278 GEOLOGY OF THE DEBTEE BASIN. 



area must have been tilted up f'om a nearly level position so as to produce 

 a relative difference of level between its eastern and western borders of 

 nearly 7,000 feet, and if this be admitted the conditions essential for a 

 slow-moving body of fluvio-laeustrine water in which the loess could have 

 been deposited, as suggested l>v (Jhamberlin, might have been fulfilled. 

 But previous to the loessial epoch, as has been already shown, there was a 

 time of more rapid erosion than that of the present day, which involved 

 a decided slope of the land; though with greatly increased volume of water, 

 under the then prevailing conditions of precipitation, this slope may not 

 have been so great as that which exists at the present day. It is conceivable 

 that with the immense freshets that may be assumed to have occurred 

 during the melting of the ice, enormous amounts of rolled gravels may 

 have swept down from the mountains, and when the slope of the stream 

 beds was nut sufficient to carry them forward the finer material may have 

 been spread out in a more or less continuous sheet over the adjoining 

 country. The deposit which, according to Hay, is so prevalent under the 

 loess of Kansas, constituting the water-bearing belt of that region, which 

 he denominates the Tertiary grit, might he perhaps contemporaneous with 

 the river drift. 



It were useless in the present state of knowledge to attempt to 

 speculate upon the age of the deposits that have just been considered, 

 relative to the supposed divisions of the (Uncial period. Gilbert's investi- 

 gations in the Great Basin have shown that two periods of great precipitation 

 prevailed during Pleistocene time, with an intermediate period of relative 

 aridity, which he assumes to have been more or less contemporaneous with 

 the Glacial period. The writer found evidence in the vicinity of Leadville 

 of two maximal extensions of the ice, separated by a warmer period, during 

 which it was partially melted and a great body of water was impounded 

 at the head of the Arkansas Valley. But it is by no means proved that 

 these maximal extensions corresponded with those of the continental ice 

 sheet. Still less is it possible to correlate either of them definitely with the 

 deposits described above, and this must be left for later investigation by 

 special students. 



