The rocks of this great class are formed by the cooling and
solidification of materials that have come up from a great depth in the
earth’s crust in a melted and highly heated condition. When the fissures
in the earth’s crust reach down to the great reservoirs of liquid rock,
and the latter wells up and overflows on the surface, forming a volcano,
then we may, as was pointed out on page 33, divide the eruptive mass
into two parts: first, that which has actually flowed out on the
surface, and cooled and solidified in contact with the air, forming a
lava flow; second, that which has failed to reach the surface, but
cooled and solidified in the fissure, forming a dike.
Lava flows or volcanic rocks and dikes or plutonic rocks are identical
in composition; but there is a vast difference in texture, due to the
widely different conditions under which the rocks have solidified. The
dike or fissure rocks solidify under enormous pressure, and this makes
them heavy and solid—free from pores. They are surrounded on all sides
by warm rocks: this causes them to cool very slowly, and allows the
various minerals time to crystallize. Other things being equal, the
slower the cooling the coarser the crystallization; and hence, the
greater the depth below the surface at which the cooling takes place,
the coarser the crystallization.
The volcanic rock, on the other hand, cools under very slight pressure;
and the steam, which exists abundantly in nearly all igneous rocks at
the time of their eruption, is able to expand, forming innumerable small
vesicles or bubbles in the liquid lava; and these remain when it has
become solid. Cooling in contact with the air, the lava cools quickly,
and has but little chance for crystallization. Hence, to sum up the
matter, we say: plutonic rocks are solid and crystalline; and volcanic
rocks are usually porous or vesicular, and uncrystalline.
As we descend into the earth’s crust, it is perfectly manifest that the
volcanic must shade off insensibly into the dike rocks, and we find it
impossible to draw any but an arbitrary plane of division between them;
but this is no argument against this classification, for, as already
stated, all is gradation in geology, and we experience just the same
difficulty in drawing a line between conglomerate and sandstone, or
between gneiss and mica schist, as between the dike rocks and volcanic
rocks.
We will now observe to what extent the distinctions between these two
great classes of eruptives can be traced in the rocks themselves,
beginning with the dike rocks. But first it is important to notice the
general fact, clearly expressed in the classification, that, with
perhaps some trifling exceptions which need not be mentioned here, all
eruptive rocks are silicates, and nearly all are feldspathic silicates.
They are of definite mineralogical composition, and, like the chemically
and organically formed stratified rocks, can be classified chemically.
But, although there are eruptives corresponding closely in composition
to the feldspathic silicates, which we have just studied, we find among
them little to represent the non-feldspathic silicates, and nothing
corresponding in composition to the limestones, dolomites, gypsum,
flint, tripolite, siliceous tufa, iron-ores, bitumens or coals.
1. PLUTONIC (DIKE) ROCKS.—These are also known as the _ancient_ eruptive
rocks, and for this reason: It is impossible, of course, for us to
observe them except where they occur on or near the earth’s surface.
But, since they are formed wholly below the surface, and usually at
great depths in the earth, it is evident that they can appear on the
surface only as the result of enormous erosion; and erosion is a slow
process, demanding, in these cases, many thousands or millions of years.
Therefore, the more ancient dike rocks alone are within our reach; those
of recent formation being still deeply buried in the earth’s crust. It
follows, as a corollary to this explanation, that the coarseness of the
crystallization of any dike rock must be a rough measure of its age and
of the amount of erosion which the region has suffered since its
eruption.
As regards composition, the dike rocks present, as already stated,
essentially the same combinations of minerals as the feldspathic
silicates of the stratified series, but occurring under different
physical conditions and having a widely different origin. The only
important difference in texture between the two classes of rocks is that
the sedimentary rocks are stratified and the dike rocks are not; and
when we consider that the dike rocks sometimes present a laminated
structure that resembles stratification, while the sedimentary rocks
frequently appear unstratified, it is easy to understand why, in the
absence of any marked difference in composition, geologists have often
found it difficult to distinguish the two classes of rocks. We also find
here the explanation and the justification of the fact that the names of
the dike rocks are in most cases the same as those of the sedimentary
rocks of similar composition.
Granite.—Granite (from the Latin _granum_, a grain) is a
crystalline-granular rock, agreeing in composition with gneiss. The
essential constituents are quartz and orthoclase; and when they alone
are present we have the variety _binary granite_. Mica, however
(commonly muscovite, sometimes biotite, and frequently both) is usually
added to these, forming _micaceous granite_ (specimen 44); and often
hornblende, forming _hornblendic granite_ (specimen 45). The orthoclase
is sometimes replaced in part by triclinic species, especially albite
and oligoclase. Accessory minerals are not so abundant in granite as in
gneiss; but, besides those named, garnet, tourmaline, pyrite, apatite,
and chlorite are most common. Orthoclase is always the predominant
ingredient; and, except when there is much hornblende present, usually
determines the color of the granite. Thus, specimens 44 and 45 are gray
because they contain gray orthoclase; while all red granites contain red
or pink orthoclase. The quartz has usually been the last of the
constituents to crystallize or solidify; and, having been thus obliged
to adapt itself to the contours of the orthoclase and mica, it is rarely
observed in distinct crystals.
In texture, the granites vary from perfectly compact varieties,
approaching petrosilex, to those which are so coarsely crystalline that
single crystals of orthoclase measure several inches in length. Of
course one of the most important things to be observed about granite,
especially in comparing it with gneiss, is the complete absence of
anything like stratification; that, as before stated, being the only
important distinction between the two rocks. Gneiss is the most abundant
of all stratified rocks, and granite stands in the same relation to the
eruptive series.
Syenite.—This is an instance where stratified and eruptive rocks,
agreeing in composition, have the same name. That rocks consisting of
orthoclase, of orthoclase and hornblende, or of orthoclase and mica,
_i.e._, having the composition of syenite, do occur in both the eruptive
and stratified series there can be no doubt. They should, however, have
distinct names on account of their unlike origins; and would have but
for the practical difficulty in determining, in many cases, whether the
rock is stratified or not. The best that we can do now, when we desire
to be specific, and have the necessary information, is to say stratified
syenite or eruptive syenite, as the case may be.
Diorite.—Here, again, we find identity of names, as well as of
composition, between the two great series. Eruptive diorite is an
abundant and well known rock, and consists of the same minerals as
stratified diorite combined in the same proportions. Diorite includes a
large part of the dike rocks commonly known as “trap” and “greenstone.”
The principal accessories are chlorite, epidote, pyrite, magnetite,
apatite, and quartz. The texture varies from perfectly compact or
felsitic to coarsely crystalline; averaging, however, less coarse than
syenite and granite.
Diabase.—By referring to the classification it will be seen that diabase
occupies the same position among the dike rocks as norite among the
stratified rocks. Like norite it consists usually of the more basic
varieties of plagioclase with or without augite, diallage, or
hypersthene. Augite, or one of its representatives, is usually present,
and is often the principal constituent. Specimen 1 shows a somewhat
equal development of the feldspar and augite. The name _gabbro_ is
sometimes applied to the coarser and more feldspathic diabases, and
especially to those containing diallage or hypersthene in the place of
common augite. In the opinion of some high authorities, however, it is
unnecessary to recognize two species here; and it makes the
classification more simple and symmetrical not to do it. The principal
accessories in diabase are biotite, chlorite, magnetite, pyrite,
calcite, and olivine. Chlorite is often an important constituent, giving
the rock a greenish aspect; but here, as well as in diorite, the
chlorite is due chiefly or entirely to the alteration of the augite and
feldspar; and the chloritic varieties of diorite and diabase together
make up the old species “greenstone.” Similarly, the more compact and
darker varieties of these two rocks, forming regular, wall-like dikes,
are known as “trap.” Specimen 46.
In consequence of their more basic composition, diabase and diorite are
usually strongly contrasted with granite and syenite in color and
specific gravity, being darker and heavier. The basic rocks, too, decay
much more readily than the acidic.
2. VOLCANIC ROCKS.—As regards composition, we shall find nothing new in
the volcanic series; for the rocks of this group present essentially the
same combination of minerals as the dike rocks. In composition, the dike
and volcanic rocks are identical; but in texture, as already explained,
there is a vast difference. The volcanic rocks differ so widely in
texture from both the dike and stratified species, that there is rarely
any difficulty in distinguishing them; and hence they have in every
instance distinct names.
Volcanic rocks are rarely found in this part of the world; and specimens
of most of them are difficult to obtain. For this reason they can only
be noticed briefly here, since it is the plan of this Guide to give
especial attention only to those portions of the subject which can be
illustrated by material within easy reach of teachers.
Rhyolite.—This rock corresponds in composition with granite and gneiss,
but is less frequently micaceous. The orthoclase in rhyolite, and
generally in volcanic rocks, is the clear, pellucid variety—_sanidine_.
It is more difficult to separate from quartz than ordinary orthoclase,
the chief distinguishing feature being its cleavage. Plagioclase and
hornblende are common, but not abundant, constituents. The mica, when
present, is usually biotite. The texture of rhyolite is often more or
less distinctly porphyritic, having a finely crystalline or granular
matrix, with interspersed crystals of sanidine and quartz. The rock has
usually a rough, harsh feel; and while the coarser varieties have the
aspect of granite, the finer approach petrosilex; but all are somewhat
porous, which is seen in the lower specific gravity of rhyolite as
compared with granite and gneiss.
Trachyte.—In texture and general aspect rhyolite and trachyte are nearly
identical. Trachyte, however, is darker, contains little or no quartz,
and more hornblende and plagioclase. In fact, it agrees in composition
with syenite. This is one of the most important of the volcanic rocks.
Obsidian.—Obsidian is sharply distinguished from all other rocks by its
perfect vitreous texture; it is a true volcanic glass. Its surface
(specimen 47) is smooth and glassy, and its fracture eminently
conchoidal. To the naked eye, and usually under the microscope, the
typical variety is perfectly homogeneous; chemical analysis, however,
shows that it has the composition, commonly of rhyolite, but sometimes
of trachyte. Obsidian is, in fact, simply rhyolite or trachyte which,
cooling quickly, has not had time to crystallize, but has remained
permanently in the amorphous or glassy state. The composition is
sometimes partially revealed where a portion of the sanidine comes out
in distinct crystals porphyritically interspersed through the glass. The
homogeneity of the texture is sometimes disturbed: by numerous minute
concentric cracks, forming what is known as perlitic structure and the
variety perlite; by numerous small spherical concretions, forming the
spherulitic structure and the variety spherulite; and also by the
banding, which is the result of flowing while in a plastic state,
whereby portions of the glass of slightly different colors are drawn out
into layers and interlaminated. The bands are rarely continuous for any
distance, being usually merely elongated lenticular streaks. The glassy
state is generally one of inferior density, and hence we find that
obsidian is lighter than the crystalline rocks of the same composition.
Obsidian is a good illustration of a non-essential color, for its
capacity and jet-black color are due entirely to impurities. In very
thin flakes it is transparent and white. It also forms a white powder
when crushed, _i.e._, it has a white streak.
Obsidian is often vesicular, from the expansion of the steam and other
gases which it contained when liquid. The most thoroughly vesicular
varieties are known as _pumice_ (specimen 48). The vesicular texture, by
rendering the rock impervious to light, conceals the impurities, and
thus we get a snow-white pumice from black obsidian. The vesicles are
frequently elongated, sometimes in a definite direction, though often
forming an irregular net-work of glassy fibres. Pumice is often light
enough to float on water, and it is transported thousands of miles by
the oceanic currents. It is employed in the arts, and good specimens can
be obtained at almost any drug-store.
Petrosilex and Felsite.—Sharply defined groups are unknown in lithology,
but all is gradation; and between rhyolite and trachyte, which are
always more or less distinctly crystalline, and obsidian, which is a
true glass and perfectly amorphous, there is no break. It is impossible
to draw a sharp line and say, Here the vitreous texture ends and the
crystalline begins; for the transition is not abrupt, but gradual. We
recognize, really, in these feldspathic rocks, an intermediate state,
which is neither crystalline nor colloid, but both; and this
lithologists have designated the _felsitic_ texture. Felsitic matter
cannot, even with the highest powers of the microscope, be resolved into
separate grains or particles; and it does not exhibit, except perhaps
very indistinctly, the phenomenon of double refraction. In other words,
it is not truly crystalline or stony, and yet it is just as clearly not
amorphous or glassy.
Feldspathic rocks exhibiting the felsitic texture in whole or in part
are known as _felsites_. Many high authorities hold that true felsites
are found only among the eruptive rocks; while others claim that they
are in part, or wholly, of sedimentary origin. The writer accepts the
former view. The felsites are in part acid lavas which have cooled too
slowly to form a true glass, like obsidian, and yet too quickly to
become truly crystalline, like rhyolite and trachyte. But they are also
in large part simply devitrified obsidian. Glass is an unstable form of
mineral matter; and every species of glass, including obsidian, tends
with the lapse of time to become crystalline or stony, the amorphous
changing to the felsitic structure. Thus, in many cases or usually, what
we now call felsites were originally true glassy obsidian. Being
perfectly intimate mixtures of the component minerals, the composition
of felsites can usually be determined with certainty only by means of
chemical analysis. By this means chiefly, it has been proved that there
are felsites agreeing in composition with both rhyolite and trachyte.
There is this general difference in composition, however, between these
crystalline rocks and the felsites; viz.: mica, hornblende, and augite
are generally wanting in the latter. From this it follows that the
felsites are, with unimportant exceptions, composed either of quartz and
feldspar or of feldspar alone.
The physical differences between the felsites of unlike composition are
not great; but they are sufficient to warrant the division of the
felsites into two species: a basic species, to which the term _felsites_
may properly be restricted; and an acidic species, for which
_petrosilex_ is a very appropriate name. According to this arrangement,
felsite is composed chiefly of orthoclase, and, as the table shows,
agrees in composition with trachyte; while petrosilex consists mainly of
orthoclase and quartz, agreeing in composition with rhyolite. We find
here nothing new in composition; but petrosilex and felsite are simply
the crystalline rocks which we have already studied, repeated under a
different texture.
The typical felsite or petrosilex is composed entirely of felsitic
matter, and is perfectly homogeneous, like flint or jasper, which it
closely resembles in hardness and other physical characteristics. As a
rule, however, the rock is not entirely homogeneous, but there is a
manifest tendency in the component minerals, and especially in the
feldspar, to separate out, usually in the form of crystals. In the
banded variety (specimen 42) the rock is built up of thin layers, which
are often alternately quartzose and feldspathic. There is not a perfect
separation of the minerals; but that the quartz is chiefly in the dark
layers, and the feldspar in the light, is shown by the way in which the
layers are affected by the weather.
One of the most common varieties is where a portion, frequently a large
portion, of the feldspar comes out in the form of distinct, separate
crystals, producing a porphyritic texture. Specimens 5, 6, and 7 are
examples of porphyritic felsite; and after examining these we can no
longer doubt that feldspar is an important constituent of the rock.
Petrosilex and felsite are more generally porphyritic than any other
rocks; and they are commonly called porphyry. It is better, however,
since almost any rock may be porphyritic, and since this texture cannot
be correlated with any particular composition, not to use porphyry as a
rock-name, but simply as the name of a very important rock-texture. The
banded and porphyritic textures are about equally characteristic of
petrosilex and felsite. In petrosilex, quartz, as well as feldspar, is
sometimes porphyritically developed, forming the variety known as
quartz-porphyry. There is no limit to the proportion of the quartz and
feldspar which may crystallize out in this way, and thus we find a
perfectly gradual passage from normal petrosilex or felsite to
thoroughly crystalline granite and syenite.
Andesite.—This rock has nearly the texture of rhyolite and trachyte, but
is darker and heavier, and corresponds in composition to diorite,
consisting of plagioclase and hornblende, with usually more or less
sanidine, quartz, augite, biotite, and magnetite.
Basalt.—The rock bearing this familiar name represents diabase among the
dike rocks. It is the most basic of the volcanic rocks, and consists of
the more basic varieties of plagioclase, especially labradorite, with
augite, magnetite, and titanic iron. Olivine is a very common and
characteristic constituent, and the plagioclase is often replaced in
part by leucite and nephelite. The basalts are usually black, and of
high specific gravity; and vary in texture from compact to coarsely
crystalline. The contraction due to cooling frequently results in the
development of a columnar structure of remarkable regularity, the
columns being normally hexagonal and standing perpendicularly to the
cooling surfaces of the mass. This structure occurs in other eruptive
rocks, but is most characteristic of basalt.
Tachylite.—Tachylite is a highly basic volcanic glass, standing in the
same relation to basalt and andesite that obsidian does to trachyte and
rhyolite. It is much heavier than obsidian, and is perfectly black and
opaque, except in the finest fibres. It is a comparatively rare rock,
for the reason that basalt and andesite crystallize more readily than
the acidic rocks on passing from the liquid to the solid state. On the
surface of the basic lava, however, where it is in contact with the air,
and congeals almost instantly, a film of glass is formed; but this may
not be more than a small fraction of an inch in thickness. Like
obsidian, tachylite is often vesicular; but the vesicular basic rocks,
as well as the solid, are usually stony. They occur in vast abundance in
many volcanic regions, and may be considered the typical lava (specimen
49).
In the more ancient lavas, the vesicles are frequently filled by various
minerals—chlorite, epidote, quartz, calcite, etc.—deposited by
infiltrating waters, and derived in most cases from the decomposition of
the original constituents of the rock. Thus the vesicular is changed to
the amygdaloidal texture, and the lava becomes an amygdaloid (specimen
50). The amygdaloidal texture is common in the basic lavas and rare in
pumice, simply because the former are more readily decomposed and
contain a greater variety of bases from which secondary minerals can be
formed.
Porphyrite and Melaphyr.—These two rocks hold essentially the same
relation as regards origin and structure to the basic lavas that
petrosilex and felsite do to the acidic lavas. Porphyrite agrees in
composition with andesite, and melaphyr with basalt. They are usually
dark-colored rocks having a compact or felsitic texture. Porphyrite is,
as the name implies, very commonly porphyritic; while melaphyr is often
vesicular or brecciated, exhibiting all the structural features of
tachylite and basalt, and being in its older forms very generally
amygdaloidal.
Volcanic Tuff and Agglomerate.—Besides the crystalline, glassy, and
felsitic lavas, already described, and due chiefly to the rate of
cooling of the liquid rock, we may recognize a fourth class to include
the very abundant lavas which, during explosive eruptions, are ejected
in the solid state, being violently blown out of the crater in the form
of dust and fragments. Falling on the slopes of the volcano or over the
surrounding country, as in the case of the buried city of Pompeii, the
fragmental lavas remain largely unstratified. But when, as frequently
happens, they fall into the sea, they are assorted by the waves and
currents and arranged in layers after the manner of ordinary sediments,
with which they are often more or less mixed. Before they become
consolidated the finer fragmental lava, of whatever composition, is
called volcanic dust, and the coarser lapilli or volcanic sand; while
the consolidated materials are known as tuff and agglomerate
respectively.
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