GEOLOGICAL SOCIETY OF LONDON. 133 
The author then endeavours to trace the series of changes which 
have taken place in the region of Lakes Erie and Ontario^ re- 
ferring, first_, to a period of emergence,, Vv^hen lines of escarp- 
ment, like that of Queenstown, and valleys like that of St. 
David's, were excavated; secondly, to a period of submergence, 
when those valleys, and w^hen the cavities of the present lake 
basins, were wholly or partially filled up with the marine boul- 
der formations ; 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. Ke also endea- 
vours to show how, during this last upheaval, the different lakes 
may have been formed in succession, 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 themselves near Queens- 
town, must have been of moderate height, and receded rapidly, 
because the lime overlying the Niagara shale was of slight thick- 
ness at its northern termination. On the further retreat of the 
sea^ a second fall would be established over lower beds of hard 
limestone, and 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 hard quartzose sandstone, which rests again on the soft 
red marl seen at the base of the river cliff at Lewistown. 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 noY^ seen descending a con- 
tinuation of the same rocks on the Genessee 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, 
by the increasing retardation of the highest cataract, in propor- 
tion 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 
