574 Geological Society: — 
never had anything more resistant than Till to act upon. From 
a given section, 5000 feet long, it has excavated 34 million cubic 
feet of Boulder-Clay, removing it from exposed banks 1600 feet long. 
Twelve years' erosion of a 500-foot length of a part of the trough 
of the stream under observation, and from banks 1000 feet long, 
gives a rate of 8450 cubic feet per annum. Therefore, the removal 
of 34 million cubic feet from the 5000-foot section would give a 
period of 2505 years. Considerations tending to lengthen the 
estimate are the former forestation of the area and the increased 
gradient in the artificial cut-off. Those tending to shorten the 
estimate are the present wider flood-plain, the time taken for forests 
to grow, and the probably greater former water-flow. 
The erosion of the Niagara Gorge began considerably later than 
that of Plum Creek, and probably dates from midway between the 
disappearance of the ice from Northern Ohio and from Quebec. If 
conditions have been uniform, the age of the Gorge would be 7000 
years. As the Niagara Limestone is thinner at the mouth of the 
Gorge, and the Clinton Limestone has dipped out of sight at the 
Whirlpool, there is nothing in the stratigraphy to indicate a slower 
recession in the past than in the present. Moreover, nearly one- 
third of the erosion has been accomplished by two pre-Glacial 
streams, one from the south and a smaller one from the north. 
Therefore, the author concludes with considerable confidence that 
the Gorge is less than 10,000 years old, and that the ice of the 
Glacial Epoch continued down to that time, to such an extent over 
the lower St. Lawrence Valley and Central New York that it 
obstructed the entire eastern drainage of the Great Lakes. 
There is nothing which would lead to a longer estimate of the 
time which has elapsed since the Kansan stage of the Glacial Epoch 
than that approved by Prof. Calvin of Iowa, and agreed to by Prof. 
Winchell. These assume 8000 years as the limit for post-Glacial 
time, and that a multiple of this by 20, amounting to 160,000, 
would carry us back to Kansan time. This, however, would still 
leave as long a period still earlier, for the advance of the ice. The 
author's impression is that the whole epoch may well have been 
compassed within 200,000 years. 
2. ' On the Application of Quantitative Methods to the Study of 
the Structure and History of Rocks.' By Henrv Clifton Sorby, 
LL.D., E.R.S., E.L.S., F.G.S. 
The knowledge of the final velocities of material subsiding in 
water is of fundamental importance ; but the relation between size 
of particles and velocity is complex, and perhaps may be partly 
explained by a thin, adherent film of water. The angle of rest 
in the case of sand-grains of varying size and quality enables us to 
ascertain approximately the velocity of current necessary to keep 
such sand drifting, and that needed to move it when at rest. The 
comparison of this angle with that observed in sedimentary rocks 
