LOWER SILURIAN. 



227 



which made shales, fine and coarse, to succeed, and alternate in 

 the interior with other limestones. But the changes of level in 

 these oscillations, although so great along the Appalachian region, 

 were but gentle movements in the thin crust of the globe ; and 

 they may have been partly oceanic as well as continental ; for the 

 water-level along the continents would sink whether there were a 

 downward movement or sinking in the bed of the ocean, or an 

 upward bending of the land. But after the Hudson period there 

 were greater changes, attended with violence ; and the rocks and 

 country bear marks of that violence, — probably more than have 

 yet been distinguished. 



In British America, near Graspe, on the Bay of St. Lawrence, 

 according to Logan, the Lower Silurian lies in tilted strata beneath 

 beds of the Upper Silurian, — showing that an upturning had oc- 

 curred before these superior beds were formed. Similar facts have 

 been observed at the eastern base of the Green Mountains, where 

 limestones of Upper Silurian and Devonian age rest uneonform- 

 ably on the altered strata of the Quebec group ; and at Montreal, 

 where the Lower Helderberg overlies unconformably the Hudson 

 beds. The origin of the Champlain valley has been referred by 

 Logan to the epoch closing the Lower Silurian. 



The following section (fig. 356) has been published by Logan, in illustration 

 of the fault, in the Appalachian series, in the vicinity of the Falls of Mont- 

 morency, just east of Quebec. It extends from the Montmorency side of the 

 St. Lawrence across the north channel and the upper end of the island of 

 Orleans. 



Fig. 356. 



F is the fault; 1, Azoic gneiss (Laurentian of Logan); 3, Trenton limestone 

 overlying the Azoic: 4«, Iltica shale, and 4 6, Hudson River shale; 2, the Que- 

 bec group; S, S, the level of the sea; M, the position of Montmorency; C, the 

 Xorth Channel ; 0, Orleans Island. The horizontal and vertical scale is one 

 inch to a mile. " The channel of the Montmorency is cut through the black 

 beds of the Trenton formation to the Laurentian gneiss on which they rest, 

 and the water, at and below the bridge, flows down and across the gneiss, and 

 leaps at one bound to the foot of the precipice, which immediately behind the 

 water is composed of this rock." The Trenton limestone at the top of the pre- 

 cipice is fifty feet thick and nearly horizontal ; at the foot of the precipice it 

 lies against the gneiss of nearly the same thickness, but dipping at an angle 

 of 57°, and is overlaid by shales with some sandstone of the Utica formation. 



