5. If it be easy to read the origin of conglomerate and sand in the external character of their ingredients, and the mode in which these have been arranged, we shall find it not less easy to discover the origin of _shale_. Shale is, like sandstone, a granular non-crystalline rock. The particles of which it is built up are usually too small to be distinguished without the aid of a lens, but when put under a sufficient magnifying power, they exhibit evident marks of attrition. In structure it differs widely from sandstone. In the latter rock the layers of deposition, though frequently numerous, are yet separated from each other by some considerable distance, it may be by a few inches or by many yards. But in shale the layers are so thin that we may split the rock into _laminæ_ or plates. Now we know that many sedimentary materials of recent origin, such as the silt of lakes, rivers, and estuaries, although when newly dug into they appear to be more or less homogeneous, and shew but few lines of deposition, yet when exposed to the action of the atmosphere and dried, they very often split up into layers exhibiting division planes as minute as any observable in shale. There is no reason to doubt, therefore, that shale is merely compacted silt and mud--the sediment deposited by water. It becomes evident, therefore, that conglomerate, sandstone, and shale are terms of one series. They are all equally sedimentary deposits, and thus, if we slightly modify our definition of conglomerate, we shall have a definition which will include the three rocks we have been considering. For they may all be described as _granular non-crystalline rocks, the constituent ingredients of which have been more or less rounded, and arranged in more or less distinct layers, by the action of water_. 6. The _Eolian_ or _Aërial_ group of rocks embraces all natural accumulations of organic or inorganic materials, which have been formed upon the land. The group is typically represented by _débris_, such as gathers on hill-slopes and at the base of cliffs, by the _sand-hills_ of deserts and maritime districts, and by _soil_. All these accumulations owe their origin to atmospheric agencies, as will be more particularly described in the sequel. As the _Sedimentary_ and _Eolian_ rocks are the results of the _mechanical_ action of water and the atmosphere, they are fitly arranged under one great class--the MECHANICALLY FORMED ROCKS.