64 



PRINCIPLES OF STRATIGRAPHY 



desert gets 14.4 cubic miles, making an average of 34,261,500 cubic 

 feet per square mile. The inland drainage areas of Eurasia aver- 

 age 25,125,040 cubic feet and those of Australia only 23,986,540 

 cubic feet per square mile. The total of these inland drainage areas 

 comprises about 11,486,350 square miles, giving an average of 

 28,729,760 cubic feet of rain per square mile of area. None of the 

 water falling there reaches the sea. Most or all of this amount is 

 returned to the atmosphere and often more, as shown by the shrink- 

 age of inland lake basins such as Great Salt Lake of LTtah. When 

 the annual rainfall is under 10 inches, deseut conditions will prevail, 

 and a "rainless" condition is assumed to exist. Thus the average 

 precipitation along the Mexican boundary of the lower Colorado 

 district is about 8 inches ; on the Yuma and Colorado deserts it is 

 only 2 or 3 inches. The total area of land thus affected at the 

 present time is about 12,200,000. square miles, and corresponds very 

 closely in extent and position to the inland drainage areas of the 

 world. These areas are distributed as follows : 



Continent 



Area in square 



miles with less 



than 10 inches of 



rainfall 



Percentage of 



total area of 



continent 



Australia and Tasmania 



Asia 



Africa 



North America 



South America 



Europe 



Antarctica 



Total 



1,196,250 

 5,892,250 

 3,221,000 

 1,432,400 

 333,050 

 139,350 



12,215,300 



39-8 

 36.3 

 28.2 

 17.0 

 4-7 

 3-7 



21.2% total land 

 area * 



The highest rainfall is recorded in Asia, where 437,500 square 

 miles, or 2.7 per cent, of its total area, receive over 75 inches an- 

 nually (averaging 113 inches), and in South America, where 1,444,- 

 000 square miles, or 20.5 per cent, of its total area, receives 100 

 inches or over. The distribution of areas receiving over 75 inches 

 of rainfall annually is as follov/s: 



* Taking the total area of the land {see ante) as 148.8 million square kilo- 

 meters or 57,460,777 square miles- 



