Rate of Niagara Lateral Erosion.—Wight. 141 
1st. That the Niagara limestone is continuous at the sur- 
face up to the falls, gradually increasing in thickness along 
the edge of the gorge. At the escarpment near Lewiston, this 
limestone is only twenty feet thick; one mile up the gorge, it 
is nearly fifty feet, and at the present cataract it is seventy or 
eighty feet thick. 
2d. The Niagara shale preserves throughout a pretty uni- 
form thickness of from sixty to seventy feet, and does not any- 
, where exhibit any very marked variations in its composition. 
3d. The Clinton limestone presents throughout the sec- 
tion a continuous and very compact stratum from fifteen to 
twenty feet in thickness. 
* 4th. Between the Clinton limestone and the quartzose 
Medina sandstone, the space is occupied by about seventy feet 
of material which is rapidly crumbling away where exposed, 
although there are in it two or three bands of limestone and 
sandstone. These, however, are not sufficiently compact to 
present any serious obstacle to the eroding forces. The lower 
part of it is especially friable. 
To get a basis for future comparison, we first measured 
from the outer base of the arch of the tunnel along the outside 
rail southward for a distance of 7,500 feet. We could not well 
mark the stations permanently; but, as the railroad is not 
likely to be shifted to any appreciable extent, or, if it is, there 
will be a record of it, future observers can find the exact points 
of our sections with little trouble by measuring again from 
our starting-point. 
At two places the opportunity was specially favorable for 
observation. As will be seen by referring to our section, the 
Medina and Clinton shales are accessible in an exposure be- 
tween 800 and 1,100 feet from the tunnel; while the Niagara 
shale is accessible between 6,000 and 7,000 feet from it. In 
the first of these places, we made sixteen profile sections, from 
which we can obtain, with considerable exactness, the rate at 
which the Clinton and Medina shales crumble away. At the 
second place we made six profile sections, which secure the 
same results for the Niagara shales. 
Section I. was made at 860 feet from the tunnel in the 
Clinton shale. Measuring from the outer rail, it was four- 
teen feet to the original foot of the cut, and twenty-eight feet 
