426 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



tory reason why this rule, derived from the experience of the pea- 

 santry, should be depended on. But the explanation is extremely 

 simple. The opening is merely an indication that a free subglacial 

 passage has been made for itself by the water, through the com- 

 bined influence of erosion and temperature, the effect of which, 

 where the current is strongest, has been suflEicient to wear through 

 to the surface. The formation of this passage shows the cessation 

 of a supply of submerged ice, and a consequent security against any 

 further rise of the river to loosen its covering for any further move- 

 ment. The opening is thus a true mark of safety. It lasts the 

 whole winter, never freezing over even when the temperature of 

 the air reaches 30° below zero of Fahrenheit ; and from its first 

 appearance the waters of the inundation gradually subs^ide, escaping 

 through the channel of which it is the index. The waters seldom 

 or never however fall so low as to attain their summer level ; but 

 the subsidence is sufficiently great to demonstrate clearly the pro- 

 digious extent to which the ice has been packed, and to show that 

 over great occasional areas it has reached to the very bottom of the 

 river. For it will immediately occur to every one, that when the 

 mass rests on the bottom its height will not be diminished by the 

 subsidence of the water, and that as this proceeds, the ice, according 

 to the thickness which it has in various parts attained, will present 

 various elevations after it has found a resting-place beneath, until 

 just so much is left supported by the stream as is sufficient to permit 

 its free escape. When the subsidence has attained its maximum, 

 the trough of the St. Lawrence therefore exhibits a glacial landscape, 

 undulating into hills and valleys that run in various directions, and 

 while some of the principal mounds stand upon a base of 500 yards 

 in length, by a hundred or two in breadth, they present a height of 

 ten to fifteen feet above the level of those parts still supported on 

 the water. 



On the banks of the St. Lawrence, in the neighbourhood of 

 Montreal, there is an immense collection of boulders, chiefly from 

 rocks of igneous origin, and among them syenite greatly abounds. 

 They are of all sizes, but many are very large, and multitudes must 

 be tons in weight. From their appearance above the surface in 

 shallow parts of the river it is very probable the bed of it teems 

 with them also ; and it is remarked by the inhabitants that the posi- 

 tions of these boulders, both in the river and on the banks, fre- 

 quently appear changed after the removal of the ice in the spring. 

 I spent several days in the autumn of last year examining the 

 boulders along shore, all the way from Montreal to Lachine, a 

 distance of nine miles; and on again looking at them in the spring 

 I missed some which had particularly attracted my attention, but as 

 I had not mapped their positions I may inadvertently have passed 

 them over. But when we consider the manner in which the ice packs 

 and subsequently moves, it cannot fail to appear a very probable 

 agent in transporting these blocks. Closely jammed together down 

 to the very bottom of the river over such extensive areas as have 

 been mentioned, and there solidified by severe frosts around the 



