THE SUCCESSION OF DEPOSITS. 33 



mantle of ice, though there must at certain periods have 

 been very extensive local glaciers on the Appalachians, the 

 Lauren tian axis and the mountainous regions of the 

 west.* The two latter have been named by Dr. G. M. 

 Dawson the " Laurentide " and " Cordilleran " glaciers 

 respectively. The former may be named the " Appala- 

 chian " glacier, and these three must have been the 

 principal sources of land ice in the height of the glacial 

 age, when large portions of the plains and valleys must 

 have been submerged. It does not, indeed, seem possible 

 that, under any conceivable meteorological conditions, an 

 area so extensive as that of Canada, if existing as a land 

 surface, should receive, except on its oceanic margins, a 

 sufficient amount of precipitation to produce a continental 

 giacier.-f- 



In the great Cordilleran ranges of the north-west the 

 changes evidenced in the east occurred in an exaggerated 

 form. The general character and probable complexity 

 of these changes may be seen from the following provi- 

 sional table taken from Dr. G. M. Dawson, and the evidence 

 for which will be found in his memoir on the " Physio- 

 graphical Geology of the Eocky Mountain Eegion of 

 Canada," | already referred to. 



* G. M. Dawson, Reports on British Columbia, and Superficial 

 Geology of British Columbia, Journal Geol. Society, 1878, Memoir 

 on Rocky Mountains, Trans. R. S. C, 1890. 



t The term "modified drift," sometimes used for the upper pleis- 

 tocene deposits, is objectionable. The gravels and sands of the Saxicava 

 sand are no more "modified" representatives of the lower beds than a 

 carboniferous sandstone or conglomerate is a modification of underlying 

 strata. The term has no proper significance, unless it could be shown 

 that the boulder clay is a deposit formed on land and subsequently 

 modified by aqueous action. 



t Trans. R. S., Canada, 1890. 

 4 



