Page:Popular Science Monthly Volume 1.djvu/583

Rh of old banks above the Rapids, that even the highest of these ancient banks did not contain a greater river than this which flows through the narrow gorge to-day.

We assume, then, from all the monuments the river has left of its own history, that the present rate of recession would be a fair measure of the past, except at the Whirlpool and Ferry Landing. Six inches a year, measured on the channel, would place the Falls at Lewiston 74,000 years ago. We have no means of knowing how long the quartzose sandstone, which forms the lowest part of the bank at the Whirlpool, would have arrested the cataract. This stratum is 25 feet thick, and, as its southward dip is 20 feet a mile, and the slope of the river-channel 15 feet a mile, the Falls would have to cut back through this rock more than half a mile. The halt may have been many thousand years. Add another period for the halt at the landing,



 No. 6, the Niagara limestone (6 also of Fig. 1); No. 7, the shaly limestone (marked 7 in Fig. 1). The Rapids have been formed by the erosion of this limestone. D, alluvial drift covering the eroded limestone. The section will show how great had been the denudation of the limestone by the river before the drift D accumulated, and before the river had found its present level.

and the age of the channel, from Lewiston to the Horseshoe, may not fall below 200,000 years. Unquestionably the channel has been excavated since the close of the glacial epoch, which science has well-nigh demonstrated occurred about 200,000 years ago. But this channel is only the last chapter in the history of Niagara.

Standing by the Whirlpool on the east, and looking over the river, we see a break in the ledges of rock which everywhere else form the bank. On the western side, around the bend of the Whirlpool, for a distance of 500 feet, bowlders and gravel take the place of ledges of rock. Many of these bowlders are granite and greenstone and gneiss, which have travelled hundreds of miles from the northeast. This mass of northern drift fills an old river-channel, which we can trace from the Whirlpool to the foot of the escarpment at St. David's—about two miles and a half. The reader will see by the map (Fig. 4) that this old channel marked 13 lies in a line with the present channel above the Whirlpool. The opening at St. David's is two miles wide. Here the Falls stood "in the beginning," wide, but not deep. They had cut back two miles and a half when the glacial period came, and lakes and rivers, and the great cataract, were buried under a colossal