nto silicates and
the carbon dioxide expelled; hydrated minerals have
their water driven out and new minerals, with less or no
water, are formed. This zone of rock flowage, in contrast
to the zone of fracture above it, has been called the zone of
anamorphism by Professor Van Hise. We may term it
the zone of constructive metamorphism.

It is chiefly in the lower part of the belt of cementation
(zone of fracture), and the upper part of the zone of rock
flowage, that the greater part of the work of metamorphism,
in the production of the metamorphic rocks as we see
them, is done. In the upper zone, the results are chiefly
those produced by dynamic shearing, and the imposing


upon the rocks of characteristic textures. Chemical
work may be done and new minerals produced, but it is
possible for new textures to be formed without change
in mineral composition. In the lower zone, the work done
is largely chemical, new and more stable mineral combi-
nations being formed; and here also characteristic tex-
tures are produced.

Minerals of Metamorphic Bocks. Just as certain min-
erals, of which nephelite and sodalite might be mentioned
as examples, are characteristic of igneous rocks, so other
minerals are peculiar to the metamorphic ones, such as
cyanite, zoisite, staurolite and talc. Other minerals are
found in both groups alike, such as quartz, feldspar, horn-
blende, pyroxene, garnet and mica. It should be remem-
bered, however, that the names just mentioned are really
names of families, under which quite a variety of individual
mineral species may be grouped, on account of certain
common properties, such as crystal form. Thus in the
hornblende group, arfvedsonite is found only in igneous
rocks; tremolite and uralite occur practically only in meta-
morphic ones; common hornblende occurs in both. Of the
pyroxenes, the normal home of augite is in igneous rocks,
of wollastonite, a pyroxene-like mineral of the composition
CaSi0 3 , in the metamorphic ones; common pyroxene in
both. Of the micas, paragonite has been found only in
metamorphic schists, biotite and muscovite are present
in both groups of rocks, but muscovite is relatively rare in
fresh, normal, igneous ones. In the garnet group, pyrope,
the magnesia-alumina garnet, is formed only in igneous
rocks very rich in magnesia and low in silica, such as the
peridotites; H occurs m them, or in the serpentines formed
from them, while grossularite, the lime-alumina garnet,
has its characteristic home in metamorphic limestones;
almandite and common garnet are found both in igneous
and metamorphic rocks. In the following list are given
the minerals which may occur in metamorphic rocks; the
first column contains those of wide distribution, and of


prime importance, as chief components; the second column,
those of lesser importance, which occur either as prominent
accessory minerals, or locally developed as chief compo-
nents; the third, occasional minerals, which may be at
times megascopically developed. But this is true only
in a general way, and over emphasis must not be laid on
these divisions.


Quartz Garnets Graphite

Feldspars Staurolite Tourmaline

Biotite Epidote Chrondrodite

Muscovite Zoisite Vesuvianite

Hornblendes Cyanite Hematite

Calcite Pyroxenes

Dolomite Magnetite

Chlorite Talc

Of these minerals, chlorite, serpentine, and talc are
specially characteristic of the upper zone, while cyanite,
staurolite, and some of the others are formed in the lower
zone. Some minerals, like quart:', and some members of
the groups may be formed in either zone, or be persistent
components of the original rocks.

Texture of Metamorphic Rocks. The metamorphic
rocks resemble the greater part of the igneous ones, in
that they possess a highly crystalline character, so much
so that they are frequently referred to as the crystalline
schists. On the other hand, they resemble the stratified
ones in possessing a parallel structure which may closely
resemble stratification. Thus they show analogies to
both of the other great rock groups. This parallel structure
expresses itself to a greater or lesser degree by a foliated,
laminated, or, as it is frequently termed, a schistose texture,
one in virtue of which the rock tends to split or cleave
more or less perfectly in the direction of a certain plane
passing through it. This direction of cleavage is called
the chief fracture, and the break of the rock at right angles


to it is termed the cross fracture. Highly crystalline rocks
exhibiting this texture are called gneisses or schists, ac-
cording to their mineral composition, as described later.
While it is the characteristic texture for the metamorphic
rocks, there are a few, such as serpentine, marble
and quartzite, that for certain reasons to be explained /
may not show any trace of it, and yet are true meta-
morphic rocks.

Observation of the gneisses and schists shows that this texture
is due to arrangements of unlike mineral grains in layers, or very
flat lenses, or to a parallel arrangement of minerals having prismatic
or tabular forms, such as hornblende or mica, or to a mixture of
both. It is a result of the orogenic forces, the shearing and pressures
to which the original rocks have been subjected, and it makes no
difference whether these were igneous or sedimentary, this texture
may be imposed upon both alike under proper conditions. The
superficial resemblance, which the gneisses and schists bear to strati-
fied rocks in their parallel laminated character, for a long time led
geologists to think that the former must have been derived wholly
from the latter, and the general recognition that they contain former
igneous ones as well has come only in the last twenty-five years
through petrographic and chemical studies. From the fact that in
places stratified rocks could be traced into metamorphic ones and
the latter into igneous ones, it was even assumed that the igneous
rocks were in part derived from sediments by extreme metamorphism.
Such cases merely represent instances where both have been meta-
morphosed in common, with a remnant at either end which is not
metamorphosed, and whose original characters may therefore be
recognized. In the light of our present knowledge we should be
no more justified in tracing out such a deduction, than we would in
reversing it, and deriving the stratified rocks from the igneous ones
by metamorphic processes!

Varieties of Texture, Three chief varieties of the schis-
tose texture may be recognized, (1) the banded, in which
unlike mineral layers are in parallel bands, as shown in
Fig. A, Plate 34. This resembles stratification, but may
be induced in igneous masses as the result of shear. (2)
the lenticular, or foliated, in which some of the components
are collected into thinner or thicker lenses, around which
the other minerals tend to be wrapped or wound, as. shown


in Fig. B, Plate 34, which shows a view of the cross frac-
ture. The surface of chief fracture in this case is apt to
be more or less lumpy, and not to show well the minerals
of the lenses Both this and the foregoing variety vary
greatly from coarse to fine. (3) The slaty texture is one
in which the mineral grains are extremely small, usually

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 28 of 35)