PEE-GLACIAL AND POST-CxLACIAL DESICCATION. 113 



ence, as affecting drainage, between simply-acting orographic forces and 

 complex ones.* 



Proofs of desiccation similar to those offered in the region of the Great 

 Basin are not wanting in any portion of the Cordilleras. But when we come 

 to examine in detail the surface geology of the country, we find that there 

 is some difficulty in drawing the line between the phenomena indicating 

 a diminution of the water surface as something now in progress, and those 

 which connect themselves with similar events belonging to a former geologi- 

 cal period. In other words, assuming for convenience the existence of a 

 distinctly marked Glacial period, we have to deal with a Pre-glacial as well 

 as a Post-glacial desiccation. All the facts to which reference has been made 

 above as being exhibited around the shores of Great Salt Lake and other 

 bodies of water in the Great Basin, we have no hesitation in referring to 

 the latest geological period, the Post-glacial ; and we even go further, and 

 unhesitatingly connect them with the present epoch, considering that the 

 drying-up is something now in progress. But, at the same time, a desicca- 

 tion which has been going on during the latest geological period, and is even 

 continuing at the present day, may have been begun considerably earlier ; 

 during the Pliocene epoch, or even before that. That this is the case 

 throughout the whole region of the Cordilleras, there is abundant evidence ; 

 and even a limited investigation of the geology of the North American 

 continent is sufficient to convince the observer that in seeking for proofs of 

 a diminution of water-area throughout the countrv, we have to do with a 

 phenomenon which belongs to the past as well as to the present. Some pre- 

 liminary observations will be necessary, however, before proceeding further 

 in this direction. 



There are evidently two quite distinct phases in connection with the dimi- 

 nution of water area on the earth's surface ; one Ave may designate as oro- 

 graphic, the other as purely climatological. But it must be borne in mind 

 that every orographic change is liable to bring about climatic ones. This 

 point will come up for discussion further on ; at present it is only necessary 

 to remember that every new aspect of the earth's surface, whether area or 

 altitude of the land-masses be thereby affected, is attended by some new 

 features of wind or ocean currents, resulting in corresponding changes of 

 climate in the regions concerned. By the orographic phase of the desicca- 

 tion process we mean, then, to indicate a local change of condition, brought 



* See ante, p. 13. 



