1875. ] 177 [Shaler. 



a time, a balance was attained. These terraces are said, among 

 other cases, to be prominent on the basin of the Great Salt Lake. 



Considering, for a moment, the case of this particular basin, we 

 notice that the shrinkage is marked by terraces, said to be so distinct 

 that I find it difficult to believe that they were formed before the 

 glacial period. If we accept, then, the opinion that they were 

 formed since the last ice time, we are driven to either of two conclu- 

 sions: that the basin was below the sea at the close of the o-lacial 

 period, or that it has had its water supply greatly diminished since 

 that time. It is manifest that the basin could not have been lowered 

 into the sea during, or since, the last glacial period ; so, granting the 

 shrinkage phenomena to be recent, we are driven to accept the 

 conclusion that terrace levels are due to a recent diminution of 

 rainfall. Much the same considerations will convince us that many 

 other of the closed lake-basins of the earth represent a shrinkage of 

 rainfall in their regions, a shrinkage go'ng on to the present day. 

 Evidences of diminished rainfall are not wanting in many regions 

 which have not been made into closed basins. The western shore of 

 South America seems to have felt the effect of this shrinkage since 

 the period of man. There are in Peru, for instance, evidences of 

 extensive cultivation in the shape of artificial terraces, where irriga- 

 tion is impossible, and where no crops could be grown with the 

 present rainfall. Similar and even stronger arguments could be 

 drawn from the well known facts given by the Caspian Sea and its 

 neighborhood. 



Without endeavoring, at present, to assemble all the evidence 

 which points to the diminution of rainfall, I propose now to consider 

 what are the forces which could bring about a change in the amount 

 of rainfall in any country. There are evidently two ways in which 

 the rainfall of a country may be modified: 1st, by the change in the 

 distribution of the total rainfall of the earth ; 2d, by the change in 

 the actual amount of that rainfall through the reduction or increase 

 of evaporation. It is evident that these two sets of causes may. and 

 doubtless do, cooperate and interact to a greater or less extent, but 

 this is a matter I do not intend to discuss. 



Considering the first of these categories of causes, we see abundant 

 evidence to show us that in the successive changes of level of the 

 land, bringing every point of its surface at various times at different 

 heights, we must have a most efficient cause of variation of rainfall 

 It may be safely said that a change of level of one hundred feet in 



PROCEEDINGS B. S. X. H. — VOL. XVIII. 12 JANUARY, 1876. 



