TRANSACTIONS OF THE SECTIONS. 45 



On the Excavation of the Rocky Channels of Rivers by the Recession of their 

 Cataracts. By G. W. Featherstonhaugh, F.R.S., F.G.S. 



The author of this communication (now Her Majesty's Consul at Havre de Grace), 

 in travelling through North America, had noticed that at some points of the course of 

 all the great rivers there was either a cataract, or evidence of the former existence of 

 one, in rapids now obstructing navigation ; and on comparing the quantity of water in 

 the rivers now, with certain marks which appeared to indicate the quantity which 

 formerly flowed in their channels, he came to the conclusion that the volume of water 

 was formerly much greater than at present, and that such a state of things was neces- 

 sary for the excavation of their rocky channels, which he considers to have been 

 effected by the recession of their cataracts. 



In the case of the St. Lawrence and its tributaries, evidence to this effect is said by 

 Mr. P'eatherstonhaugh to be very complete. The isthmus separating lakes Huron and 

 Erie is a lacustrine deposit, containing everywhere decayed freshwater shells, and the 

 land which separates the Wisconsin (a tributary of the Mississippi) from Upper Fox 

 River, a tributary of Green Bay, which is an elbow of Lake Huron, is so little above the 

 general level of the country, that it is now passed over in boats in the flood season. It is 

 therefore inferred that when these alluvial plains and lacustrine deposits were under 

 water, there was free freshwater communication between the St. Lawrence and the 

 Mississippi. 



The author then proceeded to quote the Mississippi as another example illustrating 

 his views ; and stated that that river for several hundred miles of its course south of 

 the Falls of St. x\nthony, runs through a valley, from one to two and a half miles in 

 breadth, bounded by escarpments from 200 to 450 feet high. On looking down upon 

 this valley from the heights, it appears as if the whole had been originally the bed of 

 the river. 



It is however evident that the river channel could not have been eroded to its pre- 

 sent extent by the water that now runs through it ; and Mr. Featherstonhaugh there- 

 fore suggests, that the volume of the Mississippi, v.'hich accomplished the work, was 

 much greater formerly than at present. 



The author then illustrates two methods by which he considers that the rocky chan- 

 nels of rivers may have been excavated by the recession of their cataracts ; one he 

 denominates the molar, or grinding, and the other the subtracting, or undermining 

 process. In describing the effects produced by the first, he referred to a cataract near 

 600 feet in height, called Oonaykay-amah, or the white-water, situated in the Che- 

 rokee country, on the east flank of the Alleghanies, and not hitherto described by 

 travellers. The rock here is compact gneiss, and it appears that the rush of water 

 eddying in the accidental hollows of the surface excavates cavities or pot-holes, some 

 of them of very large size, one of which measured four feet in diameter and six feet deep. 

 In many instances the rock was observed to be almost filled with these hollows, which 

 at last coalesce, and become larger, several uniting in one, until at the season of floods 

 considerable masses are detached, and precipitated below by the cataract. Immense 

 masses of rock perforated and detached in this way were found at the bottom. 



It appeared to the author, that the gorge into which this cataract fell (a gorge se- 

 veral miles long, and near 600 feet deep) had been ground out of the solid rock in 

 this way ; and it was considered to add to the interest of the case that at one spot 

 there were indications ia a circular ledge of gneiss adjacent to the cataract, and worn 

 bare for a great distance from the top, that it had at one time plunged over this semi- 

 circular ledge, at a period when the volume of the water was immensely greater than 

 it is at present. 



Of the other process, that of undermining, the cataract of Niagara was adduced as 

 an instance. The Niagara river flows upon a bed of compact limestone, overlying a 

 friable shale upwards of seventy feet thick, and the sheet of water having fallen over 

 the edge of limestone, forms a sort of screen before the shale ; while behind this 

 screen, the constant moisture, the violent concussion, and the strong current of air 

 loosen and disintegrate the shale, which falls down and is washed away, leaving the 

 limestone without support. This process continues incessantly ; and the author, in a 

 paper published in 1831, showed that the gorge beyond the fall had been cut from 

 the heights of Queenstown to the point where it now is (a distance of seven miles), by 

 a recession, depending upon this alternation of hard and soft strata. The excavation, 



