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 



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