THE DATE OF THE GLACIAL PERIOD. 607 



ice had melted back sufficiently to reopeii the natural drain- 

 age lines of the Red River Valley into Hudson Bay. Mr. 

 Upham confesses that beds of vegetal deposit which are 

 both underlaid and overlaid by till are very rarely found in 

 northern Minnesota. Still, he supposes some such are found, 

 and gives an exhaustive list of. instances.* The two which 

 he mentions as being in the area of Lake Agassiz are encount- 

 ered in digging wells, first, at Barnesville, Clay county, where 

 twelve feet of till was penetrated, then one foot of quicksand 

 " containing several sticks of tamarack up to eight inches in 

 diameter ; second, in Wilkin county, where the record is that 

 till occupied the first eight feet, then a layer of gray sand 

 one half an inch in thickness, then a much harder lower till 

 for eighteen feet, which was underlaid by sandy black mud 

 containing many snail-shells. But these two cases hardly 

 seem sufficient to establish the theory, while the correspond- 

 ing cases adduced by the Canadian geologists farther north 

 are not described with sufficient minuteness to render their 

 meaning unequivocal.! 



Another class of phenomena bearing on the questions of 

 the discontinuity and date of the great Ice age is to be found 

 in the inclosed lake-basins lying between the Rocky Mount- 

 ains and the Sierra Nevada, near the fortieth parallel. Nu- 

 merous salt lakes now occupy this region. But it is evident, 

 even upon hasty examination, that these are but insignificant 

 remnants of those which formerly occupied it. Great Salt 

 Lake is estimated to have contained at one period four hun- 

 dred times its present volume of water. The terraces mark- 

 ing its former limits are very distinctly visible, and are nine 

 hundred feet above its present level. Lake Mono has several 

 distinct terraces, the highest of which is six or seven hun- 

 dred feet above the present level. Pyramid and North Car- 

 son Lakes, in Nevada, are but the remnants of an immense 

 salt lake extending from the Oregon boundary to latitude 



* "Minnesota Geological Report for 1879," p. 48. 



f " Report of Progress, Geological Survey of Canada, 1882-84," p. 414, C. 



