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

crystalline masses. Halite, rock salt, p. 113.

C. Cleavage apparently as above. No perceptible taste. Easily

scratched by the knife. White, bluish. In crystalline
masses. Anhydrite, p. 113.

D. Apparent cleavages sometimes forming rhombs, sometimes

apparently cubic. Very hard, scratches quartz easily. In
hexagonal crystals, grains or lumps of a dark, smoky, or
bluish gray; more or less translucent. Corundum, p. 86.

SEC. 4. Has a fine fibrous or columnar structure

A. In opaque brown to black masses. Streak yellow-brown.

Limonite, p. 93.

B. In opaque red-brown to black masses. Streak brownish red.

Hematite, p. 91.



126 ROCKS AND ROCK MINERALS

C. White or reddish ; translucent. Brittle. Often radially fibrous.

Sometimes showing slender prismatic crystals. Difficultly
scratched by the knife. Occurs in cavities, veins or seams.
Natrolite, p. 103.

D. White or reddish ; translucent or transparent. Brittle. Often radi-

ally fibrous. Compound crystals often sheaf shaped. Scratched
by knife. In cavities, veins or seams. Stilbite, p. 103.

E. Shreds easily into fine, flexible fibers like cotton or silk. White

or light gray. a. Hornblende asbestus, page 65. b. White
to yellowish brown; silky; generally in veins in or associated
with serpentine. Chrysotile (serpentine) asbestus, page 101.

F. White or pale colors. Translucent. Brittle. Easily scratched

by knife but not by finger nail. In masses. Calcite, p. 105.

G. White to pale red. Silky luster, translucent. Brittle, soft,

scratched by finger nail. In masses and seams. Gypsum,
p. 111.

SEC. 5. Without good or apparent cleavage

A. Opaque, brass-yellow crystals with metallic luster. Not

scratched by the knife. Pyrite, p. 94.

B. Opaque, earthy, brown to brown-black masses. Streak yellow-

brown. Scratched by the knife. Limonite, p. 93.

C. Opaque, reddish brown to black masses, or crystals and grains,

iron black with metallic luster. Streak brownish red.
Scratched by the knife. Hematite, p. 91.

D. Opaque, iron black masses, grains or octahedrons with metallic

luster. Streak black. Magnetic. Not scratched by the
knife. Magnetite, p. 89.

E. Opaque, black grains or masses often with reddish tone. Luster

metallic to submetallic. Streak black to reddish black. Not
noticeably magnetic. Scarcely or not scratched by the knife.
Ilmenite, p. 90.

F. In garnet-shaped crystals or spherical. Usually dark red to

black and translucent. Brittle. Not scratched by feldspar.
Garnet, p. 70.

G. In garnet-shaped crystals. Colorless or white to gray white,

translucent. Not scratched by knife but by feldspar. Leucite,
p. 49, or Analcite, p. 103.

H. In transparent to translucent crystals or grains of a light yellow-
ish- or bottle-green color. Not scratched by feldspar.
Olivine, p. 67.

/. In prismatic crystals, generally slender, shiny and black with
triangular cross section. Not scratched by quartz. Tour-
maline, p. 78.



THE DETERMINATION OF ROCK MINERALS 127

/. In grains, masses or hexagonal, pyramidal crystals. Con-

choidal fracture. Greasy to glassy luster. Colorless, white,

smoky, dark; transparent to translucent. Not scratched by

feldspar. Quartz, p. 83.
K. In grains or masses, rarely in crystals with rectangular or

hexagonal sections. Conchoidal fracture. Greasy, oily

luster. White, gray or reddish; translucent. Scratched by

feldspar. Nephelite, p. 47.
L. In grains or masses, generally of a bright blue color. Sodalite,

p. 48.
M . In masses, of a dark or yellowish green, easily scratched or cut

by knife. Serpentine, p. 100.
N. In masses, often somewhat foliated. Greasy feel; very soft,

marks cloth. White, greenish, gray. Talc, p. 102.
O. In hexagonal crystals, grains or lumps. Dark smoky or bluish

gray; translucent. Very hard, not scratched by quartz,

garnet or tourmaline. Corundum, p. 86.
P. In masses, compact or chalky. Friable, very soft, easily cut by

finger nail. Rubbed between the fingers has a soft soapy feel.

Kaolin, p. 96.

TABLE 2

A. The mineral has a metallic luster or is opaque and gives a

dark or strongly colored streak. 2.*

B. The mineral is without metallic luster or is transparent or

translucent on very thin edges and its streak is white or
light-colored. 6.*

A. Heated in the blowpipe flame the mineral burns and gives

off sulphurous fumes. Has a brass yellow color. Pyrite,
p. 94.

B. Heated in the reducing blowpipe flame becomes magnetic

when cold. Not brassy in appearance. Infusible or
very difficultly so. Iron oxides. 3.

I A. Is magnetic without heating. Magnetite (and in part
Ilmenite"), p. 89.
B. Is only magnetic after heating. 4.

{A. Heated in the closed glass tube gives water. Limonite, p. 93.
B Gives little or no water. 5.

I A. Reacts for titanium. Ilmenite, p. 90.
B. No reaction for titanium. Streak brownish or Indian red.
Hematite, p. 91.

a, | A. Fusible before the blowpipe (fusibility 1-5). 7.
1 1 B. Infusible or very difficultly fusible. 17.

* The appended number in each case refers to that in front of a succeeding
section.



128 ROCKS AND ROCK MINERALS

A. Become magnetic after heating before the blowpipe in

reducing flame. 8.

B. Do not become magnetic. 11.

A. Soluble in hydrochloric acid with separation of silica, some-

times gelatinous. 9.

B. Insoluble in hydrochloric acid or only slightly acted on. 10.

A. Micaceous or foliated. Mica (Biotite or Lepidomelane),

p. 50.

B. Isometric crystals. Gelatinizes imperfectly. Garnet

(Andradite), p. 70.

C. Gelatinizes. Olivine, rich in iron Fayalite, p. 67.

A. Micaceous difficultly fusible. Biotite, p. 50.

B. Isometric crystals or spherical in shape. After fusion

gelatinizes with HC1. Dark red color. Garnet (Alman-
dite), p. 70.

C. Quietly and difficultly fusible. Greenish black or bronze-

brown. Good cleavage. Pyroxene (Hypersthene), p. 55.

D. Fuses with intumescence coloring flame strong yellow.

Black. Prismatic cleavage, angle 55 degrees. Amphi-
bole (Arfvedsonite), p. 60.

E. Fuses quietly, coloring flame yellow. Black, prismatic

cleavage, 93 degrees. Pyroxene (Aegirite), p. 55.

A. Readily and completely soluble in water; has a saline taste.

Halite, rock-salt, p. 113.

B. Difficultly soluble in water. After intense ignition colors

moistened turmeric paper brown.

a. Gives much water in closed glass tube, Gypsum, p. 1 1 1 .
6. Gives little or no water in closed tube, Anhydrite.
p. 113.

C. Soluble in hydrochloric acid without gelatinizing or separa-

tion of silica on evaporation. A drop of sulphuric acid
in the solution precipitates calcium sulphate. Apatite,
p. 95.

D. Soluble in hydrochloric acid with gelatinization.

a. Heated in closed glass tube gives off water. 12.

b. Heated as above yields little or no water. 13.

E. Soluble in hydrochloric acid, silica separates but no jelly

forms.

a. Heated in closed glass tube gives off water. 14.
6. Heated as above yields little or no water. 15.

F. Insoluble in hydrochloric acid. 16.

A. Fuses quietly to a clear transparent glass. White or

colorless ; in slender crystals or fibrous bunches. Natrolite,
see Zeolites, p. 103.

B. A fragment in warm dilute hydrochloric acid gives off

minute bubbles of CO 2 gas. Cancrinite, see Felds-
pathoids, p. 48.



THE DETERMINATION OF ROCK MINERALS 129



A.



Fuse rather easily
before the blow-
pipe, coloring
the flame strong
yellow. Dissolve
easily in very
dilute nitric
acid and gela-
tinize.



a. The nitric acid solution gives a pre-
cipitate with silver nitrate solution
(page 121). Color usually blue.
Sodalite, p. 48.

6. The nitric acid solution gives a pre-
cipitate with barium chloride solu-
tion. Hauynite-Noselite, p. 48.

c. No reaction with silver nitrate or
barium chloride. Nephelite. p. 47.



[Difficultly soluble in hydrochloric acid and colors the flame
B.\ very little. Has a good cleavage in two directions. Anor-
| thite, see Feldspars, p. 34.

A. Usually in greenish masses, compact, greasy, sometimes

fibrous. Difficultly fusible. Serpentine, p. 100.

B. Fuses quietly to a clear glass coloring flame yellow. Gen-

erally in colorless or white garnet-like crystals. Analcite,
p. 103.

C. Fuses with swelling and intumescence. Commonly in

sheaf-like or radiated crystals. Stilbite, p. 103.

D. Fuses as in (7. Crystals have a fine cleavage with pearly

luster and lozenge-shaped section. Heulandite, p. 104.

A. Fuses quietly to a glassy globule. Slowly acted on by

hydrochloric acid. Good cleavage in two directions; one
generally shows fine parallel twinning lines. Often

frayish or bluish with a play of colors. Labradorite, see
eldspars, p. 34.

B. Fuses quietly to white globule. Easily soluble in hydro-

chloric acid; solution evaporated to dryness, residue
moistened with little hydrochloric and dissolved in water
and filtered, ammonia produces little or no precipitate.
Wollastonite, CaSiOg, a variety of Pyroxene, generally of
a white color.

A. Micaceous. Cleave into thin flexible elastic plates in one

direction. Micas, p. 50.

B. Micaceous. Cleaves into thin plates, flexible but not

elastic, micaceous. Very soft and has a greasy feel.
Color white, gray or greenish. Talc, p. 102.

C. Cleavable, micaceous, but cleavage plates not elastic, though

flexible. Soft, but not so soft as talc. Color green,
usually rather dark green. Chlorite, p. 98.

D. Not micaceous. Solid and brittle. Good cleavage in two

directions at or about 90 degrees. Generally light
colored, red or gray. Hard, cannot be scratched by
knife. Difficultly fusible. Feldspar, p. 34.
E. Before the blowpipe fuses with swelling and bubbling.
Very hard, scratches feldspar. Generally in black
columnar crystals, sometimes red or green. No cleavage.
Tourmaline, p. 78.



17



130 ROCKS AND ROCK MINERALS

F. Fuses quietly. Gelatinizes with hydrochloric acid aftei

fusion. Crystals as on page 70 or in spherical forms.
Very hard. No good cleavage. Garnet, p. 70.

G. Fuses with swelling and intumescence to a black slaggy

mass which gelatinizes in hydrochloric acid. Powdered
mineral on intense heating in closed glass tube yields a
little water. Yellowish to blackish green. Epidote, p. 73.
, H. Fuses quietly or with little intumescence. Generally
scratched by feldspar.

a. Prismatic cleavage with angle of 87 degrees. Pyroxene

p. 55.
6. Prismatic cleavage with angle of 55 degrees. Amphi-

bole, p. 60.

/. Fuses with intumescence to a greenish or brownish glass
which will gelatinize with hydrochloric acid. Vesuvianite,
p. 75.

A. After intense ignition before the blowpipe gives a brown

stain when placed on moistened turmeric paper. 18.

B. Dissolves in hydrochloric acid but without gelatinizing or

yielding a residue of silica on evaporation. 19.

C. a. Dissolves in hydrochloric acid and gelatinizes. Olivine,

p. 67.
b. Reacts for fluorine (see topaz 22 F.). Chondrodite, p. 82.

D. Dissolves in hydrochloric acid, does not gelatinize but

silica separates. 20.

E. Insoluble in hydrochloric acid.

a. Can be scratched by glass or a knife point. 21.
ft. Cannot be scratched by glass or the knife. 22.

I A. Effervesces freely in cold dilute acid. Calcite, p. 105.
B. Effervesces freely in hot but not in cold acid. Dolomite,
p. 108.

A. Heated in the reducing blowpipe flame becomes magnetic.

a. Little or no water in closed tube; streak brown-red.

Hematite, p. 91.
6. Water in closed glass tube; streak yellow-brown.

Limonite, p. 93.

B. In hexagonal crystals usually. Gives reactions for phos-

phorus. A little dilute sulphuric acid gives a precipitate
of white CaSO 4 , in the cold concentrated solution of
mineral in hydrochloric acid. Readily scratched by the
knife. Apatite, p. 95.

A. Commonly in compact green masses. Sometimes fibrous.

like asbestus, then white or brownish or yellowish. Greasy
feeling, easily scratched by knife. Serpentine, p. 100.

B. In spherical or garnet-shaped crystals. White to gray

Leucite, p. 49.



19



20



21



22 -



THE DETERMINATION OF ROCK MINERALS 131

A. Micaceous. Cleavage leaves tough and elastic. Micas,

p. 50.

B. Micaceous. Cleavage leaves tough and flexible but not

elastic. Intense ignition in closed tube gives water.
Color green. Chlorite, p. 98.

C. Very soft and has a greasy feeling. Talc, p. 102.

D. Clay-like, compact or mealy. Leaves undissolved silica in

the phosphorus salt bead. Gives water in the closed
glass tube. Kaolin, p. 96.

A. Extremely hard. Scratches quartz. Generally has a

parting that looks like cleavage. Corundum, p. 86.

B. No cleavage; conchoidal fracture. Scratches feldspar.

Sometimes in hexagonal crystals with pyramid at end.
Quartz, p. 83.

C. Prismatic cleavage. Becomes black before the blowpipe

and very fine splinters fuse with difficulty. Brown to
green or greenish black. Pyroxene (enstatite-hypers-
thene), p. 55.

D. Good cleavage in two directions at 90 degrees or nearly so.

Generally light in color, red or gray. Scratched by
quartz. Fusibility = 5. Feldspars, p. 34.

E. In prismatic crystals, often twinned; scratches quartz; red-

brown to brownish black; intense ignition in closed tube
gives a little water. Staurolite, p. 76.

F. Reaction for fluorine when heated in tube with soda meta-

phosphate. With cobalt nitrate reacts for alumina (see G
below). One good cleavage. Scratches quartz. Topaz,
p. 81.

G. Powdered mineral moistened with cobalt nitrate and

intensely heated by the blowpipe on charcoal becomes blue
(alumina)', a, in stout rectangular prisms, often full of
impurities, not scratched by knife. Andalusite, p. 77 ;b,
in bladed, generally blue, crystals; scratched by knife
parallel to cleavage, but not at right angles to it. Cyanite,
p. 78.

H. No crystal form or structure. Effervesces in Na 2 CO 3 bead.
Yields a little water in closed tube on intense ignition.
Opal, etc., p. 86.



PART III.

ROCKS.



CHAPTER VI.
GENERAL PETROLOGY OF IGNEOUS ROCKS.

IT has been previously stated that all rocks may be
divided into three great natural groups, the igneous, the
sedimentary and the metamorphic. The igneous are
those which have been formed by the solidification of
molten masses from within the earth. With reference to
their origin they have also at times been called the primary
rocks because the material which composes the other two
classes has been originally derived from igneous rocks
which, from time to time, have been formed either in or on
the upper part of the earth's crust or from the earth's
original crust itself. And if we follow the view that the
earth was once molten, the original cooling crust must
have been of the nature of igneous rock. Hence in this
sense the igneous rocks are the primary ones.

Distinguishing Characters of Igneous Rocks. The char-
acters of the igneous rocks, by which they may be
distinguished from the sedimentary and metamorphic
ones, are of two kinds field characters and specimen
characters. The field characters are those which can only
be observed in the field by studying the mass of rock in
its relation to surrounding masses, or in other words its
mode of occurrence, that is, whether it is a dike, a laccolith,
a lava sheet, etc. These modes of occurrence will be
presently described. If that of a given rock mass can be

132



GENERAL PETROLOGY OF IGNEOUS ROCKS 133

clearly determined it indicates, more definitely than any-
thing else, if it is igneous in origin or not.

But very often it happens that the boundaries of a rock
mass are so covered or obscured that its relations to the
surrounding rocks and its mode of occurrence cannot be
told, or it is often necessary to determine the nature of a
specimen which has been removed from a parent mass
which is not accessible for study. In this case we are
compelled to fall back upon those characters of the rock
which are inherent, and to be observed by an examination
of the material of the outcrop or specimen. Of these
there are three principal ones which distinguish the
igneous from the sedimentary and metamorphic rocks.
They are:

a. Entire absence of fossils.

b. The material composition.

c. The arrangement of the material, texture or structure.

The first character is an obvious one, but it is largely of
negative value since many sedimentary and most meta-
morphic rocks do not contain fossils.

The second refers to whether the rock contains glass
or is wholly made up of mineral grains, and if the latter,
the kinds and relative amounts of the minerals present.
If a rock is made up wholly or in part of glass it is cer-
tainly of igneous origin. The presence of certain minerals
is also proof of igneous origin, but no general rule by which
a rock may be certainly stated to be of igneous origin from
its mineral composition can be given. This would have
to be done from a knowledge of the different kinds of
igneous rocks themselves, as they are described in a fol-
lowing chapter. The third character is that the igneous
rocks present a homogeneous appearance; that a surface
of the rock in one direction is like a surface in any other
direction that they do not show the stratified, barfded,
or foliated structures which are characteristic of the
sedimentary and metamorphic rocks. In addition there



134 ROCKS AND ROCK MINERALS

are certain minor structures which sometimes appear in
igneous rocks, such as the amygdaloidal, which are
highly characteristic and will be described later.

There are exceptions to the rules given above in a and c, but at
.the outset it is better for the student to consider them as if absolute,
and the exceptions, which will be discussed in their appropriate
places, will take care of themselves.

Occurrences of Igneous Rocks.

There are two chief modes of occurrence of igneous
rocks recognized by geologists, the extrusive and the
intrusive. In the extrusive the molten mass or magma
rising from depths below has attained the surface, come
out upon it, solidified and formed the rock. They are
also called effusive and sometimes volcanic rocks, though
they are not always connected with volcanoes. In the
case of the intrusive rocks the magma has stopped before
attaining the surface and has cooled and solidified, sur-
rounded by other rock masses of the earth's upper crust.
Each of these cases has a number of recognized sub-
divisions; with the extrusive rocks depending on the
conditions under which the magma was ejected and with
the intrusive rocks on the relation which the mass bears
to the rocks which surround it. Following the course of
the magma upward we will describe first the intrusive
and then the extrusive modes of occurrence.

Intrusive Modes of Occurrence. These are dikes,
sheets, laccoliths, necks, stocks and bathyliths. Various
other modes have been recognized and described, but for
simplicity's sake they may be regarded as modifications
of these which have just been mentioned. The simplest
form of intrusion is that of the dike, and this will be
described first.

Dikes. A dike is the result of the simple filling of a
fissure in rock masses by molten magma from below
which there solidified. In shape, its extension in length
and breadth is great as compared with its thickness. It



PLATE 2.




03 <



GENERAL PETROLOGY OF IGNEOUS ROCKS 135

may " cut, " that is, pass through, other igneous rocks or
through sedimentary or metamorphic ones, whatever the
material was in which the fissure was formed. In passing
through sedimentary rocks it always cuts at some angle
across the planes of stratification; if parallel to them it
becomes an intrusive sheet. A dike may be of all sizes
from a fraction of an inch in thickness up to half a mile;
from two or three feet up to twenty are the ones most
commonly observed; it may be but a yard or two long as
exposed on the surface, or it may be many miles; a great
dike in the north of England has been traced over a
hundred miles. The plane of extension of a dike in most
cases appears to be vertical or nearly so; often it is inclined
at varying angles to the vertical plane. This angle of
inclination is called the hade of the dike, and the direction
which its outcrop takes in intersecting the horizontal
plane is called its trend. Dikes may have attained the
surface and given rise to lava outflows, or they may not
and have been exposed only by subsequent erosion. In
the processes of erosion they may have resisted better
than the surrounding rock and thus project as walls, a
common feature; or they may have resisted less well and
have become ditches, which is less common. Dikes very
often show pronouncedly the columnar structure described
later, the columns lying at right angles to the walls.
Where dikes have cut through sedimentary rocks they
have often changed and altered them for some distance in
the manner described under contact metamorphism. A
view of a dike cutting a sheet of igneous rock and stratified
beds is seen in Plate 2.

Intrusive Sheets. It frequently happens that where
molten magma is being forced upward through, or into,
stratified rocks, that it attains a place where the con-
ditions are such that it is easiest for it to spread out in a
layer between the sedimentary beds. This frequently
happens where the beds are weak and easily penetrated,
as in shales, thinly bedded sandstones, etc. The form of



136 ROCKS AND ROCK MINERALS

such a mass is like that of a dike, but unlike the latter it
lies concordantly along the planes of stratification. Such
sheets may be only a foot or less in thickness, and from
this up to several hundred feet or even more: they may
spread over many miles in extent. Like dikes they often
show a columnar structure, the columns being perpen-
dicular to the upper and lower surfaces and thus often
vertical. Sometimes they break, dike-like, across the
strata and are continued along a new horizon. They are
usually of the same hard, firm rock at top and bottom,
and to some extent have altered and changed the sedi-
mentary beds both above and below them: these char-
acters distinguish them from surface flows of lavas which
have been buried by later deposits of sediment upon
them. They are most apt to occur in connection with
larger and more important intrusions of magma, such as
stocks, laccoliths, etc., as accompanying or dependent
features. In regions where thick extensive sheets occur
and the strata have been dislocated, faulted, and upturned
they often give rise, through erosion, to prominent topo-
graphic features as illustrated in the trap ridges of southern
New England, northern New Jersey, and in Scotland at
Edinburgh and in many other places. In Great Britain
and frequently in Canada an intrusive sheet is called a
sill. Examples are shown on Plates 2 and 3.

Laccoliths. These are great lenticular masses of igneous
rock lying between stratified beds which infold them. If
in the forming of an intrusive sheet the supply of material
from below goes on faster than it can spread at a given
thickness laterally, the strata above will begin to arch up,
and if the process continues a great thick half lens,
flat below and rounded above, of liquid rock, will be
formed.* A cross section of such a one is shown in Fig.
63. They are apt to run out into intrusive sheets or be
accompanied by them. Also on the flanks of folding,
uplifting mountain ranges where the folding strata are
subjected to horizontal pressure they may tend to open,
* Increased viscosity of magma also helps in this result.



PLATE 3.




LACCOLITH, AND INTRUSIVE SHEETS OF BASALT, IN

SANDSTONES, SHONKIN SAG, MONTANA.

(U. S. Geological Survey.)



GENERAL PETROLOGY OF IGNEOUS ROCKS 137



and such openings be filled with magma from below in

measure as they open, as illustrated in Fig. 64. In

general, laccoliths are

more or less circular or

oval in ground plan,

and while sometimes

symmetrical as in the

diagrams they are apt

not to be so but more




Fig. 63. Cross Section of a Laccolith




or less distorted in shape. The floor may be flat .or tilted
as in the figures. They differ from intrusive sheets only
in being extremely thick in comparison with their lateral

extension, and all gra-
dations between the two
may be found. They
may be a mile or more
in thickness and a num-
ber of miles in diameter,
or but a few hundred

Fig. 64. Section of an inclined Laccolith yardg ^^ The bedg

above are usually stretched, thinned and broken in the
process of formation. Like intrusive sheets they alter
and change the strata above and below by contact
metamorphism. They are most apt to occur in weak
beds of shale, etc., the stronger, thicker beds of sand-
stone and limestone being up-arched. The best exam-