Geology of the Valley of the St. Lawrence, 8fc. 317 



may have been produced in the ancient beds of these vast inland bodies 

 of fresh water, while the modern deposit and the subjacent Silurian stra- 

 ta may to the eye appear perfectly horizontal. 



The author then endeavors to trace the series of changes which have 

 taken place in the region of Lakes Erie and Ontario, referring first to a 

 period of emergence when lines of escarpment like that of Queenston, 

 and when valleys like that of St. David's were excavated ; secondly, to 

 a period of submergence when those valleys and when the cavities of 

 the present lake-basins were wholly or partially filled up with the marine 

 boulder formation ; and lastly, to the re-emergence of the land, during 

 which rise the ridges before alluded to were produced, and the boulder 

 formation partially denuded. He also endeavors to show, how during 

 this last upheaval the different lakes may have been formed in succes- 

 sion, and that a channel of the sea must first have occupied the original 

 valley of the Niagara, which was gradually converted into an estuary 

 and then a river. The great Falls, when they first displayed them- 

 selves near Queenston, must have been of moderate height, and re- 

 ceded rapidly, because the limestone overlying the Niagara shale was 

 of slight thickness at its northern termination. On the further retreat of 

 the sea a second fall would be established over lower beds of hard lime- 

 stone and sandstone previously protected by the water ; and finally, a 

 third fall would be caused over the ledge of hard quartzose sandstone 

 which rests on the soft red marl, seen at the base of the river-cliff at Lew- 

 iston. These several falls would each recede further back than the 

 other in proportion to the greater lapse of time during which the higher 

 rocks were exposed before the successive emergence of the lower ones. 

 Three falls of this kind are now seen descending, a continuation of the 

 same rocks on the Genesee River at Rochester. Their union, in the 

 case of the Niagara into a^single fall, may have been brought about in 

 the manner suggested by Mr. Hall, (Boston Journ. Nat. Hist., 1841,) 

 by the increasing retardation of the highest cataract in proportion as the 

 uppermost limestone thickened in its prolongation southwards, the lower 

 falls meanwhile continuing to recede at an undiminished pace, having 

 the same resistance to overcome as at first. 



Mr. Lyell considers the time occupied by the recession of the Falls 

 from the Whirlpool to be quite conjectural, but assigns a foot rather 

 than a yard a year as a more probable estimate ; thus he shows the 

 Mastodon, found on the right bank near Goat Island, though associated 

 with shells of recent species, to have claim to a very high antiquity, 

 since it was buried in fluviatile sediment before the Falls had receded 

 above the Whirlpool. 



