^ GEOLOGY OF SOIL. 25 



"will probably define that which will limit grain cultivation 

 in America. 



Tracing a line through the points above named, it is the 

 northern boundary of all the cereals, or grains. A little 

 beyond this line is the boundary of the potato, and the belt 

 between the two is remarkable. It is the zone between 

 agriculture, and fishing, and hunting ; between races of men, 

 subsisting on animal and on vegetable diet, and those whose 

 chief food is animal. The northern cultivation of barley is 

 bounded, if its course be traced, by a very curved line. Is 

 this determined by geological causes, or do causes purely 

 physical erect a barrier to its further ncrthward advance? 

 The answer will be found in tracing the temperature of the 

 seasons of the different places, through which the limit of the 

 northern cultivation of barley passes. It will be evident 

 that the line of this limit is isotheral, for the mean tempera- 

 ture, Fahrenheit, is as follows: 



Casting the eye on this table, it is evident that the annual 

 and the winter temperature have little influence on the barley 

 limit, and that a mean summer temperature from 46*^ to 47° 

 is the only indispensable physical condition to the cultivation 

 of barley. On the Atlantic islands, a mean temperature 

 from 3 to 4 degrees 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 geologi- 

 cal structure. In that it agrees with the fertile shores of 

 the Mediterranean. It is volcanic. So far as nitrogen, car- 

 2 



m 



