SOIL OK SURFACE-MATTER. 225 



Boulder-clay drift has in general been formed out 

 of the denudation of various rocks ; and we find fertile 

 soils more often over it than in places where the 

 surface is due to the disintegration of only one kind 

 of rock. There are, however, exceptions to this rule ; 

 as some rocks, especially if belonging to the Kaino- 

 zoic epoch, may be so constituted that they form 

 excellent soils. Drift, moreover, may be formed 

 almost solely of one substance, such as clay or sand, 

 when it will weather very slowly ; or it may contain 

 subordinate layers or beds of an impervious character, 

 such as layers of bog iron-ore, or beds saturated 

 with other minerals, which will retard, or even pre- 

 vent, the growth of soil in places, or even over large 

 tracts. In many of the estuaries of South-east 

 Ireland a sandy marl has been deposited, which, if 

 exposed to atmospheric influences, would freely 

 weather into a rich soil ; this, however, in most 

 places is covered by a ferriferous clayey or sandy 

 deposit, that will not weather freely or support 

 vegetation. The good marl was evidently deposited 

 while the water was deep, all the iron brought 

 down in solution by the streams from the adjoining 

 country being carried out to sea ; but when the 

 estuaries began to fill up, and the muds to be ex- 

 posed twice daily, more or less of the water evapo- 

 rated, depositing the iron it contained. In the north 

 mud-lands of Wexford estuary there is a rich blue 



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