1917.] NATURAL SCIENCES OF PHILADELPHIA. 49 



climatic factors, chief of which are precipitation and temperature, 

 evaporation being an expression of the relations of temperature to 

 humidity. Even within the short period of occupation by the white 

 race, notable changes in the area of Great Salt Lake and other lakes 

 of the region have been recorded, but these changes have been 

 insignificant when compared with those shown by the geological 

 record. In Pleistocene time there were at least two periods of 

 enormous expansion and contraction. During these expansions the 

 Oquirrh and some of the other mountain ranges were peninsulas 

 projecting far out into the lake, while the southern part of Promontory 

 Range and many other small ranges were islands. Thus during 

 the periods of great extension the direct intercommunication of the 

 land-snail faunas of the several mountain ranges was either partialh- 

 or wholly prevented by the intervening waters. During the recession 

 periods between the major expansions, as well as during the long 

 periods before the first expansion and after the last contraction, 

 many of these mountain areas were nearly as well insulated by wide 

 expanses of territory apparently quite unfavorable to land snails, 

 especially the larger snails of the genus Oreohelix. It is well to note, 

 however, that as work upon this genus progresses, flourishing colonies 

 of the living snails are found from time to time in unexpected places, 

 where the cover does not seem at all good. It should also be kept 

 in mind that very few of the isolated mountain ranges have been 

 explored at all, and none of them thoroughly explored, by concholo- 

 gists. Hence the last word on this phase of the subject cannot be 

 written, even aside from the possibility of the accidental trans- 

 portation of snails by birds and other agencies. Turning now to the 

 fresh-w^ater mollusks, direct intercommunication of the faunas of 

 such streams as the Bear River, flowing into Great Salt Lake from 

 the north, and the Jordan, entering from the south, has been pre- 

 vented ever since the last contraction of Lake Bonneville by the 

 extremely saline waters of the lake, which form a complete barrier, 

 as effectual as a land barrier. This salinity is due to the fact that 

 the lake is land-locked. The streams pour their waters, containing 

 salts dissolved from the soil and rocks, into the lake. There, in the 

 absence of an outlet, the loss is wholly by evaporation, which leaves 

 the salts in the water, thus maintaining a concentrated solution, 

 w^hile Utah Lake, Bear Lake and others with outlets remain fresh. 

 However, during what is known as the ''Lake Epoch," Lake Bonne- 

 ville once or twice reached such depth that it established an outlet 

 at the north end of the Cache Valley in Idaho, resulting in the 

 4 



