OF NORTH AMERICA. 



73 



doubtless afforded ample havens for the mollusks which were driven 

 from the frozen territory. 3 



To adequately understand the rapidity with which the newly 

 opened territory was occupied by the Lymnaeas, it must be borne in 

 mind that these mollusks are able to withstand a very low temperature. 

 Many of the species today live in ice-cold water and may be collected 

 in ponds and streams when the surface of the water is frozen over. 

 They were thus enabled to follow the retreating ice very closely and 

 so to quickly occupy the new territory as it became available. This 



FIG. 4. 



Glacial Lake Iroquois showing Mohawk River outlet. (After Gilbert, U. S 

 Geol. Surv.) 



is clearly indicated by the discovery of the remains of Lymnaeas, as 

 well as of other boreal mollusks, in various inter-glacial deposits, as 

 at Cayuga Lake 1 and in the Don Valley at Toronto. 2 As there is 

 believed to have been a period between each ice sheet, during which 

 time there was a comparatively mild climate, it is to be supposed that 

 the Lymnaeas migrated into this territory, which they occupied at each 

 successive inter-glacial period. 



TERTIARY LAKES OF THE GREAT BASIN. Figures 2 and 5. 



During the Glacial Period the western area known as the Great 

 Basin again developed several large lakes, the largest known as Lake 

 Bonneville, attaining a maximum depth of 1000 feet and an area of 



3 See Chamberlain, Geol. of Wis., Vol. 1, plates 9 and 10. 

 1 Maury, Journ. Geol. XVI, p. 565, 1908. 



2 Coleman, Interglacial Periods In Canada, 1906. Also, Journal Geol. IX. p. 

 285, 1901; Amer. Geol., XIII, p. 85, 1894. 



