,3,3. GREAT LAKES. 119 



first sight to be any resemblance between the isolated lakes and seas 

 of western Asia and the Mediterranean ; but when we trace back 

 the geological history of the basin to early Pliocene times we meet 

 with quite different conditions. The commencement of the Pliocene 

 period was heralded in the Old World by the formation of brackish- 

 water lakes on a scale unknown at the present day. Congeria-heds — 

 so called from the numerous species of the brackish-water mollusc 

 Conger ia (or Dreissena) found in them — extend from Spain eastward 

 into Asia. They are the deposits of the large lakes which in early 

 Pliocene times seem to have occupied most of the Mediterranean 

 basin. The Mediterranean does not seem ever to have formed a 

 single great lake, for at various times during the Tertiary period 

 there was apparently a connection between Europe and Africa, not 

 only at the Strait of Gibraltar, but also across the ridge near Sicily 

 and Malta. 



If the movement which transformed the Miocene sea of the 

 Mediterranean basin into a chain of brackish-water lakes had 

 continued, and the barriers at the Strait of Gibraltar and the 

 Isthmus of Suez had remained unbroken, we should have expected 

 the lakes gradually to become smaller and smaller and more and 

 more salt, as long as the inflow was less than the evaporation. But 

 if the rainfall and inflow from rivers exceeded the evaporation, the 

 lakes must increase, and find outlet across the barrier. Thus the sea 

 would become gradually transformed into a series of freshwater lakes, 

 like those of Central Africa. As far, however, as we can judge, the 

 Mediterranean never reached this last stage, for after long ages of 

 brackish-water conditions, and the differentiation of a certain number 

 of peculiar species, the sea again broke in, and a copious marine 

 fauna re-conquered its ancient territory. Thus the Mediterranean 

 region is occupied by basins which may at any time easily be 

 transformed into large freshwater lakes, though, as it happens, the 

 sea has broken in on several occasions when the lakes were gradually 

 forming. The continuity of the process was thereby stopped before 

 the great sheets had become thoroughly fresh, and before a large 

 lacustrine fauna had appeared. 



Turning now to the second region of extinct lakes to which we 

 have alluded, we find in the Great Basin of North America a 

 district of which the geology has been worked out quite recently 

 by the Geological Survey of the United States. It would be impos- 

 sible in a few words to do justice to the valuable monographs by 

 Messrs. G. K. Gilbert and I. C. Russell, but their main conclusions 

 are these. An enormous area in Nevada, Utah, Oregon, and Cali- 

 fornia forms an enclosed basin, now occupied by deserts and by salt 

 lakes with no outlet towards the sea. The basin seems to have 

 originated in late Tertiary times, through irregular earth movement?^ 

 which raised certain tracts and depressed others. The region was 

 apparently altogether above the sea-level when the formation of the 



