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

the arrangement and regular internal structure of the
ovoids precludes such an idea and it is now generally held
that they are due to some process of differentiation or
aggregation of material in the magma with subsequent
crystallization, though in some cases it is thought that
they may represent inclusions of other rocks which have



212 ROCKS AND ROCK MINERALS

been melted up and recrystallized. Granites of this kind
are called orbicular and though not common they have
been described from Sweden, Finland, Corsica, Canada
and Rhode Island. Similar structures have also been
found in diorites and gabbros.

Miarolitic Structure. The older and deeper seated
granites and especially those which have been subjected
to heavy mountain making pressures show little or nothing
of the miarolitic structure described on page 159. The
conditions have been unfavorable for the formation of
such cavities or under the pressure they have been oblit-
erated. In other occurrences and in the younger, higher
or unsqueezed granites these cavities may occur, and on
their drusy surfaces fine crystallizations of the minerals
may be seen. The crystals from such cavities in the
granite of the Pike's Peak region in Colorado, from the
Mourne Mountains in Ireland, from Baveno on the Lago
Maggiore in Northern Italy and other localities are well
known in mineral collections.

Pegmatite Dikes. These are very common in granites,
so much so, that when this word is used a granite pegmatite
is usually understood unless the rock is otherwise specified.
They have the general characters described on page 175
and the following ones. The chief minerals are quartz
and feldspar, the latter being mostly orthoclase or the
variety of it called microcline, though albite also occurs.
The quartz and feldspar are apt to be intergrown in such
a manner that the interstices of a spongy quartz crystal
are filled by an equally spongy feldspar crystal, the two
sponges thus mutually filling each other's interstices and
interclasping. As the quartz has no cleavage while the
feldspar has, the cleavage through the intergrown mass
is that of the feldspar and upon such surfaces the quartz
appears, repeating its tendency to crystal form again and
again and thus producing figures which recall the script
used in Arabic writings. This arrangement is called
graphic granite and a figure of it is seen on Plate 16. It



PLATE 16.



. .




GRAPHIC-GRANITE, OR PEGMATITE.



DESCRIPTION OF IGNEOUS ROCKS 213

shows that these two minerals have crystallized simul-
taneously. The minerals occur also separately and often
in huge crystals so that such dikes are mined for com-
mercial purposes, the quartz and feldspar being used in
several technical processes such as the manufacture of
china, porcelain ware, etc. The large crystals of mus-
covite mica which occur in them are the source of this
material as used in stove windows, lamp chimneys, paper
making, etc. In addition to these chief minerals a great
variety of accessory ones are found, some of the more
common of which are tourmaline, garnet, beryl, and
spodumene among the silicates, apatite, triphylite, and a
series of related phosphates, and a variety of kinds con-
taining rare earths. Some of these minerals like the
colored tourmalines, topaz, beryl, etc., are valuable for the
material suitable for cutting into gems which they afford;
others are useful as sources of the rarer elements used in
chemical and some technical processes such as the making
of Welsbach mantles. A full list of all the minerals known
to occur in these pegmatites would cover a large proportion
of all the kinds known in mineralogical science.

Inclusions. Schlieren. It is not uncommon to find
in granite the various kinds of inclusions described on
page 163 and the following. Sometimes the composition
and form of these show that they are fragments of pre-
viously existing rock formations broken off and engulfed
in the granite magma. These are most common near the
border of the mass. They may vary in size from an inch
across or less to masses a number of yards long. When
they are found in the center of the mass they may be
suspected of having sunk into it from the former overlying
roof of other rocks.

In other cases the apparent inclusions are the schlieren
described. They may consist wholly or nearly so of
quartz and feldspar and thus be very light in color or
extremely rich in biotite or hornblende or both, with
iron ore, and thus be very dark in color. Such dark



214 ROCKS AND ROCK MINERALS

streaks may at times be due to melted up inclusions but
in other cases they may be caused by aggregations of the
normal dark minerals of the granite and in general are
ascribed to processes of differentiation.

Complementary Dikes. Very frequently it will be found
that bodies of granite are cut by complementary dikes
as described on page 167. The leucocratic ones are com-
monly composed almost solely of quartz and feldspar
with which is usually associated a little white mica.
The rock has a granular appearance and this variety of
granite is called aplite. Sometimes small black specks
of biotite or hornblende or of black tourmaline may be
seen in them but always the dark minerals play a very
subordinate role. The color of these rocks is nearly con-
stantly very light, white, flesh-color, pale yellow, gray or
brown being common. Sometimes these rocks are so
fine of grain that they pass into felsites of the colors
mentioned, and sometimes they are porphyritic with
phenocrysts of quartz or feldspar or both and are thus
granite or felsite porphyries. But most commonly they
are even-granular with a grain about like that of loaf
sugar and the dike is the characteristic mode of occurrence.
They are mostly noticed cutting the granite mass, less
commonly the surrounding rocks. They are of all sizes,
from a fraction of an inch to a number of yards in breadth.
If the larger ones are traced along the outcrop it may
sometimes be found that they change into pegmatite
dikes.

The melanocratic dikes, sheets, etc., complementary to
the quartzo-feldspathic aplites described above, are dark
to black heavy rocks of basaltic aspect. They are com-
posed chiefly of biotite-mica, hornblende, pyroxene and
iron ore with feldspars, but very commonly the grain is
too fine for these minerals to be distinguished and they
are to be classed as basalts, or, in allusion to their mode
of occurrence, they may be termed lamprophyric basalt.
In many cases, however, when biotite is the prominent



DESCRIPTION OF IGNEOUS ROCKS 215

ingredient they have a characteristic glimmering appear-
ance or the plates of biotite may be distinctly seen, and in
this case they are known as mica traps. The most char-
acteristic color of these rocks is a dark stone gray. Occa-
sionally porphyritic crystals of hornblende or of feldspar,
as well as of biotite, appear in them and not uncommonly
fragments of the granite which they cut and of other
rocks. They also at times contain sulphurets of the
heavy metals, usually pyrite, and on this account have
been prospected or mined as if ore veins, generally without
much result. They alter and weather down into soft
greenish material full of chlorite or into brown earthy
masses. The earlier stages of alteration by the elements
are marked by the formation of carbonates and they then
effervesce freely when treated with acid.

They occur characteristically in dikes, usually of but a
few feet in width, but as previously mentioned, also in
intrusive sheets, small laccoliths, etc. While they often
cut the granite they are more apt to be found in the outer
zone of rocks surrounding it and sporadic occurrences may
be discovered a number of miles distant from the parent
mass. The origin of these complementary dikes has been
already discussed on pages 167 and 178.

Contact Phenomena. It is especially around great
granite intrusions that the contact phenomena described
on page 180 are seen in their greatest development and
perfection. In the endomorphic form the granite may
become a felsite or granite porphyry at the contact, or it
may show a differentiated border zone (see page 165) and
become so enriched in the dark silicates as to pass into a
diorite or dolerite border facies, or, more rarely, on the
other hand, be so poor in these as to present a marginal
facies of aplite, quite like that seen in the complementary
dikes. The first cases mentioned are purely textural
modifications ; the second are chemical and mineralogical.
More rarely cases are known where granites have a border
of pegmatite. With respect to exomorphism the changes



216 ROCKS AND ROCK MINERALS

described in the previous chapter, on account of the
common occurrence of granites, are more frequently seen
and have been more extensively studied in connection with
them than with any other variety of igneous rock. Around
the great granite batholiths these effects are often profound
and far reaching, involving tracts of possibly several miles
in width. Such areas are often of great interest and im-
portance, not only from the geological standpoint, but
because they are frequently the site of important ore
deposits. If granite comes directly against sedimentary
rocks with vertical contact and the latter show no
evidence of metamorphism, it may be safely assumed
that faulting or dislocation has brought them together.

Weathering of Granite into Soil. Through the action
of the atmosphere, of water, of heat and cold, granite
breaks down into soil. In northern and in temperate
regions, such as eastern North America, as Merrill has
shown, this change is at first largely a mechanical dis-
integration and the resultant material differs in its general
chemical composition but slightly from the original rock.

In appearance, however, as it changes granite may alter
considerably. The mica tends to bleach and lighten, and
ferrous compounds tend to become ferric and the iron
oxide to leach out, staining the rock red to dark brown.
At the same time its firm texture is lost and it becomes
more or less friable and crumbly. Finally it falls into an
angular gravel or sand, composed mainly of particles of
quartz and feldspar, called gruss. See Plate 17.

From this stage as the change into soil becomes more
complete, the most important process is the conversion
of the feldspar into kaolin, according to the following
reaction.

Orthoclase Water Carb.diox. Clay Quartz Potas. Carb.

2KAlSi 3 O 8 + 2 H 2 O + CO 2 = H 4 Al 2 Si 2 O 9 + 4 SiO 2 + K 2 CO 3

This reaction begins as soon as the rock is exposed; it has
only partially taken place when the rock crumbles, but



PLATE 17.




GNEISSOID GRANITE, THROUGH WEATHERING, PASSING
FROM FIRM ROCK BELOW INTO ALTERED

ROCK AND SOIL ABOVE.
(Merrill, Bulletin Geological Society of America.)



DESCRIPTION OF IGNEOUS ROCKS 217

after that goes on slowly but steadily until the feldspar is
eventually wholly changed into clay. As a result, a soil
consisting of a mixture of clay and quartz sand, stained
reddish or yellowish by the iron compounds, is formed.
Such a soil is called a loam. Usually the process is not
entirely complete and the soil contains more or less
small particles of feldspar undergoing alteration. This
has an important bearing on the self-renewal of its
fertility.

This reaction is one of the most important that takes
place in the great laboratory of Nature, for by means of it,
not only is the solid rock converted into soil, but one of the
most essential of plant foods, the potash, is converted
into soluble form in which it can be assimilated. At the
same time the other essentials of plant food, the silica,
magnesia, lime, etc., are also unlocked from the rocks and
rendered available. Thus by their aid plant life is able
to grow and produce from water, carbon, dioxide, etc.,
those substances upon which all animal life ultimately
depends.

In tropical regions the decay of granite gives rise to a
red or yellow-brown ferrugineous earth to which the name
of laterite is given. It has been shown to consist of a
mixture of quartz sand with hydrargillite, a clay-like sub-
stance with the composition A1(OH)3, colored by iron
oxides. But the name has also been applied in India to
soils formed from the basalts of the great Deccan plateaux
mentioned later.

In tropical deserts the surface of granites becomes
coated by a brownish or black skin, sometimes with a
luster like varnish, due to the alteration of the iron-
bearing components and the formation of iron and man-
ganese compounds. This also occurs with other kinds of
rocks as well.

Occurrence of Granites. Granite is one of the most
common and widely occurring of igneous rocks, and plays
a prominent role in the formation of the continental



218 ROCKS AND ROCK MINERALS

masses. In the form of great stocks and batholiths it
forms the central core of many of the great mountain
ranges and is revealed by later erosion. In those parts of
the earth's surface which have been subjected to repeated
disturbances of the crust and profound erosion granites
are common rocks. Thus great stocks of different ages
of intrusion are found in eastern Canada, in New England
and generally along the region of the Piedmont plateau
from southern New York into Georgia. They occur
again in Missouri, Wisconsin, etc., in isolated areas, but in
general, until the Rocky Mountains region is approached,
the central states, which compose the Mississippi Valley,
being covered with stratified rocks, are devoid of them,
though it may be inferred by analogy that they form a
large part of the basement on which these later rocks lie.
In the Rocky Mountains and in the far western states
they are of importance. Likewise in Europe, in western
and southern England, in Ireland and Scotland, in various
places in France and Germany and in the Alps they are of
common occurrence and their exposures form considerable
areas. Such a list of occurrences might be almost in-
definitely extended but enough has been said to show
their importance and wide distribution.

SYENITE.

Composition. Syenites are granular rocks composed
chiefly of feldspars. They differ from granites in that
they contain no quartz or only a negligible quantity.
They may consist entirely of feldspar, but usually more or
less hornblende, mica or pyroxene is present. These
however are subordinate in amount to the feldspar, since
if they are equal to or exceed it, the rock becomes a
diorite. If the rock is fairly coarse-grained, occasional
particles of magnetite and other minerals may be seen,
but these are only accessory and not of the importance of
the ones mentioned. Occasional minerals which produce
varieties will be mentioned presently.



DESCRIPTION OF IGNEOUS ROCKS 219

In strict petrographic classification founded on microscopic
examination a distinction is made in these rocks based upon the
kind of feldspar. If the latter is predominantly an alkalic feldspar,
without lime, the rock is called a syenite, as above, but if it is a lime-
soda feldspar or plagioclase the rock is termed a diorite without
reference to the quantity of dark minerals present. This dis-
tinction, however, cannot, except in certain exceptional cases, be
carried out by megascopic examination and therefore no attempt is
made to separate them in this work.

From what has been said it may be seen that the mineral
composition of syenite may vary considerably; there may
be a mixture of feldspars present or only one kind, either
alkalic or lime-sodic, and there may be variations among
the ferromagnesian minerals. According to the pre-
dominant kind of the latter, the rock is spoken of as horn-
blende syenite, mica syenite or augite syenite. All of
these are here treated under the general heading of syenite,
but in two cases the rock may have a particular mineral
composition which makes it of especial interest and there-
fore deserving of separate description. Generally these
two varieties may be identified by observing with care the
special features which they present and which are described
beyond, otherwise they cannot be distinguished and must
be classed in the general group of syenites. These two
are as follows: (A) the rock contains in addition to the
feldspars and other minerals a notable amount of nephelite
or this and its congener sodalite; (B) the rock consists
entirely, or very nearly so, of soda-lime feldspar (labra-
dorite). We may then divide the group of syenites as
follows :

a. Syenite, in general or common syenite consisting

chiefly of feldspars, without quartz.
6. Nephelite Syenite, consisting chiefly of alkalic

feldspars with nephelite.
c. Anorthosite, consisting almost wholly of labradorite.

Properties of Syenite. The texture of syenites is usually
even granular, but sometimes a tendency may be noticed



220



ROCKS AND ROCK MINERALS



for the feldspar to assume a flattened tabular form like
that of a book, its cross sections on the rock surface are
then elongated and often arranged more or less parallel,
an arrangement which is thought to be due to movements
of the fluid mass during crystallization. This variety of
texture occurs practically only when the feldspars are of
the alkalic variety. Porphyritic varieties also occur as in
granite and these grade into syenite porphyry. The color
is variable like that of granites; white to pink or red, or
gray or yellow tones are common, gray especially so. The
specific gravity varies with the minerals and their propor-
tions; it may extend from 2.6-2.8. In a tendency to
miarolitic structure, in jointing, in erosion forms, in altera-
tion into soil, inclusions, and in contact metamorphism, etc.,
what has been said in regard to granite, applies also to
syenites and need not be repeated. They are also accom-
panied by pegmatite dikes, but these are not so common
nor so well known as the granite pegmatites. They also
often yield a great variety of minerals.

Chemical Composition. Chemically, the syenites are
distinguished from the granites by a lesser amount of
silica, which accounts for the absence of the quartz; in
other respects they resemble them. These characters
may be seen in the following table of analyses.



ANALYSES OF SYENITES.





SiO 2


A1 2 3


Fe 2 3


FeO


MgO


CaO


Na 2 O


K 2


H 2 O


XyO


Total.


I


60.7


19.6


1.5


3.0


0.8


2.3


4.9


5.9


0.3


0.6


99.6


II


60.2


20.4


1.7


1.9


1.0


2.0


6.3


6.1


0.3


0.4


100.3


III


61.6


15.1


2.0


2.2


3.7


4.6


4.3


4.5


0.7


1.0


99.7


IV


62.5


16.5


2.4


2.0


1.9


4.2


4.4


4.6


0.6


1.3


100.4



I, Belknap Mountains, New Hampshire ; II, Fourche Mountain,
Arkansas; III, Little Belt Mountains, Montana; IV, Plauen by
Dresden, Germany. Xyo = Small amounts of other oxides.



DESCRIPTION OF IGNEOUS ROCKS 221

Occurrence of Syenites. Syenites are not very common
rocks, and, while they sometimes occur in independent
masses, they are very apt to be connected with larger
bodies of granite, which by the diminishing of quartz
passes into syenite. In the United States they occur in
several places in New England, at Mount Ascutney, in the
White Mountains and adjacent region, on the coast north
of Boston, also in Arkansas, in Montana and in a number
of other localities. They are found in several places in
Germany and in the Alps. An important area of them
exists in South Norway. In comparison with the great
batholiths and stocks of granite, distributed so generally
in the continental masses, they are, geologically speaking,
of relatively small importance.

Uses of Syenite. For all constructional and other
commercial uses syenite has the same value as granite.
On account of its relative rarity, compared with the latter
rock, it is however little used. Its crushing strength is
equal to that of granite and from experiments by J. F.
Williams on syenite from Arkansas it may be even greater.
Its weight per cubic foot is about the same. The absence
of the quartz, which is harder than feldspar, should make
it an easier stone to dress and polish, and practically it
resists weathering as well, if not better. The absence of
the quartz makes it also a better stone in resisting the heat
of fires (compare granite, page 209) and it would be in
consequence a more advantageous material for building
in our large cities. If these advantages over granite
were more generally understood it is probable that the
accessible occurrences in New England would be more
extensively exploited. The beautiful dark gray syenite
of South Norway with pearly blue reflections is con-
siderably used in northern Europe as an ornamental
stone.

Nephelite Syenite. This variety is distinguished by the
fact that in addition to the feldspars, which are almost
wholly alkalic in composition, a considerable amount of



222 ROCKS AND ROCK MINERALS

nephelite is present. This mineral is sometimes flesh-
colored but usually it is smoky gray. In its lack of good
cleavage and oily, greasy luster it resembles quartz, but
can be readily distinguished from it by the gelatinization
test (page 115). It is generally present in formless grains
mixed with the feldspars, but sometimes shows the outlines
of a crystal form. It is apt to be accompanied by sodalite,
which is often of a bright blue color, in grains or, if the rock
is very coarse-grained, in lumps and masses; if it is thus
present it is useful in aiding to distinguish this rock from
common syenite. Nosean and cancrinite may also be
present. Mica (lepidomelane), hornblende (arfvedsonite)
and pyroxene (aegirite) are usually present in variable
amounts, in plates, grains or prisms, of a black color, and
containing considerable soda and iron. The presence of
soda in the minerals of this rock is readily understood
from a consideration of the chemical analyses, given
beyond, which show the composition of the magma from
which they crystallized.

The color of nephelite syenites is variable but commonly
gray. The texture is granular, sometimes rather por-
phyritic. The book-shape of the feldspars mentioned
above is common. The rock is liable to contain many
accessory minerals but usually only in microscopic sizes;
some of these are of especial interest on account of the
rare earths they contain. It is also prone to exhibit in
places great variations of the constituent minerals giving
rise to different facies. Many of these varieties have
received special names. Usually it is cut by comple-
mentary dikes and these are of a different character from
those found associated with granites and common syenites;
one is a pale brown or pink felsite, another a bright to
dark green rock called tinguaite which owes its color to
microscopic needles of aegirite ; it usually contains nephelite
and gelatinizes with acid. The complementary lam-
prophyres to these are heavy dark rocks of basaltic aspect
often showing distinct to large phenocrysts of biotite,



DESCRIPTION OF IGNEOUS ROCKS



223



augite or hornblende; they are particular varieties of
melaphyre (basalt-porphyry).

The chemical composition is illustrated in the two
following analyses of nephelite syenites.





Si0 2


A1 2 3


Fe 2 3


FeO


MgO


CaO


Na 2 O


K 2


H 2 O


XyO


Total.


I


58.8


22.5


1.5


1.0


0.2


0.7


9.6


4.9


1.0


0.3


100.5


II


53.1


21.2


1.9


2.0


0.3


3.3


6.9


8.4


1.4


2.0


100.5



I, Salem Neck, Mass.; II, Magnet Cove, Arkansas. XyO, small
quantities of various oxides.

It will be seen that the most striking thing in respect
to these magmas are the high amounts of alumina and
alkalies, with moderate silica. It is this which causes the
formation of nephelite (Na 2 . A1 2 O 3 . 2 SiO 2 ) rather
than albite (Na 2 O . A1 2 3 . 6 Si0 2 ), there being not
enough silica to convert all the alumina and alkalies into
feldspar. From this it may be seen that free quartz and
nephelite cannot crystallize from the same magma; the
silica would convert the nephelite into albite, and therefore
these two minerals are not found in the same rock. Some-
times nosean is present; cancrinite may also occur, and of
the associated minerals zircon is perhaps the most charac-
teristic. In this connection the description of the felds-
pathoid group in the part dealing with minerals should
be read.

Pegmatite dikes occur in connection with nephelite
syenites and those of South Norway and Greenland are
especially interesting for the great variety of minerals,
many of them composed in part of the rarer elements,