Soil-Makers 37 



rarely attains any great depth, for one thing, as the 

 rock below is protected more or less from frost ; and 

 then, again, generally speaking, one kind of rock alone 

 does not contain all that is necessary to make a really 

 fertile soil rich in all the various mineral matter re- 

 quired for luxuriant crops. If we look at those soils 

 which are acknowledged to be the richest in the world 

 we shall find that, as a rule, they have been much 

 mixed. We say, as a rule, because most of the lavas 

 are rich enough in the minerals which plants require, 

 and are also so well drained, thanks to the cracks and 

 fissures within, that they do form most productive soil 

 when simply crumbled down. 



With the granites, however, the case is very 

 different : they are poor in the necessary minerals to 

 begin with, and what they do possess is, as we have 

 seen, dissolved, and in great part washed away. 

 Granite slopes are poor and sandy, therefore, while 

 the clay deposited at their feet is too stiff and compact 

 to be fertile ; and nature seems to tell the farmer as 

 plainly as she can that in most cases he will not find it 

 worth his while to try and grow wheat either on the 

 hills or in the dales of a granite district. Of course, 

 where granite decays on the level, and its various 

 minerals remain, all crumbled down and mixed to- 

 gether, it is naturally more fertile than where the best 

 of them are washed away; and thus we find that 

 the granite soils of the Scilly Isles are far more pro- 

 ductive than those of the Scotch hills, and are capable 

 of bearing good crops of corn, in part, at least, because 

 less of the potash has been washed away from them. 

 But some thanks are also due to the more genial 

 climate, for on the granite highlands of Dartmoor 



