Sir W, E^ Logan on the Quebec Group, Sfc, 203 



This equivalency and the disturbance which brings the Quebec 

 group to the surface (the course of which disturbance has already 

 been given in the communication addressed to Mr. Barrande) 

 suggest the following considerations. 



From the occurrence of wind-mark and ripple-mark on closely 

 succeeding layers of the Potsdam sandstone where it rests imme- 

 diately upon the Laurentian series, we know that this arenaceous 

 portion of the formation must have been deposited immediately 

 contiguous to the coast of the ancient Silurian sea, where part of 

 it was in some places even exposed at the ebb of tide. No want 

 of conformity is known to exist between the Potsdam and Calci- 

 ferous formations, and the Quebec group being of Calciferous age 

 and 7000 feet thick, it follows that during the Potsdam period, 

 while the typical sands of the formation were on a level with the 

 surface of the sea, there must have existed a depth of water of at 

 least 7000 feet over the area on which were subsequently depo- 

 sited the strata of the Quebec group. 



As constituting the great metalliferous formation of the conti- 

 nent this group is traceable under various designations from 

 Gasp^ to Alabama, thence sweeping round on the west side of the 

 Mississipi through Kanzas to Lake Superior, without suffering 

 any diminution in its volume, thus forming the measure of a 

 deep sea in the course indicated, and probably still farther to the 

 Arctic ocean. Within this line northward in so far as Canada 

 is concerned, we find a marginal outcrop of these rocks of only a 

 few feet in thickness on the north coast of Lake Huron from 

 Lake Superior to Lacloche. Including the Potsdam sandstone 

 they are altogether absent between Lake Huron and the neigh- 

 bourhood of Kingston. In the area between the Laurentide and 

 Adirondac Mountains, from a line between Lake Ontario and the 

 Lac des Chats eastward to Lake Champlain on the one hand and 

 the St. Maurice on the other, the united thickness of the Potsdam, 

 Calciferous and Chazy formations scarcely attains 1000 feet- 

 With the exception of a small mass of the Potsdam sandstone at 

 St. Ambroise near Quebec, we have no evidence of a marginal 

 outcrop of the formation between the St. Maurice River and the 

 Mingan Islands, while a similar outcrop of the Calciferous and 

 Chazy formations has not been observed from the longitude of 

 Lake St. Peter to the same group of islands ; in these islands 

 themselves the thickness of the three formations does not exceed 



