Elements of agricultural chemistry and geology / [Jas. F.W. Johnston].

  • James Finlay Weir Johnston
Date:
1844
Catalogue details

Licence: Public Domain Mark

Credit: Elements of agricultural chemistry and geology / [Jas. F.W. Johnston]. Source: Wellcome Collection.

    72/248 (page 56)
    GENERAL COMPOSITION OF ROCKS, sufficient depth, we always come sooner or later to th? solid rock. In many places the rock actually reaches the surface, or rises in cliffs, hills, or ridges, far above it. The surface (or crust) of our globe, therefore, consists evei-ywhere of a more or less solid mass of rock, overlaid with a covering, generally thin, of loose materials. The upper or outer part of these loose materials forms the soil. The geologist has travelled over great part of the earth’s surface, has examined the nature of the rocks which everywhere repose beneath the soil, and has found them to be very unlike in appearance, in hardness, and in composition—in different countries and districts. In some places he has met with a sandstone, in other places a limestone, in others a slate or hardened rock of clav. But a careful comparison of all the kinds of rock he has observed, has led him to the general conclusion, that they are all either sandstones, limestones, or clays of diffe- rent degrees of hardness, or a mixture in different pro- portions of two or more of these kinds of matter. When the loose covering of earth is removed from the sui-face of any of these rocks, and this surface is left ex- posed, summer and winter, to the action of the winds and rains and frosts, it may be seen gradually to crumble away. Such is the case even with many of those which, on account of their greater hardness, are employed as build- ing-stones, and which, in the walls of houses, are kept ge- nerally dry; how much more with such as are less hard, or lie beneath a covering of moist earth, and are continu- ally exposed to the action of water. The natural crumb- ling of a naked rock thus gradually covers it with loose materials, in which seeds fix themselves and vegetate, and which eventually fonu a soil. The soil thus produced partakes necessarily of the chemical character and com- position of the rock on which it rests, and to the crumb- ling of which it owes its origin. If the rock be a sand-