FIELD GEOLOGY. 223 



is not by any means proportionate to its rainfall. The 

 water-bearing beds are great distributors, and by them 

 the supply is to a great extent equalised. The supply 

 to be obtained by boring down to deep-seated springs 

 is practically inexhaustible > being scarcely, if at all, 

 affected by drought, and these springs form the only 

 source on which can be placed a full reliance. 



SOILS. 



All soil or mould has been produced during the lapse 

 of many ages by the disintegration arising chiefly from 

 the action of rain, frost, and other atmospheric influences 

 of the exposed surface of the strata which form the 

 base or subsoil. It has been increased in depth by the 

 annual growth and decay of vegetable matter, assisted 

 by the apparently trifling, but still ceaseless, action of 

 earthworms, working into and turning up the subsoil. 

 It is thus evident that the nature of the soil of a district 

 must strictly depend on that of the subsoil from which 

 it is directly derived, and as the strata, or rocks, forming 

 the subsoil, present many varieties, they give rise to 

 subsoils and soils that differ in a corresponding degree. 



The composition of soil is very variable is greatly 

 dependent on that of the subsoil, by the decomposition 

 of which it has been formed and bears to the subsoil 

 a nearly constant relation. An average soil consists of 

 silica, alumina, lime, magnesia, oxide of iron, small 

 quantities of ammonia, carbonic acid, and alkaline and 

 earthy salts, with a portion of decaying organic matter. 

 The nature and fertility of the soil varies as these con- 

 stituents are present in a greater or less degree, all 

 being to some extent necessary for the proper develop- 



