_Muscovite_ = _Biotite_ =
Acidic mica. Basic mica.
Non-ferruginous mica. Ferruginous mica.
Potash mica. Magnesian mica.
White mica. Black mica.
Non-dichroic mica. Dichroic mica.
_Feldspar Family._—Like mica, _feldspar_ is the name of a family of
minerals; and these are, geologically, the most important of all
minerals. They are, above all others, the minerals of which rocks are
made, and their abundance is well expressed in the name,—feldspar being
simply the German for field-spar, implying that it is the common spar or
mineral of the fields.
Chemically, the feldspars are silicates of aluminum and potassium,
sodium or calcium. They crystallize in the monoclinic and triclinic
systems; and all possess easy cleavage in two directions at right angles
to each other, or nearly so. The general physical characters, including
the cleavage, are well exhibited in the common species, orthoclase
(specimen 22).
In hardness the feldspars range from 5 to 7, being usually near 6, and
almost always distinctly softer than quartz. Sp. gr. varies from
2.5-2.75; lustre, from vitreous to pearly; color, from white and gray to
red, brown, green, etc., but usually light. Streak, always white; rarely
transparent. By exposure to the weather, feldspars gradually lose their
alkalies and lime, become hydrated, and are changed to kaolin or common
clay. A similar change takes place with the micas, augite, and
hornblende; but these species, being usually rich in iron, make clays
which are much darker colored than those derived from feldspars. The
fact that the feldspars contain little or no iron undoubtedly explains
their low specific gravity and light colors, as compared with the other
minerals just named. The only common minerals for which the feldspars
are liable to be mistaken are quartz and the carbonates. From the latter
they are easily distinguished by their superior hardness and
non-effervescence with acids; and from the former, by possessing
distinct cleavage, by being rarely transparent, by being somewhat
softer, and by changing to clay on exposure to the weather.
The feldspars of greatest geological interest are five in number, and
may be classified chemically as follows:—
Orthoclase,—silicate of aluminum and potassium, or
potash feldspar.
Albite,—silicate of aluminum and sodium, or
soda feldspar.
Anorthite,—silicate of aluminum and calcium, or
lime feldspar.
Oligoclase,—silicate of aluminum and sodium, and calcium, or
soda-lime feldspar.
Labradorite,—silicate of aluminum and calcium, and sodium, or
lime-soda feldspar.
This appears like a complex arrangement, but it can be simplified.
Orthoclase crystallizes in the monoclinic system, and all the other
feldspars in the triclinic system. With the exception of albite, which
is a comparatively rare species, the triclinic feldspars all contain
less silica than orthoclase; _i.e._, are more basic. This is shown by
the subjoined table giving the average composition of each of the
feldspars:—
SiO₂ Al₂O₃ K₂O Na₂O CaO Total.
Orthoclase, 65 18 17 -- -- = 100
Albite, 68 20 -- 12 -- = 100
Oligoclase, 62 24 -- 9 5 = 100
Labradorite, 53 30 -- 4 13 = 100
Anorthite, 43 37 -- -- 20 = 100
As we should naturally expect, the triclinic feldspars occur usually
with other basic minerals, while the monoclinic species, orthoclase, is
acidic in its associations; furthermore, the triclinic feldspars are
often intimately associated with each other, but are rarely important
constituents of rocks containing much orthoclase. In other words, the
distinction of orthoclase from the basic or triclinic feldspars is
important and comparatively easy, while the distinction of the different
basic feldspars from each other is both unimportant and difficult.
Hence, in lithology, we find it best to put all these basic feldspars
together, as if they were one species, under the name _plagioclase_,
which refers to the oblique cleavage of all these feldspars, and
contrasts with _orthoclase_, which refers to the right-angled cleavage
of that species.
This statement of the relations of the feldspars is, of course, beyond
the comprehension of many children, and yet it should be understood by
the teacher who would lead the children to any but the most superficial
views.
15. Orthoclase.—This is the common feldspar, and the most abundant of
all minerals, being the principal constituent of granite, gneiss, and
many other important rocks. The most characteristic colors are white,
gray, pinkish, and flesh-red. Specimen 22.
16. Plagioclase.—Like orthoclase, these species may be of almost any
color; yet these two great divisions of the feldspars are usually
contrasted in this respect. Thus, bluish and grayish colors are most
common with plagioclase, and white or reddish colors with orthoclase.
Specimen 23 is labradorite, and, in every respect, a typical example of
plagioclase. On certain faces and cleavage-surfaces of the plagioclase
crystals we may often observe a series of straight parallel lines or
bands which are often very fine,—fifty to a hundred in a single crystal.
These striæ are due to the mode of twinning, and are of especial
importance, since, while they are very characteristic of plagioclase,
they never occur in orthoclase. As stated, these twinning striæ in
plagioclase are often visible to the naked eye; and when they are not,
they may usually be revealed by examining a thin section under the
microscope with polarized light. Plagioclase decays much more rapidly
when exposed to the weather than orthoclase. This point becomes
perfectly clear when we compare weathered ledges of diabase (or any
trap-rock, see specimen 2) and granite; for plagioclase is the principal
constituent of the former rock, and orthoclase of the latter.
_Hydrous Silicates._—Many silicates contain water, and some of these are
of great geological importance. What has been stated on a preceding page
concerning the softness and lightness of hydrated minerals is especially
applicable here; for all the geologically important hydrous silicates
are distinctly softer and lighter than anhydrous minerals of otherwise
similar composition. Furthermore, they usually have an unctuous or
slippery feel; and, with one exception (kaolin), are of a green or
greenish color.
17. Kaolinite (Kaolin).—Hydrous silicate of aluminum: silica (SiO₂), 46;
alumina (Al₂O₃), 40; and water (H₂O), 14; = 100. Orthorhombic system, in
rhombic or hexagonal scales or plates, but usually earthy or clay-like.
Hardness, 1-2.5; sp. gr., 2.4-2.65. The pure mineral is white; but it is
usually colored by impurities, the principal of which are iron oxides
and carbonaceous matter. Kaolin is the most abundant of all the hydrous
silicates, and it is the basis and often the sole constituent of common
clay,—a very common mineral, but rarely pure. We have already (p. 11)
noticed the mode of origin of kaolin or clay. It results from the
decomposition of various aluminous silicate minerals, especially the
feldspars. Under the combined influence of carbon dioxide and moisture,
feldspars give up their potassium, sodium, and calcium, and take on
water, and the result is kaolin. This mineral is believed to be always a
decomposition product. Perhaps the best, or at least the most
convenient, test for kaolin is the argillaceous odor, the odor of
moistened clay. Specimen 24.
18. Talc.—Hydrous silicate of magnesium: silica (SiO₂), 63 (acidic);
magnesia (MgO), 32; water (H₂O), 5; = 100. Orthorhombic system, but
rarely in distinct crystals. Cleavage in one direction very perfect; the
cleavage lamellæ are flexible, but not elastic, as in mica. Hardness, 1;
see scale. Sp. gr., 2.55-2.8. Lustre, pearly. Color, apple-green to
white; and streak, white. The feel is very smooth and greasy; and, in
connection with the color and foliation, affords the best means of
distinguishing talc from allied minerals. Talc sometimes results from
the alteration of augite, hornblende, and other minerals, but it is not
always nor usually an alteration product.
19. Serpentine.—Hydrous silicate of magnesium: silica (SiO₂), 44
(basic); magnesia (MgO), 44; water (H₂O), 12; = 100. Essentially
amorphous. Hardness, 2.5-4; sp. gr., 2.5-2.65. Lustre, greasy, waxy, or
earthy. Color, various shades of green and usually darker than talc, but
streak always white. Feel, smooth, sometimes greasy. Distinguished from
talc by its hardness, compactness, and darker green. Sometimes results
from the alteration of olivine and other magnesian minerals, but usually
we are to regard it as an original mineral. Specimen 25.
20. Chlorite.—This is, properly, the name of a group of highly basic
minerals of very variable composition, but they are all essentially
hydrous silicates of aluminum, magnesium, and iron; and the average
composition of the most abundant species, prochlorite, is as follows:
silica (SiO₂), 30; alumina (Al₂O₃), 18; magnesia (MgO), 15; protoxide of
iron (FeO), 26; and water (H₂O), 11; = 100. The chlorites crystallize in
several different systems, but in all there is a highly perfect cleavage
in one direction, giving, as in talc, a foliated structure with flexible
but inelastic laminæ. The cleavage scales, however, are sometimes
minute, and the structure massive or granular. Hardness of prochlorite,
1-2; between talc and serpentine. Sp. gr., 2.78-2.96. All the chlorites
have a pearly to vitreous lustre. Color usually some shade of green; in
prochlorite a dark or blackish green, darker than serpentine, as that is
darker than talc. Streak, a lighter, whitish green. Less unctuous than
talc, but more so than serpentine. The chlorites are produced very
commonly, but not generally, by the alteration of basic anhydrous
silicates, like augite and hornblende. Specimen 26.
21. Hydro-mica.—This, too, is properly the name of a group of minerals;
but for geological purposes they may be regarded as one species. Taking
a general view of the composition, these are simply the anhydrous or
ordinary micas, which we have already studied, with from 5 to 10 per
cent. of water added. In crystallization and structure they are
essentially mica-like. Although not distinctly softer than the common
micas, they are lighter, always more unctuous and slippery, and usually
of a greenish color. The micaceous structure with _elastic_ laminæ
serves to distinguish the hydro-micas from other hydrous silicates.
22. Glauconite.—Hydrous silicate of aluminum, iron, and potassium:
silica (SiO₂), 50; alumina (Al₂O₃), protoxide of iron (FeO), and potash
(K₂O), together, 41; and water (H₂O), 9; = 100. Amorphous, forming
rounded and generally loose grains, which often have a microscopic
organic nucleus. It is dull and earthy, like chalk, and always soft,
green, and light, but not particularly unctuous. Glauconite is the
principal, often the sole, constituent of the rock greensand, which
occurs abundantly in the newer geological formations, and is now forming
in the deep water of the Gulf of Mexico and along our Atlantic
sea-board. Specimen 27.
This completes our list of minerals occurring chiefly as _essential_
constituents of rocks; and following are three of the more common and
important minerals occurring chiefly as _accessory_, rarely as
essential, rock-constituents.
23. Chrysolite (Olivine).—Silicate of magnesium and iron: silica (SiO₂),
41; magnesia (MgO), 51; protoxide of iron (Fe₂O₃), 8; = 100.
Orthorhombic system; but usually in irregular glassy grains. Hardness,
6-7. Sp. gr., 3.3-3.5. Lustre, vitreous; color, usually some shade of
green; and streak, white. Chrysolite sometimes closely resembles quartz,
but its green color usually suffices to distinguish it. It is a common
constituent of basalt and allied rocks. By absorption of water it is
changed into serpentine and talc. See examples in specimen.
24. Garnet.—The composition of this mineral is extremely variable; but
the most important variety is a basic silicate of aluminum and iron:
silica (SiO₂), 37; alumina (Al₂O₃), 20; and protoxide of iron (FeO), 43;
= 100. Isometric system, usually in distinct crystals, twelve-sided
(dodecahedrons) and twenty-four-sided (trapezohedrons) forms being most
common. Hardness, 6.5-7.5; average as hard as quartz. Sp. gr., 3.15-4.3;
compare with the high percentage of iron. Lustre, vitreous; colors,
various, usually some shade of red or brown; and streak, white. Some
varieties contain iron enough to make them magnetic. Garnet is easily
distinguished by its form, color, and hardness from all other minerals.
It is a common but not an abundant mineral, occurring most frequently in
gneiss, mica schist, and other stratified crystalline rocks. See
examples in specimen.
25. Pyrite.—Sulphide of iron: sulphur, 53.3; iron, 46.7; = 100.
Isometric system, occurring usually in distinct crystals, the cube and
the twelve-sided form known as the pyritohedron being the most common.
Hardness, 6-6.5, striking fire with steel. Sp. gr., 4.8-5.2; heavy
because rich in iron. Lustre, metallic and splendent. Color, pale,
brass-yellow, and streak, greenish or brownish. Pyrite is sometimes
mistaken for gold, but it is not malleable; while its color, hardness,
and specific gravity, combined, easily distinguish it from all common
minerals. As an accessory rock-constituent, pyrite occurs usually in
isolated cubes or pyritohedrons. Specimen 10.
_Textures of Rocks._
_Texture_ is a general name for those smaller structural features of
rocks which can be studied in _hand specimens_, and which depend upon
the _forms_ and _sizes_ of the _constituent particles_ of the rocks, and
the _ways_ in which these are _united_.
By “constituent particles” we mean, not the atoms or molecules of matter
composing the rocks, but the _pebbles_ in conglomerate, _grains of sand_
in sandstone, _crystals of quartz_, _feldspar_, and _mica_ in granite,
etc. The four most important textures are:—
(1) _Fragmental texture._—The rock is composed of mere irregular,
angular, or rounded, but visible, fragments. Examples: sand, sandstone,
gravel, conglomerate, etc. Specimens 30, 31, 28, 29.
(2) _Crystalline texture._—The constituent particles are chiefly, at
least, distinctly crystalline, as shown either by external form, or
cleavage, or both. Examples: granite, diabase, gneiss, etc. Specimens
45, 1, 41.
(3) _Compact texture._—The constituent particles are indistinguishable
by the naked eye, but become visible under the microscope, appearing as
separate crystalline grains or as irregular fragments. In other words,
if, in the case of either the granular or crystalline textures, we
conceive the particles to become microscopically small, then we have the
compact texture. Examples: clay, slate, many limestones, basalt, etc.
Specimens 34, 35, 39.
(4) _Vitreous texture._—The texture of glass, in which the constituent
particles are absolutely invisible even with the highest powers of the
microscope, and may be nothing more than the _molecules_ of the
substance, which thus, so far as our powers of observation are
concerned, presents a perfectly continuous surface. Examples: obsidian,
glassy quartz, and some kinds of coal. Specimens 47, 15.
These four textures, which, it will be observed, are determined by the
forms and sizes of the constituent particles, may be called the
_primary_ textures, because every rock _must_ possess one of them. We
cannot conceive of a rock which is neither fragmental, crystalline,
compact, nor vitreous. But in addition to one of the primary textures, a
rock may or may not have one or more of what may be called _secondary_
textures. These are determined by the way in which the particles are
united, the mode or pattern of the arrangement, etc. Following are
definitions of the principal secondary textures:—
(1) _Laminated texture._—This exists where the particles are arranged in
thin, parallel layers, which may be marked simply by planes of division,
or the alternate layers may be composed of particles differing in
composition, form, size, or color, etc. Among the laminated textures we
thus distinguish: (_a_) the _banded_ texture, where the layers are
contrasted in color, texture, or composition, but cohere, so that there
is no cleavage or easy splitting parallel with the stratification; and
(_b_) the _schistose_ or _shaly_ texture, where such fissility or
stratification-cleavage exists. If a fragmental, compact, or vitreous
rock is fissile, we use the term _shaly_; but a fissile, crystalline
rock is described as _schistose_. The banded texture may occur with the
fragmental,—banded sandstones, etc.; with the crystalline,—many
gneisses, etc. (specimen 41); with the compact,—many slates, limestones,
felsites, etc. (specimens 34, 42); with the vitreous,—banded obsidian,
furnace slags, and some coal. The schistose texture may occur with the
crystalline,—mica schist, etc. (specimen 43); and the shaly texture with
the compact and fragmental, but rarely with the vitreous.
(2) _Porphyritic texture._—We have this texture when _separate_ and
_distinct crystals_ of _any_ mineral, but most commonly of feldspar, are
enclosed in a _relatively_ fine-grained base or matrix, which may be
either crystalline, compact, or vitreous, but rarely fragmental.
Specimens 5, 6, 7 are examples of the porphyritic compact texture.
(3) _Concretionary texture._—When one or more constituents of a rock
have the form, in whole or in part, not of distinct angular crystals,
but of rounded concretions, the texture is described as concretionary,
the concretions taking the place in this texture of the isolated
crystals in the porphyritic texture. This texture occurs in connection
with all the primary textures, but the most familiar example is oölitic
limestone.
(4) _Vesicular texture._—A rock has this texture when it contains
numerous small cavities or vesicles. These are most commonly produced by
the expansion of steam and other vapors when the rock is in a plastic
state; and hence the vesicular texture is found chiefly in volcanic
rocks. Except rarely, it is associated only with the compact
texture,—ordinary stony lavas (specimen 49); and with the vitreous
texture,—pumice (specimen 48).
(5) _Amygdaloidal texture._—In the course of time the vesicles of common
lava are often filled with various minerals deposited by infiltrating
waters, giving rise to the amygdaloidal texture, from the Latin
_amygdalum_, an almond, in allusion to a common form of the vesicles, or
amygdules, as they are called, after being filled. The amygdaloidal
texture is thus necessarily preceded by the vesicular, and is limited to
the same classes of rocks. Specimen 50.
Besides the foregoing, there are many minor secondary textures. The
rocks known as tufas have what may be called the _tufaceous_ texture.
Then we have kinds of texture depending on the _strength_ of the union
of the particles, as _strong_, _weak_, _friable_, _earthy_, etc.
_Classification of Rocks._
Having finished our preliminary observations on the characteristics of
rocks, we are now about ready to begin a systematic study of the rocks
themselves; but it is needful first to say a few words about the
classification of rocks, since upon this depends not only the order in
which we shall take the rocks up, but also the ideas that will be
imparted concerning their relations and affinities. The classifications
which have been proposed at different times are almost as numerous as
the rocks themselves. Some of these are confessedly, and even
designedly, artificial, as when we classify stones according to their
uses in the arts, etc. But we want something more scientific, a
_natural_ classification; that is, one based upon the natural and
permanent characteristics of rocks. Rocks have been classified according
to chemical composition, mineralogical composition, texture, color,
density, hardness, etc.; but these arrangements, taken singly or all
combined, are inadequate.
A _natural_ classification may be defined as a concise and systematic
statement of the natural relations existing among the objects
classified. Now the most important relations existing among rocks are
those due to their different origins. We must not forget that lithology
is a branch of geology, and that geology is first of all a _dynamical_
science. The most important question that can be asked about any rock
is, not What is it made of? but _How_ was it made? What were the general
forces or agencies concerned in its formation? Rocks are the material in
which the earth’s history is written, and what we want to know first
concerning any rock is what it can tell us of the condition of that part
of the earth at the time it was made and subsequently.
_Classification of Rocks._
+--------------------------------------------------------------------------+
| Sedimentary or Stratified Rocks. |
+--------------------------------------------------------------------------+
| MECHANICALLY FORMED. |
+-----------------------+-------------------------+------------------------+
| | _Unconsolidated._ | _Consolidated._ |
+-----------------------+-------------------------+------------------------+
| _Conglomerate | Gravel. | Conglomerate. |
| group._ | | |
+-----------------------+-------------------------+------------------------+
| _Arenaceous | Sand. | Sandstone. |
| group._ | | |
+-----------------------+-------------------------+------------------------+
| _Argillaceous | Clay. | Slate. |
| group._ | | |
+-----------------------+-------------------------+------------------------+
| CHEMICALLY AND ORGANICALLY FORMED. |
+-----------------------+-------------------------+------------------------+
| _Coal | _Iron-ore |_Calcareous | _Metamorphic group (Silicates)._ |
| group._ | group._ | group._ | Acidic. Basic. |
| | | |/----/\----\ /-------/\-------\ |
| | | |85 80 70 60 50 40 30|
+-----------+-----------+------------+-------------------------------------+
| Feldspathic. |
+-----------+-----------+------------+-------------------------------------+
| Peat. | Limonite. | Limestone. | Gneiss. Diorite. : |
| Lignite. | Hematite. | Dolomite. |........................... : |
|Bit. Coal. |Magnetite. | Gypsum. | : : : : : |
|Anthracite.| Siderite. | Rock-salt. | : :Syenite. Norite. : |
| Graphite. | | Phosphate | : : .............. : |
|Asphaltum. | | Rock. | : : : : : |
+-----------+-----------+------------+-------------------------------------+
| Non-Feldspathic. |
+-----------+-----------+------------+-------------------------------------+
| |_Siliceous | | : Mica Schist. : : |
| | group._ | |........................... : |
| +-----------+ | : : : : : |
| |Tripolite. | | :Hornbl. Schist. Amphibolite. |
| | Flint. | | :.............. ........: |
| | Siliceous | | : : : : : |
| | Tufa. | | : Talc Schist. :Chl. Schist. |
| |Novaculite.| | : :........ : ..........|
| | | | : : : : : |
| | | | : : Greensand. Serpentine. |
| | | | : : ........... ........ |
| | | | : : : : : |
+-----------+-----------+------------+-------------------------------------+
| Eruptive or Unstratified Rocks. |
+------------------------------------+-------------------------------------+
| | PLUTONIC. |
| | Feldspathic. |
| +-------------------------------------+
| | : Granite. : Diorite. : |
| |.................. ........ : |
| | : : : : : |
| | : : Syenite. Diabase. |
| | : : ....... .......: |
| | : : : : : |
| +-------------------------------------+
| | VOLCANIC. |
|This part of the classification | Feldspathic. |
|is a blank, for the reason that +-------------------------------------+
|no eruptive rocks are known | :Rhyolite. : Andesite. : |
|which are chiefly composed of |........................... : |
|minerals belonging to the classes | : : : : : |
|of Native Elements, Chlorides, | : : Trachyte. Basalt. |
|Oxides, Sulphates, or Carbonates; | ...............: |
|_i.e._, all eruptive rocks, so | : : : : : |
|far as known, are principally | :Obsidian. : Tachylite.: |
|composed of minerals belonging |............................ : |
|to the class of Silicates. | : : : : : |
| | Petrosilex. :Porphyrite.: |
| |............................ : |
| | : : : : : |
| | : : Felsite. Melaphyr. |
| | : : ....... .......: |
| | : : : : : |
+------------------------------------+-------------------------------------+
The geological agencies, as we have already learned, may be arranged in
two great classes: first, the aqueous or superficial agencies
originating in the solar heat, and producing the sedimentary or
stratified rocks; and, second, the igneous or subterranean agencies
originating in the central or interior heat, and producing the eruptive
or unstratified rocks. Hence, we want to know first of any rock whether
it is of aqueous or igneous origin. Then, if it is a sedimentary rock,
whether it has been formed by the action chiefly of mechanical forces,
or of chemical and organic forces. And, if it is an eruptive rock,
whether it has cooled and solidified below the earth’s surface in a
fissure, and is a dike or trappean rock, or has flowed out on the
surface and cooled in contact with the air, and thus become an ordinary
lava or volcanic rock.
Here we have the outlines of our classification, and it will be observed
that we have simply reached the conclusion, in a somewhat roundabout
manner, that there should always be a general correspondence between the
classification of rocks and the classification of the forces that
produce them. The general plan of the preceding scheme of the
classification must now be clear, and the details will be explained as
we go along.
_Descriptions of Rocks._
1.—Sedimentary or Stratified Rocks.
1. MECHANICALLY FORMED OR FRAGMENTAL ROCKS.—These consist of materials
deposited from _suspension_ in water, and the process of their formation
is throughout chiefly mechanical. The materials deposited are mere
fragments of older rocks; and, if the fragments are large, we call the
newly deposited sediment gravel; if finer, sand; and, if impalpably
fine, clay. These fragmental rocks cannot be classified chemically,
since the same handful of gravel, for instance, may contain pebbles of
many different kinds of rocks, and thus be of almost any and very
variable composition. Such chemical distinctions as can be established
are only partial, and the classification, like the origin, must be
mechanical. Accordingly, as just shown, we recognize three principal
groups based upon the size of the fragments; viz.:—
(1) Conglomerate group.
(2) Arenaceous group.
(3) Argillaceous group.
This mode of division is possible and natural, simply because, as we
observed in an early experiment, materials arranged by the mechanical
action of water are always assorted according to size. When first
deposited, the gravel, sand, and clay are, of course, perfectly loose
and unconsolidated; but in the course of time they may, under the
influence of pressure, heat, and chemical action, attain almost any
degree of consolidation, becoming _conglomerate_, _sandstone_, and
_slate_, respectively. The pressure may be vertical where it is due to
the weight of newer deposits, or horizontal where it results from the
cooling and shrinking of the earth’s interior. The heat may result from
mechanical movements, or contact with eruptive rocks; or it may be due
simply to the burial of the sediments, which, it will be seen, must
virtually bring them nearer the great source of heat in the earth’s
interior, on the same principle that the temperature of a man’s coat, on
a cold day, is raised by putting on an overcoat. The effect of the heat,
ordinarily, is simply drying, coöperating with the pressure to expel the
water from the sediments; but, if the temperature is high, it may bake
or vitrify them, just as in brick-making. Sediments are consolidated by
chemical action when mineral substances, especially calcium carbonate,
the iron oxides, and silica are deposited between the particles by
infiltrating waters, cementing the particles together. This principle is
easily demonstrated experimentally by taking some loose sand and wetting
it repeatedly with a saturated solution of some soluble mineral, like
salt or alum, allowing the water to evaporate each time before making a
fresh application. The interstices between the grains are gradually
filled up, and the sand soon becomes a firm rock. But the student should
clearly understand that, in geology, gravel, sand, and clay are just as
truly _rocks_ before their consolidation as after. It is plain then that
in each of the principal groups of fragmental rocks we must recognize an
unconsolidated division and a consolidated division.
(1) _Conglomerate group._—The rocks belonging in this group we know
before consolidation as _gravel_, and after consolidation as
_conglomerate_.
Gravel.—The pebbles, as we have already seen, are usually, though not
always, well rounded or water-worn; and they may be of any size from
coarse grains of sand to boulders. As a rule, however, the larger
pebbles, especially, are of approximately uniform size in the same bed
or layer of gravel, with, of course, sufficient fine material to fill
the interstices. Although the same limited mass of gravel may show the
widest possible range in chemical and mineralogical composition, yet
hard rocks are evidently more likely than soft rocks to form pebbles;
and hence quartz and quartz-bearing rocks usually predominate in
gravels. Specimen 28.
Conglomerate.—Consolidated gravel. Children should be led to an
appreciation of this point by a careful comparison of the forms of the
pebbles in the gravel and conglomerate. The conglomerate seems to
contain a larger proportion of fine material than ordinary gravel. But
this is because the gravel is usually, as with our specimen, taken from
the _surface_ of the beach, where, of course, the pebbles are clean and
separate; but if it had remained there to be covered by a subsequently
deposited layer, enough fine stuff would have been sifted into the holes
to fill them. And in the finished gravel, just as in the conglomerate,
the pebbles are usually closely packed, with just sufficient sand and
clay, or _paste_, as the material in which the pebbles are imbedded is
called, to fill the interstices. The paste is usually similar in
composition to the pebbles, with this difference: hard materials
predominate in the pebbles and soft in the paste.
Stratified rocks generally show the stratification in parallel lines or
bands differing in color, composition, etc.; but nothing like this can
be detected in our specimens of conglomerate; and the question might be
asked, How do we know that this is a stratified rock? In answer, it can
be said that our hand-specimens appear unstratified simply because the
rock is so coarse; but when we look at large masses, and especially when
we see it in place in the quarry, that parallel arrangement of the
material which we call stratification is usually very evident; and we
often see precisely the same thing in gravel banks. It is, however,
wholly unnecessary that we should _see_ the stratification in order to
know certainly that this is a stratified or aqueous rock, because the
forms of the pebbles show very plainly that they have been fashioned and
deposited by moving water; and we have in the smallest specimen proof
positive that our conglomerate is a consolidated sea-beach.
Conglomerate shows the same variations in composition and texture as
gravel; it may be composed of almost any kind of material in pebbles of
almost any size. We recognize two principal varieties of conglomerate
based on the forms of the pebbles; if, as is usual, these are well
rounded and water-worn, the rock is true _pudding-stone_ (specimen 29);
but, if they are angular, or show but little wear, it is called
_breccia_.
(2) _Arenaceous Group._—The conglomerate group passes insensibly into
the arenaceous group; for, from the coarsest gravel to the finest sand,
the gradation is unbroken, and every sandstone is merely a conglomerate
on a small scale.
Sand.—Like gravel, sand may be of almost any composition, but as a rule
it is quartzose; quartz, on account of its hardness and the absence of
cleavage, being better adapted than any other common mineral to form
sand. Where the composition of a sand is not specified, a quartzose sand
is always understood. By examining a typical sand with a lens, and
noting the glassy appearance of the grains, and then testing their
hardness on a piece of glass, which they will scratch as easily as
quartz, the pupil is readily convinced that each grain is simply an
angular fragment of quartz. Specimen 30.
Sandstone.—Consolidated sand. In proving this, children will notice
first the granular or sandy appearance of the sandstone; and then, with
the lens, that the grains in the sandstone have the same forms as the
sand-grains. The stratification cannot be seen very distinctly in our
hand-specimens, but in larger masses it is usually very plain, as may be
observed in the blocks used for building, and still better in the
quarries. However, even if the stratification were not visible to the
eye, we could have no doubt that sandstone is a mechanically formed
stratified rock; because the form of the grains, just as in the
conglomerate, tells us that. Many sandstones, too, contain the fossil
remains of plants and animals, and these are always regarded as
affording positive proof that the rocks containing them belong to the
aqueous or stratified series.
There are many varieties of sandstone depending upon differences in
composition, texture, etc., but we have not space to notice them in
detail. In sandstone, just as in sand, quartz is the predominant
constituent, although we sometimes find varieties composed largely or
entirely of feldspar, mica, calcite, or other minerals. Specimen 31 is
an example of the architectural variety known as freestone, which is
merely a fine-grained, light-colored, uniform sandstone, not very hard,
and breaking with about equal freedom in all directions. The
consolidation of sandstones is due chiefly to chemical action. The
cementing materials are commonly either: _ferruginous_ (iron oxides),
giving red or brown sandstones; _calcareous_, forming soft sandstones,
which effervesce with acid if the cement is abundant; or _siliceous_,
making very strong, light-colored sandstones. Ferruginous sandstones are
the most valuable for architectural purposes; for, while not excessively
hard, they have a very durable cement. Siliceous sandstones are too
hard; and the calcareous varieties crumble when exposed to the weather
because the cement is soluble in water containing carbon dioxide, as all
rain-water does. Specimen 32 is a good example of a ferruginous
sandstone, and it is coarse enough so that we can see that each grain of
quartz is coated with the red oxide of iron. The mica scales visible
here and there in this specimen are interesting as showing that the
grains are not necessarily all quartz; and it is important to observe
that the mica was not made in the sandstone, but, like the quartz, has
come from some older rock.
Quartzite.—This rock is simply an unusually hard sandstone. Now the
hardness of any rock depends upon two things: (1) the hardness of the
individual grains or particles; and (2) the firmness with which they are
united one to another. Therefore, the hardest sandstones must be those
in which grains of quartz are combined with an abundant siliceous
cement; and that is precisely what we have in a typical quartzite, such
as specimen 33. Quartzite is distinguished, in the hand-specimen, from
ordinary quartz by its granular texture (compare specimens 15 and 33);
and of course in large masses the stratification is an important
distinguishing feature.
3. _Argillaceous group._—Just as the conglomerate group shades off
gradually into the arenaceous group, so we find it difficult to draw any
sharp line of division between the arenaceous group and the
argillaceous, but we pass from the largest pebble to the most minute
clay-particle by an insensible gradation. For the sake of convenience,
however, we draw the line at the limit of visibility, and say that in
the true clay and slate the individual particles are invisible to the
naked eye; in other words, these rocks have a perfectly compact texture,
while the two preceding groups are characterized by a granular texture.
Although clay, like sand and gravel, may be of almost any composition,
yet it usually consists chiefly, often entirely, of the mineral kaolin.
The reason for this is easily found. Quartz resists both mechanical and
chemical forces, and is rarely reduced to an impalpable fineness; but
all the other common minerals, such as feldspar, hornblende, mica, and
calcite, on account of their cleavage and inferior hardness, are easily
pulverized; but it is practically impossible that this should happen
without their being broken up chemically at the same time. Decomposition
follows disintegration; and, while calcite is completely dissolved and
carried away, the other minerals are reduced, as we have seen, to
impalpable hydrous silicates of aluminum, _i.e._, to kaolin. Hence, we
find that the fragmental rocks are composed principally of two minerals,
quartz and kaolin,—the former predominating in the conglomerate and
arenaceous groups, and the latter in the argillaceous group.
Clay.—That kaolin is the basis of common clay is proved by the
argillaceous odor, which is so characteristic of moist clay. Pure kaolin
clay is white and impalpable, like China clay; but pure clays are the
exception. They often become coarse and gritty by admixture with sand,
forming _loam_; and they also usually contain more or less carbonaceous
matter, which makes black clays; or more or less _ferrous_ oxide, which
makes blue clays; or more or less _ferric_ oxide, which makes red,
brown, and yellow clays. By mixing these coloring materials in various
proportions, almost any tint may be explained. Clays are sometimes
calcareous, from the presence of shells and shell-fragments or of
pulverized limestone. These usually effervesce with acid, and are
commonly known as _marl_. It is the calcareous material in a pulverulent
and easily soluble condition that makes the marls valuable as soils.
Slate.—Consolidated clay. The compact texture and argillaceous odor are
usually sufficient to prove this. To get the odor we need simply to
breathe upon the specimen, and then smell of it. We find all degrees of
induration in clay. It sometimes, as every one knows, becomes very hard
by simple drying; but this is not slate, and no amount of mere drying
will change clay into slate; for, when moistened with water, the dried
clay is easily brought back to the plastic state. To make a good slate,
the induration must be the result of pressure, aided probably to some
extent by heat. True slate, then, is a permanently indurated clay which
will not soak up and become soft when wet.
Slate is usually easily scratched with a knife, and it is distinguished
from limestone by its non-effervescence with acid. As we should expect,
it shows precisely the same varieties in color and composition as clay.
A good assortment of colors is afforded by the roofing-slates. Specimen
34 is a typical slate, for it not only has a compact texture and
argillaceous odor, but it is very distinctly stratified. The
stratification is marked by alternating bands of slightly different
colors, and is much finer and more regular than we usually observe in
sandstone, and of course entirely unlike the stratification of
conglomerate. These differences are characteristic. Some slates,
however, are so homogeneous that the stratification is scarcely visible
in small pieces. Thus the roofing-slates (specimen 35) rarely show the
stratification; for it is an important fact that the thin layers into
which this variety splits are entirely independent of the
stratification. This is the structure known as slaty cleavage; it is not
due to the stratification, but is developed in the slate subsequently to
its deposition, by pressure. Some roofing-slates, known as
ribbon-slates, show bands of color across the flat surfaces. These bands
are the true bedding, and indicate the absolute want of conformity
between this structure and the cleavage. Few rocks are richer in fossils
than slate, and these prove that it is a stratified rock. Slate which
splits easily into thin layers _parallel with the bedding_ is known as
_shale_.
Porcelainite.—This is clay or slate which has been baked or partially
vitrified by heat so as to have the hardness and texture of porcelain.
2. CHEMICALLY AND ORGANICALLY FORMED ROCKS.—We have already learned that
from a geological point of view the differences between chemical and
organic deposition are not great, the process being essentially chemical
in each case; and since the limestones and some other important rocks
are deposited in both ways, it is evidently not only unnatural, but
frequently impossible, to separate the chemically from the organically
formed rocks. Unlike the fragmental rocks, the rocks of this division
not only admit, but require, a chemical classification. This is natural
because they are of chemical origin; and it is practicable because, with
few exceptions, only one class of minerals is deposited at the same time
in the same place,—a very convenient and important fact. Therefore our
arrangement will be mineralogical, thus:—
(1) Coal Group.
(2) Iron-ore Group.
(3) Siliceous Group.
(4) Calcareous Group.
(5) Metamorphic Group (Silicates).
Most of the silicate rocks are mixed, _i.e._, are each composed of
several minerals; but some silicate rocks and all the rocks of the other
divisions are simple, each species consisting of a single mineral only.
(1) _Coal Group._—These are entirely of organic origin, and include two
allied series, which are always merely the more or less extensively
transformed tissues of plants or animals; viz.:—
Coals and Bitumens.—At the first lesson we examined a sample of peat
(specimen 8), and considered the general conditions of its formation,
peat being in every instance simply partially decayed marsh
vegetation. It was also stated that, as during the lapse of time the
transformation becomes more complete, the peat is changed in
succession to _lignite_, _bituminous coal_, _anthracite_, and
_graphite_. The coals, indeed, make a very beautiful and perfect
series, whether we consider the composition—there being a gradual,
progressive change from the composition of ordinary woody fibre in the
newest peat to the pure carbon in graphite,—or the degree of
consolidation and mineralization—since there is a gradual passage from
the light, porous peat, showing distinctly the vegetable forms, to the
heavy crystalline graphite, bearing no trace of its vegetable origin.
This relation is easily appreciated by a child, if a proper series of
specimens is presented. The coals also make a chronological series,
graphite and anthracite occurring only in the older formations, and
lignite and peat in the newer, while bituminous coal is found in
formations of intermediate age.
Bituminous coal is the typical, the representative coal; and from a good
specimen of this variety we may learn two important facts:—
(1) That true coals, no less than peat, are of vegetable origin. To see
this we must look at the flat or charcoal surfaces of the coal. These
soil the fingers like charcoal, and usually show the vegetable forms
distinctly.
(2) That coals are stratified rocks. These dirty charcoal surfaces
always coincide with the stratification, being merely the successive
layers of vegetation deposited and pressed together to build up the
coal; and when we look at the edge of the specimen the stratification
shows plainly enough.
The bitumens form a similar though less perfect series, beginning with
the organic tissues, and ending, in the opinion of some of the best
chemists and mineralogists, with diamond. In fact the coals and bitumens
form two distinct but parallel series. The coals are exclusively of
vegetable origin, while the bitumens are largely of animal origin. The
organic tissues in which the two series originate are chemically
similar,—the animal tissues, which produce the lighter forms of bitumen,
however, containing more hydrogen and less carbon and oxygen than
vegetable tissues; while the final terms, as just shown, are probably
chemically identical, being pure carbon,—graphite for the coals and
diamond for the bitumens; so that the entire process of change in each
series is essentially carbonization, a gradual elimination of the
gaseous elements, oxygen and hydrogen, until pure solid carbon alone
remains.
The principal differences between the coals and bitumens are the
following:—
Coals are rich in carbon, with some oxygen and little
hydrogen.
Bitumens are rich in hydrogen, with some carbon and little
or no oxygen.
Coals are entirely insoluble.
Bitumens are soluble in ether, benzole, turpentine, etc.,
and the solid forms are soluble in the more fluid,
naphtha-like varieties.
Coals are never liquid, and cannot be melted or, with
trifling exceptions, even softened by heat.
Many bitumens are naturally liquid, and all become so on the
application of heat.
The coals partake of the characteristics of their chief
constituent element, carbon, the most thoroughly solid
substance known; while the bitumens similarly show the
influence of hydrogen, the most perfectly fluid
substance known.
The two bitumens of the greatest geological importance are asphaltum or
mineral pitch and petroleum; but these substances are too familiar to
require any farther description here.
(2) _Iron-ore Group._—These interesting and important stratified rocks
include the three principal oxides of iron,—limonite, hematite, and
magnetite,—as well as the carbonate of iron, siderite; and the rocks
have essentially the same characteristics as the minerals. In economical
importance they are second only to the coals; and the history of their
formation through the agency of organic matter is one of the most
interesting chapters in chemical geology (see page 26). The three oxides
are easily distinguished from each other by the colors of their powders
or streaks, and the magnetism of magnetite, and from all other common
rocks by their high specific gravity. Magnetite is the richest in iron,
and limonite the poorest. As regards the degree of crystallization and
order of occurrence in the formations, they form a series parallel with
the coal series, thus:—
Limonite, never crystalline, and found in recent formations.
Hematite, often crystalline, and found in older formations.
Magnetite, always crystalline, and found in oldest
formations.
Siderite effervesces with strong acid; and this separates it from all
other rocks, except limestone and dolomite; and from these it is
distinguished by its high specific gravity. As a mineral, siderite is
often light colored; but as a rock it is always dark, and usually black,
from admixture chiefly of carbonaceous matter. In studying dynamical
geology, we have learned (page 28) the reason for the intimate
association of siderite with beds of coal, and this accounts equally for
the carbon contained in the rock itself. The connection of this rock
with the coal-formations adds much to its value as an ore of iron.
Finally, the iron-ores, at least where of much economical importance,
are truly stratified. This can often be seen in hand-specimens; and is
well shown by their relations to other rocks, in quarries and mines; and
in many cases, for limonite and hematite, by the fossils which they
contain.
(3) _Siliceous Group._—These rocks are composed of pure silica in the
forms of quartz and opal. When first deposited, whether organically,
like tripolite, or chemically, like siliceous tufa, the siliceous rocks
are soft and light, and the silica is in the form of opal. Subsequently
it changes to quartz, and the rocks assume the much harder and denser
forms of chert and novaculite, respectively.
Tripolite or Diatomaceous Earth.—This interesting rock is soft, light,
and looks like clay; but it is lighter, and the argillaceous odor is
faint or wanting. It does not effervesce with acid. Hence, it is neither
clay nor chalk. Notwithstanding its softness, it is really composed of a
hard substance, viz., silica, in the form known as opal. By rubbing off
a little of the dust, and examining it under the microscope, we easily
prove that the silica is mainly or entirely of organic origin; for the
dust is seen to be simply a mass of more or less fragmentary organic
remains, occurring in great variety, and of wonderful beauty and
minuteness. There are few rocks so unpromising on the exterior, and yet
so beautiful within. We have already learned that these organic bodies
are principally Diatom cases, Radiolaria shells, and Sponge spicules. We
can form some idea of their minuteness from Ehrenberg’s estimate that a
single cubic inch of pure tripolite contained no less than
41,000,000,000 organisms.
The lightness of tripolite (sp. gr., 1-1.5) is due to the facts that
opal is a light mineral (sp. gr., 1.9-2.2), and that many of the shells
are hollow. Tripolite is a good example of a soft rock composed of a
hard mineral; and it owes its value as a polishing material to the fact
that it consists of a hard mineral in an exceedingly fine state of
division. Tripolite, when pure, is snow-white; but it is rarely pure,
being commonly either argillaceous or calcareous. This rock is now
forming in thousands of places, in both fresh water and the ocean.
Flint and Chert.—During the course of geological time, beds of tripolite
are gradually consolidated, chiefly by percolating waters, which are
constantly dissolving and re-depositing the silica; and, finally, in the
place of a soft, earthy rock, we get a hard, flinty one, which we call
_flint_ if it occurs in the newer, or _chert_ if it occurs in the older,
geological formations. Besides forming beds of nearly pure silica, which
we call tripolite, the microscopic siliceous organisms are diffused more
or less abundantly through other rocks, especially chalk and limestone.
In such cases the consolidation of the silica implies its segregation
also; _i.e._, the silica dissolved by percolating water is deposited
only about certain points in the rock, building up rounded concretions
or nodules. Thus, a siliceous limestone becomes, by the segregation of
the silica, a pure limestone containing nodules of chert, which are
usually arranged in lines parallel with the stratification. Specimen 16
is a fragment of a flint-nodule from the chalk-formation of England.
Siliceous Tufa.—Hot water, and especially hot alkaline water,
circulating through the earth’s crust, is always charged with silica
dissolved out of the rocks; and when such water issues on the surface in
a hot spring or geyser, it is cooled by contact with the air, its
solvent power is diminished thereby, and a large part of the silica is
deposited around the outlet as a snowy-white porous material called
_siliceous tufa_. Silica deposited in this way is, like organic silica,
always in the form of opal. Siliceous tufa is distinguished from clay,
slate, chalk, and limestone by the same tests as tripolite, and from
tripolite itself by the absence of microscopic organisms.
Novaculite.—Through the action of percolating water and pressure,
siliceous tufa, like tripolite, becomes harder and denser and is thus
changed to _novaculite_, which holds the same relation to chemically
deposited silica that chert and flint do to organically deposited
silica. The white novaculite obtained at the Hot Springs of Arkansas,
and commonly known as Arkansas stone, is a typical example of this rock.
The rock which, on account of the use to which it is put, is known as
buhr-stone, is also an excellent example of chemically deposited silica.
It is usually somewhat porous and fossiliferous.
(4) _Calcareous Group._—These are the lime-rocks, including the
carbonate of lime, in limestone and dolomite, the sulphate of lime, in
gypsum, and the phosphate of lime, in phosphate rock. These rocks are
even more closely connected in origin than in composition; and it is for
this reason that rock-salt, which of course contains no lime, is also
included in this group. Limestones are formed abundantly in the open
sea, through the accumulation of shells and corals; but when portions of
the sea become detached from the main body and gradually dry up, like
the Dead Sea and Great Salt Lake, dolomite, gypsum, and rock-salt are
deposited in succession as chemical precipitates. Phosphate rock may be
regarded as a variety of limestone, resulting from the accumulation of
the skeletons and excrement of the higher animals.
Limestone.—This is the lithologic or rock form of carbonate of lime or
calcite, and one of the most important, interesting, and useful of all
rocks. Although so simple in composition,—calcite being the only
essential constituent,—limestone embraces many distinct varieties, and
is really equivalent to a whole family of rocks. A highly fossiliferous
limestone, such as specimen 38, is, perhaps, the best variety with which
to begin the study of the species. The softness of the fossil shells of
which the rock is so largely composed, and the fact that they effervesce
readily with dilute acid, proves that they are still carbonate of lime;
and by applying the acid more carefully, it can be seen that the compact
matrix of the rock also effervesces, consisting of shells more finely
broken or comminuted and mixed with more or less clay and other
impurity, almost the entire rock being of organic origin.
On the coast of Florida, and in many other places, we find beautiful
examples of shell-limestone now in process of formation. These are at
first very open and porous, because the interstices between the nearly
entire shells are not yet filled up with smaller fragments and sand. But
when that is done, we shall have a rock similar to the old fossiliferous
limestone. Specimen 37.
The shells and fragments, and the grains of calcareous sand, are, as a
rule, quickly cemented together by the deposition of carbonate of lime
between them; so that limestone is nowhere observed occurring abundantly
in an unconsolidated form.
Limestone, as a rule, is not distinctly stratified in hand-specimens,
but of course that it is a true sedimentary rock is abundantly proved by
the fossils; and it goes almost without saying that limestone, being
necessarily mainly composed of organic remains, must be to a greater
extent than any other rock the great store-house of fossils; and in no
other rock are the fossils so well preserved and perfect as in
limestone.
Nevertheless, there are extensive formations of limestone containing no
discernible traces of fossils. And some of these non-fossiliferous
limestones, too, are of very recent formation. Some of the modern
coral-reefs, for example, are composed of limestone which was formed
only yesterday, as it were, and which, from its mode of formation, must
consist entirely of corals; and yet it shows no trace of its organic
origin, but is perfectly compact, or, possibly, crystalline. This
frequent obliteration of the organic remains, as well as the perfect
consolidation of the rock, is attributed to its solubility. The calcium
carbonate is gradually dissolved by the water, and then deposited in the
interstices in other parts of the rock.
Specimen 39 is that variety of limestone known as _chalk_. It is soft
and earthy, resembling both clay and tripolite, but differing from the
former in lacking the distinct argillaceous odor, and from both by its
lively effervescence with acids. It appears to be entirely destitute of
organic remains, but this is a defect of our vision and not of the rock;
for, like the tripolite, it often appears under the microscope to be
little else than a mass of shells. Tripolite is a deposit built up of
the siliceous shells of Diatoms and Radiolaria, while chalk is chiefly
composed of the similar but calcareous shells of Foraminifera. Our
specimen is from the Cretaceous formation of England; but we have good
reason to believe that chalk is _now forming_ on a very extensive scale.
There are millions of square miles in the deeper parts of the ocean
where the dredge brings up little else but a perfectly impalpable, gray,
calcareous slime or ooze. When examined microscopically, this is seen to
be composed chiefly of Foraminifera shells, and among these the genus
Globigerina predominates; so that the deposit is frequently called
Globigerina ooze. Now this gray, calcareous ooze, when dried and
compacted by pressure, makes a soft, _white_ rock which can scarcely be
distinguished from chalk; in fact, it is a modern chalk. And there seems
no good reason to doubt that the deposition of chalk has gone on
continuously since Cretaceous time—for several millions of years at
least.
Specimen 40 is also a white rock, easily scratched with the knife, and
effervescing freely with acid, and therefore a variety of limestone. But
its texture is very different from the other varieties we have studied.
It has a sparkling surface, which we explain by saying that the rock is
crystalline. It is, in fact, a mass of minute crystals of calcite. The
crystalline limestones have not always been crystalline, but it is safe
to assume that they were originally entirely uncrystalline, and in many
cases rich in fossils; but the fossils have been mainly obliterated by
the crystallization.
Crystallization generally in rocks is an indication of great age, so
that we usually say crystalline rocks must be older than uncrystalline
rocks of the same composition; and this is mainly true with the
limestones. When the crystallization is rather fine, as in our specimen,
resembling granulated sugar, we have what is commonly called
saccharoidal limestone. This is the typical marble. Marble is not a
scientific name, and the term is usually applied to any calcareous rock
which will take a polish, and sometimes even to rocks which are not
calcareous at all.
In the section on dynamical geology, we learned that the carbonate of
calcium or calcite is deposited from the sea-water, and limestones
formed, in two ways: first, in a purely chemical way, where the water
becomes saturated with calcite; and, second, organically, where the
calcium carbonate is taken from the water by marine organisms to form
their shells and skeletons, and the gradual accumulation of these on the
ocean-floor builds up a limestone. As before stated, the difference
between these two methods of deposition is not so great as it often
seems, because we know that the animals never make the carbonate of
calcium which they secrete, but it comes into the sea ready made with
the drainage from the land.
The limestones forming at the present time are almost wholly organic;
but the rock known as _calcareous tufa_ is an exception. This is formed
under the same general conditions as siliceous tufa, but much more
abundantly, and in cold water as well as warm; because calcite is far
more soluble (especially in water containing carbon dioxide) than opal
or quartz. It is deposited, not only around the mouths of springs, but
also along the beds of the streams which they form, enveloping stones,
roots, grasses, etc., and building up usually a loose, spongy mass
having a very characteristic turfaceous texture.
The principal accessory minerals occurring in limestone are: (1)
_kaolin_, forming argillaceous or slaty limestone, which may be
recognized by the argillaceous odor and dark color; (2) _quartz_,
forming siliceous or cherty limestone, known by its hardness or by the
nodules of flint or chert; (3) _dolomite_, forming dolomitic or
magnesian limestone, which effervesces less freely with acid; and (4)
_serpentine_, forming serpentinic limestone, which is sharply
distinguished by the green grains of serpentine mingled with the white
calcite. A concretionary texture is common with limestone. If the
concretions are small, like mustard-seed, we call the rock _oölite_; if
larger, like peas, _pisolite_.
Dolomite.—If for calcite, which is the sole essential constituent of all
limestone, we substitute the allied mineral dolomite, we have the rock
dolomite. As might be inferred from its composition, dolomite is very
closely related to limestone, although there are some important
differences. Physically, the two rocks differ about as the two minerals
do. Dolomite is harder than limestone, and being also less soluble, it
resists the action of the weather more. Dolomite, if pure, effervesces
feebly, or not at all, with cold dilute acid. Here, however, we have to
recognize the fact that dolomite is rarely pure; but there exists, in
consequence of the admixture of calcite, a perfectly gradual passage
from pure dolomite to pure limestone, and parallel with this every
degree of vigor in the reaction with acid. Hence, it is entirely an
arbitrary matter as to where we shall draw the line between dolomitic
limestone and calcareous dolomite. Dolomite is a very much less abundant
rock than limestone, and, unlike limestone, it rarely contains many
fossils, and is never of organic origin; _i.e._, there are no organisms
which secrete the mineral dolomite to form their hard parts or
skeletons. Like gypsum and rock-salt, dolomite is probably never
deposited in the open ocean, but only in closed basins. Like limestone,
it occurs with both the compact and the crystalline textures.
Gypsum.—When pure, this rock (specimen 36) is identical with the mineral
gypsum (specimen 17), except that it is rarely crystalline. It is
usually, however, not only perfectly compact, but more or less
dark-colored from the admixture of clay and other impurities. Its most
notable characteristics are its softness, the absence of the
argillaceous odor, except where it contains much clayey impurity, and
its non-effervescence with acids. The first two usually serve to
distinguish it from slate, while the acid test separates it readily from
limestone and all other carbonate rocks. The deposition of gypsum is
purely chemical, and it occurs under about the same physical conditions
as the deposition of salt; _i.e._, in drying-up portions of the sea.
Hence we usually find gypsum associated with beds of rock-salt; and,
since drying-up seas are few in number, and small compared with the
whole extent of the ocean, we can easily understand why neither
rock-salt nor gypsum are abundant rocks, except in a few localities.
Rock-Salt.—This interesting and useful rock, as we have already learned,
is deposited in a purely chemical way, and only in drying-up portions of
the sea, like the Dead Sea, Great Salt Lake, etc. In some parts of
Europe there are beds of solid rock-salt over a hundred feet thick.
Phosphate Rock.—Although not specially abundant or attractive, this rock
is of great economic interest and importance on account of its extensive
use as a fertilizer. Under the general head of phosphate rock are
included: (1) the typical guano, which is the consolidated excrement of
certain marine birds inhabiting in great numbers small coral islands in
the dry or rainless regions of the tropics; (2) the underlying coral
rock, which is often changed to phosphate rock through the percolation
of the rain-water falling on the guano; (3) accumulations of the bones
and coprolites of the higher animals; (4) phosphatic limestones from
which the carbonate of lime has been largely dissolved away, leaving the
more insoluble phosphate of lime.
(5) _Metamorphic Group_ (stratified silicates).—All the chemically and
organically formed rocks which we have studied up to this point are
simple, _i.e._, they consist each of only one essential mineral; but
most of the rocks in this great group of silicates are mixed, or consist
each of several essential minerals. Quartz is the only important
constituent of these rocks which is not, strictly speaking, a silicate,
but in a certain sense it is also not an exception, since it may always
be regarded as an excess of acid in the rock.
This group of stratified rocks composed of silicate minerals is of
exceptional importance, first, on account of the large number of species
which it includes, and, second, on account of the vast abundance of some
of the species. These are, above all others, the rocks of which the
earth’s crust is composed. With unimportant exceptions, all the rocks of
this group are crystalline; and they constitute the principal part of
what is generally included under the term _metamorphic rocks_—a general
name for all stratified rocks which have been so acted upon by heat,
pressure, or chemical forces as to make them crystalline. Although the
crystalline limestone, dolomite, iron-ores, etc., show us that
metamorphic rocks are not wanting in the other groups.
As already explained, the metamorphic or crystalline stratified rocks
are usually older than the corresponding uncrystalline rocks; but a
point of greater importance here is this: the development in the
silicate rocks of crystalline characters has usually made it impossible
to determine the method of their deposition, whether mechanical or
chemical. In a few cases, as with the rock greensand, we know that the
deposition is chemical; while it is equally certain that such common
silicate rocks as gneiss, mica schist, and many others, often result
from the crystallization of ordinary mechanical sediments, like
sandstone and conglomerate. We classify all these rocks as of chemical
origin, however, without considering the mode of their deposition,
because the subsequent crystallization is itself essentially a chemical
process; and that justifies us in saying that these rocks are made what
they now are chiefly by the action of chemical forces. Whatever they
were originally, they have become, through their crystallization, rocks
having a definite mineral composition which can be classified
chemically.
Some of the details of the classification of this group, as shown in the
table, require explanation. In studying the silicate minerals it was
stated to be important to recognize two classes—the _acidic_ and the
_basic_—the dividing line falling in the neighborhood of 60 per cent. of
silica. This division is important simply because Nature has in a great
degree kept the acidic and basic minerals separate in the rocks; and few
things in lithology are more important than the distinction of the
silicate rocks in which acidic minerals predominate from those in which
basic minerals predominate. The amount of silica which any rock of this
group contains is shown at a glance by the chart. The vertical broken
lines, with the figures at the top, indicate the proportion of silica,
which increases from 30 per cent. on the right to 85 per cent. on the
left; so that the percentage of silica which a rock contains determines
its position, the acidic species being on the left, and the basic on the
right. As most of these rocks are composed of two or more minerals mixed
in very various proportions, there is usually a wide range in the
percentage of silica which the same species may contain; and this is
expressed in each case by the length of the dotted line under the name
of the rock. Thus, in syenite, the silica ranges from 55 per cent. to 65
per cent. The horizontal line in the chart separates the gneisses,
containing feldspar as an essential constituent, from the schists, in
which feldspar is wanting, except as an accessory constituent. We will
take up the gneisses first.
Gneiss.—This is the most important of all rocks. It forms not far from
one-half of New England, and a very large proportion of the earth’s
crust. The name (pronounced same as _nice_) is known to have originated
among the Saxon miners, but its precise derivation is lost in obscurity.
To find out what this very important rock is, we will consult specimen
41. The first glance shows us that it is not, like the rocks we have
just been studying, composed of a single mineral, but of several
minerals, the most conspicuous of which is the pink feldspar—orthoclase.
This we recognize as a feldspar: (1) by its hardness, which is a little
less than that of quartz, and distinguishes it from calcite, a mineral
having the general appearance of feldspar; (2) by its color, which
separates it from hornblende and augite; and (3) by its cleavage, which
distinguishes it easily from quartz. Finally, we know it is orthoclase,
and not plagioclase, by its general aspect, and by its association with
an abundance of quartz, which is the next most important constituent of
the rock. The quartz is less abundant than the orthoclase, and more
easily overlooked, yet anyone familiar with the mineral will not fail to
recognize it. It forms small, irregular, glassy grains, entirely devoid
of cleavage, and scratching glass easily. On weathered surfaces of the
rock the orthoclase becomes soft and chalky, while the quartz remains
clear and hard, and then the two minerals are very easily distinguished.
Besides these, there are numerous black, thin, glistening scales, which
we can easily prove to be elastic, and recognize as mica.
In most books on the subject, these three minerals—orthoclase, quartz,
and mica—are set down as the normal or essential constituents of gneiss.
But it is now recognized by the best lithologists that we may have true
gneiss without any mica; or we may have hornblende in the place of mica.
Quartz and orthoclase are the only essential constituents of gneiss; and
when these alone are present, we have the variety known as binary
gneiss. The addition to these essential constituents of mica, gives
micaceous gneiss; and of hornblende, hornblendic gneiss. Of these three
principal varieties, the micaceous gneiss is by far the most common and
important. The mica may be either the white species, muscovite, or the
black species, biotite; but it is usually the former.
Orthoclase is the predominant constituent in all typical gneiss, usually
forming at least one-half of the rock. The orthoclase may, however, be
replaced to a greater or less extent by albite, or even by oligoclase.
But we frequently see the term _gneiss_ carelessly, or ignorantly,
applied to rocks which are destitute of feldspar, though having the
general aspect of gneiss.
Augite rarely occurs in gneiss; and hence, when we observe a gneiss
containing a black mineral which we know is either augite or hornblende,
it is pretty safe to call it the latter.
Mica and hornblende, although the principal, are not the only, accessory
minerals in gneiss; but the following species are also of common
occurrence: garnet, cyanite, tourmaline, fibrolite, epidote, and
chlorite. Gneisses, as the table indicates, exhibit a wide range in the
proportion of silica which they contain, varying from 60 to 85 per
cent.; and there is a concomitant variation in specific gravity, from
about 2.5 in the most acidic to 2.8 in the most basic varieties.
That gneiss is a true, stratified rock is very clearly shown in specimen
41; but, unfortunately, the stratification is not always so evident as
in this case. The mica-scales, it will be observed, lie parallel with
the stratification, and assist very materially to make it visible; and
gneisses containing little or no mica, as well as some that are rich in
mica, frequently appear almost or quite unstratified. These obscurely
stratified varieties are commonly known as granitoid gneiss, having the
texture and general aspect of granite. The sedimentary origin of gneiss
is also clearly proved by its interstratification with undoubted
sedimentary rocks, such as limestone, iron-ores, graphite, quartzite,
etc.
Syenite.—This is a much abused term, but, as now employed by the best
lithologists, it is the name of a rock having a single essential
constituent, viz., orthoclase. Syenite in its simplest variety contains
nothing but orthoclase; but in addition we usually have either
hornblende, forming hornblendic syenite, or mica, forming micaceous
syenite.
Syenite, it will be observed, is equivalent to gneiss with the quartz
removed; but, while gneiss is the most abundant of all rocks, syenite is
a comparatively rare rock; and this is simply another way of saying that
nearly all orthoclase is associated with quartz. By admixture of quartz
we get a perfectly gradual passage from syenite to gneiss. The
orthoclase in syenite is more frequently replaced by plagioclase than it
is in gneiss. In syenite, too, hornblende is much more abundant than
mica; although just the opposite is true in gneiss. And, again, in
gneiss the mica is principally muscovite; but in syenite it is almost
exclusively biotite. Augite is a common accessory in the more basic
syenite; but garnet, tourmaline, and the other accessory minerals,
occurring so frequently in gneiss, are almost unknown in syenite. The
specific gravity of syenite varies from 2.7 to 2.9.
Diorite.—This is a more important rock than syenite; but it is of
analogous, though more basic, composition, containing a single essential
constituent, viz., plagioclase. Any of the triclinic feldspars may occur
in this rock, but oligoclase is most common. Like syenite, diorite
usually contains hornblende, often in large proportion, forming
hornblendic diorite, which sometimes passes into rocks composed entirely
of hornblende. It also, but less frequently, contains mica, forming
micaceous diorite. The mica is usually biotite, rarely muscovite. Mica
and hornblende also often occur together in diorite, and the same is
true of syenite and gneiss. Quartz is of common occurrence in the more
acidic varieties of diorite, and augite in the more basic.
This is a good example of a basic rock, for all its normal constituents
are basic; but the percentage of silica varies from 45 in those
varieties richest in labradorite and augite to 60 or more in those
containing more or less quartz and orthoclase. There is a corresponding
change of color from dark to light, and of specific gravity from 2.7 to
3.1.
Diorite is not rich in accessory minerals; besides those already
mentioned, the most important are chlorite, epidote, pyrite, and
magnetite.
Few rocks are more clearly stratified than diorite, whether we consider
the hand-specimen, or its relations to other formations. It is an
abundant rock in New England.
Norite.—Like diorite, this is essentially a plagioclase rock; but there
are, nevertheless, important differences. The plagioclase in diorite is
mainly the more acidic species, like oligoclase; while in norite the
more basic species, such as labradorite and anorthite, predominate.
Hornblende, which we have observed to be an important and rather
constant constituent of diorite and syenite, is much less abundant in
norite; but its place is taken by augite and the allied minerals,
hypersthene and enstatite. Black mica is common in norite; but white
mica, orthoclase, and quartz rarely occur.
Norite is the most basic of all the feldspathic rocks, as gneiss is the
most acidic; while syenite and diorite stand as connecting links,
forming a gradual passage between the two extremes. Thus, in passing
from gneiss to norite, we have observed a gradual diminution of the
quartz, a gradual change in feldspar from orthoclase to the most basic
plagioclase; at first a gradual increase in hornblende, and then a
gradual change from hornblende to augite; and, finally, a gradual
substitution of black mica for white. The amount of silica has decreased
over 40 per cent.; and the specific gravity has increased from 2.5 in
the lightest gneiss to at least 3.2 in the heaviest norite. We have also
passed from light colored rocks to dark; and from those resisting
atmospheric action to those easily decomposed.
The most characteristic accessory constituents of norite, besides those
already mentioned, are magnetite and chrysolite; though garnet,
serpentine, and pyrite often occur. In texture, this rock varies from
compact to very coarsely crystalline. The specimen of labradorite (No.
23), from the norite of Labrador, affords some idea of the coarseness of
the crystallization in much of this rock. It is not a common rock,
except in certain regions, the best known of which in eastern North
America are the coast of Labrador, various points in Canada north of the
St. Lawrence, and the eastern border of the Adirondack Mountains. In
hand-specimens, norite rarely appears stratified; but in the solid
ledges the stratification is often as distinct as could be desired.
Many lithologists call the rocks here designated norite _gabbro_, and
class them all in the eruptive division as essentially a coarse variety
of diabase. In a similar manner, diorite and syenite are denied a place
in the sedimentary series. But the stratified plagioclase rocks seem to
have as strong a claim to recognition as gneiss.
We turn now to the important and interesting division of the
non-feldspathic rocks or schists.
Mica Schist.—This is, next to gneiss, the most abundant rock in New
England. Specimen 43 is a typical example, and from it we can readily
learn what mica schist is. A glance suffices to show that it is chiefly
composed of mica, but not entirely; for, on carefully examining the
edges of the specimen, we cannot fail to see thin layers of hard, glassy
quartz interwoven with the mica. The quartz layers are short and
overlapping, and we have here a good illustration of the schistose
texture; this is, in fact, a typical schist.
Mica schist usually consists, as in this instance, of mica and quartz;
but it may be composed of mica alone; and sometimes kaolin or clay takes
the place of the quartz, forming argillaceous mica schist. The mica in
the latter is usually in very fine scales and rather inconspicuous, and
the rock often passes into ordinary clay slate. Similarly, when the mica
becomes deficient in the quartzose mica schist, a passage into ordinary
quartzite is the result. A little feldspar is sometimes present in the
rock, which thus passes into micaceous gneiss. Specimen 43 contains
several crystals of red garnet, giving the variety garnetiferous mica
schist. There is no other rock that contains such a large variety of
beautiful accessory minerals as mica schist; and for the mineralogist it
is one of the most attractive rocks. Few rocks are more distinctly
stratified; and the stratification can usually be observed in
hand-specimens. The mica in these rocks may be either muscovite or
biotite, or both; but the former is most common. No rock shows a greater
variation in the percentage of silica which it contains than mica
schist, as we pass from varieties which are nearly all quartz to those
which are nearly all mica.
Closely related to mica schist is the rock now known as hydromica
schist, in which the ordinary anhydrous micas are replaced by hydromica.
It is distinguished from mica schist by being somewhat softer, less
harsh to the touch, and less lustrous. It is to be regarded usually as
an incipient mica schist, which has not yet become anhydrous; though it
may sometimes be just the reverse; viz.: an old mica schist which has
become hydrated through the action of meteoric waters. It contains fewer
accessory minerals than mica schist.
Hornblende Schist.—This is a stratified aggregate of hornblende and
quartz. The quartz is granular and in thin layers, as in mica schist;
but the micaceous structure is wanting, and consequently the rock does
not cleave readily in the direction of the bedding. The hornblende is
mostly finely crystalline, but sometimes occurs in large, bladed
crystals. Garnet and some other minerals are of common occurrence in the
rock; but it is not rich in accessories like mica schist. The chief
difficulty in recognizing this rock consists in determining whether the
white mineral is all quartz or partly feldspar. In the latter case, of
course, it becomes a hornblendic gneiss.
Amphibolite (Hornblende Rock).—This is the name applied to a rock having
hornblende as its sole essential constituent. Hornblende schist
sometimes passes into amphibolite, through the absence of quartz; and so
does diorite, when the feldspar is deficient or wanting. Specimen 20,
though small, is a typical example of this rock. The physical and
chemical characteristics are essentially the same as for the mineral
hornblende. The texture varies from coarsely to finely crystalline. The
crystals are usually short and thick, and lie in all directions in the
rock, which is thus very massive, the schistose texture being entirely
wanting, and the stratification rarely showing in small masses. Biotite
is a common accessory in amphibolite, and garnet and magnetite
frequently occur.
By the substitution of augite for hornblende, in the description of
amphibolite, we get the much rarer, but otherwise very similar, rock,
_pyroxenite_.
Talc Schist (Steatite or Soapstone).—Although not abundant, this is a
useful and familiar rock. The composition is implied in the name; and by
comparing it with the specimen of talc (No. 58) we can readily see that
they are essentially identical. Typical talc schist is pure talc; but
the talc is often mixed with more or less quartz or feldspar; and mica,
chlorite, hornblende, garnet, and other minerals are of common
occurrence.
This rock embraces two distinct varieties, the massive and the
schistose, or foliated. The former is the common soapstone (specimen
71), which is a confused mass of crystals lying in all directions, and
with no visible stratification in the small mass. In the latter, as in
specimen , the talc scales lie in parallel planes, giving the rock a
micaceous structure, and causing it to split easily in the direction of
the stratification. The cleavage surfaces are often wavy or corrugated;
and the same is true of all schistose rocks. Talc schist is easily
distinguished from all other rocks by its light-grayish or greenish
color, combined with its extreme softness, and its smooth, slippery
feel.
Chlorite Schist.—The one essential constituent of this rock is chlorite,
and the mineral specimen (No. 26) answers equally well as an example of
the rock. As with talc schist, quartz, feldspar, and hydromica are
rarely entirely absent. Besides these, the principal accessories are
hornblende, magnetite, garnet, and epidote. This rock also agrees with
talc schist in presenting two principal varieties, the massive and the
schistose. It is easily distinguished from talc schist by its darker
color and streak, which are very characteristic; while its green color,
softness, and unctuous feel separate it from all other rocks.
This is the most basic of all the silicate rocks; but, in consequence of
containing a large proportion of water, it is not the heaviest. It is,
in fact, interesting and important to observe that all these hydrous
silicate rocks—talc schist, chlorite schist, greensand, and
serpentine—are distinctly lighter in each case than anhydrous rocks
containing the same proportion of silica. They are also notable, as a
class, for their softness, smooth feel, and green color.
Serpentine.—As the name implies, this rock is simply the mineral
serpentine occurring in large masses, and its characteristics are
precisely the same. It is fine-grained, massive, compact, rather soft,
but very tough, and varies in color from very dark green to light
greenish-yellow. The dark colors predominate, and specimen 25 is a
typical example.
Serpentine is often intimately associated with limestone and dolomite.
The white veins running irregularly through the variety known as Verd
Antique Marble, however, are not calcite, as commonly supposed, but
magnesite. They do not effervesce freely with cold, dilute acid, for the
entire rock is magnesian, and it is probable have been at one time
simply cracks along which water holding carbon dioxide has penetrated,
changing the magnesia from a silicate to a carbonate.
Geologists were, at one time, almost unanimous in the opinion that all
serpentine is of eruptive origin; but now it is conceded by the great
majority to be in some cases a sedimentary rock. It is found
interstratified with gneiss, limestone, all the schists, and many other
stratified rocks. When occupying the position of an eruptive it is never
an original rock; but has been formed by the alteration, _in situ_, of
some basic anhydrous rock, most commonly olivine basalt.
Greensand.—This rock (specimen 27) consists chiefly of the mineral
glauconite, mingled usually with more or less sand, clay, or calcareous
matter. It is usually very friable, or in an entirely unconsolidated
state. It is most abundant in the newer geological formations,
especially the Cretaceous and Tertiary; and is, perhaps, the only one of
the stratified silicate rocks now forming on an extensive scale in the
ocean. Its value as a fertilizer, for which purpose it is extensively
employed, is due to the potash that it contains.
Following is a systematic summary of the mineralogical composition of
the rocks of this great division of silicates; and this, combined with
the classification on page 69, presents in a condensed form all the more
important facts contained in the preceding descriptions. Only the more
constant and normal constituents of the species are enumerated in each
case:—
====================+====================================
Names of Species. | Constituent Minerals.
--------------------+------------------------------------
⎧| Orthoclase and Quartz.
Gneiss ⎨| Orthoclase, Quartz, and Mica.
⎩| Orthoclase, Quartz, and Hornblende.
--------------------+------------------------------------
⎧| Orthoclase.
Syenite ⎨| Orthoclase and Hornblende.
⎩| Orthoclase and Mica.
--------------------+------------------------------------
⎧| Plagioclase (chiefly Oligoclase).
Diorite ⎨| Plagioclase and Hornblende.
⎩| Plagioclase and Mica.
--------------------+------------------------------------
⎧| Plagioclase (chiefly Labradorite).
Norite ⎨| Plagioclase and Augite (Diallage).
⎩| Plagioclase and Mica.
--------------------+------------------------------------
⎧| Mica.
Mica Schist ⎨| Mica and Quartz.
⎩| Mica and Kaolin.
--------------------+------------------------------------
Hornblende Schist | Hornblende and Quartz.
--------------------+------------------------------------
Amphibolite | Hornblende.
Pyroxenite | Pyroxene.
--------------------+------------------------------------
Talc Schist | Talc.
--------------------+------------------------------------
Chlorite Schist | Chlorite.
--------------------+------------------------------------
Serpentine | Serpentine.
--------------------+------------------------------------
Greensand | Glauconite.
====================+====================================