84 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 23, 



Gneiss. — The gneiss is of the kiad prevailing in the Canadas. It is 

 reddish-grey and greyish-red, coarsely granular, rarely porphyritic ; the 

 quartz very abundant, mica scanty. It sphts into large leaves, and 

 is traversed irregularly by seams of white quartz. Now and then we 

 find a chloritic powder disseminated in it. It has a general trend N.E. 

 and S.W. We have only to deal with it in two places ; at Jeune or 

 Indian Lorette, eight miles, and at the Falls of Montmorenci, nine 

 miles from Quebec. It is here as outlying portions of the round- 

 topped mountains on the immediate north already referred to. 



Potsdam Sandstone. — The Potsdam sandstone* only occurs di- 

 stinctly, as far as is yet known, in this neighbourhood at the Falls of 

 Montmorenci, but at Jeune Lorette there are traces of it. 



The sandstone of Montmorenci has been unhesitatingly declared 

 to represent the Potsdam rock by the New York State geologists, as 

 well as by Sir Charles Lyell and INIr. Logan. With these high 

 authorities I have only to concur. Its geological position is that of 

 the Potsdam sandstone ; but its aspect and mineral condition are 

 very different from that of Lake Superior and of the Thousand 

 Islands near Kingston, Upper Canada. We know, however, that 

 this rock changes its constituents singularly even in the same basin. 

 It will be described when treating of the River Montmorenci. 



Trenton Limestone. — The important strata, which have been named 

 by American geologists Calciferous sandstone and Chazy limestone, 

 do not hereabouts succeed Potsdam sandstone, as might have been ex- 

 pected. The former is found fifty miles to the N.E., at St. Paul's 

 Bay (Logan). We find about Quebec Trenton limestone reposing 

 horizontally either on the Potsdam rock, or directly on gneiss. Its 

 position and fossil contents clearly proclaim its age ; which none 

 dispute. The description by Professor Emmons of the typical rock 

 in the State of New York is literally that of the horizontal limestone 

 of the north side of the St. Lawrence, — not only in the vicinity of 

 Quebec (Beauport), but for great distances above and below that 

 city ; and it is curious to observe, thirty miles and more below Quebec, 

 patches and shreds of this rock adhering to the naked river-face of 

 Cape Tourment, which dips into the St. Lawrence in shattered clifFs 

 of gneiss 700 and 800 feet high. 



Dr. Gerard Troost found the Trenton limestone of Montmorenci 

 to have nearly the same chemical composition as the aluminous lime- 

 stone of Quebec (Hudson River Group) ; but with less alumina and 

 carbon {vide infra, p. 91). The accidental minerals of this rock 

 are various common forms of calc-spar, purple and colourless fluor 

 spar, small rock-crystals, bitumen, both concrete and fluid, and 

 massive blende, — all, except the last, being in druses. 



The Trenton limestone is here confined to the north side of the 

 St. Lawrence. All the eight streams which flow from the adjacent 

 mountains into the St. Lawrence cut through more or less of this 

 rock. The various quarries in the intervals show that it is there the 

 uppermost rock. It is in bare ledges for a couple of miles together 



* The equivalent probably, according to Sir Roderick Murchison, of the 

 " Lingula beds," at the bottom of his Silurian series of Welsh rocks. 



