" • GEOLOGY OF SOIL. 27 



of northern wheat cultivation, and this is an isotheral line 

 of 57.4 degrees. Yet it is found that there are places 

 where, as iu Russia, the mean of spring and autumn, both 

 depending on that of winter in part, are too low to allow 

 wheat to be raised under this line of 57.4 degrees. In 

 truth, the relation of climate to cultivation cannot be accu- 

 rately determined without observations on the mean tem- 

 perature of the days which elapse between sowing and 

 harvest, and to this point the philosophic farmer should 

 direct his attention. In our country, the isotheral line of 

 57.4 degrees, starting from Labrador, 51 degrees, and pass- 

 ing between Hudson's Bay and Lakes Superior and Huron, 

 50 degrees, then turns north and approaches 58 degrees. At 

 Cumberland House, 54 degrees north, Capt. Franklin 

 found fields of barley, wheat, Indian corn. When the line 

 approaches the Pacific Ocean, it turns more southerly to 

 compensate the increasing humidity. As the limits of barley 

 mark the boundary between the races of shepherds and 

 hunters and fishers, and thus presents itself in a moral 

 view, so the limit of wheat becomes interesting, from coin- 

 ciding in some parts with that of fruit trees, as apples and 

 pears, and also with that of the oak. The whole aspect, not 

 only of agriculture, but also of the orchard and forest, 

 changes at once on approaching the isotheral line of 57.4 

 degrees, the northern limit of wheat. It would be easy to 

 extend these remarks to rye, still the staple food of a large 

 part of the population of Europe, and to oats, little used for 

 food for man out of the " land o' cakes," yet growing in 

 Norway, as high as latitude 65 degrees. Each of these 

 grains has a distinct isotheral line parallel to that of wheat 

 and barley. Indian corn and the potato have each its 

 isotheral line. Turning to the equatorial limits of the grains 

 it will be found, that extreme heat arrests their cultivation. 



