492 J. G. Goodchilcl — On the Origin of Counts. 



Those who have read the paper thoughtfully will at least admit 

 that if the ice-sheet really accomplished as much denudation as is 

 therein claimed as its work, the very nature of the agent would lead 

 us to expect results different in many important respects from those 

 accomplished hy purely subaerial means. Unlike a river, which 

 can transport the rocky material it has removed only along some 

 part or other of its bed, much of the debris detached from the old 

 pre-glacially weathered and shattered rock surface by the ice that 

 was slowly moving over it gradually worked its way into the body 

 of the ice ; where, being practically unaffected by the force of gravity, 

 it quietly floated away in the higher and swifter flowing strata of 

 the ice, towards the outer margin of the ice-sheet, leaving compara- 

 tively little detritus between the sole of the ice-sheet and the rock 

 surface. A little consideration will convince any one that an agent 

 acting in this manner would erode as freely at an elevation lower 

 than the rock surface a little further down the valley — in other 

 words, in a rock basin — as at any other part of its bed ; because, as 

 fast as the detritus was removed from the rock, it tended more or 

 less to work upwards into the body of the ice, where the more 

 quickly flowing strata would soon remove it seawards. In the case 

 of a river the force of gravity comes more strongly into play, so 

 that, except in a pot-hole, when once a large stone gets much below 

 the general level of the bed of the river, there it must lie, until 

 some accident brings it again to the level of the river's bed. 



Mr. Croll's theory of the " Physical Cause of the Motion of 

 Glaciers," published in the Phil. Mag. for March, 1869, enables us 

 to understand how ice, whilst possessing many of the properties of 

 a fluid, may yet at the same time behave in many respects as a semi- 

 solid. A proper appreciation of Mr. Croll's theory ; of the theory 

 put forward in the first instance by J. D. Forbes, and since extended 

 by Mr. James Geikie, on the up-travelling of boulders ;* and of the 

 actual thickness that it can easily be shown that the ice really had ; 

 is, I think, all that is necessary to convince the most sceptical that 

 the theory of the origin of rock basins by glacial erosion is the only 

 theory that really accords with the facts. In all probability the 

 greatly diminished rate of flow of the lower strata of the ice as 

 compared with the flow of the strata near the surface, is quite com- 

 pensated, so far as the erosive power of the ice is concerned, by the 

 enormously increased pressure. Hence it is far from unlikely that 

 the actual amount of erosion accomplished by the bottom ice may 

 not be far short of, if it does not equal, or even exceed, the amount 

 of erosion effected by the comparatively swift-flowing ice of the 

 higher parts of a glacier's sides. 



In order to rightly understand the theory that I have elsewhere 

 given 2 to account for the origin of coums, it will be well to sum- 

 marize a little of the evidence relating to the behaviour of a thick 

 mass of land ice in motion that can be gathered from the sources at 



1 On the Occurrence of Erratics at Higher Levels than the Bock Masses from 

 •which they have been derived, Trans. Geol. Soc. Glasgow, vol. iv. pt. 3, p. 235. 



2 Quart. Journ. Geol. Soc. for Feb. 1875 (read 24th June, 1874). 



