196 Kindle — Note on Bottom Currents in Lake Ontario. 



would compare with the assemblage observed in the bottom 

 current zone off Wolf Island. 



The presence in the deep waters of the great lakes of the 

 marine molluscan genus Mysis, and the marine fish Triglop- 

 sis thompsoni, has led some naturalists to infer a long period 

 after the lakes had been separated from the sea by elevation, 

 during which fresh water is assumed to " occupy entirely the 

 upper stratum, while the heavier sea water remained at the 

 bottom."* The activity at frequent intervals of powerful cur- 

 rents in the lower levels of the lake which has been pointed 

 out, would, however, make such a separation of fresh and salt 

 water for any considerable period very improbable. The read- 

 justment of such creatures from a salt to a fresh water environ- 

 ment has more probably been due to a long period of brackish 

 water conditions, during which the whole body of the lake 

 water slowly approached freshness. 



Geological Survey, Ottawa, Canada. 



*Dr. William Stimpson, On the deep-water fauna of Lake Michigan, Am. 

 Nat., vol. iv, p. 404, 1871. 



