PLEISTOCENE INTERGLACIAL PERIODS 357 



thought of as imits, though there is reason to believe that they were in- 

 terrupted by warmer interglacial periods. There is, however, much diver- 

 sity of opinion in regard to the importance of these interglacial episodes, 

 some geologists, such as Professor Wright and Mr Warren Upham in 

 America, Mr Lamplugh in England, and Doctor Geinitz in Germany, 

 minimizing their importance ; while others hold that in one or more inter- 

 glacial periods of the Pleistocene the ice-sheets were completely removed, 

 to be reformed later. 



The evidence for at least one interglacial period in northeastern Amer- 

 ica, with a complete disappearance of the ice and a climate several degrees 

 warmer than the present, seems to the present writer absolutely convinc- 

 ing. Those who wish to follow up the matter in detail will find the evi- 

 dence in various reports of a committee of the British Association*" on 

 the Toronto formation, and in a paper read by the present writer before 

 the Mexican Geological Congress.*^ The importance of the Toronto inter- 

 glacial formation is very well shown also in Chamberlin and Salisburj^s 

 Geology.*^ Here it is unnecessary to do more than call attention to the 

 more salient ^joints in connection with this interglacial period. 



The interglacial deposits began after the ice had retreated some hun- 

 dreds of miles, to a point far to the north of Toronto, and after streams 

 had cut channels through the boulder clay into the underlying rock. The 

 beds laid down on the eroded surface contain leaves and wood of a rich 

 forest, including an assemblage of trees which now grow 150 miles farther 

 south. There were also unios which are not now foimd in Ontario, but 

 inhal)it the Mississippi. A great river flowing from the north into the 

 rising waters of an interglacial lake Ontario built up a delta 180 feet 

 thick and 25 miles wide. The interglacial lake was drained and wide 

 river valleys were excavated to a depth of more than 150 feet. Finally 

 the ice covered the region again, burying the whole formation imder thick 

 sheets of boulder clay which extend hundreds of miles southwest of 

 Toronto. 



Interglacial beds with wood, peat, and brown coal are found on the 

 Hudson Bay slope also, showing that the surface was free from ice 400 

 miles north of Toronto. 



From the series of events just outlined it can hardly be doubted that 

 this interglacial period lasted for tens of thousands of years, during which 



« Canadian Pleistocene fauna and flora, 1898-1899 and 1900. 

 " Interglacial periods in Canada. 

 "Vol. iii, pp. 490-493. 



