PHYSIOGRAPHIC HISTORY OF NORTHEASTERN ONTARIO 209 



which it derived most, if not all, its pebbles and bowlders. This 

 basement is in part composed of schistose volcanics and in part of 

 gray granite. Its extent, away from the Dore series, has not been 

 worked out and will be very difficult to determine; but there is 

 quite enough to indicate that a third granite, older than the Killar- 

 ney and the pre-Huronian granites, exists, and had been laid bare 

 in large volume in pre-Dorean time.' Was this pre-Dorean granite 

 indicative, Kke its successors, of a third mountain-building activity 

 that took place in a time too remote for the mind to compre- 

 hend? 



Regarding the physiographic conditions governing the deposi- 

 tion of the Dore series the authors of the report mentioned specu- 

 late as follows:^ 



The writers are inclined to visualize Michipicoten district at the commence- 

 ment of Dorean time as a region of rugged relief and waning volcanic activity. 

 During early Dorean time this land surface, devoid of vegetation, was eroded 

 rapidly, some of the deep-seated granite masses being uncovered to provide 

 the great quantities of granite pebbles and bowlders found in the Dore con- 

 glomerate. The rude granite and porphyry conglomerate and associated 

 formations in the lower part of the series were formed nearly in place by rock 

 disintegration, with a consequent aggradation, or wearing down of summits 

 and filling in of depressions in the surface of the country. 



SUMMARY OF PHYSIOGRAPHIC HISTORY 



1. The Dore series, the earliest known Precambrian sediment 

 in northeastern Ontario, was deposited as a continental formation 

 upon a surface of rugged topography. An earlier period of 

 mountain-building and erosion is suggested by the presence, sub- 

 jacent to the Dore conglomerate, of an older granite mass, appar- 

 ently of large dimensions. 



2. From Dorean time until the end of Timiskaming time the 

 region was apparently a land area of considerable, if not high rehef, 

 and the seat of prolonged volcanic activity. 



3. A period of mountain-building (pre-Huronian) and complete 

 reduction to a peneplain followed, affecting the whole region. A 



'W. H. Collins and T. T. Quirke, Geol. Surv., Can., Memoir in course of 

 preparation. 



^ Loc. cit. 



