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.

Pseudomorphs.-Pseudomorphism is the occurrence of one mineral in the crystalline form of another mineral. This may take place either by alteration or substitution. In the one case, the first mineral has been gradually changed into another by the addition or exchange of constituents. In the second case, the new mineral is formed by the gradual replacement of the first mineral particle by particle. Both processes are accomplished by the action of carbonated water, containing solutions of various minerals obtained from the rocks through which it has percolated. Penetrating deeply into the earth, acquiring a higher temperature, and helped by pressure, such water becomes an active agent in altering the constitution of minerals and rocks.

1

Some of these changes have been imitated in the laboratory of the chemist. Thus " Stein converted a crystal of gypsum (sulphate of lime) into carbonate of lime by leaving it for several weeks in a solution of carbonate of soda at a temperature of 122° F." The sulphuric acid of the gypsum united with the soda to form sulphate of soda, which was dissolved and carried away by the water, while the lime united with the carbonic acid, and remained in the crystalline form of gypsum. The result is only attainable under certain conditions, of which the most essential is slow action, otherwise the original form is lost; but these conditions are eminently fulfilled in all natural operations. Again, Calcite is frequently found as a pseudomorph after other minerals.

The process by which such pseudomorphs are produced is a very important one, since it is precisely that of petrifaction, or the mode in which organic forms are mineralized, and their external form, with more or less of their internal structure, preserved for our examination.

1 Bischof, "Chemical Geology,” vol. i. chap. ii.

CHAPTER II.

CLASSIFICATION OF ROCKS.

MEANING of the word Rock.-Before attempt

ing 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; and a sandstone which is hard grit in one part, in another 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.-Every kind of rock possesses a special structure and texture; its structure depending on the nature and character of its component particles, its texture on the comparative size of these particles, and the manner in which they are arranged. These qualities of a rock can be determined from small

samples or hand specimens, and the study of them is a part of Mineralogy or Lithology.

When rocks are considered as rock-masses, and studied in their natural positions, other points of difference present themselves; the manner in which they occur, and the shape of the layers, blocks, or masses into which they split, are found to depend largely on the manner in which they have been formed and the materials of which they consist. The study of these relationships is that of Field Geology or Geognosy.

Lithological Classification of Rocks.-It is evident, then, that rocks may be classified either according to their internal structure or their external 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. With these may be grouped certain rocks which consist partially or wholly of a glassy substance. 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. 85) 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 heat. If then rapidly cooled they solidify into a glass, if slowly they form a crystalline mass. 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, From the broken, chipped, and rounded aspect of their component particles, such rocks are often termed Clastic, from kλaoros, broken. 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 sand and sandstones. 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,

Jukes' "Manual of Geology," second edition, p. 50.

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 neighbours of organic origin. It is evidently, then, desirable that we should ascertain whether rocks do not exhibit some differences which are more closely in relation to the conditions under which they have been formed.

Tectonic Classification of Rocks.-Rocks may be classified by the differences in the tectonic structure, or the manner in which they are built up into rock-masses. Those which have been deposited as sediments on the bottoms of seas, lakes, and rivers, are arranged in regular layers, beds, or strata. Those rocks, on the contrary, which have consolidated from a state of igneous fusion are always unstratified and massive in their structure. Hence we have a primary distinction into Stratified and Unstratified rocks, and it will also be found that most of the stratified rocks are fragmental (and derivative), while most of the unstratified rocks are crystalline.

Certain rocks will, however, be met with, which present a combination of these characters, possessing a more or less crystalline texture, yet often showing signs of having once been arranged in regular strata. Other rocks exhibit a peculiar fissile structure, which causes them to split along planes which are more or less oblique to the planes of bedding. It is natural, therefore, to conclude that such rocks have undergone a considerable amount of alteration since the period of their original formation, and that new structures have been superinduced upon that of stratification. This kind of alteration is called Metamorphism, and the rocks so affected are called Metamorphic rocks.