252 THE HISTORY OF THE NIAGARA RIVER. 



It has already been stated that the rocks at the falls lie in level 

 lajers. The order of succession of the layers has much to do with the 

 nature of the cataract's work. Above all is a loose sheet of drift, but 

 this yields so readily to the wash of the water that we need pay no 

 attention to it at present. Under that is a bed of strong limestone. 

 This is called the Niagara limestone, and in thickness is 80 feet. 

 Beneath it is a shale, called the i^iagara shale, with a thickness of 50 

 feet; and then for 35 feet there is an alternation of limestone, shale, 

 and sandstone, known collectively as the Clinton group. This reaches 

 down very nearly to the water's edge. Beneath it and extending down- 

 ward for several hundred feet is a great bed of soft, sandy shale, inter- 

 rupted, so far as we know, by a single hard layer, a sandstone ledge, 

 varying in thickness from 10 to 20 feet. These are the Medina shales 

 and the Medina sandstone. The profile in the figure indicates that the 

 hard layers project as shelves or steps, and that the softer layers are 

 eaten back. I have been led so to draw them by considerations of anal- 

 ogy only, for underneath the center of the great cataract no observations 

 have been made. We only know that the river leaps from the upper 

 surface of the Niagara limestone and strikes upon the water of the pool. 

 The indicated depth of the pool, too, is a mere surmise, for in that com- 

 motion of waters direct observation is out of the question. But where 

 the United States Engineers were able to lower their plummet, a half a 

 mile away, a depth was discovered of nearly 200 feet, and I have 

 assumed that the cataract is scouring as deeply now as it scoured at 

 the time when that i)art of the gorge was dug. 



It is a matter of direct observation that from time to time large 

 blocks of the upper limestone fall away into the pool, and there seems 

 no escai)e from the inference that this occurs because the erosion of the 

 shale beneath deprives the limestone of its sui)port. Just how the shale 

 is eroded and what is the part played by the harder layers beneath 

 are questions in regard to which we are much in doubt. In the Cave 

 of the Winds, where one can pass beneath and behind one of the thin- 

 ner segments of the divided fall, the air is filled with spray and heavier 

 masses of water that perpetually dash against the shale, and though 

 their force in that place does not seem to be violent, it is possible that 

 their continual beating is the action that removes the shaly rock. The 

 shale is of the variety known as calcareous, and as its calcareous ele- 

 ment is soluble, it may be that solution plays its part in the work of 

 undermining. What goes on beneath the water of the pool must be 

 essentially different. The Niagara River carries no sediment, and there- 

 fore can not scour its channel in the manner of most rivers, but the 

 fragments of the limestone bed that fall into the pool must be moved 

 by the i)lunging water, else they would accumulate and impede its 

 work, and being moved, we can understand that they become power- 

 ful agents of excavation. Water plunging into a pool acquires a gyra- 

 tory motion, and, carrying detritus about with it, sometimes bores deep 



