GEOLOGY OF SOIL. 15 



higher is necessary, which compensates for excessive 

 humidity. It is remarkable, that all the cereals have 

 failed in Iceland, though its mean temperature is 

 above that necessary for barley. Nor is this owing 

 to its geological structure. In that, it agrees with 

 the fertile shores of the Mediterranean. It is vol- 

 canic. So far as nitrogen, and carbonic acid, and 

 ammonia, may be supposed, to be evolved from the 

 earth, and to contribute to the growth of grain, Ice- 

 land should equal fertile Italy. But such is not the 

 fact, and it goes to prove that rocks affect very little 

 the crops grown over them, even when the great 

 physical element, temperature, is as high as is neces- 

 sary. That grains fail in Iceland, is due to the exces- 

 sively tempestuous rains with which that country is 

 visited. If then, the limits of barley are defined by 

 an isotheral line of 46 1-2° in Europe, that will also 

 limit its cultivation in America. So far as observa- 

 tion has extended, this is true, and the line of bound- 

 ary is equally curved, and winding. If a similar 

 table for the limits of wheat is constructed, by 

 drawing a line through the most northern places, 

 where this grain has been cultivated, the physical 

 conditions, essential to its cultivation, will be found 

 as follows : 



