26 



GEOLOGY OF SOIL. 



bonic acid, and ammonia, may be supposed to be evolved 

 from the earth, and to contribute to the growth of grain, 

 Iceland 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 necessary. That grains fail in 

 Iceland, is due to the excessively tempestuous rains ^vith 

 which that country is visited. If then the limit of barley is 

 defined by an isotheral line of 46^ degrees in Europe, that 

 will also limit its cultivation in America. So far as obser- 

 vation has extended, this is true, and the line of boundary is 

 equally curved, and winding. If a similar table for the limit 

 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 : 



Latitude. 



Scotland, (Inverness) 68*^ 



Norway, (Drontheim) 64° 



Sweden, 62° 



St. Petersburgh, . . 60°25 



Mean Temperature, Fahrenheit, of the 



Year. 



-)-46o3 

 +39°5 

 +39°5 

 +38° 



Summer. 



+57°3 

 +59° 

 +59° 

 +60°8 



Winter. 

 +36°5 



+23^5 

 +2S05 

 +15°6 



North latitude 64 degrees appears, then, to be the utmost 

 limit of wheat. It is evident by inspection, that this is not 

 determined by the cold of winter ; for spring wheat would 

 not be affected by it ; and even if sown in autumn, in these 

 far northern regions, the seeds would be effectually pre- 

 served from the rigors of winter, by that thick mantle of 

 snow which becomes thicker and more lasting towards the 

 north. The temperature of the air exerts no influence on 

 seeds of plants buried under snow. Nor does the mean 

 temperature of the year exert any effect ; it is seen ranging 

 9 degrees, while the summer temperature varies only 3^ 

 degrees. The summer temperature alone defines the limit 



