Soil-Makers 29 



course, penetrate the lava, but they can and do make 

 their way into every crack and cranny within reach, and 

 as they grow and swell they break up the rock into frag- 

 ments. 



As to what the plant hves upon in the absence of soil, 

 it must be remembered that often a very little mineral 

 food is enough for a plant, if only it is able to make the 

 most of what there is and has plenty of water; then we 

 must remember, too, that lava is especially rich in the 

 materials required by plants, and that water flowing over 

 it, or draining through it, would certainly dissolve some 

 of these materials and bring them within reach of the 

 roots, which would obtain them in this way quite as well 

 as from soil. 



But we have now to consider other means by which 

 nature prepares the soil. Hitherto we have confined our 

 attention to what is done with the rocks on the spot, the 

 soil being left where it is made; but this proceeding is 

 attended by certain disadvantages: the soil rarely attains 

 any great depth, for one thing, as the rock below is pro- 

 tected 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 required 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: 



