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



extensive one which occurred on the Maskinonge in 1840 is described 

 in this part of Mr. Logan's paper. 



The waters of that river, after passing through a series of lakes, are 

 precipitated from the granitic region in a beautiful cascade, and then 

 flow along a deep valley in the terrace, the only interruption to their 

 course being a collection of boulders combined with a mill-dam, pro- 

 ducing a fall of about fifteen feet. The valley has a uniform breadth, 

 the distance between the summit of the banks being about 200 yards, 

 and the height of the banks is 120 feet. 



The point at which the landslip occurred is nine miles from the 

 granitic hills, and where the river, ten to twenty yards wide, changes 

 its direction from south to west, for 700 yards. The movement com- 

 menced about eight o'clock on the morning of the 4th of April 1840, 

 and when the winter snow was still on the ground. The mass of 

 marly clay, first detached, was about 200 yards in breadth and 700 in 

 length ; and it was followed, at intervals of a few minutes, by four 

 others. The whole of the area thus affected amounted to about 

 eighty-four acres, and the total length was 1300 yards; but the breadth 

 varied, the narrowest part being nearest to the river, and the widest, 

 equalling 600 yards, a considerable way from it. The moving mass 

 first crossed the stream, and then splitting against the opposite 

 bank, where it averaged a thickness of seventy-five feet, one-half 

 turned up the valley for about three-quarters of a mile, and the other 

 half down it for an equal distance, forming a dam half a league in 

 extent. The whole operation was completed in about three hours. 

 For some time after the movement began the surface of the great 

 masses remained unbroken, and the sugar maple-trees, with which 

 they were covered, preserved, for the greater part, an erect position. 

 Two farm-steads were also carried away, and though the people 

 escaped, the cattle, and other live stock perished with the falling 

 buildings. The masses which moved along the valley had a height 

 of about sixtv feet, and their surface was slightly raised, but the 

 front of each terminated in a blunt point which projected in the 

 middle and in the lower part. As these great double-acting plough- 

 shares advanced, they turned up, Mr. Logan says, the soft mud from 

 the bed of the river, casting it on the banks, and producing so into- 

 lerable a stench that no one could approach within 100 yards. This 

 odour, he conceives, arose from the sulphuretted hydrogen produced 

 by the decay of vegetable matter. 



No sooner was this dam formed than the waters of the Maskinonge' 

 began to rise, and the houses, with every other thing composed of 

 wood throughout the whole of the nine miles to the granitic hills, were 

 set afloat. It was two days, however, before the lake thus formed 

 overtopped the barrier; but by October it had transported into Lake 

 St. Peter so great a portion of the dibdcle that the river was not 

 more than ten feet above its ordinary level. 



When Mr. Logan examined, in the subsequent autumn, the spot 

 where the slip took place, the bottom of the widest part of the chasm 

 was thirty feet below the level of the surrounding country ; and about 

 400 yards from the river, where the disturbed district narrowed, there 



