Shaler.] 176 [October 6, 



Notes on the Cause and Geological Value of Variation 

 in Rainfall. By Prof. N. S. Shaler. 



The rainfall of the earth's surface is the most variable of all its 

 conditions. Great as are the variations of temperature, they are rel- 

 atively less considerable than the variation in the amount of moisture 

 deposited. Jn the range from less than ten to over eight hundred 

 inches, which can be found in one region within a few hundred miles, 

 and in the considerable, though less striking, variation at many points 

 on the earth's surface, we have a rate of difference many times greater 

 than that which occurs in the range of temperature between the equa- 

 tor and the poles. There are two kinds of variation, both of geologi- 

 cal interest, but belonging to different categories of facts. The first 

 of these classes includes the variation due to change in the distribu- 

 tion of the rainfall of the earth, the quantity of that rainfall being 

 the same; the second class includes the variations which arise from 

 the differences in the amount of the total evaporation. The existence 

 of such variations may be regarded by some as a questionable matter; 

 the evidence, however, is great in quantity, and of the most distinct 

 kind, going to show that in the immediate past the rainfall of the 

 earth's surface was greater than now. On every continent, save 

 Europe and South America, we find closed basins, which show dis- 

 tinctly that there has been a gradual and progressive shrinkage of 

 the waters within the time that has elapsed since the end of the 

 rdacial period. I do not mean to maintain that the shrinkage of a 

 salt lake after its separation from the sea, is a necessary consequence 

 of a diminution of the rainfall. There are, at the present time, 

 many regions of the ocean where the supply from the clouds is not 

 sufficient to balance the evaporation. Many parts of the Mediter- 

 ranean, if closed by some barrier from the general ocean, Avould 

 begin rapidly to shrink into the state and dimensions of the Dead 

 Sea; but a lake separated from the ocean by a barrier, and shrinking 

 from a gradual abstraction of its water through evaporation, the 

 climate remaining the same, would probably retire slowly, and with 

 a certain steadiness from the time of its formation until it found 

 itself, so to say, balanced, the evaporation area just equaling the rain- 

 fall. But many, if not most, of these areas of excessive evaporations 

 cut off from tbe general supply, show us a series of terraces which 

 probably represent a succession of stages in the shrinkage when, for 



