Page:VCH Cornwall 1.djvu/49

 GEOLOGY waste water from the china clay works. This deposit is being utilized for the manufacture of bricks and the coarser class of earthenware. The vast amount of material thus collected in our estuaries and spread out on the sea floor to mingle with the deposits derived from the disintegration of the coast, is the product of the denudation of our slopes by the action of rain. The wash of the soil from steep slopes to their base is familiar to the farmer, who periodically collects the accu- mulations of the lower ground and respreads them on the denuded land to preserve its fertility; operations which entail considerable toil. While we are apt to regard the functions of rain from a standpoint of irrigation, its mechanical action is largely lost sight of. In the extensive valley system into which Cornwall is broken up, such mechanical action has the fullest play. While the floors of our valleys are lined deeply with loam or clay, the converging slopes are more sparingly covered with a mantle of soil, among which stony fragments are abundantly inter- spersed. In extreme cases the sides of the valleys contain stretches of ground in which soil is altogether absent and the bare rock protrudes. It requires indeed little observation to discern a very close relationship between the depths of our soils and the surface configuration, our low- lying basins supporting the deepest deposits, while on the slopes which converge to them the depth of the underlying rock depends upon the angle of slope, so that very steep situations are barren in consequence of the entire absence of soil. This varying soil cap, due to the form of the ground, produces marked divergence in the agricultural value of the land, apart from the nature of the parent rock from which the soil has been derived. While the richness of the soil depends upon the ever-varying nature of the rock, the quantity of soil dependent upon physical situation is a factor of equal importance in its bearing on the fertility of the county. The perpetual creep of the soil to lower levels is well illustrated in some of our upland valleys, where alluvial basins have been so encroached upon by the downward creep that their ancient margins have been completely obliterated. The stream-tin valley of Porkellis in the parish of Wendron_is fringed with gentle slopes in which the granite is completely hidden by a soil-cap made up of its own dis- integration, and corresponding precisely with the granite alluvium which floors the valley ; and the one shades imperceptibly into the other. The high ground in Cornwall occupied by the granite presents frequent in- stances of basins in which the original flat is gradually being obliterated by the soil creep from the slopes. Some of the moors which so frequently occur at the head of our valleys have thus been formed. Largely composed of clay, the want of fall renders drainage difficult, and their bottoms are frequently lined with marshes. Being on this account unsuitable for cultivation they are given over to scrub and gorse and afford coverts for game, just as many of the rocky slopes in the granite districts consist of moorland stretches, clothed with gorse and bracken, the recesses of which form the congenial haunts of the fox. Large tracts on the seaboard of Cornwall owe their existence to the 5