AN INTERGLACIAL FAUNA 567 



trary, two-thirds of the Unios in the Don valley beds withdrew per- 

 manently from Lake Ontario after the destruction of their colonies. 

 These forms required a milder climate and could only live in Ontario 

 during the warm interglacial period when, as Coleman has shown, 

 the climate of Toronto was so mild as to permit the growth of the 

 paw-paw and the osage orange. 



In addition to the similarity of faunas of the Don and Cayuga 

 valley deposits, there is a great similarity of levels. According to 

 Mr. Coleman the lowest of the Don valley beds was formed when the 

 water stood 19 feet above the present level of Lake Ontario. The 

 Cayuga valley deposit began to be formed with the water about 

 twenty feet above the present lake level. 



It is unfortunate that the plant remains in the Cayuga valley 

 interglacial bed are too decayed and fragmentary for identification, 

 but the lowest layers of the fossiliferous blue clay, like the lowest of 

 the Don valley beds, are formed of sheets of vegetable matter con- 

 sisting of twigs, fragments of leaves, and reeds. 



In conclusion we may say that (i) the Cayuga interglacial colony 

 was established by Mississippi and St. Lawrence molluscs which came 

 down from the north and west; (2) this would have required a very 

 considerable period of time after their widespread destruction in the 

 preceding ice invasions; (3) the colony could not have been estab- 

 lished during a slight oscillation northward of the ice (as might have 

 been the case if the species had come in from the Susquehanna 

 system) ; (4) more than one-half the molluscs found in the Cayuga 

 valley deposit are reported from the Don valley beds of the Toronto 

 formation; (5) the Cayuga and Don valley fossiliferous beds both 

 began to be formed when the water stood about twenty feet above the 

 present lake levels; (6) it is very probable that the Cayuga fossiliferous 

 deposit corresponds approximately in time with the Don valley, or 

 warm climate beds, of the Toronto Pleistocene formation which is 

 regarded as representing the Peorian, or fourth, interglacial period. 



