LOWER SILURIAN. 



215 



The following section (Fig. 395 D) has been published by Logan, in illustration of the 

 fault in the vicinity of the Falls of Montmorency, just east of Quebec. It extends from 

 the Montmorency side of the St. Lawrence across the north channel and rtie upper end 

 of the island of Orleans. 



Fig. 395 D. 



Fis the faidt; 1, Archaean gneiss (Laurentian of Logan); 4 a, Trenton limestone over- 

 lying the Archaean ; -i b, Utica shale, and ± c, Hudson River shale ; 3, the Quebec group ; 

 S, S, the level of the sea; M, the position of Montmorency; C, the North Channel; 0, 

 Orleans Island. The horizontal and vertical scale is one inch to a mile. "The chan- 

 nel 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 precipice, is fifty feet thick and nearly horizontal ; at the foot of the preci- 

 pice, it lies against the gneiss, of nearly the same thickness, but dippping at an angle of 

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

 bec group and the beds of the Trenton and Hudson River groups are represented as 

 having been originally laid down in conformable strata, and as having been involved 

 together in the folding and faulting here illustrated. 



2. Evidence as to the time of the Epoch of Disturbance. — This 

 epoch is proved to have been between the Lower and Upper Silurian 

 eras, as Logan first observed, by the fact that unaltered and uncon- 

 formable Upper Silurian formations overlie in some places the up- 

 turned Lower Silurian beds. This is the case, as Logan states, near 

 Gaspe, on the Bay of St. Lawrence ; also near Montreal, on St. 

 Helen's Island and Beloeil Mountain, and at Becraft's Mountain, near 

 Hudson, west of the Hudson River, in each of which cases the Lower 

 Helderberg beds overlie unconformably Lower Silurian slates ; and 

 near Lake Memphremagog, where the Niagara limestone occurs with 

 its characteristic fossils, and also beds of Devonian corals. Again, on 

 the eastern side of the mountains, in the Connecticut valley, there are 

 unconformable Lower or Upper Helderberg beds at Bernardston, 

 Mass., and Littleton, N. H. It is therefore certain that the upturning 

 antedated the Lower Helderberg, and almost equally so that it closed 

 the Lower Silurian era. The earlier formations of the Upper Silurian 

 are scarcely represented, or are very thin, in the eastern part of New 

 York State, and this is apparently owing to the previous elevation of 

 the Green Mountain region. After this epoch, this region was part of 

 the solid continent, like the older Laurentian hills, yet of less elevation 

 than now above the ocean, and still undergoing some oscillations of 

 level. 



3. Other effects of the Disturbance. — Lake Champlain valley 



