1. Quartz, SiO₂.
2. The Feldspars:
2a. Orthoclase. Anhydrous silicate of alumina with varying amounts of lime, potash, or soda and rarely barium.
2b. Microcline. „
2c. Albite. „
2d. Oligoclase. „
2e. Andesite. „
2f. Labradorite. „
2g. Bytownite. „
2h. Anorthite. „
3. The Amphiboles:
3a. Hornblende. Anhydrous silicates of lime and magnesia with iron and alumina in the dark varieties.
3b. Tremolite. „
3c. Actinolite. „
3d. Arfvedsonite. „
3e. Glaucophane. „
3f. Smaragdite. „
4. The Monoclinic Pyroxenes:
4a. Malacolite. Anhydrous silicates of magnesia and lime with alumina and iron in the dark varieties.
4b. Diallage. „
4c. Augite. „
4d. Acmite. „
4c. Aegerite. „
5. The Rhombic Pyroxenes:
5a. Enstatite (bronzite). Silicates of magnesia and iron.
5b. Hypersthene. „
6. The Micas:
6a. Muscovite. Anhydrous silicates of alumina with potash, soda, and iron.
6b. Biotite. „
6c. Phlogopite. „
7. Calcite,
8. Dolomite.
9. Gypsum.
10. Olivine.
11. Beryl.
12. Tourmaline.
13. Garnet, variable common form.
14. Vesuvianite.
15. Epidote.
16. Zoisite.
17. Allanite.
18. Andalusite.
19. Staurolite.
20. Fibrolite.
21. Cyanite.
22. Scapolite.
23. Apatite.
24. Elaeolite and Nepheline.
25. Leucite.
26. Cancrinite.
27. The Sodalite Group:
27a. Sodalite.
27b. Haüyn (noseau).
28. Zircon.
29. Chondrodite.
30. Cordierite.
31. Topaz.
32. Corundum.
33. Titanite (sphene).
34. Rutile.
35. Menaccanite.
36. Magnetite.
37. Hematite.
38. Chromite.
39. The Spinels:
39a. Pleonast.
39b. Picotite.
40. Pyrolusite.
41. Halite (common salt).
42. Fluorite.
43. The Elements:
43a. Graphite.
43b. Carbon.
43c. Iron.
43d. Copper.
44. The Metallic Sulfids:
44a. Galena.
44b. Sphalerite.
44c. Pyrrhotite.
44d. Marcasite.
44e. Pyrite.
44f. Chalcopyrite.
44g. Arsenopyrite.
SECONDARY MINERALS.
1. Quartz:
1a. Chalcedony.
1b. Opal.
1c. Tridymite.
2. Albite.
3. The Amphibole Group:
3a. Hornblende.
3b. Tremolite.
3c. Actinolite.
3d. Uralite.
4. Muscovite (sericite).
5. The Chlorites:
5a. Jefferisite.
5b. Ripidolite.
5c. Penninite.
5d. Prochlorite.
6. Calcite (and aragonite).
7. Wollastonite.
8. Scapolite.
9. Garnet.
10. Epidote.
11. Zoisite.
12. Serpentine.
13. Talc.
14. Kaolin.
15. The Zeolites:
15a. Pectolite.
15b. Laumontite.
15c. Prehnite.
15d. Thomsonite.
15e. Natrolite.
15f. Analcite.
15g. Datolite.
15h. Chabazite.
15i. Stilbite,
15k. Heulandite.
15l. Harmotome.
16. Magnetite.
17. Hematite.
18. Limonite.
19. Siderite.
20. Pyrite.
21. Pyrrhotite.
ROCKS AND ROCK DECAY.
=27. Types Of Rocks.=—Rocks may be divided in reference to their
structure into four types: First, crystalline; second, vitreous; third,
colloidal; fourth, fragmental.
Of these classes there may be selected, as types of the first order,
granite and crystalline limestone.
The second class is typically represented by obsidian. Rocks of this
kind are confined to a volcanic origin.
The third class of rocks is completely amorphous in its structure and is
less common than the others. It is found only in rocks of chemical
origin. Types of this class are the siliceous sinters, opals, flint
nodules, and many serpentines.
Of the fourth class of rocks, sandstone is typical, being comprised
wholly of fragments of rocks pre-existing. The particles may be held
together either by cohesion or by a cement composed of silica, iron
oxids, carbonate of lime or clayey matter.
=28. The Microscopical Structure of Rocks.=—A great deal more light is
thrown upon the nature of rock materials by microscopical study than by
their study in bulk. The requisites for a microscopical study of rock
are that the material should be cut into extremely thin laminae with
parallel sides and polished so as to transmit the light freely. The
study of the crystalline structure of the material is then conducted by
means of a microscope furnished with polarizing and analyzing
appliances. The light before passing through the mineral film is
polarized by a Nicol prism. After passing through the film it is
analyzed by a second Nicol prism. In this way the crystalline structure
of the rock as affecting polarized light is distinctly brought out. The
thickness of the films examined should be from ¹⁄₅₀₀ to ¹⁄₆₀₀ of an
inch.
[Illustration:
FIG. 1. Microstructure of granite.
FIG. 2. Microstructure of micropegmatite.
FIG. 3. Microstructure of quartz porphyry.
FIG. 4. Microstructure of porphyritic obsidian.
FIG. 5. Microstructure of trachyte.
FIG. 6. Microstructure of serpentine.
]
The method of rock study by thin microscopic sections is one of
comparatively recent origin. It is scarcely more than a dozen years
since the process was fairly adopted by mineralogists. The value of the
method is based upon the fact that every crystalline mineral has certain
definite optical properties. Therefore, when a crystalline mineral is
distorted or misshapen so as to be incapable of identification by the
ordinary method, it can be at once identified by its optical examination
in the manner just described. In this way not only can one mineral be
distinguished from another, but the crystalline system to which it
belongs can be accurately pointed out. The value of the method is well
summed up by Merrill,[21] who says that it is not merely an aid in
determining the mineralogical composition of a rock, but also, which is
often much more important, its structure and the various changes which
have taken place in it since its first consolidation. Rocks are not the
definite and unchangeable mineral compounds they were once considered,
but are rather ever varying aggregates of minerals which even in
themselves undergo structural and chemical changes almost without
number.
Another valuable result of such a study is illustrated by the discovery
that the structural features of a rock are not dependent upon its
chemical composition or geologic age, but upon the conditions under
which it cooled from the molten magma. Portions of the same rock may
vary all the way from a wholly crystalline to a pure vitreous form.
Some typical microstructures of crystalline rocks are shown in the
accompanying figures 1–6.[22]
Although this method of study has thus far been confined mainly to
crystalline rocks, its efficiency is by no means limited to them. The
fragmental rocks and their decomposed débris to which the name soil is
given are equally worthy of study by this method. Indeed, the full value
of a chemical analysis of any rock or soil can not be ascertained unless
such an analysis is accompanied by a microscopic examination. It is
desirable to know not merely what there is in any soil, but in what form
these compounds exist. To this latter question the chemical analysis as
ordinarily made will give no clew. In Germany a beginning has been made
in this line of work, and American scientists are beginning to realize
its importance. An outline of this method of analysis will be given in
the proper place.
=29. Specific Gravity.=—Much information in regard to the properties of
a rock, or mineral constituent thereof, may be derived from its specific
gravity.
The internal structure of a rock may have much to do with its apparent
specific gravity. As an instance of this, it may be stated that an
obsidian pumice will float upon water, buoyed up by the air contained in
its vesicles, while a compact obsidian of the same composition will sink
immediately. A careful discrimination must, therefore, be made between
apparent and true specific gravity. In general it may be said that
crystalline rocks have a higher specific gravity than those of a
vitreous nature. The specific gravity is, therefore, largely dependent
upon chemical and crystallographic properties; for instance, among
siliceous rocks those which contain the largest amount of silica are the
lightest, while those with a comparatively small amount, but rich in
iron, lime, and magnesia, are heaviest.
=30. Chemical Composition of Rocks.=—Rocks are often classified with
respect to the chief mineral constituent which they contain. Rocks which
are composed largely of lime are termed calcareous; of silica,
siliceous; of iron, ferruginous; and of clay, argillaceous. In respect
of eruptive rocks, it is customary to speak of those which show above
sixty per cent of silica as acidic, while those containing less than
fifty per cent of silica and a correspondingly larger amount of iron,
lime, and magnesia, are spoken of as basic. Illustrations of the
classification of rocks on the above principles are given below.[23]
STRATIFIED ROCKS.
──────────────────────────┬──────────────┬─────────────────────────────
Kind. │ Specific │ Composition.
│ Gravity. │
──────────────────────────┼──────────────┼─────────────────────────────
Calcareous: │ │
Compact limestone │ 2.6 to 2.8 │ Carbonate of lime.
Crystalline limestone │ „ │ „
│ │
Compact dolomite │ 2.8 to 2.95 │ Carbonate of lime and
│ │ magnesia.
Crystalline dolomite │ „ │ „
│ │
Siliceous: │ │
Gneiss │ 2.6 to 2.7 │ Same as granite.
Siliceous sandstone │ 2.6 │ Mainly silica.
Schist │ 2.6 to 2.8 │ 60 to 80 per cent silica.
│ │
Argillaceous: │ │
Clay slate (argillite) │ 2.5 │ Mainly silicate of alumina.
──────────────────────────┼──────────────┼─────────────────────────────
│ │
ERUPTIVE ROCKS.
──────────────────────────┬──────────────┬─────────────────────────────
│ Specific │ Per cent silica.
│ Gravity. │
──────────────────────────┼──────────────┼─────────────────────────────
Acidic Group: │ │
Granite │ 2.58 to 2.73 │ 77.65 to 62.90
Liparite │ 2.53 to 2.70 │ 76.06 to 67.61
Obsidian │ 2.26 to 2.41 │ 82.80 to 71.19
Obsidian pumice │ Floats on │ 82.80 to 71.19
│ water. │
│ │
Intermediate Group: │ │
Syenite │ 2.73 to 2.86 │ 72.20 to 54.65
Trachyte │ 2.70 to 2.80 │ 64.00 to 60.00
Hyalotrachyte │ 2.4 to 2.5 │ 64.00 to 60.00
Andesite │ 2.54 to 2.79 │ 66.75 to 54.73
│ │
Basic Group: │ │
Diabase │ 2.66 to 2.88 │ 50.00 to 48.00
Basalt │ 2.90 to 3.10 │ 50.59 to 40.74
Peridotite │ 3.22 to 3.29 │ 42.65 to 33.73
Peridotite (iron rich) │ 3.86 │ 23.00
Peridotite (meteorite) │ 3.51 │ 37.70
──────────────────────────┴──────────────┴─────────────────────────────
=31. Color Of Rocks.=—The color of rocks is determined chiefly by the
oxids of metals which they contain and the degree of oxidation of the
mineral in each particular case. There are, however, many colors of
rocks which seem to depend not upon any particular mineral ingredient
which they contain, but upon some particular crystalline structure or
physical condition.
The chief coloring matters in minerals are those which form colored
bases such as iron, manganese, chromium, etc. The yellow, brown, and red
colors, common to fragmental rocks, are due almost wholly to free oxids
of iron. The gray, green, dull brown, and even black colors of
crystalline rocks are due to the presence of free iron oxids or to the
prevalence of silicate mineral rich in iron, as augite, hornblende, or
black mica. Rarely copper and other metallic oxids than those of iron
are present in sufficient abundance to impart their characteristic hues.
As a rule, a white or light-gray color denotes an absence of an
appreciable amount of iron in any of its forms. The bluish and black
colors of many rocks, particularly the limestones and slates, are due to
the presence of carbonaceous matter.
In still other cases, and particularly the feldspar-bearing rocks, the
color may be due in part to the physical condition of the feldspar.
Inasmuch as the color of rocks is due so largely to metallic oxids, it
is easy to see that they may undergo changes when exposed to weathering,
or the degree of oxidation may change, and either, together with changes
in the physical structure of the rock, may cause a distinct change in
color. Luster is often considered in connection with color, and is due
almost exclusively to physical conditions.
=32. Kinds of Rocks.=—The rocks which form any essential part of the
earth’s crust are grouped under four main heads, the distinction being
based upon their origin and structure.[24] Each of the main divisions
may be subdivided into groups or families, the distinction being based
mainly upon chemical composition, structure, and mode of occurrence. The
four chief families are:
First, aqueous rocks, formed mainly through the agency of water as
chemical precipitates or as sedimentary beds.
Second, aeolian rocks formed from wind-drifted materials.
Third, metamorphic rocks, changed from their original condition through
dynamic or chemical agencies, and which may have been partly of aqueous
and partly of igneous origin.
Fourth, igneous or eruptive rocks, which have been brought up from below
in a molten condition, and which owe their present structural
peculiarities to variations in composition and conditions of
solidification.
=33. Aqueous Rocks.=—Aqueous rocks may be divided into the following
general classes:
First, rocks formed as chemical precipitates.
Second, rocks formed as sedimentary deposits and fragmental in
structure. The second class may again be subdivided into rocks formed by
mechanical agencies and mainly of inorganic materials; and second, rocks
composed mainly of the débris of plant and animal life.
In regard to the first form of aqueous rocks, namely, those formed as
chemical precipitates, it may be said that while their quantity is not
large they are yet of considerable importance from an agricultural point
of view. They embrace those substances which, having once been in a
condition of vapor or aqueous solution, have been deposited or
precipitated, either by cooling or by the evaporation of the liquor
holding them in solution, or by coming in contact with chemical
substances capable of precipitating them. The influence of water as a
solvent is perhaps not fully appreciated. Its solvent influence will be
noted particularly under the head of weathering or decay of rocks. Its
importance, however, in producing stratified rocks has been very great.
Water, especially when under great pressure and at a high temperature,
has the power of dissolving many minerals. This power is often greatly
increased by the mineral matter previously in solution in the water or
by the gases which it may contain. As an illustration of the latter
property, the solvent action of water charged with carbon dioxid on
limestone may be cited.
When mineral matters have been dissolved by the water in the ways
mentioned and carried with the water beyond the condition where the
solution has taken place, new conditions are found favorable to the
precipitation of the dissolved matters. The water, which before may have
been very hot, may reach a place where it cools, and being a
supersaturated solution, the excess of the material is thrown down as
the water cools.
On the other hand, if the solution be due to the presence of carbon
dioxid and the water reach a place where it is exposed to the air or
where the pressure under which the abundance of the gas has been due is
diminished, the carbon dioxid will escape and the mineral matters which
have been dissolved thereby will be precipitated.
The incrustations which often appear round the mouth of springs and the
occurrence of stalagmites and stalactites in caves are illustrations of
this action.
In respect of the formation of rocks as precipitates from a state of
vapor we have scarcely any illustrations excepting in volcanic regions.
Rocky materials with which we are generally acquainted are practically
non-volatile at the highest temperature which can be secured on the
earth’s surface, but it is possible that in the interior of the earth
the temperature may be so high as to maintain many substances in a state
of vapor.
They may, in this case, become disassociated so that the compounds or
elements exist distinctly in a vaporous condition. Such a vapor
transported to regions of diminished temperature would first of all on
cooling permit a union of the chemical elements forming new compounds
less volatile, which, of course, would be at once precipitated.
The rocks and minerals formed in this way which are of some agricultural
importance may be classified as follows:
Oxids, carbonates, silicates, sulfur, sulfids, sulfates, phosphates,
chlorids, and hydrocarbon compounds, the most important from an
agricultural point of view being the phosphates.
The second group of rocks, namely those formed as sedimentary deposits,
differ from those just described in that they are comprised mainly of
fragmental materials derived from the breaking down of pre-existing
rocks. The formation of fragmental rocks includes, therefore, the same
processes as are active in the formation of arable soil. They are
deposited from water, and are as a rule distinctly stratified.
Through the action of pressure and the heat thereby generated, or simply
through the chemical action of percolating solutions, such rocks pass
over into the crystalline sedimentary forms known as metamorphic. All
metamorphic rocks, however, are not of a sedimentary origin. For
instance, by pressure, heat, and the chemical changes thereby induced,
granite may be changed into gneiss and the latter would then be a
metamorphic rock.
This group of sedimentary rocks and of sedimentary material, either
unchanged or metamorphosed, is of vast extent and includes materials of
widely varying chemical and mineralogical nature. They form by far the
greater portion of the present surface of the earth, even the mountain
ranges being composed mainly of this sedimentary material. Indeed, in
the whole of this country there is only a comparatively very small
extent of igneous or irruptive rocks. They are of great importance from
a purely scientific, as well as agricultural standpoint, since they
contain the fossil records of past geologic ages. From them it is
possible to study the variations in climate, the meteorological
conditions in circumstances and periods far remote, and thus form some
idea of the process by which the crust of the earth has been modified by
natural forces from its original form to the present time.
The sedimentary rocks may be divided, with sufficient accuracy for our
purposes, into two great classes: First, rocks formed by mechanical
agencies and mainly of inorganic materials. These are subdivided again
as follows:
(a) The arenaceous group.
(b) The argillaceous group.
(c) The volcanic group.
The second class of sedimentary rocks is formed largely, or in part at
least, by mechanical agencies, but is comprised chiefly of the débris of
plant and animal life. It may be subdivided as follows:
(a) The siliceous group, such as infusorial earth.
(b) The calcareous group, fossiliferous formations, limestone, etc.
(c) The carbonaceous group, such as peat, lignite, coals, etc. The
different classes of rock described above are distinguished by special
qualities represented largely by the name. The first division, the
arenaceous group, is composed mainly of the siliceous or coarsely
granular materials derived from the disintegration of older crystalline
rocks, which have been rearranged in beds of varying thickness through
the mechanical agency of water. They are, in short, consolidated or
unconsolidated beds of sand and gravel. In composition and texture they
vary almost indefinitely. Many of them having suffered little during the
process of disintegration and transportation are composed essentially of
the same materials as the rocks from which they were derived.
The sandstones, which are the type of these rocks, vary greatly in
structure as well as in composition, in some the grains being rounded
while in others they are sharply angular.
The microscopic structure of sandstone is shown in figure 7.[24]
The material by which the individual grains of a sandstone are bound
together is usually the material of some of the other classes. The
calcareous, ferruginous, and siliceous cements being the chief ones.
This cementing substance is deposited among the granules forming the
sandstone by percolating water.
The colors of sandstone are dependent usually upon iron oxids.
Especially is this true of the red, brown, and yellow colors. In some of
the light grey varieties, the color is that of the minerals comprising
the stone. Some of the darker colored sandstones contain organic matter.
[Illustration:
FIG. 7.
Microscopic Structure of Sandstone.
]
The rocks of the argillaceous group are composed essentially of a
hydrous silicate of alumina, which is the basis of common clay, and
varying amounts of free silica, oxids of iron and manganese, carbonates
of lime and magnesia, and small quantities of organic matter. They may
have originated _in situ_ from the decomposition of feldspars or as
deposits of fine mud or silt at the bottom of large bodies of water. The
older formations of these rocks are known as shales, argillites, and
slates and the fissile structure which enables this to be split into
thin sheets is probably due to the conditions under which they have been
formed and not to any properties of the clays themselves.
One of the purest forms of this rock is kaolin, which is almost a pure
hydrous silicate of alumina formed from the decomposition of feldspathic
rocks from which the alkalies, iron oxids and other soluble constituents
have been removed by water.
Under the volcanic group are included the materials ejected from
volcanic vents in a more or less finely comminuted condition and which
through the drifting power of atmospheric currents may be scattered over
many miles of territory. Various names are applied to such products,
names dependent in large part upon their state of subdivision. Volcanic
dust and sand, or ashes, includes the finer dust-like or sand-like
materials, and lapilli, or rapilli the coarser. The general name tuff
includes the more or less compacted and stratified beds of this
material, while trass, peperino, and pozzuolano are local varietal names
given to similar materials occurring in European volcanic regions.
The second division, namely sedimentary rocks composed of the débris of
plant and animal life includes many forms of great agricultural
importance.
The first subdivision of this group is the infusorial or diatomaceous
earth. It forms a fine white or yellowish pulverulent rock composed
mainly of minute shells, or tests of diatoms, and is often so soft and
pliable as to crumble readily between the thumb and fingers. According
to Whitney the beds are of comparatively limited extent and for this
reason are of little agricultural value, although the weathering of this
diatomaceous material gives rise to a light yellow clay forming very
fertile agricultural lands.
The second subdivision of this group includes the rocks of a calcareous
nature derived from animal life; that is to say, what are properly
called limestones. They vary in color, structure, and texture almost
indefinitely, and include all possible grades of materials from those
which can be used only as a flux, or for lime burning, through ordinary
building materials to the finest marbles. These rocks are world-wide in
their distribution and limited to no one particular geologic horizon,
but are found in stratified beds among rocks of all ages from the most
ancient to the most recent.