PLEISTOCENE OF EUROPE, NORTH AFRICA, AND NORTH AMERICA 447 



quirtxl several hundred thousand years, and the recent times in which we 

 are hving may represent only the first thirtl of another interglacial interval. 

 He discusses the two opposing theories in regard to interglacial conditions, 

 namely (1) that the central ice sheet diminished and expanded according 

 to climatic changes, but never disappeared completely until the end of the 

 Wisconsin (or fifth period), (2) that the melting was complete so that the 

 successive sheets represent the work of separate glaciations. The writer 

 adopts the latter view. This is rendered probable by the fact that the rich 

 forest flora and the temperate insect fauna found in beds of the Toronto 

 Formation, corresponding perhaps to the Sangamon or third interglacial 

 interval, indicate that at this interglacial period, at least, eastern North 

 America was entirely free from glaciers and enjoyed climatic conditions 

 similar to the present. This interglacial period alone appears to have lasted 

 from 20,000 to 100,000 years. 



Loess. — It is a striking fact that the vast deposits of loess in North 

 America, as in Europe, belong to glacial times, although the loess extends 

 far south of the glacial drift and to the west of the Mississippi River. The 

 aquatic theory of origin has been replaced by the aeohan, according to which, 

 while the river floods poured dovm the materials of the loess these were 

 whipped up by the winds and redeposited on the adjacent uplands, being 

 held after deposition by vegetation. Thus the loess is found along the 

 sides of streams. The deposits seem to be related to periods of aridity. 



The thin glacial drift of Nebraska, rarely more than a foot or two in 

 thickness, belongs to the Kansan epoch. ^ The loess bluff deposit, or ' yel- 

 low clay ' of Nebraska, belonging probably to a later epoch than the ' Kan- 

 san, ' covers the eastern part of the state, averaging in thickness 100 feet, 

 distributed evenly over hills and hollows alike, and is thus of aerial 

 rather than aquatic origin. It contains remains of the mammoth E. co- 

 lumbi and numerous species of terrestrial molluscs. 



The geologic conditions in Kansas will be considered in connection with 

 the fauna (p. 461). 



Climate of the Great Mountain Basin. — The periodic changes of climate in 

 the mountain region are best recorded in the Great Basin (Fig. 214) between 

 the Wasatch and Sierra Nevada ranges which witnessed the rise and fall 

 of Lake Bonneville^ the 'greater Salt Lake' of Pleistocene times, and of 

 Lake Lahontan,^ which lay on the eastern slopes of the Sierras. These 

 fluctuations are of great importance because connected at one point with 

 the Equus Zone fauna of Silver Lake in the old Lahontan Lake terraces. 

 The climatic succession is as follows : 



' Barbour, E. H., Nebraska Geological Survey, Vol. I, 190.3. Report of the State Ge- 

 ologist. 



2 Gilbert, G. K., Lake Bonneville. U.S. Geol. Surv., Monogr., Vol. I, 1890. 



" King, C., Systematic Geology. U.S. Geol. Explor. 40(h parallel, Clarence King geologist 

 in charge, 1878. 



