PETROLOGY.[C]
MECHANICALLY FORMED ROCKS.
17. (A.) Sedimentary Class.—Three of the most commonly occurring rocks of this class have already been described, but a few details are added here.
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.