1866.] DAWSON — ON POST-PLIOCENE PLANTS. 75 



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 refrigeration may be assumed to correspond 

 with the requirements 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 Lcda, and fre- 

 quented by capelin, the diminution 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. 



III. Age or the deposits. 



I have arranged elsewhere the Post-pliocene deposits of the 

 central part of Canada, as consisting of, in ascending order ; (1) 

 The Boulder Clay ; (2) A deep-water deposit, 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 truncata, 

 the most exclusively Arctic shell of these deposits. In other 

 words I regard the plants above mentioned as probably belonging 

 to the period of greatest refrigeration of which we have any 

 evidence of course not including that mythical period of universal 

 incasement in ice, of which, as I have elsewhere endeavoured to 

 show, in so far as Canada is concerned, there is no evidence 

 whatever. 



The facts above stated in reference to Post-pliocene plants, 

 concur with all the other evidence I have been able to obtain, in 

 the conclusion that the refrigeration of Canada in the Post-pliocene 



