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



bright green, in well-defined layers. It contains very numerous 

 nodules or fragments, chiefly of white translucent quartz, varying 

 from the size of a pin's head to that of a horse-bean. These are 

 cemented by a calcareous, opaque, friable matrix. On the west bank, 

 below the bridge, this sandstone has, both imbedded in it and resting 

 on the naked gneiss, large boulders of the same gneiss, — some weigh- 

 ing many hundred weight. One of these Sir Charles Lyell found by 

 admeasurement to be 8 feet long. 



"When the water is low, an interesting fact is seen near the east 

 bank, 350 yards above the bridge. On the prolonged flank of a 

 mound of gneiss rest some horizontal layers of Potsdam sandstone. 

 They gradually thin ofi" as they ascend the mound and cease (see 

 fig. 1), but the superincumbent hmestone is continued, and, in im- 

 mediate contact with the gneiss mound, covers its little crest and its 

 opposite side. 



Fig. 2. — Section showing the superposition of beds of Potsdam sand- 

 stone and Trenton limestone on gneiss. River Montmorenci. 



a. Gneiss. h. Potsdam sandstone. e. Trenton limestone. 



The Potsdam sandstone at this place has not yet been found to 

 contain organic remains. 



The Trenton limestone, which reposes conformably on the Potsdam 

 sandstone, is amply displayed. It constitutes the whole mural gulley 

 at the Natural Steps, and lines both banks of the river either in slopes 

 or clifi's from thence to the brink of the great Falls. On the west 

 side of the brink the clifi" is 40 feet high, chiefly consisting of Trenton 

 limestone with the rocks beneath well exposed. 



The Trenton limestone at Montmorenci is fetid, moderately hard, 

 finely granular, black, bluish, or blackish brown. Certain thin 

 layers at every level are abruptly crystalline and pale brown ; others 

 again take on a quasi-conglomerate appearance, with rounded con- 

 cretions, from the size of an orange downwards. At irregular intervals, 

 a dark calcareous shale, from 6 to 18 inches thick, is intercalated; 

 and this, as we shall find, is soon, under the name of Utica slate, 

 wholly substituted for the Trenton rock. 



The sides of the clifi^ over which the Montmorenci plunges into 

 the River St. Lawrence show the manner in which the Trenton lime- 

 stone passes into the next and upper division of Silurian rocks, Utica 

 slate (represented in Wales by Llandeilo flags), and the Hudson 

 River group ; and it is thus : — on both sides of the chasm (the middle 

 being wholly of gneiss) the Trenton limestone becomes rapidly both 



