34 The Soils of the British Isles 



with, the newer sands and friable sandstones. One feature 

 in which these old rocks generally differ from the newer 

 is that the soils they form are on the whole shallower, 

 owing to the greater resistance to disintegration of the 

 harder rocks, and this certainly affects the vegetation in 

 an important manner. But we are still very ignorant of 

 the effects of these soils upon plants, and a great deal of 

 work is required before we shall be in a position to formu- 

 late a scientific classification of soils in their relations to 

 vegetation. 



The problem is also complicated by the fact that the 

 older rocks are exclusively developed in the north and 

 west of the British Isles, where the climate differs de- 

 cidedly, as we have seen, from that of the south and east, 

 and we cannot as yet present a satisfactory analysis of 

 the factors due to climate as it affects the vegetation, not 

 only directly, but through the soils. The combination of 

 soils very poor in lime with high atmospheric humidity 

 undoubtedly leads to the marked acidity which is so char- 

 acteristic a feature of many of the soils of the north and 

 west and has a most important influence in determining 

 the vegetation. The natural drainage channels become 

 clogged with mosses, impeding free drainage and tending 

 to keep the soil constantly damp, while aeration is hin- 

 dered, and acid peaty humus accumulates. 



5. Calcareous soils, i.e. soils containing a fairly large 

 proportion of lime, are mainly derived from limestones, 

 calcareous sandstones, and highly calcareous marls^ They 

 are mostly light open soils, except in the case of the 

 heavier marls, but become sticky when wet, somewhat 

 like clay. The influence of lime on the soil is certainly 

 very great, though its effect upon vegetation and its 

 relation to other factors are still very obscure. 



' Marl is properly a mixture of olay and lime, but the term is some- 

 times loosely used in geology for ferruginous clays which may contain 

 ■very little lime, e.g. the Keuper marl and other Triassio marls. 



