The distinction between a rock and a mineral is not always an easy one for the beginner to grasp, yet it is essential that he should do so. A Rock is any extensive constituent of the earth's crust, which may consist, though rarely, of a single mineral, but in the great majority of cases is a mechanical mixture of two or more minerals. A rock thus has seldom a definite chemical composition, or homogeneous internal structure. An examination with the microscope almost always shows that a rock is an aggregate of distinct mineral particles, which may be all of one kind, or of many different kinds, in varying proportions. Rocks, then, are mechanical mixtures, and their properties vary in proportion to their various ingredients, while minerals are chemical compounds (see p. 6).

In ordinary speech the term rock is held to imply a certain degree of solidity and hardness, but in geological usage the word is not so restricted. Incoherent masses of sand and clay are regarded as being rocks, quite as much as the hardest granites.

The classification of rocks is a very difficult and obscure probfern, and would be so, even were our knowledge much more complete and exhaustive than it is. There are, therefore, great diversities in the various schemes of classification which have been proposed and which are still in use, and all such schemes require modification to meet continually advancing knowledge.

Bearing in mind the principle, already emphasized so often, that geology is primarily a historical study, the most logical scheme of classification is obviously one that, so far as possible, is genetic, that is to say, one which expresses in brief the history and mode of formation of the rocks. Other criteria, such as texture and chemical and mineralogical composition, must be employed for the minor subdivisions. On this genetic principle we may divide all rocks into three primary classes or groups.

A. Igneous Rocks, those which were melted and have solidified by cooling. Texture glassy or crystalline.

B. Sedimentary Rocks, those which have been laid down (most commonly) under water, by mechanical, chemical, and organic processes. Rocks composed of more or less pounded and worn fragments, seldom crystalline.

C. Metamorphic Rocks, those which have been profoundly changed from their original sedimentary or igneous character, often with the formation of new mineral compounds in them. Texture fragmental or crystalline.