Lesley.] "tj [Nov. 4, 



The most difficult of all these questions lias naturally excited the most 

 strenuous discussion :— the excavatory power of ice. 



Every geologist knows that an uncertain amount of erosion must be ex- 

 plained by ancient ice movements ; for, the eroding action of glaciers may 

 be studied in Alpine valleys as it is now going on. But some think this 

 erosion to be so insignificant as to be justly compared to the sandpaper 

 smoothing-ofi of a roughly-planed board ; while others please their imagi- 

 nations with its incredible force and magnitude, and describe it as plough- 

 ing out Alpine valleys, and excavating American lakes. Recent works on 

 the Glacial Age might be quoted to show that conjectures of all grades be- 

 tween these two extremes are accepted by their geological authors — vague 

 postulates, or general propositions, taken for granted, without being sub- 

 jected to any mathematical analysis — as a groundwork for the considera- 

 tion and description of old and new local facts. 



It is needless to say that no personal sentiment on the subject can have 

 a scientific value. For my own part, I entertain a lively persuasion in 

 favor of the .sandpaper end of the series of hypotheses ; but lean assign no 

 higher value to this persuasion, or personal opinion, nor do I think it can 

 any more efficiently secure scientific results, than an impulse towards the 

 opposite, or lake excavating prejudice. It is after all merely a prejudice, 

 but a prejudice in favor of the preponderance of a multitude of facts which 

 bear upon the subject under discussion ; facts which I think have never 

 yet been placed in the strongest light ; facts of topograph}', especially 

 abundant in regions near to but outside of glaciated regions. 



There are two principal lines of investigation, it seems to me, which may 

 lead us to a hopeful elucidation of the question of how much of our topog- 

 raphy has been effected hy ice. 



1. We may take up one feature of topography after another, and by a 

 process of exclusion, narrow down the field of ice-action until what is left 

 shall remain reasonably certain to be due to ice alone ; and 



2. We may study, directly and mathematically, by number, weight, 

 bulk and velocity, the work actually done by an existing glacier, and infer 

 by strict comparison the possible limits of ice-work over any given glaciated 

 region. 



Thus, to take the last first, let os ask wiiat is the potential of eroding 

 energy in the case of a glacier? 



Pure ice, of course, has no scratching power. The facility* with which 

 it moulds itself upon surfaces, is shown, in an astonishing manner, by 

 grooves on the underside of a moving glacier, produced by large stones 

 lying quite loose upon the bed-rock, and prevented from slipping forward 

 with the ice by some slightly obstructive irregularity of the bed-rock sur- 

 face. The common notion is that all such stones are necessarily embedded 

 by the ice and used as scratchers, or eroding tools. Rut at least some of 

 them are not so taken up by the ice, which slips smoothly over them, re- 

 taining as a groove the shape of their cross section, for many yards after 

 passing their position. 



