LOGAN ON THE PACKING OF ICE IN THE ST. LAWRENCE. 431 



greatest diameter. From the shape of the mounds, it appeared that 

 the motion of the landslip must have been down the middle of the 

 chasm towards the river, and from the sides to the centre. 



A circumstance connected with the form of the area affected by 

 the slip appears singular. The ground moved constituted part of 

 the general plain of the neighbouring country ; but on all sides of 

 it, with the exception of the northern extremity, there was a depres- 

 sion in the surface, between which and the chasm produced, there 

 remained after the slip a narrow ridge at the original height, form- 

 ing a bounding rim to the cup which the chasm presented. The 

 depression on the east side was formed by the slope of the right bank 

 of the river and an offset or bay of lower land than the general plain 

 of the country, and on the west by a dingle furnished with a brook. 

 The rim was not many feet wide on the top, but its parallelism to 

 the depression was remarkable, and it was only broken through in 

 one place where a tributary dingle had joined the one on the west. 



It would not be very difficult to prove that there is scarcely any 

 other mode of satisfactorily accounting for the movement of this 

 mass of land than its pressure on an inclined surface, assisted by the 

 action of water on some bed below. The layers of the deposit itself 

 appeared all perfectly horizontal : the slip therefore could not have 

 been on one of them. But the dip of the underlying limestone, 

 wherever I could detect its appearance for miles around, was pre- 

 cisely in the direction of the slip, with an inclination of about 4°; 

 and although none of it was visible near the spot, I am persuaded it 

 could not be very deep below the bottom of the riven It is highly 

 probable that the surface of one of its beds presented the plain which 

 gave occasion to the launch. Supposing any boulders to exist at 

 the bottom of a deposit moved in the manner described, it is easy 

 to see that parallel grooves and a polish on ^surfaces of rock may 

 not in all cases be attributable to the agency of ice. 



The age of the deposit constituting the extensive plains on the 

 banks of the St. Lawrence is a question of great interest. Its ma- 

 rine origin is demonstrated by sixteen species of fossil shells collected 

 by Captain Bayfield in the neighbourhood of Quebec, and described 

 by Mr. Lyell (Geol. Trans, vol. vi. series 2), who appears inclined 

 to consider them of the newer pliocene period, though he does not 

 think the evidence sufficient to remove all doubt of a more modern 

 date. Unable to add anything to the evidence in respect to age, the 

 discovery in the neighbourhood of Montreal of four of the species 

 mentioned by Mr. Lyell, at a higher elevation than that of the Que- 

 bec fossils, puts it in my power to extend the probable range of the 

 deposit through Canada, and widen the boundaries of the ancient 

 sea of which it was the bottom. The greatest heiglit given for any 

 of the Quebec fossils is 300 feet above the Gulf of St. Lawrence. 

 But those of Montreal occur on the neighbouring mountain at an 

 elevation of 430 feet by barometrical measurement above the river 

 in the city harbour. This, as near as I have been able to ascertain, 

 is about 460 feet above the Atlantic ; and as the locality bears very 

 much the character of a raised beach, it may possibly mark the 



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