_Conglomerate._--This is a consolidated mass of more or less water-worn and rounded stones. These stones may be of any size. When they are very large, the rock is called a _coarse conglomerate_; the finer varieties, in which the stones are small, are known as _pebbly conglomerates_. The ingredients of a conglomerate may consist of any kind of rock, or of a mixture of many different kinds. When they consist entirely of quartz, the rock becomes _quartzose_. The finer-grained conglomerates usually shew lines of deposition or bedding, but in some of the coarser sorts it is often difficult to detect any kind of arrangement. The stones are usually imbedded in a matrix of quartzose grit and sand, but sometimes this is very scanty. When the nature of the material which binds the stones together is very well marked, the rock becomes _ferruginous_, _calcareous_, _arenaceous_, or _argillaceous_, according as the binding or cementing material is _iron_, _lime_, _sand_, or _clay_. _Breccia_ is a rock in which the included fragments are _angular_. 18. _Sandstone_ is, as already remarked, merely consolidated sand. The coarser varieties, in which the grains are as large and larger than turnip-seeds, are termed _grit_. From these coarse varieties the rock passes insensibly, in one direction, into a fine or pebbly conglomerate, and in another into a rock, so fine-grained that a lens is needed to distinguish the component particles. Quartz is the prevailing ingredient--sometimes clear, at other times white. Frequently, however, the grains are coated with an oxide of iron, which gives the resulting rock a red colour. The other colours assumed by sandstone--such as yellow, brown, green, &c.--are also in like manner due to the presence of some compound of iron. When mica or felspar occurs plentifully, we have, in the one case, _micaceous sandstone_, and in the other _felspathic sandstone_. A sandstone in which the grains are cemented by carbonate of lime is said to be _calcareous_. _Freestone_ is a sandstone which can be worked freely in any direction. In most sandstones, the lines of bedding are distinct; when they are so numerous as to render the rock fissile, the sandstone is said to be _shaly_. _Shale_ is a more or less indurated fissile or laminated clay. When the rock becomes coarse by the admixture of sand, it gradually passes into a _shaly sandstone_. There are many other varieties of clay-rocks--such as _fire-clay_, _pipe-clay_, _marl_, _loam_, &c.--which are sufficiently familiar. 19. (_B._) EOLIAN or AËRIAL CLASS.--_Blown-sand_ is found at many places on sea-coasts. It generally forms smooth rounded hummocks, which are sometimes arranged in long lines parallel to the trend of the coast, as, for example, in the Tents Moor, near St Andrews. The _sand-hills_ of deserts also belong to this class.