2. _Rocks._--Every one knows that the crust of the earth is composed of very various substances, some of which are hard and crystalline in texture, like granite; others less indurated and non-crystalline, such as sandstone, chalk, shale, &c.; while yet others are more or less soft and incoherent masses, as gravel, sand, clay, peat, &c. Now, all these heterogeneous materials, whether they be hard or soft, compact or loose, granular or crystalline, are termed _rocks_. Blowing sand-dunes, alluvial silt and sand, and even peat, are, geologically speaking, rocks, just as much as basalt or any indurated building-stone. The variety of rocks is very great, but we do not study these long before we become aware that many kinds which present numerous contrasts in detail, yet possess certain characters in common. And this not only groups these diverse species together, but serves also to distinguish them from other species of rock, which in like manner are characterised by the presence of some prevalent generic feature or features.
_Classification of Rocks._--All the rocks that we know of are thus capable of being arranged under _five_ classes, as follows:
I. MECHANICALLY FORMED. II. CHEMICALLY FORMED. III. ORGANICALLY DERIVED. IV. METAMORPHIC. V. IGNEOUS.
3. The MECHANICALLY FORMED class comprises a considerable variety of rocks, all of which, however, come under only two subdivisions--namely, _Sedimentary_, and _Eolian_ or _Aërial_, the former being by far the more important. Of the _Sedimentary_ group, there are three rocks which may be taken as typical and representative--namely, _conglomerate_ or _puddingstone_, _sandstone_, and _shale_. A short examination of the nature of these will sufficiently explain why they come to be grouped together under one head. _Conglomerate_ consists of a mass of various-sized rounded stones cemented together; each stone has been well rubbed, and rolled, and rounded. It is quite obvious that the now solid rock must at one time have existed in a loose and unconsolidated state, like gravel and shingle. Nor can we resist the conclusion that the stones were at one time rolled about by the action of water--that being the only mode in which gravel-stones are shaped. Again, when we have an opportunity of examining any considerable vertical thickness of conglomerate, we shall frequently observe that the stones are arranged more or less definitely along certain lines. These, there can be no question, are _lines of deposition_--the rounded stones have evidently not been formed and accumulated all at once, but piled up gradually, layer upon layer. And since there is no force in nature, that we know of, save water in motion, that could so round and smooth stones, and spread them out in successive layers or beds, we may now amplify our definition of conglomerate, and describe it as a _compacted mass of stones which have been more or less rounded, and arranged in more or less distinct layers or beds, by the action of water_.