114 THE DESICCATION OF LATER GEOLOGICAL TIMES. 



about primarily by crust movements, in contradistinction to such as might 

 result from more general causes, of a nature to affect the whole earth's sur- 

 face. For instance, we might conceive of such an elevation of a part or the 

 whole of the plateau of Central Africa as to cause the draining off of a large 

 portion of the many thousand square miles of area at present covered by w.ater 

 in that region. Such a change of altitude of a considerable mass of land would 

 imdoubtedly produce climatic changes to some extent ; but the diminution 

 in the lake area would not have had primarily a climatological cause. If, on 

 the other hand, we conceive the surface of the land to remain at its present 

 height and to be otherwise unchanged, but that, for some reason, the rainfall 

 in the tropical regions is diminished, then evaporation would gain on precipi- 

 tation and the area covered by water would diminish permanently, instead of 

 varying temporarily, as it would othervyise do, in harmony with the ordinary 

 fluctuations of the seasons. 



Thus, then, if we find that there has been a great diminution in the area 

 covered by water on the western side of this continent during the later geo- 

 logical ages, we have to inquire how far this may have been due to an up- 

 heaval of the land, and how far to a purely climatological cause. This is by 

 no means an easy tasic, and it is tliis very fact which makes the kind of evi- 

 dence in regai'd to desiccation furnished by a region like that of the Great 

 Basin, or of Central Asia, as will be stated further on, of so much value, as 

 indicative of a change of climate independent of any orographic cause. 

 Bearing these considerations in inind, we may now proceed one step further 

 in the present investigation. 



The combined investigations of all the geological surveys carried on west 

 of the Mississippi during the past twenty years make it evident that thci'e 

 have been great changes in the relief of the surface in that region during the 

 later geological periods. The present writer, however, is clearlj- of opinion, 

 after a careful review of the whole body of published fticts and a personal 

 inspection of a considerable portion of the area in question, that there has 

 been no essential alteration in the configuration or topography of the western 

 side of the continent since the Glacial epoch, — that is, since the time when 

 the crests of the highest ranges were, to some extent, covered with snow and 

 ice. Therefore, no part of the desiccation proved to have taken place since 

 that time can be due to orographic changes; the phenomenon must have 

 been a climatological one. 



But during the Tertiary period vast areas of the region between the 



