Silicates (D.) (Hydrous).

Talc is a hydrated silicate of magnesia and its percentage composition is generally as below:

When crystallized it is ortho-rhombic, but it usually occurs in a massive condition, splitting into thin lamina, which are flexible, but not elastic as the plates of mica are. In colour it varies from white to apple-green. Steatite or

Soapstone is a coarse earthy variety of talc, usually of a dull white or yellowish colour, and always found in a massive form. Both talc and steatite have a soft soapy feel. Meerschaum is a silicate of similar composition.

Serpentine is another hydrated silicate of magnesia, with the following composition in typical specimens:

It is generally compact and massive, of a dark green colour, and is greasy to the touch; but it varies much in colour, and is sometimes yellow, brown, red, or black, from the presence of iron. Serpentine, like steatite, results from the decomposition of magnesian minerals, and gives its name to a particular kind of rock, which is almost entirely composed of it.

Chlorite appears to be a hydrated silicate of magnesia and iron, with some sesquioxide of alumina; like all products of alteration, its chemical composition varies, but the following is the average percentage:

Chlorite, like mica, is the name of a group rather than of a single mineral; it generally occurs in earthy or scaly

granules of a leek-green colour; sometimes it forms fibrous radiating crystals.

Glauconite is a hydrated silicate of iron, alumina, and potash, of a dull green colour; it is found in cavities of igneous rocks as a result of decomposition, and also occurs in the form of loose grains in marls and sandstones (see p. 227).

The Zeolites are hydrated double silicates having many varieties, to which separate names are given; most of them consist of silicate of alumina with silicates of lime, soda, or potash. They are, therefore, closely related to the Felspars, and may indeed be regarded as hydrated varieties of Felspar. The name Zeolite is derived from their intumescence or boiling up under the blow-pipe as the water is driven off. Mesotype, Prehnite, Chabasite, Analcime, Stilbite, and Thomsonite are among those most frequently met with.

CHAPTER II.

CLASSIFICATION OF ROCKS.

EANING of the word Rock.-Before attempting to classify the various kinds of rocks which enter into the composition of the earth's crust, it is necessary in the first place to understand what geologists mean by the term rock. In ordinary language a rock means a hard and massive stone, but geologists know that the character of hardness is an accidental one. The very same beds of limestone may be soft chalk in one place and hard marble in another. The same beds of clay may be harder than the hardest brick in one place, and soft enough to mould into bricks in another. The same beds of sandstone may be hard grit in one part, while in another they could be dug out with a spade. Geologists apply the word "rock," then, as a general term, to any considerable mass of mineral matter, whether hard or soft, or whatever may be its form or character, provided it be of sufficient importance to be spoken of as a constituent part of the crust of the earth.

A rock, therefore, may be defined as a mass of mineral matter, consisting of numerous crystalline or fragmentary particles which may belong to one or several different kinds of minerals, and which may be loosely coherent or firmly compacted together.

Texture and Structure of Rocks.-The texture of a rock is the manner in which the component particles are arranged, so as to produce differences in the grain or texture. Texture, therefore, is a matter which can be determined from hand specimens, and is part of the study of Mineralogy or Lithology. The structure of a rock is the manner in which its different parts are built up into a rock-mass, so as to produce differences in the shape of the pieces or blocks into which it can be broken up. Structure therefore is a matter which can be best determined from

an examination of rocks in the field, and comes under the head of Petrology, or the study of rock-masses.

Lithological Classification of Rocks. It is evident, then, that rocks may be classified either according to their textural or structural relations. Let us, in the first place, see to what conclusions a lithological arrangement would lead us. The words in which we have defined a rock suggest a primary lithological difference, whereby all rocks may be divided into two classes. Some are composed of definite crystalline particles, which, if not perfect crystals, yet possess some of the external faces and angles of perfect crystals, and have clearly been formed in the position which they now occupy. Other rocks consist of a congeries of particles which have not grown together, but are fragments which have been broken off their parent masses and brought together by some external agency; their coherence being caused either by mechanical compression or by a cement of some other substance.

By these lithological differences, therefore, rocks may be primarily distinguished into: 1, Crystalline; 2, Fragmentary.

Crystalline Rocks." All crystals are built up by the successive external addition of minute crystalline particles. It is clear, then, that these particles must have been free to move and arrange themselves; in other words, they must have been the result either of solution in water or other liquids, or of fusion by heat. Whenever, then, we find a crystal or a mineral particle that has an internal crystalline structure, we may feel assured that this structure has been produced either by solution or fusion; in other words, that the crystal has been either dissolved or melted. But if this be true as regards individual crystals, or crystalline particles, it must also be true of rocks that are made up of such crystals or particles." 1

It has been already stated (see p. 68), that some minerals are soluble in water containing carbonic acid, and that under certain circumstances they may be deposited directly from such chemical solution. Other minerals, again, such as those which compose volcanic rocks, though practically insoluble in water, can be made fluid by the influence of

Jukes' "Manual of Geology," 2nd Edition, p. 50.

heat. Crystalline rocks therefore are all chemically-formed, and are divisible into two sections-a, Aqueous crystalline rocks, and, b, Igneous crystalline rocks.

2. Fragmentary Rocks.-These consist of particles which have been derived from the disintegration and detrition of pre-existent rocks, or from the decay and disintegration of organic bodies. They may be divided into two groups or sections, according to the manner of their formation, viz., a, Mechanically-formed rocks; and b, Organically-formed rocks.

The particles of mechanically-formed rocks bear evident marks of mechanical fracture and attrition, most of them having been more or less rounded and worn by currents of water or of wind. This detritive and fragmental origin is very clear in the case of such rocks as are chiefly made up of pebbles or rounded fragments of other rocks, and is hardly less obvious in the case of sands and sandstones which have been derived from the continued attrition of such pebbles. Even igneous rocks have their mechanical accompaniments, in the shape of the dust, ashes, and fragments ejected from volcanic craters, and these may be compacted into solid rocks, whether they fall on the land or into the water.

The organically-derived rocks are wholly or in great part composed of fragments of the hard structures secreted by certain plants and animals, and the manner of their formation has already been fully described. The fragments may be little altered from their original condition or else they may be greatly altered and partly mineralized. In the latter case they become allied to the aqueouslyformed crystalline rocks.

A lithological arrangement of rocks therefore results in our making four groups, thus:

This, however, is not quite satisfactory, for it classes together aqueous and igneous rocks, and separates the chemically-formed aqueous rocks from their closely allied