224 FIELD GEOLOGY. 



ment of plants, but some, in excess, are injurious to 

 vegetable growth ; for instance, soils containing much 

 sulphate of iron are invariably unproductive. 



A base of gravel produces a light soil abounding in 

 silica that substance not unfrequently forming more 

 than four-fifths of its whole weight and which varies 

 from a fine sandy mould to a stony soil as the particles 

 of the gravel beneath are in size fine and uniform or 

 coarse and irregular. Sand makes a similar but some- 

 what finer soil. Clay gives rise to a stiff, heavy, and 

 sometimes tenacious soil, consisting mainly of alumina, 

 varying in quality and appearance perhaps more than 

 any other kind, but being generally more productive. 



The nature of the subsoil itself also bears to the 

 geology of a district a relation equally constant. The 

 subsoil of any locality situated on a formation of reason- 

 able thickness must partake of the nature of that for- 

 mation, whether it be Lime, Sand, Clay, or any other 

 rock. And this applies not merely regarding those 

 substances as generally understood, but also when they 

 occur in a compact state, as Lime stone, Sandstone, Shale, 

 &c. Therefore, the geology of a place being known, the 

 subsoils and soils are more or less understood. 



There are certain other natural causes which modify ? 

 to some extent the nature of soil ; the results of the 

 influence exerted by these causes being comparatively 

 small, but economically important. They are 1. The 

 rain-wash, which removes the particles of soil from 

 higher to lower ground. In a flair country the effects 

 of this action are not very perceptible; but where 

 the surface is broken by hills ' and small valleys, accu- 

 mulations, several feet thick, may often be seen. Also, 



