232 THE GEOLOGICAL HISTORY OF PLANTS. 



land molluscs whose shells have been found in the beds 

 containing the plants, and which are species still occur- 

 ring in Canada, perfectly coincide. 



The climate of that portion of Canada above water at 

 the time when these plants were imbedded may safely be 

 assumed to have been colder in summer than at present, 

 to an extent equal to about 5 of latitude, and this re- 

 frigeration may be assumed to correspond with the re- 

 quirements of the actual geographical changes implied. 

 In other words, if Canada was submerged until the 

 Ottawa valley was converted into an estuary inhabited by 

 species of Leda, and frequented by capelin, the diminu- 

 tion of the summer heat consequent on such depression 

 would be precisely suitable to the plants occurring in 

 these deposits, without assuming any other cause of 

 change of climate. 



I have arranged elsewhere the Post-pliocene deposits 

 of the central part of Canada, as consisting of, in ascend- 

 ing order : (1) The boulder clay ; (2) a deep-water de- 

 posit, the Leda clay ; and (3) a shallow-water deposit, the 

 Saxicava sand. But, although I have placed the boulder 

 clay in the lowest position, it must be observed that I do 

 not regard this as a continuous layer of equal age in all 

 places. On the contrary, though locally, as at Montreal, 

 under the Leda clay, it is in other places and at other 

 levels contemporaneous with or newer than that deposit, 

 which itself also locally contains boulders. 



At Green's Creek the plant-bearing nodules occur in 

 the lower part of the Leda clay, which contains a few 

 boulders, and is apparently in places overlaid by large 

 boulders, while no distinct boulder clay underlies it. 

 The circumstances which accumulated the thick bed of 

 boulder clay near Montreal were probably absent in the 

 Ottawa valley. In any case we must regard the deposits 

 of Green's Creek as coeval with the Leda clay of Montreal, 

 and with the period of the greatest abundance of Leda 



