No. 4.] CLAYPOLE — PRE-GLACIAL GEOGRAPHY. 223 



to retain a glacier of sufficient mass to produce sucli markings, 

 and moreover we are not driven to violate any law of glacial 

 physics in trying to explain the phenomena. 



The argument, however, is not yet quite complete. Supposing 

 we admit all the premises which Dr. Newberry lays down — that 

 the continental glacier during its retreat was capable of extending 

 tongues of ice several hundred miles long and only 40 to 80 miles 

 wide, and that these tongues of ice lying on a nearly level surface 

 and rising above it to a height of several hundred feet without 

 side walls to confine them, persisted in pushing forward their 

 ends where the resistance was greatest instead of spreading later- 

 ally where it was less — let us enquire next whether these tongues 

 could possibly accomplish the task assigned to them. 



In a lecture on New York Island and Harbour, published in 

 the Popular Science Monthly for October, 1878, Dr. Newberry 

 estimates the mass of material worn off the surface of that part 

 of the State during the ice age and by the action of the ice as 

 not improbably " one hundred feet."' This estimate seems rather 

 high, especially as that region consists of the hard primitive 

 rocks. We have no reason to believe that so large an amount 

 was removed from the surface of northern Ohio by the same 

 agent. Probably the ice age in Ohio did not last quite so long 

 as in New York, New England, and Lower Canada. We have 

 fortunately, however, a gauge, though at present a somewhat 

 rough one, of the amount eroded in this region. Deposition is 

 the true measure of erosion, and if we can form an estimate with 

 tolerable accuracy of the mass of the drift clays, &c., that cover 

 our own and neighbouring States, we shall then have to that 

 extent a key to the amount of degradation they suffered at the 

 hands of the northern ice. Observation shows that as a rule the 

 material was not transported very far, but that what was eroded 

 from one locality was pushed on to another a little south or south- 

 east of it. The deposit on one spot may therefore be used to a 

 great extent to measure the denudation a few miles to the north- 

 ward. Now it is on the whole very rare to find the drift much 

 exceeding 100 feet in thickness in this or adjoining States, 

 Such districts are never very extensive. The bulk of the trans- 

 ported material was not carried on the top of the ice but shoved 

 along underneath it, constituting a ground moraine. There can 

 have been few spots to the northward from which superficial 

 moraine-matter could be obtained during the greatest extension 



