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

ciation with other rocks, the retained form of pheno-
crysts, and the good amygdaloidal structure, rarely seen
in the most common felsites, may help one to recognize
the original character of the rock.

The normal weathering of basalt gives rise to chlorite,
serpentine, and carbonates, with clay and iron ores; the
rock often turns green and becomes soft when much
chlorite is developed. In other cases it turns brown
through oxidation and eventually falls away into brownish
ferrugineous soil, to which various names are given, as
laterite in India, wacke in Germany, etc. Sometimes from
such deposits all but the hydroxides of iron and alumina
are leached, forming one variety of the so-called beauxite.

Under processes of metamorphism the basalts act like
the gabbros and dolerites previously described, and give
rise to " greenstone " and to greenstone schists and
amphibolite.

Occurrence of Basalt. As intrusive rocks, sheets, and
especially dikes, of basalt of various types, both plain and
porphyritic in texture, are so common in all regions where
igneous rocks occur that they need no further mention.
As extrusive lavas, in the form of flows and extended
sheets, they are of much greater geological interest and
importance. There is scarcely any volcanic region in the
world which does not exhibit them in greater or lesser
amount, and in some regions, as in the lava fields of the
Columbia in western America, and in western India, they
have been poured out in stupendous masses, so that tracts
of country nearly 200,000 square miles in extent have
been covered thousands of feet deep. A similar great
field existed in northern Great Britain, and its remnants,
portions yet saved from the eroding edge of the Atlantic,
form in great part the northern British Isles.

Leuclte Rocks. Basaltic rocks in which the feldspathoid minerals,
nephelite or leucite, are present, either accompanying the feldspar



260 ROCKS AND ROCK MINERALS

or replacing it, while not common, have in certain regions a consider-
able local development. Ordinarily these minerals are in the
groundmass, and only to be detected by the microscope, and such
rocks in the field must be classed as regular basalts. In central
Italy, however, the leucite rocks have a great development, and in
many cases the leucite crystals appear as phenocrysts as large
as peas, or larger at times, and are easily recognized. They are
leucite-basalt-porphyries or leucite-melaphyre. For the properties
of leucite its description under rock minerals should be con-
sulted. According to the other minerals present, several different
types of these rocks are distinguished and named. Some of them
are so light colored they would be classed as varieties of felsites.
Outside of Italy these leucite rocks are very rare, occurrences being
known in the Rhine district, in central Montana, western Wyoming
and a few other localities, but since the well-known lavas of Vesuvius
are composed of them, they are mentioned here.

Glassy Rocks.

In the felsites and basalts the use of the microscope on
thin sections would show in many cases that a certain
amount of glass, uncrystallized and solidified magma, is
present in them, acting as a cement to hold the mineral
grains together. This cannot be detected megascopically,
and under the term of glassy rocks, as here used, is meant
only such as are entirely of glass, or if partly crystalline,
those containing it in such amounts and in such circum-
stances, that it is visible and evident to the eye.

The conditions which will cause a magma to solidify as
a glass are evidently those which are unfavorable to crys-
tallization, extremely quick cooling in the first place, and
probably to some extent the rapid loss of mineralizers in
the second. This has been already discussed in connection
with the texture of igneous rocks. These conditions are
best realized when the magmas are poured out on the
surface as effusive lavas, and just as we associate a
coarse-textured, entirely crystalline, granular rock, such
as granite, with an intrusive or deeply seated origin, so
conversely we associate glassy rocks with an extrusive
one. Indeed while it is true that dikes may sometimes



DESCRIPTION OF IGNEOUS ROCKS 261

show glassy selvages along the contact, when they have
been intruded into cold rocks, or may indeed be wholly
of glass when the exposure is near the original surface, as
in recently denuded dikes in volcanic regions, this is so
uncommon and so inconsiderable an affair, that in general
we may regard the fact, that a rock is composed partly or
wholly of evident glass, as a proof of its extrusive origin,
that it was originally a surface lava, although it may have
been buried under later formations.

Any of the different magmas, varying as to composition,
may form glassy rocks if chilled with sufficient rapidity,
but petrographical research has shown that, while glassy
forms of the felsite lavas are common, those corresponding
to basalt are much rarer and relatively of inconsiderable
volume. The reason for this appears to be that the mag-
mas, which furnish felsites, or the granite and syenite which
correspond to them, have a relatively high freezing point,
and as the magma cools down and approaches this, it
becomes so enormously viscous that the free movement
of molecules necessary for crystallization is prevented.
This is due to the large amount of silica that such magmas
contain, which has a strong effect in promoting viscosity.
The presence of water in the magma tends to neutralize
this, and to make the magma more fluid and thus to help
crystallization, but when it is poured out on the surface
the water is rapidly lost with increase in viscosity. On
the other hand the basaltic magmas, or those corresponding
to gabbro or diorite in part, which contain relatively
low silica and high iron and magnesia, have a much lower
freezing point and remain liquid as they approach it,
thus permitting easy crystallization and the assumption
of stony texture and appearance. Consequently those
glasses, which have the highest percentage of silica and
correspond to granite in composition, are the most common
ones.

Classification of Glassy Bocks. As already stated in
the classification of igneous rocks, we may divide the



262 ROCKS AND ROCK MINERALS

glassy rocks into two groups, one containing distinct
crystals or phenocrysts embedded in a glass base, or
porphyritic varieties in short, and second, those without
distinct phenocrysts, consisting of either pure glass, or
glass more or less filled with spherulites or lithophysae, as
described later. The second group is again subdivided
according to luster and structure. In accordance with
this we have as follows:

(Obsidian, strong bright vitreous luster.

Glass with few or nol Pitchstone, dull pitchy or resinous luster,
phenocrysts j Perlite, apparently made of small spheroids .

I Pumice, cellular structure, glass froth.
Glass more or less r

filled with pheno- J Vitrophyre, glass porphyry,
crysts [

Obsidian. This is pure, solid, natural glass, devoid of
all apparent crystal grains, or nearly so. It has a bright
luster like that of artificial glass. It usually has a jet-
black color, but when the edges of thin chips are examined
against the light it is generally seen to be transparent or
translucent with a more or less smoky color, and it can be
often observed with a lens that the coloring matter is
more or less collected into fine parallel streaks, bands, or
threads, as if drawn out in the flowage. Less commonly
the glass is gray, or Indian red, or rich brown, and this is
sometimes mixed with the black in bands and strings,
which kneaded through it produce a marbled effect. The
microscope shows the black glass as colorless and filled
with tiny, black, dust-like particles; they are probably
specks of magnetite, which represent the beginnings of
crystallization, and diffused through the glass, they act as
a pigment, coloring it black. In other cases they have
been oxidized to hematite dust and the color is then red
or brown.

Obsidian has a remarkable conchoidal fracture, illus-
trated in Figure 4, page 29, due to its homogeneity and
lack of structure. It was this quality that made the



DESCRIPTION OF IGNEOUS ROCKS



263



substance so highly valued by primitive peoples, for it
enabled them by chipping to work it into desired forms,
knives, spearheads and other implements and weapons,
while long, slender flakes possessed, for cutting pur-
poses, knife-edges of razor-like keenness. The ancient
Mexicans were especially skilful in working it, and were
able to spring off blades of bayonet-like cross-section,
half an inch in breadth by six inches or more in length.

While obsidian corresponding to the various kinds of
igneous rocks is known, it usually has a composition
similar to that of granite, as may be seen from the analysis
of a typical specimen from the Yellowstone Park.



Si0 2


A1 2 3


Fe 2 3


FeO


MgO


CaO


Na 2 O


KjO


H 2 O


FeS 2




74.7


13.7


1.0


0.6


0.1


0.8


3.9


4.0


0.6


0.4


=99.8



It can be readily shown by calculation that had this
magma crystallized, it would have produced a rock con-
sisting of 35 per cent of quartz, 60 per cent of feldspar,
with 5 per cent of other minerals, that is to say, a granite.
The specific gravity varies from 2.3-2.7, depending on the
composition; of the most common variety, 2.3-2.4. The
hardness is greater than that of ordinary window glass,
which it scratches. Before the blowpipe a splinter of
black obsidian fuses readily, with bubbling, to a vesicular
gray or white enamel, which, after the removal of the
water, becomes exceedingly infusible. This experiment
is very instructive in showing the effect of water in lower-
ing the fusing point of magmas and in increasing their
liquidity. The water in the obsidian is not the product
of alteration, for it is present in what the microscope
reveals as the purest and clearest glass, nor are there
cavities to contain it; it appears to be chemically a part of
the mixture, like Na 2 O and K 2 O.



264 ROCKS AND ROCK MINERALS

Spherulites. In many obsidians may be seen rounded,
sometimes perfectly spherical, bodies of white, gray or red
color, varying in size from those of microscopic dimen-
sions up to those of an egg, or even larger; usually from
the size of fine shot to peas. If closely examined with a
lens, it can generally be seen that they are composed of
fibers radiating from a common center; at uniform dis-
tances from the center the fibers are apt to change color,
or to be saturated with a differently colored material, and
the body appears built of successive concentric shells.
These bodies are called spherulites and are composed of
fibers of feldspar. They are indicative of sudden cooling
and a very rapidly induced crystallization, the fibers
shooting outward from some center where crystallization
commences, and branching as they grow, until checked by
the viscosity of the rapidly cooling magma. They should
not be confused with phenocrysts which are single,
individual crystals. An example is shown on Plate 21.
They are sometimes formed by accident in artificial glass,
as seen on Plate 21; in this case the artificial mineral
forming them is wollastonite, CaSiOa.

Frequently the spherulites form before the lava has
come to rest and are thus drawn out, so that they are
dotted along the rock in lines. When in great num-
bers, and minute, they may coalesce; some streaks of the
rock are then composed of them, while other bands are
of dark, solid glass, as shown in Plate 22.

Lithophysae. Closely connected with the spherulites
there occur also in glassy rocks peculiar formations known
as lithophysae (stone bubbles). These consist of a series
of concentric shells of crystalline material, resembling some-
what nested watchglasses, which surround a central cavity,
and are more or less separated from each other. They
consist of adherent crystals, and are very fragile. When
exposed by the breaking of the rock they appear much
like flowers with concentric layers of petals. They vary
in size from very small to several inches in diameter. The



PLATE 21.




A. Spherulites in Obsidian.



B. Spherulites in Glass.




C. Lithophysae. D. Lithophysae.

STRUCTURES IN GLASSY ROCKS.



DESCRIPTION OF IGNEOUS ROCKS 265

walls of the cavities are coated with minute but beautiful
crystals of quartz, tridymite and feldspar, and sometimes
fayalite, topaz, garnet and tourmaline are found in them.
Sometimes they are more or less flattened and strung
along the flowage planes of the rock. They occur, not
only in the pure glassy lavas, but also in those which by
more or less crystallization have assumed megascopically
a stony texture and appearance. They are illustrated on
Plate 21. Their origin is ascribed to repeated shells of
crystallization, with consequent liberation of water vapor,
and expansions of the cavities through its influence under
high temperature. The formation of topaz and other
minerals points to the presence of fluorine and other
accompanying gases. Thus the lithophysae seem to
bear a certain analogy to miarolitic cavities in the intrusive
rocks, as described elsewhere.

Pitchstone. This may be regarded as merely a variety
of obsidian in which the luster, instead of being bright and
glassy, is duller and the rock appears resinous or pitchlike.
There is also a chemical difference in that, while the
water contained in obsidian is rarely so much as one
per cent and may sink to mere traces, pitchstone contains
much more, as much as 5 or 6 per cent or even greater.
It is this which probably influences the luster. They are
also variable in color, black, gray, red, brown, and green,
and are translucent to transparent on thin edges.

Perlite. This is a peculiar variety of glassy rock which
is composed of small spheroids, usually varying in size
from small shot to peas. It is generally of a gray to blue-
gray color, rarely red, has a soft, pearly, or wax-like luster
and resembles enamel. The spheroids either lie separated
in a sort of cement and are then round, or they may be
closely compressed and are then polygonal. They tend
to have a concentric, shelly structure and are the result of
a contraction phenomenon in the cooling glass, which pro-
duces a spherical, spiral cracking, as shown in thin sections.
Analyses of perlites prove them to have a rather constant



266 ROCKS AND ROCK MINERALS

percentage of combined water, between 3 and 4 per cent,
and there may be a connection between this amount of
water and the peculiar method of cracking. To the casual
observer they somewhat resemble oolites and pisolites of
the concretionary sedimentary rocks. Perlite is produced
only by felsitic magmas, especially by those high in silica;
it does not occur in basaltic glasses.

Pumice, Scoria, etc. Pumice is highly vesicular glass
produced by the extravasation of the water vapor at high
temperature, through relief of pressure, as the magma comes
to the surface. It is best described as glass froth. Its
color is white, gray, yellowish, or brownish, rarely red. It
sometimes has a somewhat silky luster. Examined with
the lens it is seen to be composed of a mass of silky glass
fibers of a cottony appearance, full of pores, and separated
by larger holes like a sponge. If drawn out by flowage
the fibers are parallel, otherwise they are interwound.
The chemical composition of typical pumice is like that
of the highly siliceous obsidian, or in other words like
that of granite. Pumice does not form independent rock-
masses, it occurs as the upper crust of flows of felsite lava,
or in fragments among the explosive material ejected by
volcanoes. On account of its light, porous nature, and its
content of sealed glass cells, it floats almost indefinitely
on water, and the material ejected by volcanoes near or
in the sea is borne by currents all over the world, and
drifts ashore everywhere. Its use as an abrasive and
polishing agent, and for toilet purposes, is due to the sharp
cutting edges of the thin films and fibers of glass; nearly
all that is used comes from the Lipari Islands off the coast
of Sicily. Other places of occurrence are mentioned
later.

Scoria. While all magmas, whatever their chemical
composition, at times and under proper conditions, form
pumiceous rocks, typical pumice, as stated above, is most
characteristic of the felsitic ones, while basaltic pumices
are of local development and of inconsiderable impor-



PLATE 22.




A. FLOW STRUCTURE IN GLASSY LAVA.




B- SLAGGY STRUCTURE OF BASALTIC LAVA



DESCRIPTION OF IGNEOUS ROCKS 267

tance. Nevertheless the basaltic magmas develop through
the expansion of gases vesicular forms, as described under
basalt. These pass, especially on the upper surface of
basalt flows, and in the material thrown out by volcanoes,
into more or less glassy, partly stony, dark or reddish,
loosely compacted, spongy, cindery or slag-like modifica-
tions known as volcanic scoria. This form is illustrated
on Plate 23.

A peculiar modification of what may be considered basaltic
pumice occurs in the crater of Kilauea in Hawaii, where drops of
lava flying up from the boiling lava lakes pull out thin, hair-like
threads of glass after them. These threads, drifted by the wind,
collect in tow-like masses, called by the natives " Pele's hair " after
the titulary goddess of the islands.

Vitrophyre. Either pitchstone or obsidian may contain
embedded crystals or phenocrysts which can be recognized.
As in felsite porphyries the amount may vary widely from
cases where they are rare and widely scattered to those in
which the rock is thickly strewn with them. Such por-
phyries, consisting of a glass base and phenocrysts, are
called vitrophyre. Perlite porphyries are known, but are
rare. The glassy base of vitrophyre has the properties
of the obsidian or pitchstone previously described; it
often contains spherulites in addition to the phenocrysts.
Of the latter felclspar is the most common; it is very apt to
be limpid with a glassy habit; the cleavage distinguishes
it from quartz, which may also occur, sometimes alone and
sometimes with the feldspar. If phenocrysts of a ferro-
magnesian mineral are present it is usually biotite, less
commonly hornblende, while pyroxene, though known, is
rare. The varieties are usually named according to the
prevailing phenocryst without regard to the character of
the groundmass; so we have quartz-vitrophyre, feldspar-
vitrophyre, quartz-biotite-vitrophyre, etc. The phenocrysts
are generally rather small.

The chemical composition of the vitrophyres is similar
on the one hand to the felsites and on the other to the pure



268



ROCKS AND ROCK MINERALS



glasses; they represent an intermediate stage of develop-
ment, as may be seen from the following table, which shows
the relations of all these varieties of extrusive rocks or
lavas to one another.



Conditions under which Magma Cooled
and Solidified.


No Formation of
Crystals in the
Depths Before
Extrusion, no
Phenocrysts.


Crystals Formed
in Depth Before
Extrusion and
Brought up by
Magma; Pheno-
crysts.


No crystallization of magma on
surface, on account of rapid
cooling. Glassy texture.


Pitchstone and
Obsidian


Vitrophyre.


Crystallization of magma on sur-
face; slower cooling.
Stony texture.


Felsite


Felsite-por-
phyry.



Tachylite. As previously stated, basaltic magmas crystallize
easily and rarely form glass, or only in relatively small volume.
Basaltic glass is, however, seen occasionally as a thin marginal facies
or selvage in dikes, on lava flows, or among the products of basalt-
yielding volcanoes, like those in Hawaii. It is known by the name
of tachylite.

Occurrence of Glassy Bocks. The glasses are found in
those regions which are, or have been in the past, scenes of
volcanic activity. While obsidian and pitchstone occur
as independent flows and masses near volcanic vents, the
glassy rocks in general form only the upper surface of
lava sheets, which become crystalline as they are penetrated
downward; they are also found, especially in pumiceous
forms, in the fragmental material ejected by volcanoes.
To attempt to name all the different occurrences would be
impracticable, but it may be mentioned that obsidian in
large masses is found in the Yellowstone Park and is
known for its beautiful spherulites and lithophysae; at
Mono .Lake in California; Glass Butte, Oregon; White



PLATE 23.




A. VOLCANIC BOMB.




B. SCORIA.



DESCRIPTION OF IGNEOUS ROCKS 269

Mountains, Utah; Tewan Mountains, New Mexico, and
various other places in the United States; in Mexico,
Iceland, Lipari Islands, Italy, Hungary, New Zealand,
Transcaucasia, etc. Pitchstone occurs in Colorado near
Georgetown and at Silver Cliff; well known localities are
on the Island of Arran off the west coast of Scotland, in
Ireland, and at Meissen and Tharandt near Dresden,
Germany. Perlites and pumice are also found in the
Yellowstone Park; in Hungary, Italy, Iceland, Japan, etc.
Basaltic glasses occur on the west coast of Scotland, in
Iceland and especially in the Hawaiian Islands.

Alteration of Glassy Bocks. It has been found by
microscopic and field study that ancient lavas, in a variety
of places, were once glassy, though not so at present. It
appears that when the natural glasses are exposed to the
various agencies which tend to alter rocks, such as pressure,
heat, action of water, etc., they undergo a slow change,
the glass is converted into an intimate mixture of exces-
sively fine particles of quartz and feldspar, and loses
entirely its vitreous character. It then assumes the stony
texture and becomes a dense felsite. This change is
called devitrification. While the former glassy condition
of many felsites cannot be proved, even microscopically,
it may often be suspected in them, from the presence of
chains of spherulites, flow structures and lithophysae,
which may be seen megascopically, and give strong hints
of their former character. Ancient altered lavas of this
kind have been described from the coast of Maine; from
South Mountain, Pennsylvania; from Wisconsin; from
Sweden and other places. In Sweden they have been
called hdlleflinta, though this name is also used to
designate somewhat similar rocks of a different origin.

Fragmental Volcanic Rocks.

Origin. The fragmental igneous rocks represent the ma-
terial thrown out by volcanoes during periods of activity.
The explosive action is due to vapors, chiefly that of



270 ROCKS AND ROCK MINERALS

water, which is contained under pressure in the magma,
and as the latter rises to the surface and the pressure is
relieved, departs with violence. While the major part is
passing off in great volumes, which rush upward and carry
the solid or liquid materials to great heights, a minor part
is also expanding in the liquid, converting it into cellular
vesicular forms. Consequently the solid particles as they
fall are commonly found to be of spongy consistency, but
mixed with them are often seen compact pieces of lava
and other rocks, parts of the solid lava crust formed by
cooling after a previous eruption, mingled with fragments
torn from the rock walls of the conduit. As the lava con-
tinues rising, the greater volume of the gases may pass off,
the explosive activity ceases, and the projection of material
may be succeeded by quiet outflows of liquid rock. Hence
it is very common to find the beds of fragmental material
interspersed with layers of compact lava, felsite or basalt.
In this connection it should be also mentioned that the
chemical composition of the magma plays a considerable
part in explosive activity. Those magmas which corre-
spond to felsite, and are high in silica are, as has been men-
tioned, very viscous at temperatures where those low in
silica, such as the basalt magmas, which are rich in iron,
magnesia and lime, are still relatively very liquid. From
the former the vapors, on account of their thick viscous
condition, escape with difficulty, and with explosive
violence; from the latter they pass off readily and easily
without explosive activity. While there are many ex-
ceptions to this, it may be accepted as a general rule, and
we therefore find that vents yielding felsite lavas generally
build high and steep cones, composed chiefly of frag-
mental materials, while basaltic ones are built up largely
of liquid outflows and are therefore low and broad. Many
volcanoes, like Vesuvius, are of intermediate character in
which explosion and projection of material is succeeded