454 ON THE CONNECTION OF GEOLOGY 



becomes obvious, when we consider many facts ascertain- 

 ed in Agriculture and Forestry. If, therefore, as the poet 

 advises, our object be to determine what each particular 

 region can produce, and what it cannot, our attention 

 ought in the first place to be directed to the physical cir- 

 cumstances which exert their influence over vegetation. 



All plants that are the subject of cultivation are fixed 

 in the ground. By one of their parts, through which 

 they derive their principal nourishment, they penetrate 

 into the soil, which serves them as a basis, and affords 

 them the means of procuring subsistence ; by the other 

 part they raise themselves into the atmosphere, which is 

 not only necessary in itself for their existence, but is also 

 the medium through which they derive the warming and 

 vivifying influence of the solar rays. Hence we can un- 

 derstand how much the existence of plants must be in- 

 fluenced by differences in the condition of the soil and 

 air. 



The superficial crust of the globe is formed of soil ca- 

 pable of producing vegetables. This productive soil, 

 however, is not everywhere continuous, being inter- 

 rupted on the one hand by the watery covering of 

 the earth, and on the other by perennial snow and bare 

 rock. Where soil does occur, it separates the solid 

 mass of the earth from the atmosphere, and is the po- 

 rous medium through which the gaseous and watery 

 parts of the latter may act in a greater or less degree 

 upon the former. It is very seldom that strata of vege- 



of the work. They will, we think, be useful to students of agriculture and 

 geology, and interesting to the general reader. 



