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 21 of 35)
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of the classification of igneous rocks, these dense varieties
are divided into two groups, on the basis of color, into the
dark to black basalts and the lighter colored felsites; each
of these has a porphyritic subdivision. Of these the
felsite will be considered first.



The felsites include all those dense igneous rocks which
are of stony texture and not evidently glassy, of all colors
except dark gray, dark green or black, these latter belong-
ing to basalts. They normally and commonly show light
shades of color; white, which is not very common, light to
medium gray, light pink or red to dark red, pale yellow or
brown, purple or light green. With the lens it can be fre-
quently seen that they consist of minute mineral grains, too
small for determination, and the texture is then very fine
granular. In other cases the grains may be entirely too
fine to be seen; the rock has then a dense, horn-like or
flinty aspect, appearing like a homogeneous substance.
In this latter case it is very apt to have a smooth con-
choidal fracture. In other cases, especially in surface
lavas, the texture is more or less porous and the fracture
surface of the rock rough and hackly, with a harsh feeling.
A pronounced cellular or vesicular structure, common in
basalts and in glassy rocks, and illustrated on Plate 8 is not
very common in this group. The surface lavas not infre-
quently show fluidal bandings and streakings, more or less
flat lenticular, and often curved or curled, due to flowage,
and often clearly brought out on weathered surfaces, as
illustrated on Plate 22.

The division of the dense igneous rocks into felsites and basalts
is based on color, and not on mineral composition, since the latter
cannot be determined. Nevertheless the felsites as classed above
are, in general, feldspathic rocks, and they represent in dense form
and often as lavas those magmas, which under different geologic and
physical conditions, would have produced granites and syenites,
while the basalts correspond to diorites, gabbros and dolerites, as
already explained under classification.

In many cases, where these rocks are of medium gray or drab
color, it is difficult to know whether to assign them to the felsites or
basalts. This happens especially when they are very dense. In
this case, if the rock be sharply examined with a good lens, it may be
seen that many tiny chips and flakes, only partly formed and yet in
the main a part of the mass, lie upon its surface, their thin edges


separated from it by a flat underlying crack. It will be observed
that their thin edges are very much lighter in color than that of the
rock upon which they lie and are translucent to light; indeed in
many cases they will appear practically white, even when the rock
is a dark gray or stone color. This is a peculiarity of those dense
rocks which are chiefly composed of feldspar and are, therefore, to
be classed as felsites; it is not shown at all, or only in a very small
degree, by the basalts. The reason for this is that, although they
may be colored by a pigment, the feldspars are transparent to
translucent minerals and a rock composed mostly of them is also
megascopically translucent on thin edges, while one composed
mostly of ferromagnesian mineral particles is not, since these minerals
are either opaque, or practically so, from the megascopic standpoint.
The same effect may be observed on the very thin edge of a flat chip
broken from the rock. In case the rock is not so dense, but that the
individual grains can be seen with the lens, if these are transparent
or translucent with light colors, it may be assumed in general that
they are mainly feldspar, the ferromagnesian mineral grains being
mostly dark, dull, to opaque. Olivine, with its yellow-green color,
however, is an exception and must not be confused with feldspar.
It will also be noted that under a good lens the mineral grains, or
many of them, appear much lighter in color than does the rock in its
general effect to the eye.

These tests, while they cannot be held to be exact, will of ten prove
of service in helping to decide, in doubtful cases, whether a rock
belongs to the felsites or basalts, for the division between them,
while based primarily on color, is due also to a mineralogical dif-
ference as well.

It is assumed in what has been said regarding these rocks that
one is dealing with reasonably fresh, unaltered material, not those
rocks which have been long exposed to atmospheric agencies and
are weathered into dull ferrugineous material, or green masses of

Varieties of Felsite. From the megascopic standpoint
the different varieties of felsite which can be recognized
are those which are due to color and texture alone; the
petrographer by the use of the microscope on thin sections
is, however, able to determine the different kinds of miner-
als, which form the minute grains, and to divide and classify
these rocks on a mineralogical basis, just as was done with
the coarse-grained rocks, whose grains could be seen and
determined by the eye.



This is done on the consideration of whether the rock contains
quartz or not, whether the predominant feldspar is alkalic or a soda-
lime variety, or if it contains a feldspathoid, such as nephelite, in
addition to the feldspar. According to this we have the following
kinds recognized by petrographers.

Equivalent Coarse-

Equivalent Coarse

Chief Component Minerals.


Grained Rock in

Rock in Field
Class, of this



Alkalic feldspars and quartz.


Granite . . .


Lime-soda feldspars and


Quartz diorite .



Alkalic feldspars, little or


Syenite . . .

Syenite, mostly.

no quartz.

Soda-lime feldspars, little


Diorite ....

Syenite and Dio-

or no quartz.


Alkalic feldspars and neph-


Nephelite Syenite

Nephelite Syenite.


A variety containing lime-soda feldspars and a feldspathoid is
known, but is very rare. The ferromagnesian minerals are present
in variable amounts and, according to the dominant one of these, we
have such terms as mica-trachyte, hornblende-dacite, augite-andesite,
etc., etc.

These are terms which are constantly seen in geological literature
and in general it can be understood that the rock so designated has
been subjected to microscopic study. They distinguish varieties of
the felsites which cannot be accurately made without such study,
but on the other hand the following points will serve in a rather
vague and general way to indicate megascopically, to which of the
above divisions a given felsite probably belongs. If the rock con-
tains phenocrysts of free quartz, as mentioned later in the descrip-
tion of the porphyritic varieties of felsite, it almost certainly contains
a considerable amount of quartz in the dense groundmass and is
either a rhyolite or dacite; of these rhyolite is more common than
dacite. If the rock on being tested yields gelatinous silica, according
to the method recommended on page 115, it is almost certainly a
phonolite. The distinction between rhyolite and dacite, and between
trachyte and andesite, cannot be made megascopically, since it
depends on the determination of the kind of feldspar, and in dense
rocks this can only be done by optical means.


Felsite Porphyry or Leucophyre. While felsites occur
which contain no porphyritical crystals it is much more
common for them to contain phenocrysts. These may
be very few, scattered and isolated, or they may be abun-
dant. They may be quite evenly scattered, or they may
be collected in groups. By increasing abundance, when
they form half the bulk of the rock or more, they cause
transitions into the group of determinable granular por-
phyries previously described. The phenocrysts may be
salic, in which case they are quartz or feldspar, or they may
be ferromagnesian, biotite, hornblende or pyroxene. For
the appearance and properties of these phenocrysts
reference may be had to the description of them in the
former group of porphyries, page 245.

In the association of these phenocrysts, while all of the
above minerals occur at times alone, there are apt to be
two or more present. Quartz and feldspar with the
others is not uncommon, if a dark mineral is associated
with the quartz it is most apt to be biotite ; hornblende is
less common and pyroxene very rare. Feldspar and horn-
blende, pyroxene and feldspar are very common. In
general feldspar is the most common phenocryst.

In naming these rocks the general term of felsite por-
phyry may be given to them or this may be contracted to
leucophyre, from the Greek (A.CVKOS white) meaning
light-colored porphyry.* If no mineral qualifier is used
with this it is understood that the phenocrysts are of
feldspar, since this is most general. If they are of quartz,
biotite, hornblende, etc., we have as follows:

Quartz-felsite-porphyry or quartz-leucophyre.

Hornblende- felsite-porphyry or hornblende-leucophyre.

Biotite-felsite-porphyry or biotite-leucophyre.

Augite-felsite-porphyry or augite-leucophyre.

Hornblende feldspar-leucophyre.

Quartz- feldspar-leucophyre.

* As suggested in the " Quantitative Classification of Igneous
Rocks." Messrs. Cross, Iddings, Pirsson and Washington, p. 184.


These examples are sufficient to show how mineral
qualifiers may be used in designating the phenocrysts of
these rocks, and how these may be combined, if desired, to
give expression to considerable descriptive detail. Other
examples will readily suggest themselves.

The same distinctions are made by petrographers among these
rocks, as in the felsites proper, and as described above. Thus we
find such terms in use as " rhyolite-porphyry," " mica-andesite-
porphyry," " augite-trachyte-porphyry," etc., resulting from the
study of thin sections and accurate determination of the different
kinds of minerals.

General Properties. The chemical composition of felsites
is variable, depending, like the subvarieties enumerated
above, on the prevailing minerals. Some correspond with
the analyses of granite already given; these contain free
quartz; others are like the analyses of syenites and such
must contain little or no quartz. If the amount of lime
is small, the silica below 60 per cent, and the alkalies high
the rock must be mostly composed of alkalic feldspar and
probably nephelite is present. The specific gravity ranges
from 2.4-2.65 and is usually lower than that of granite
and syenite. The jointing is platy, or in small blocks;
columnar structure also occurs, but is not so common nor
so perfect as in basalts. In normal weathering to soil
these rocks become discolored, brownish, reddish, etc.
The ferromagnesian mineral generally disappears, leaving
a rusty spot or cavity; the rock crumbles into debris, at
first largely through mechanical disintegration. Finally
the feldspars change to clay, as described under granite,
and the change to soil is complete. Where hydrothermal
action takes place, as in the vicinity of ore veins, they are
often changed to soft clay-like masses consisting some-
times of clays, and sometimes of sericite, the fine scaly
form of muscovite.

Occurrence. Felsites occur intrusively as dikes and
sheets, and sometimes as the endomorphic contact facies
of larger intrusive masses, whose main character is that of
granite, granite-porphyry, syenite, etc. They indicate


intrusions of magma into cold rocks, and when found in
intrusions, these are usually relatively small, or narrow.

They are much more common extrusively, as lava flows
and sheets, and they sometimes cover very large areas,
many hundreds and even thousands of square miles in
extent. In these cases, and especially in volcanic cones
or their eroded remnants, they are usually interbedded
with tuffs and breccias.

While it would be impossible to give any adequate list
of actual occurrences it may be mentioned that felsites as
intrusives occur extensively in eastern North America,
among the older rocks in the Atlantic border states, along
the eastern front of the Appalachian uplift; as extrusives
they occur in Maine, in the White Mountains, in Penn-
sylvania and to the southward. They occur also in
Wisconsin. Much more extensive in western America is
their effusive occurrence throughout the whole of the
Cordilleran tract, where they play an important r61e in the
upbuilding of many of the ranges, and sometimes occupy
large areas. Here all the different varieties are found, as
for instance rhyolite in the Yellowstone Park, in Colorado
and elsewhere; andesite in the lofty volcanoes of the
Coast Ranges, and in those in South America, and in the
eroded ones of the main chain of the Rocky Mountains;
the rare variety phonolite is found at Cripple Creek,
Colorado, and in the Black Hills of South Dakota.

The felsites are just as extensively found in other parts
of the world. In Europe they are well distributed as
effusive lavas. So in Great Britain they occur in northern
Wales, in the Lake district of England, and in northern
Ireland. These are the varieties called rhyolite and
andesite. They are found in western Germany, in France,
Italy, and in Hungary. Wherever volcanic outbreaks
have taken place extensively, felsite lavas occur.

Felsites and felsite porphyries are often found, in the form of
narrow dikes and sheets, traversing larger stocks or intrusions of
coarse-grained rocks, such as granite, syenite, etc., or the rocks


in their immediate vicinity. They are generally complementary in
this case to dark basaltic dikes, mentioned later as lamprophyres,
and have received special names from petrographers. Thus we
have bostonite (allied to trachyte), tinguaite (allied to phonolite) and
many others. Note in this connection paragraphs on complemen-
tary rocks, on aplite, and on lamprophyres.


The basalts include those dense igneous rocks, of very
dark color, whose fabric is so fine that the constituent
grains either cannot be perceived by the eye or with the
lens, or, if seen, are too small to be recognized, and which
are of stony but not of glassy texture. The color varies
from grayish black or dark stone color, greenish or purplish
black, to pure black. In the great majority of cases they
do not show translucency on the edges of thin flakes as
described under felsite. When not cellular, and very
dense, they have a uniform dull, soft, almost velvety
appearance, and do not show the horny, flinty, or greasy
luster of many dense felsites.

The study of thin sections of these rocks shows that in general
they are composed of minute crystal grains of soda-lime feldspar
generally labradorite, pyroxene and iron ore, very often with
more or less olivine, and sometimes biotite or hornblende. In some
cases' nephelite or leucite may accompany the feldspar, or replace it,
giving rise to varieties which have received special names. These
varieties, although very interesting from the standpoint of theoretical
petrography, are comparatively rare and relatively of small impor-
tance in a general geological way.

Being composed of the same minerals, these rocks represent, in
dense form and generally as surface lavas, those magmas which,
under different physical conditions, would have solidified as gabbros,
peridotes, dolerites and (in part) diorites. A large part of the
dolerites in fact are transition rocks between them and gabbros, as
previously mentioned in the description of that rock, and instances
may be found in the same rock mass where the gradation into
dolerite may be seen.

Basalt Porphyry. While porphyritic varieties of basalt
are not uncommon rocks it may be said in general, that this
type of texture plays a far less important role in this


A. Labradorite-Porphyry.

B. Augitophyre.


group than in the felsites previously described, owing
probably to the low freezing point and easy crystallization
of the magma. One exception to this, however, is in the
presence of olivine, which is very apt to occur scattered
through the basalt in transparent yellowish or bottle-
green porphyritic grains, averaging about the size of
moderately coarse shot. The mineral is so common
(indeed at one time a rock was not considered a basalt
unless it contained olivine) and produces in general so
little of a striking porphyritic effect, that it is rather the
custom to ignore it in this respect, and term such rocks
oli vine-basalt rather than olivine-basalt-porphyry.

The chief minerals as phenocrysts, when such occur, are feldspar
and pyroxene; hornblende and mica are much less common. The
feldspar is commonly labradorite; it occurs in elongate tabular forms,
either singly, or in twinned groups. The pyroxene is the variety
augite ; it is black, sometimes shining, sometimes dull, and is in short
thick prisms or prismoids, as illustrated under pyroxene. The
hornblende is also black and has its usual shining and good cleavage.
Biotite is in six-sided tablets.

These rocks would be named in accordance with the
prevailing phenocryst, so for example augite-basalt-
porphyry. Instead of the term basalt-porphyry the name
melaphyre, meaning " black porphyry," may be used as
more convenient * and we should then have feldspar-
melaphyre, augite-melaphyre, biotite-melaphyre, etc.

General Properties of Basalt. The chemical composition
of basalt varies with its mineral composition; in general
it is of the same nature as that of gabbro previously given,
as may be seen from the following analysis of one from
California, which will serve as an example.

SiO 2 A1 2 O 3 Fe 2 O 3 FeO MgO CaO Na 2 O K 2 O H 2 O XyO Total
51.9 15.3 3.1 3.6 8.7 7.4 3.3 2.5 2.5 1.7 = 100.0

The specific gravity is high, about 3.0 (2.9-3.1). The
jointing is platy or columnar; the best examples indeed of

* Quantitative Classification of Igneous Rocks, p. 185


this structure are found in basalt and many notable
examples of it are found in all parts of the world, the
Giants' Causeway on the north coast of Ireland being one
of the best known. This structure is seen in Plate 11.
Sometimes basalt on weathering develops a singular "pil-
low" structure by which there is formed spheroidal masses.

Varieties. In the dense non-porphyritic basalts there
is little opportunity for variation, save that which is based
on a change from the compact into the porous or cellular
structure. This last is particularly common in surface
lavas, especially in their upper portion, and has been
illustrated on Plate 8. It is particularly in these basalts
that the amygdaloidal structure occurs, also illustrated on
the same plate. The minerals filling the cavities in
basalt are commonly quartz, calcite or zeolites; among
the latter minerals analcite, natrolite, stilbite and heu-
landite may be particularly mentioned. Such rocks are
termed amygdaloidal basalt. In a number of places, and
particularly in western America, basalts have been found
as surface lavas which contain visible grains of quartz.
One of the most noted of these is the basalt flow from the
Cinder Cone, near Lassens Peak in northern California,
which is filled with angular pieces of quartz of varying
sizes. As many of these correspond in composition to
gabbros and dolerites, the presence of the quartz in them
appears anomalous, since magmas so low in silica, as may
be seen by referring to the analyses of gabbro, would not
be expected to develop free quartz on crystallizing. Some
petrographers therefore think that these are fragments of
quartz rock in the depths, which have been torn loose and
distributed through the magma, while others regard them
as a primary crystallization, produced under exceptional
conditions of pressure and mineralizers. These rocks
have been called quartz-basalts.

The porphyritic varieties have been described above,
but it may be mentioned that a variety containing distinct
and sometimes large crystals of labradorite feldspar has


been called labradorite-porphyry. The greenish-black por-
phyry from Greece, employed by the ancients (porfido
verde antico), is a somewhat altered example of this type.
A variety containing rather large and distinct crystals of
augite has been termed augitophyre.

The name trap has been used in a general way as a field
term to designate the rocks called here basalts, and also
dolerites. As thus employed it would mean any dark-
colored, heavy, igneous rock of undetermined mineral
composition. Thus the dolerites and basalts of the
Newark formation along the Atlantic coast have been
termed " Triassic traps;" the great effusives of western
India are known as the " Deccan traps."

Lamprophyres. The ferromagnesian complementary
rocks, occurring in dikes and sheets in or around stocks
of granite, syenite, etc., and often called " trap " dikes,
etc., belong for the most part under this heading of
basalt. Their origin and relations have been discussed
in the preceding part of this work and they have been
mentioned again under granite. They are very apt to
contain phenocrysts of the ferromagnesian minerals,
olivine, augite, hornblende and biotite, either separately
or together, and sometimes these phenocrysts are of very
large size. These are embedded in a groundmass that is
usually dense and basaltic. According to the variations
in the minerals, as shown by the microscope, a large
number of different types have been named by petrogra-
phers, distinctions which ordinarily cannot be made mega-
scopically. For field work they may be treated simply as
basalt-porphyries as described above, and termed augite
melaphyre, biotite melaphyre, etc. It may be mentioned
that biotite melaphyre is a rock which is frequently found
in dikes with granite, and has been called mica trap or
minette. A hornblende melaphyre occurs in the same way
with many syenites and nephelite syenites; it has quite
a wide distribution in New England and has been termed


Olivlne Nodules. It frequently happens that basalts, in
addition to the ordinary crystals of oh'vine, contain yellow-
ish, or green lumps, or nodules, made up of grains of this
mineral. Grains of other minerals, such as pyroxene,
spinel, etc., may be present in them. These lumps may
vary in size from a pea to masses as large as one's fist,
or even larger. They are generally rounded, but often
distinctly angular in shape. Their origin is somewhat
problematical; some hold that they are merely agglomera-
tions of the earlier formed crystals in the liquid magma,
while others regard them as fragments of rock (dunite)
torn off below and brought up in it.

Exotic Minerals. Basalts sometimes contain unusual
minerals, which do not appear in the ordinary rock, and
whose origin in them must be ascribed to unusual condi-
tions, or composition of the magma. The quartz basalt
mentioned above is one of these. Another case is seen in
the iron-bearing basalts of Greenland, which contain
small to large masses of native iron, which is much like
the iron found in meteorites. By the use of a solution of
copper sulphate specks of native iron have been found in
basalts from other places. The Greenland basalts also
contain graphite. Corundum, in the form of sapphire, has
also been found in basalts, and a dike in Montana has
furnished a quantity of valuable gems. In this connection
also, may be mentioned the occurrence in places of native
copper, especially in the Lake Superior district, where the
metal occurs in dolerites and basalts and in connection
with them, in quantities which have made it one of the
most important sources of the world's copper supply.

Weathering and Alteration. In many volcanic regions,
where basalts have been subjected to exhalations of steam
or to heated water, the minerals containing ferrous oxide,
such as magnetite and olivine, become reddened through
change to ferric oxide. Sometimes the olivines alone are
reddened; in other cases the whole rock becomes deep red
to reddish brown. Such rocks may be difficult to dis-


tinguish in the field from red felsites, and may even have
to be classed with them. Sometimes, however, the asso-

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 21 of 35)