20 THE ROYAL SOCIETY OF CANADA 



above the lowland which adjoins it on the south. Since this feature 

 attains its greatest prominence in Grenville township to the north of 

 the town of Grenville it may be appropriately named for purpose of 

 reference the Grenville escarpment. Between Montebello and the^ 

 village of Fasset at the west end of this escarpment a group of promi- 

 nent hills lies along the margin of the plateau but the linear scarp 

 feature that characterizes the plateau border from Fasset eastward is 

 not well developed. At its eastern end, on the other hand, the 

 escarpment although only from 100 to 200 feet high maintains its 

 linearity to its termination a few miles west of St. Jerome. 



St. Jerome to Quebec From the town of St. Jerome eastward to 

 Quebec a distance of approximately 130 miles the highland border 

 exhibits the same indefinite relationships observed in most other 

 localities. It is true that the surface of the Pre-Cambrian highlands 

 is much more irregular than that of the Palaeozoic lowlands and that 

 these irregularities, when viewed from points in the Palaeozoic low- 

 lands at a distance from the highlands, appear as a continuous line of 

 elevation, but this is an optical illusion for the greater part of the 

 hills that appear to be in alignment in reality lie may miles within the 

 Laurentian plateau and have no relationship to the plateau margin. 



Discussion 



Factors Determining the Present Relationship of the Palœozoic 

 to the Pre-Cambrian 



It is evident from the general data previously cited that the Palae- 

 ozoic formations occurring in the lower Ottawa and St. Lawrence 

 valleys stand at an elevation ranging from several hundred to several 

 thousand feet below the elevation of the Pre-Cambrian highlands 

 that adjoin them on the north. Three hypotheses that might explain 

 these relationships of the Palaeozoic to the Pre-Cambrian suggest 

 themselves: (1) That the depression in which the Palaeozoic sediments 

 occur, is pre- Palaeozoic in age, or (2) that the Palaeozoic sediments 

 have been downwarped or downfolded, or (3) downfaulted into their 

 present position. 



If the Palaeozoic sediments occurring in the lower St. Lawrence 

 valley were deposited in a depression, as the first of these hypotheses 

 assumes, then successively younger formations would overlap one 

 another in the direction of the Laurentian highlands; and as regards 

 the older Palaeozoic formations the Potsdam, Beekmantown, and 

 Chazy, these relationships hold, for, as previously pointed out, these 

 formations are present in the lower Ottawa and St. Lawrence valleys 

 in the region extending from the Three Rivers district on the east to 



