WITH AGRICULTURE AND PLANTING. 455 



table soil lie beneath strata of other matters ; and where 

 they occur in this position, the overlying strata are either 

 of volcanic or of alluvial origin. Of the former case, a 

 very remarkable example occurs in the Isle of Bourbon, 

 in which large tracts covered with vegetables and even 

 trees, have been laid waste and overwhelmed by streams 

 of lava ; and large rivers in their overflowings occasion- 

 ally leave deposits of various characters, over the produc- 

 tive soil containing remains of formerly existing plants. 



Productive soil, as well in regard to its situation as to 

 its constitution ^ depends upon the nature and condi- 

 tion of the rocks which form the solid mass of the 

 earth. It is always of secondary formation, compared 

 with the rock on which it rests, its principal parts usually 

 originating from the decomposition of this rock. While 

 the forms of the surface of the solid mass of the earth, 

 have much influence upon the action of the atmosphere, 

 they also in some degree modify that of climate. From 

 these circumstances it would appear that the solid sub- 

 strata of productive soil exert an influence in various 

 ways upon vegetables ; whence it follows that, in order 

 to obtain a more intimate knowledge of the conditions 

 which operate upon their existence, it is necessary to call 

 in geology to our assistance. 



Although the scientific study of agriculture has made 

 great progress in our times, the relations which exist be- 

 tween the constitution of the solid crust of the earth, 

 and the formation and nature of vegetable soil, present a 

 wide field for investigation. Geologists have hitherto 

 too much neglected the examination of the productive 

 covering of the earth, and those who have treated scien- 



