460 Geological Society : Mr. Logan on the St. Lawrence, 



water is still, it is immediately cased over. The first barrier completed 

 across the river below Montreal is usually formed about Christmas, at 

 the entrance of Lake St. Peter's, where the St. Lawrence is divided 

 into a multitude of channels by low alluvial islands. This barrier is 

 rapidly increased by extensive fields of drift-ice, enormous quantities 

 of which are heaped upon, or forced under, the stationary mass. The 

 space left for the water to flow being thus greatly diminished, a per- 

 ceptible rise in the river takes place, and by the time that the ice 

 becomes stationary at the foot of St. Mary's current, opposite Mont- 

 real, the waters in the harbour have usually risen several feet, and as 

 the packing rapidly proceeds, they soon attain the height of twenty, 

 and sometimes twenty-six feet, above the summer level. It is at 

 this period that the grandest glacial phenomena are presented. 

 In consequence of the packing and piling of the ice, as well as the 

 accumulation of the moistened snow of the season, and the freezing 

 of the whole into a solid body, sometimes more than twenty feet 

 thick, the water suddenly rises, and lifting a wide expanse of the en- 

 tire covering of the St. Lawrence, urges it forward with terrific vio- 

 lence, piling up the rended masses on the banks of the narrower 

 parts of the river to the height of forty or fifty feet. In front of 

 Montreal is a newly built revt-tement, the top of which is twenty- 

 three feet above the summer level of the river ; but the ice broken by 

 it, accumulates on the surmounting terrace, and before the wall was 

 erected the adjacent buildings were endangered, the ice sometimes 

 breaking in at the windows of the second floor, even 200 feet from 

 the margin of the river. In one instance, a warehouse of considerable 

 strength and magnitude, having been erected without due protection, 

 the great moving sheet of river-ice pushed it over as if it had been a 

 house of cards ; and in another case, where a similarly situated and 

 equally extensive warehouse, four or five stories high, had been provided 

 with a range of oaken piles placed at an angle of less than 45°, the drift 

 ice rose up the inclined plane, and after meeting the walls of the 

 building, fell back, and formed, in a few minutes, an enormous but pro- 

 tecting rampart. In some years the ice accumulated nearly as high 

 as the roof of the warehouse. 



Several of these grand glacial movements take place, sometimes 

 at intervals of many days, but occasionally of only a few hours, the 

 permanent setting being indicated by a longitudinal opening of con- 

 siderable extent in some part of St. Mary's current. This opening, 

 which is never afterwards frozen over, even when the temperature is 

 30° below zero of Fahrenheit, is due to the water having formed a free 

 subglacial as well as superficial passage, in consequence of its own 

 action and the cessation in the supply of drifting ice. From this 

 period the waters gradually subside, but seldom or never to their 

 summer level ; and when they have attained their minimum, the trough 

 of the St. Lawrence exhibits, Mr. Logan states, a glacial landscape 

 of undulating hills and valleys. 



On the banks of the river, near Montreal, is an immense accumu- 

 lation of boulders, chiefly of igneous rocks, the most abundant con- 

 sisting of syenite ; and multitudes of them are stated to be " tons in 



