LOCAL GEOLOGY AND LOCAL DRIFT. 19 



8, Differences of soils arising from their geological origin. 

 Value of local geology and physical geography. 



But the relative proportions in which all the mineral consti- 

 tuents are found in a soil depend very much on the nature of 

 the rocks from which the soils have been formed, or upon the 

 physical and meteorological conditions of the district in which 

 it lies. 



1. In a limestone or chalk country, the soil maybe abundant 

 in lime. From 20 to 30 per cent of carbonate of lime is not a 

 rare proportion in such circumstances. In the neighbourhood 

 of magnesian limestones, (dolomites,) magnesia is usually plenti- 

 ful in the soil ; in red sandstone districts, and especially in the 

 valleys, common salt and gypsum ; and, among the green sand 

 rocks, phosphoric acid is sometimes found in comparatively large 

 proportion ; while in the lias, oolite, and coal countries, alumina 

 and stiff clays extensively prevail. 



2. An acquaintance with the local driftis of especial importance 

 in reference to the character and composition of the soil, and of 

 the waters which flow from, or which lodge in it. A geological 

 map tells us, through the eye, the general nature of the rocks 

 which lie immediately beneath the surface in a given district. 

 But, in very many cases, the actual surface is formed, not from 

 the underlying rocks themselves, but from the debris of other 

 rocks, perhaps at no great distance which debris has been 

 drifted from its place of origin, and spread over the surface of 

 the adjoining country in the direction of the current which car- 

 ried it, and has thus become the material out of which the 

 existing soil and subsoil have been formed. Thus chalk and other 

 drift overspreads a large part of Norfolk, forming the marls and 

 chalky clays by means of which the cultivated surface has in that 

 country been so much improved. The clays of Huntingdon 

 (Oxford clay) are in many places covered with the drift of the 

 chalk, the green sand, the lias clay, and the oolite sandstones, 

 especially in the direction of the prevailing valleys. And here, 

 while the soils in the bottoms are gravelly, and on the slopes freer 

 and more easily worked and drained, they are richer also in lime ; 

 and the accidental accumulations of chalk form in places deposits 



