QUATERNARY AGE. 263 



calcareous tufas were produced. In reference to this, King re- 

 marks: " The occurrence of such a tremendous formation of alka- 

 line carbonates, necessitates a very long period, during which the 

 surface of Lake Lahontan was some distance below its level of 

 outlet. To account for the existing p resence of the weak solutions 

 of the residual hikes, it is necessary, after the formation of gay-lus- 

 site and its pseudomorphism into thinolite, to suppose a flood-period 

 during which the lake had free drainage over its outlet, and which 

 continued long enough, practically, to wash out the saline contents 

 of the great lake." Now in a way somewhat similar, it is possible 

 that in immediately pre-Loess times, the great Quaternary lake of 

 Nebraska and western Iowa, may have become so reduced in vol- 

 ume by climatic change as to lo'se more by evaporation than by 

 overflow, and then through the interaction of other chemical 

 agents, precipitated its alkalies to the bottom. That some such 

 agency was here at work for a long time, is evident from the extent 

 and great thickness of these alkaline deposits. When finally this 

 condition of things was drawing to a close, the finer silicious de- 

 posits commenced to form, which shaded into the Loess or next 

 deposit above. As already observed, these transition beds can be 

 seen in the Republican Valley, and with still greater distinctness in 

 some of the small canyons in the region of the Loups, where often 

 it is impossible to tell with exactness where* the Loess or next de- 

 posit above begins. 



Resimie of Geological History between the Glacial and Loess Pe- 

 riods. — We have seen that the retreating ice sheet of the Glacial 

 Period left in its path huge beds of blue clay and other Drift ma- 

 terials, which in their upper portions were modified by water 

 agency. The land was flooded, and over the great lake or interior 

 sea thus formed icebergs floated and dropped their loads of sand, 

 gravel and boulders on the bottoms, and where they were stranded 

 left this debris in enormous heaps. This period of depression and 

 floods was followed by one of slow elevation, when the waters 

 were drained off and a new forest bed was formed to the shores of 

 the retreating lakes, or to the foot of the glacier mass. As the 

 period of glaciation was a time of great relative humidity, this 

 must also have been the character of the climate all through the 

 flood and Old Forest Bed period. The ice sheet again advanced 

 and destroyed these magnificent forests before it. Newberry, who 

 first directed attention to this Old Forest Bed, found no evidences 



