. and on the Modern Deposits of its Valley. 463 



was a sudden additional descent of fifteen feet. From this point the 

 ground sloped gently to the water's edge. The only vestiges of the 

 original surface then visible were a few patches of grass, and occa- 

 sionally twenty or thirty yards of wooden fence, the superficies being 

 composed of parallel mounds three or four feet high, and which over 

 the central portion of the area ranged at right angles to the axis of 

 the slip, but along the sides conformably with the bounding outline. 



A circumstance connected with the form of the disturbed district 

 Mr. Logan considers worthy of attention. Around the whole of the 

 area, except the most northern extremity, there was, previous to the 

 slip, a depression of the surface, due on the eastern side to the slope 

 of the right bank of the river, and a tract of low land ; and on the 

 western side to a dingle traversed by a brook. After the slip, a ridge, 

 not many feet wide, remained between these lower surfaces and the 

 chasm, forming a marginal rim which was broken through at only 

 one point, where it was intersected by a dingle which united with the 

 one on the western side. 



The cause of the slip, Mr. Logan is of opinion, was pressure on an 

 inclined plain, assisted by water ; and though he was not able to de- 

 termine the nature of the subsoil, he is of opinion, from a. survey of 

 the surrounding country, that it consists of the Silurian limestone, 

 the dip of which, where visible, is in the direction of the slip. 



If boulders were at the bottom of a mass moved in the manner of 

 the Maskinonge slip, it is easy to see, Mr. Logan observes, that 

 parallel grooves and a polish on the surface of rocks may not, in all 

 cases, be due to the agency of ice. 



3. Marine Shells on Montreal Mountain. — After alluding to Mr. 

 Lyell's account of the fossil shells collected by Capt. Bayfield* in the 

 neighbourhood of Quebec, Mr. Logan proceeds to describe briefly the 

 circumstances under which four of the same species of mollusks were 

 found near Montreal. The spot from which they were principally pro- 

 cured is stated to bear very much the character of a raised beach, and 

 was determined barometrically to be 430 feet above the Montreal 

 harbour, or about 460 feet above the Atlantic, the greatest height 

 at which Captain Bayfield's specimens were found, being 300 feet 

 above the level of the Gulf. The above altitude of the Montreal 

 deposit is further stated to be 240 feet above the level of Lake On- 

 tario and 7o feet above the Falls of Niagara, but to fall short of 

 Lake Erie by about 1 05 feet. The four ascertained species obtained 

 by Mr. Logan are the following : — 

 ' 1. Saxicava rugosa, very abundant to the north of the road to the 

 C6te de Nieges, in a bed of coarse sand inclined conformably with 

 the side of the hill, and which has, above it, a layer of pebbles and 

 small boulders. The altitude of this position is 430 feet. The shell 

 occurs also, but not abundantly, above the village of St. Henry, on 

 the road to Lachine, on the top of an elevated terrace along the bank 

 of the St. Lawrence, and 120 feet above the river ; it has been also 

 obtained on the same terrace at Logan's Farm. 



* See Phil. Mag. S. 3. vol. xv. p. 399, and Geol. Trans., 2nd Series, 

 vol. vi. p. 135. 



