Part 3

Before proceeding to study particularly the various kinds of rocks, a little more preliminary work should be done. As already intimated, the more important characteristics of rocks may be grouped under two general heads,—_composition_ and _texture_.

_Composition of Rocks._

Rocks are properly defined as large masses or aggregates of mineral matter, consisting in some cases of one and in other cases of several mineral species. Hence it is clear that the composition of rocks is of two kinds: chemical and mineralogical; for the various chemical elements are first combined to form minerals, and then the minerals are combined to form rocks.

Of course those minerals and elements which can be described as principal or important rock-constituents must be the common minerals and elements. Now it is a very important and convenient fact that although chemists recognize about sixty-five elementary substances, and these are combined to form nearly one thousand mineral species, yet both the _common_ elements and the _common_ minerals are few in number.

So that, although it is very desirable and even necessary for the student of lithology to know something of chemistry and mineralogy, it by no means follows that he or she must be master of those sciences. A knowledge of the chemical and physical characteristics of a few common minerals is all that is absolutely essential, though it may be added that an excess of wisdom in these directions is no disadvantage.

Chemical Composition of Rocks.

The elementary substances of which rocks are chiefly composed, which make up the main mass of the earth so far as we are acquainted with it, number only fourteen:—

_Non-Metallic or Acidic Elements._—Oxygen, silicon, carbon, sulphur, chlorine, phosphorus, and fluorine.

_Metallic or Basic Elements._—Aluminum, magnesium, calcium, iron, sodium, potassium, and hydrogen.

The elements are named in each group in about the order of their relative abundance; and to give some idea of the enormous differences in this respect it may be stated that two of the elements—oxygen and silicon—form more than half of the earth’s crust.

Silicon, calcium, and fluorine, although exceedingly abundant, are also very difficult to obtain in the free or uncombined state, and specimens large enough to exhibit to a class would be very expensive. With these exceptions, however, examples of these common rock-forming elements are easily obtained.

My purpose in calling attention to this point is simply to suggest that the proper way to begin the study of minerals and rocks with children is to first familiarize them with the elements of which they are composed. The most important thing to be known about any mineral is its chemical composition; and when a child is told that a mineral—corundum, for example—is composed of oxygen and aluminum, he should have a distinct conception of the properties of each of those elements, for otherwise corundum is for him a mere compound of names.

It is very important, too, if the pupil has not already studied chemistry, that he should be led to some comprehension of the nature of chemical union and of the difference between a chemical compound and a mechanical mixture. For this purpose a few simple experiments (the details of which would be out of place here) with the more common and familiar elements will be sufficient. Mrs. Richard’s “First Lessons in Minerals” should be introduced here.

Mineralogical Composition of Rocks.

The fourteen elements named above are combined to form about fifty minerals with which the student of geology should be acquainted; but not more than one-half of these are of the first importance. It is desired to lay especial emphasis upon the importance of a perfect familiarity with these few common minerals. There is nothing else in the whole range of geology so easily acquired which is at the same time so valuable; for it is entirely impossible to comprehend the definitions of rocks, or to recognize rocks certainly and scientifically, unless we are acquainted with their constituent minerals.

With one or two exceptions, these common rock-forming minerals may be easily distinguished by their physical characters alone, so that their certain recognition is a matter of the simplest observation, and entirely within the capacity of young children. Furthermore, being common, specimens of these minerals are very easily obtained, so that there is no reason why teachers should not here adopt the best method and place a specimen of each mineral in the hands of each pupil. Typical examples, large enough to show the characteristics well, ought not to cost, on the average, over two cents apiece.

A MINERAL is an inorganic body having theoretically a definite chemical composition, and usually a regular geometric form.

THE PRINCIPAL CHARACTERISTICS OF MINERALS.—These may be grouped under the following general heads:—

(1) _Composition_, (2) _Crystalline form_, (3) _Hardness_, (4) _Specific gravity_, (5) _Lustre_, (6) _Color and Streak_.

1. _Composition._—This, according to the definition of a mineral, ought to be _definite_, and expressible by a chemical formula. When it is not so, we usually consider that the mineral is partially decomposed, or that we are dealing with a mixture of minerals. It is well to impress upon the mind of the pupil the important fact that the more fundamental properties of the elements, such as specific gravity and lustre, are not lost when they combine, but may be traced in the compounds. In other words, the properties of minerals are, in a very large degree, the average of the properties of the elements of which they are composed; minerals in which heavy metallic elements predominate being heavy and metallic, and _vice versa_.

To fully appreciate this point it is only necessary to compare a mineral like galenite—a common ore of lead, and containing nearly 87 per cent. of that heavy metal; or hematite (specimen 13), containing 70 per cent. of another heavy metal, iron—with quartz (specimen 15), which is composed in nearly equal parts of oxygen and silicon, two typical non-metallic elements. Many minerals contain water, _i.e._, are hydrated. Now water, whether we consider the liquid or solid state, is one of the lightest and softest of mineral constituents; and it is a very important fact that hydrated minerals are invariably lighter and usually softer than anhydrous species of otherwise similar composition. Other striking illustrations of this principle will be pointed out in the descriptions of the minerals which follow.

2. _Crystalline form._—A crystal is bounded by plane surfaces symmetrically arranged with reference to certain imaginary lines passing through its centre and called axes. Crystals of the same species are always constant in the angles between like planes, while similar angles usually vary in different species; so that each species has its own peculiar form.

“Besides external symmetry of form, crystallization produces also regularity of internal structure, and often of fracture. This regularity of fracture, or tendency to break or cleave along certain planes, is called cleavage. The surface afforded by cleavage is often smooth and brilliant (see specimens 17, 18, and 21), and is always parallel with some external plane of the crystal. It should be understood that the cleavage lamellæ are not in any sense present before they are made to appear by fracture.”—(Dana.)

Crystals are arranged in six systems, based upon the number and relations of the axes, as follows:—

Isometric System.—Three equal axes crossing at right angles. Example, cube.

Tetragonal System.—Two axes equal, third unequal, all crossing at right angles. Example, square prism.

Orthorhombic System.—Three unequal axes, but intersections all at right angles. Example, rhombic prism.

Monoclinic System.—Three unequal axes, one intersection oblique. Example, oblique rhombic prism.

Triclinic System.—Three unequal axes, all crossing obliquely. Example, oblique rhomboidal prism.

Hexagonal System.—Three equal axes lying in one plane and intersecting at angles of 60°, and a fourth axis crossing each of these at right angles and longer or shorter. Example, hexagonal prism.

By the truncation and bevelment of the angles and edges of these fundamental forms a vast variety of secondary forms are produced. The limits of the guide will not permit us to follow this topic farther; but it may be added that for the proper elucidation of even the simpler crystalline forms the teacher should be provided with a set of wooden crystal models and Dana’s “Text-Book of Mineralogy.”

The crystallization of a mineral may be manifested in two ways: first, by the regularity of its internal structure or molecular arrangement, as shown by cleavage and the polarization of transmitted light; and, second, by the regularity of external form which follows, _under favorable conditions_, as a necessary consequence of symmetry in the arrangement of the molecules.

When a mineral is entirely devoid of crystalline structure, both externally and internally, it is said to be _amorphous_.

Perfect and distinct crystals are the rare exception, most mineral specimens being simply aggregates of imperfect crystals. In such cases, and when the mineral is amorphous, the _structure_ of the _mass_ may be:—

Columnar or fibrous.

Lamellar, foliaceous, or micaceous.

Granular.—When the grains or crystalline particles are invisible to the naked eye the mineral is called _impalpable_, _compact_, or _massive_.

And the _external form_ of the mass may be:—

Botryoidal, having grape-like surfaces.

Stalactitic, forming stalactites or pendant columns.

Amygdaloidal or Concretionary, forming separate globular masses in the enclosing rock.

Dendritic, branching or arborescent.

3. _Hardness._—By the hardness of a mineral we mean the resistance which it offers to abrasion. But hardness is a purely relative term, calcite, for example, being hard compared with talc, but very soft compared with quartz. Hence mineralogists have found it necessary to select certain minerals to be used as a standard of comparison for all others, and known as the _scale of hardness_. These are arranged at nearly equal intervals all the way from the softest mineral to the hardest, as follows:—

_Scale of Hardness._

(1) Talc. (2) Gypsum. (3) Calcite. (4) Fluorite. (5) Apatite. (6) Orthoclase. (7) Quartz. (8) Topaz or Beryl. (9) Corundum. (10) Diamond.

If a mineral scratches calcite and is scratched by fluorite, we say its hardness is between 3 and 4, perhaps 3.5; if it neither scratches nor is scratched by orthoclase, its hardness is 6; and so on. There are very few minerals harder than quartz, and hence the first seven members of the scale are sufficient for all ordinary purposes; and these are all included in the series of specimens accompanying this Guide.

Although it is desirable to be acquainted with the scale of hardness, and to understand how to use it, still the student will learn, after a little practice, that almost as good results may be obtained much more conveniently by the use of his thumb-nail and a good knife-blade or file. Talc and gypsum are easily scratched with the nail; calcite and fluorite yield easily to the knife or file, apatite with more difficulty; while orthoclase is near the limit of the hardness of ordinary steel, and quartz is entirely beyond it.

4. _Specific Gravity._—The specific gravity of a mineral, by which we mean its weight as compared with the weight of an equal volume of water, is determined by weighing it first in air and then in water, and dividing the weight in air by the difference of the two weights. Minerals exhibit a wide range in specific gravity; from petroleum, which floats on water, to gold, which is nearly twenty times heavier than water. Although this is one of the most important properties of minerals, yet, being more difficult to measure than hardness, it is less valuable as an aid in distinguishing species. One can with practice, however, estimate the density of a mineral pretty closely by lifting it in the hand.

5. _Lustre._—Of all the properties of minerals depending on their relations to light the most important is lustre, by which we mean the quality of the light reflected by a mineral as determined by the character or minute structure of its surface. Two kinds of lustre, the _metallic_ and _vitreous_, are of especial importance; in fact all other kinds are merely varieties of these.

The metallic lustre is the lustre of all true metals, as copper and tin, and characterizes nearly all minerals in which metallic elements predominate. The vitreous lustre is best exemplified in glass, but belongs to most minerals composed chiefly of non-metallic elements. Metallic minerals are always opaque, but vitreous minerals are often transparent.

Other kinds of lustre are the _adamantine_ (the lustre of diamond), _resinous_, _pearly_, and _silky_. When a mineral has no lustre, like chalk, it is said to be dull.

It should be made clear to children that lustre and color are entirely distinct and independent. Thus, iron, copper, gold, silver, and lead are all metallic; while white or colorless quartz, black tourmaline, green beryl, red garnet, etc., are all vitreous. Generally speaking, any color may occur with any lustre.

6. _Color and Streak._—The colors of minerals are of two kinds,—_essential_ and _non-essential_. By the essential color in any case we mean the color of the mineral itself in its purest state. The non-essential colors, on the other hand, are chiefly the colors of the impurities contained in the minerals.

Metallic minerals, which are always opaque, usually have essential colors; but vitreous minerals, which are always more or less transparent, often have non-essential colors. The explanation is this: In opaque minerals we can only see the impurities immediately on the surface, and these are, as a rule, not enough to affect its color; but in diaphanous minerals we look _into_ the specimen and see impurities below the surface, and thus bring into view, in many cases, sufficient impurity so that its color drowns that of the mineral.

To prove this we have only to take any mineral (serpentine is a good example in our series) having a non-essential color, and make it opaque by pulverizing it or abrading its surface, when the non-essential color, the color of the impurity, immediately disappears; just as water, yellow with suspended clay, becomes white when whipped into foam, and thus made opaque.

What we understand by the _streak_ of a mineral is its essential color, the color of its powder; and it is so called because the powder is most readily observed by scratching the surface of the mineral, and thereby pulverizing a minute portion of it. The streak and hardness are thus determined at the same time. The streak of soft minerals is easily determined by rubbing them on any white surface of suitable hardness, as paper, porcelain, or Arkansas stone.

ESSENTIAL AND ACCESSORY MINERALS.—Lithologists, regarding minerals as constituents of rocks, divide them into two great classes: the _essential_ and the _accessory_. The essential constituents of a rock are those minerals which are essential to the definition of the rock. For example, we cannot properly define granite without naming quartz and orthoclase; hence these are essential constituents of granite; and if either of these minerals were removed from granite it would not be granite any longer, but some other rock. But other minerals, like tourmaline and garnet, may be indifferently present or absent; it is granite still; hence they are merely accidental or accessory constituents. They determine the different _varieties_ of granite, while the essential minerals make the _species_.

This classification, of course, is not absolute, for in many cases the same mineral forms an essential constituent of one rock and an accessory constituent of another. Thus, quartz is essential in granite, but accessory in diorite.

PRINCIPAL MINERALS CONSTITUTING ROCKS.—Having studied in a general way the more important characteristics of minerals, brief descriptions of the chief rock-forming species are next in order. We will notice first and principally those minerals occurring chiefly as _essential_ constituents of rocks.

1. Graphite.—Essentially pure carbon, though often mixed with a little iron oxide. Crystallizes in hexagonal system, but usually foliated, granular, or massive. Hardness, 1-2, being easily scratched with the nail. Sp. gr., 2.1-2.3. Lustre, metallic; an exception to the rule that acidic elements have non-metallic or vitreous lustres. Streak, black and shining (see pencil-mark on white paper). Color, iron-black. Slippery or greasy feel. Every _black-lead_ pencil is a specimen of graphite. Specimen 9.

The different kinds of mineral coal are, geologically, as we have seen, closely related to graphite, but they are such familiar substances that they need not be described here.

2. Halite (common salt).—Chloride of sodium: chlorine, 60.7; sodium, 39.3; = 100. Isometric system, usually forming cubes. Hardness, 2.5, a little harder than the nail. Sp. gr., 2.1-2.6. Lustre, vitreous. Streak and color both white, and hence color is essential. Often transparent. Soluble; taste, purely saline. In specific gravity and lustre it is a good example of a mineral in which an acidic element predominates. Specimen 11.

3. Limonite.—Hydrous sesquioxide of iron: oxygen, 25; iron, 60; water, 15; = 100. Usually amorphous; occurring in stalactitic and botryoidal forms, having a fibrous structure; and also concretionary, massive, and earthy (yellow ochre). Hardness, 5-5.5. Sp. gr., 3.6-4. Lustre, vitreous or silky, inclining to metallic, and sometimes dull. Color, various shades of black, brown, and yellow. Streak, ochre-yellow; hence color partly non-essential. Specimen 12.

4. Hematite.—Sesquioxide of iron: oxygen, 30; iron, 70; = 100. Hexagonal system, in distinct crystals, but usually lamellar, granular, or compact,—columnar, botryoidal, and stalactitic forms being common. Hardness, 5.5-6.5; good crystals are harder than steel. Sp. gr., 4.5-5.3. Lustre, metallic, sometimes dull. Color, iron-black, but red when earthy or pulverized (red ochre). Streak, red, and color, therefore, mainly non-essential; sometimes attracted by the magnet. Specimen 13.

Hematite has the same composition as limonite, minus the water; and by comparing the hardness and specific gravity of these two minerals we see that they are a good illustration of the principle that hydrous minerals are softer and lighter than anhydrous minerals of analogous composition. Limonite and hematite are two great natural coloring agents, and almost all yellow, brown, and red colors in rocks and soils are due to their presence.

5. Magnetite.—Protoxide and sesquioxide of iron: oxygen, 27.6; iron, 72.4; = 100. Isometric system, usually in octahedrons or dodecahedrons. Most abundant variety is coarsely to finely granular, sometimes dendritic. Hardness, 5.5-6.5, same as hematite. Sp. gr., 4.9-5.2. Lustre, metallic. Color and streak, iron-black, and hence color essential. Strongly magnetic; some specimens have distinct polarity, and are called loadstones. Specimen 14.

The three iron-oxides just described—limonite, hematite, and magnetite—are all important ores of iron, and form a well-marked natural series. Thus limonite is never, hematite is usually, and magnetite is always, crystalline. Again, limonite with 60 per cent. of iron is never magnetic, hematite with 70 per cent. is sometimes magnetic, while magnetite with 72.4 per cent. is always magnetic. As the iron increases so does the magnetism. We have here an excellent illustration of the principle that the properties of the elements can be traced in those minerals in which they predominate. Iron is the only strongly magnetic element: magnetite contains more iron than any other mineral, and it is the only strongly magnetic mineral.

These three iron-ores are easily distinguished from each other by the color of their powders or streak,—limonite yellow, hematite red, and magnetite black,—and from all other common minerals by their high specific gravity.

6. Quartz.—Oxide of silicon or silica: oxygen, 53.33; silicon, 46.67; = 100. Hexagonal system. The most common form is a hexagonal prism terminated by a hexagonal pyramid. Also coarsely and finely granular to perfectly compact, like flint; the compact or cryptocrystalline varieties often assuming botryoidal, stalactitic, and concretionary forms. It has no cleavage, but usually breaks with an irregular, conchoidal fracture like glass. Hardness, 7, being No. 7 of the scale; scratches glass easily. Sp. gr., 2.5-2.8. Lustre, vitreous. Pure quartz is colorless or white, but by admixture of impurities it may be of almost any color. Streak always white or light colored. Quartz is usually, as in specimen 15, transparent and glassy, but may be translucent or opaque. It is almost absolutely infusible and insoluble.

The varieties of quartz are very numerous, but they may be arranged in two great groups:—

1. _Phenocrystalline_ or _vitreous_ varieties, including rock-crystal, amethyst, rose quartz, yellow quartz, smoky quartz, milky quartz, ferruginous quartz, etc.

2. _Cryptocrystalline_ or _compact_ varieties, including chalcedony, carnelian, agate, onyx, jasper, flint, chert, etc. Only three varieties, however, are of any great geological importance; these are: common glassy quartz (spec. 15), flint (spec. 16), and chert.

Quartz is one of the most important constituents of the earth’s crust, and it is also the hardest and most durable of all common minerals. We have already observed (p. 12) that it is entirely unaltered by exposure to the weather; _i.e._, it cannot be decomposed; and, being very hard, the same mechanical wear which, assisted by more or less chemical decomposition, reduces softer minerals to an impalpable powder or clay, must leave the quartz chiefly in the form of sand and gravel. This agrees with our observation that sand (spec. 30), especially, is usually merely pulverized quartz.

_Opal_ is a mineral closely allied to quartz, and may be mentioned in this connection. It is of similar composition, but contains from 5 to 20 per cent. of water, and is decidedly softer and lighter. Hardness, 5.5-6.5; sp. gr., 1.9-2.3.

7. Gypsum.—Hydrous sulphate of calcium: sulphur trioxide (SO₃), 46.5; lime (CaO), 32.6; water (H₂O), 20.9; = 100. Monoclinic system. Often in distinct rhombic crystals; also foliated, fibrous, and finely granular. Hardness, 1.5-2; the hardest varieties being No. 2 of the scale of hardness. Sp. gr., 2.3. Lustre, pearly, vitreous, or dull. Color and streak usually white or gray. The principal varieties of gypsum are (_a_) _selenite_, which includes all distinctly crystallized or transparent gypsum; (_b_) _fibrous gypsum_ or _satin-spar_; (_c_) _alabaster_, fine-grained, light-colored, and translucent. Gypsum is easily distinguished from all common minerals resembling it by its softness and the fact that it is not affected by acids. Specimen 17.