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_. 4. _Sandstone_ may at the outset be described as a _granular non-crystalline rock_. This rock shews every degree of coarseness, from a mass in which the constituent grains are nearly as large as turnip-seed, down to a stone so fine in the grain that we need a lens to discover what the particles are of which it is composed. When these latter are examined, they are found to exhibit marks of attrition, just like the stones of a conglomerate. Sharp edges have been worn off, and the grains rounded and rubbed; and whereas lines of deposition are often obscure, and of infrequent occurrence in conglomerate--in sandstone, on the contrary, they are usually well marked and often abundant. We can hardly doubt, therefore, that sandstone has also had an _aqueous_ origin, or in other words, that it has been formed and accumulated by the force of water in motion. In short, sandstone is merely compacted sand.