GEOLOGY OF SOIL. 19 



in October. It is this line of equal winter temper- 

 ature, or rather that of the coollest months, which 

 allows the grains to be cultivated in many places 

 within the torrid zone, and the line of 68 to 70 de- 

 grees, which constitutes the tropical limits of wheat 

 culture, varies between 20 and 23 degrees latitude. 

 The other grains enduring from 5 to 7 degrees lower 

 temperature, are found in higher latitudes. 



27. The wide belt of our globe, comprised with- 

 in these limits, extending from 20 to 70 degrees 

 north latitude, presents every variety of geological 

 structure ; yet, nowhere, in all this space is the 

 quantity cr quality of crops affected, by the chem- 

 ical nature of the underlaying rocks. 



28. A similar principle governs the growth and 

 cultivation of the grain-bearing plants on mountains. 

 Their limits are found at heights, which correspond 

 to the latitude, which marks the isotheral line. In 

 the Swiss Alps, the grains cease growing at the fol- 

 lowing heights. 



Wheat at 3400 ft. corresponding to lat. 64 deg. 

 Oats " 3500 " " " 65 " 



Rye " 4600 " " " 67 " 



Barley " 4800 " " " 70 " 



This shows a beautiful correspondence between 

 latitude and altitude, and leads a step farther in 



