CHEMICAL WORK OF SOLUTIONS. 459 



The average annual temperature near the Arctic Circle in northern 

 North America and northern Asia is about —15° C; and from this temperature 

 the average temperature varies to about — 18° C. in northern Greenland. In 

 the Temperate Zone the average temperature varies from about — 15° C. in 

 its northern part to about 22° C at its southern part. The Tropical Zone 

 varies in temperature from about 20° C. to about 27° C. In central North 

 America and central Asia the normal average temperature is about 5° C, 

 in southern North America and southern Asia it is from 15° to 20° C. 

 For the present purpose the average temperatures, rather than the changes 

 of temperature or the extremes of temperature, are the data of first 

 consequence; for the average action of solutions upon the rocks, extending 

 through years, is controlled by these average temperatures. In the arctic 

 and north temperate regions, during the time the temperature is below 0° C, 

 it is highly probable that the decomposition of the rocks practically ceases; 

 as the temperature rises above 0° C. decomposition begins, and this goes on 

 with increasing- speed in proportion as the temperature is high. Therefore, 

 in the above facts as to temperatures we have in large part the explanation 

 of the slowness of the decomposition of rocks in the arctic regions and 

 the rapidity of the decomposition in the tropical regions. 



It is to he noted that the activity of the solutions in the warm climates, 

 as compared with the cold climates is much greater than would be inferred 

 from the absolute temperatures. The temperature of northern Greenland 

 (—18° C.) is 255° absolute: that of the Tropics (27° C.) is 300° absolute. 

 Thus the ratios of the absolute temperatures between the extreme cold of 

 the arctic regions and the warmth of the Tropics is as 255 to 300, a 

 difference of less than one-fifth: but it is certain that the activity of the 

 solutions in the Tropics is manvfold greater than in the arctic regions. 



While the average temperature is the matter of most significance in this 

 connection, the changes of temperature have an important effect upon the 

 chemical decomposition of the rocks as well as upon the mechanical disin- 

 tegration. From the tables given above it appears that while the average 

 temperatures in the arctic regions are very low for certain seasons of the 

 year the temperature for a whole month may average as high as 10° C, 

 and during the day the temperature ma}* rise as high as 20° C At such 

 temperatures it is certain that the process of decomposition of the rocks 



