ON SOILS. 27 



That such disintegration is constantly going 

 on, is evident from many causes — and one 

 which is, perhaps, the most convincing of any 

 is, that the lavas in Sicily, which have been 

 thrown out from Mount Etna within a compa- 

 ratively recent period, are now found covered 

 with a fertile soil, which could only have arisen 

 from this source — and which is proved to be 

 derived from this cause by the chemical analy- 

 sis of the rock itself, which yields the sub- 

 stances found under other modifications in the 

 soil itself 



This fertility is owing to the alkalis which 

 are contained in the lava, and which, by expo- 

 sure to the combined action of the air and mois- 

 ture, are reduced to a state capable of being 

 absorbed by plants. 



A soil vvliich has been subjected to this influ- 

 ence for ages, and from which no crops have 

 been removed, will be enabled to support a 

 luxuriant vegetation for many seasons succes- 

 sively, without any supply of manure, simply 

 because the soil is full of the alkaline matter, 

 phosphates, and other ingredients necessary for 

 the growth of plants; but when under cultiva- 

 tion and when the crops are removed from it, 

 it must become gradually exhausted, and un- 

 less the alkaline property removed with the 

 crop is restored in the shape of manure to the 

 land, the soil will be eventually quite' unable to 

 support a crop and bring it to perfection. But 

 if when such exhaustion of the land has oc- 

 curred the soil is allowed to lie fallow or 

 cropped only with such plants as do not require 



