204 Sir W, E, Logan on the Quebec Groupf Sfc, 



500 feet, but beyond this we are not yet sufficiently acquainted 

 with the Lower Silurian rocks to make any statement. 



From these facts however it would appear probable that during 

 the Potsdam period the older rocks which formed the coast of 

 the Lower Silurian sea extended under comparatively shallow 

 water south-eastwardly from the St. Lawrence and Ottawa to the 

 position of the fault which brings the Quebec group to the sur- 

 face between Gaspe and the Mohawk river, and south-westwardly 

 from a line between the Mohawk and Lake Superior as far as 

 Alabama. From this shallow area they descended quickly into 

 deep water all around, thus constituting a subaqueous pro- 

 montory from the so-called azoic rocks of the north-east, and 

 forming with them what Professor Dana I believe has termed the 

 nucleus of the North American continent. 



But though the volume of the Quebec group makes it apparent 

 that over the area occupied by it and the subjacent black shales, 

 there must during the Potsdam period have existed a deep sea, it 

 is yet to be remarked that many of the members both of the 

 lower and upper parts of the group have by no means the char- 

 acter of deep sea deposits. To obtain the conditions required for 

 the accumulation of the coarser members of the series, which 

 commence near the bottom of the group, it must be supposed 

 that about the beginning of the Calciferous period, a great conti- 

 nental elevation occurred, carrying the shallow water deposits of 

 the Potsdam high above the sea and bringing the area at the 

 base of the Quebec group comparatively near the surface. The 

 successive coarse deposits of the group indicate a subsequent 

 gradual subsidence at unequal intervals until the early shallow 

 water strata were again submerged, to be first partially covered 

 over by deposits of the Chazy formation, and then almost univer- 

 sally by those of the Birdseye, Black River and Trenton. 



In this way may be accounted for the break which occurs in 

 the succession of life between the Calciferous and Chazy, in the 

 development of the latter formation between the Allumette 

 Island and Montreal, as well as among the Mingan Islands ; and 

 the break in the succession of deposits between the Potsdam and 

 Birdseye at St. Ambroise, between the Laurentian and Birdseye 

 from the north shore of Lake Huron to Kingston, in the vicinity 

 of Bay St. Paul and of Murray Bay, and in Lake St. John on the 

 Saguenay, 



