14 GEOLOGY OF SOIL. 



A little beyond this line is the boundary of the po- 

 tato, 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 bound- 

 ed, if its course is traced, by a very curved line. Is 

 this determined by geological causes, or do causes 

 purely physical erect a barrier to its farther north- 

 ward advance ? The answer will be found, in trac- 

 ing the temperature of the seasons of the different 

 places, through which the limit of the northern cul- 

 tivation of barley passes. It will be evident that 

 the line of this limit is isotheral, for the mean tem- 

 perature, Fahrenheit, is as follows : 



Latitude. Year. Winter. Summer. 



Ferae Isles, 61— 62° +45° +39° +51° 



W.Lapland, 70 +33*8 +21-2 +46-2 



Russia, at the mouth of the White Sea, 



66—68° +32 +10-2— 8-8 +46-3 



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

 annual or the winter temperature has little influence 

 on the barley limit, and that a mean summer temper- 

 ature from 46 to 47° is the only indispensable phys- 

 ical condition, to the cultivation of barley. On the 

 Atlantic islands, a mean temperature from 3 to 4° 



