J. G. Goodchild — On the Origin of Coums. 491 



give rise to a rudely semicircular hollow with steep craggy walls ; 

 hut it is quite impossible that this, or any other vertically acting 

 force could ever give rise to tiers of rock shelves with the unbroken 

 sweeping curves that actually occur. Lastly, many, perhaps the 

 greater number of coums occur in such situations that, if they had 

 ever been tenanted by a glacier, this, in consequence of the nearness of 

 the hollow to the snow-shedding line, must rarely have exerted much 

 pressure upon its bed ; even supposing that, in such a situation, the 

 neve was sufficiently consolidated to take on the properties of ice in 

 any form. If, then, perfect coums exist where, under glacier con- 

 ditions, there could hardly have been sufficient pressure to consolidate 

 the neve in the bottom of the coum into ice, much less near its upper 

 margin, it seems clearly impossible that an ordinary glacier, or, 

 indeed, ice in any form moving in the way that a glacier does, could 

 give rise to the crater-like recesses whose origin is here discussed. 



The widely different elevations that adjoining coums are found at, 

 and the entire absence of any marks of erosion in their neighbour- 

 hood that can clearly be shown to be the work of the sea, are objec- 

 tions of sufficient weight to convince most field geologists that veiy 

 few indeed of these pot-hole-like hollows have received their present 

 form by marine action. 



All who have followed Professor Eamsay in collecting facts relat- 

 ing to Glacial Erosion have remarked upon the association of well- 

 glaciated rock surfaces with coums and rock basins ; and, probably, 

 they have all felt more or less convinced that this association is 

 something more than accidental. Tire close resemblance of many of 

 the coums to gigantic pot-holes, such as, on a small" scale, a mountain 

 torrent drills in its bed, seems to point to some analogy in their 

 modes of formation ; and this view is considerably strengthened 

 when it is found that the position of not a few of the coums bears 

 the same relation to the direction, position, and relative sizes of the 

 adjoining valleys that the position of their smaller analogues do with 

 regard to the direction, position, and relative sizes of the rock 

 channels that cause the eddies in a river. 



In the paper on Glacial Erosion an attempt has been made to 

 prove that all the minor features of the scenery in the north-western 

 part of the Dale District are due to mechanical erosion by land ice, 

 which in all probability rose to a level of at least 2400 feet above 

 the sea, and may have had a greater thickness even than that. It 

 was pointed out that the erosive powers had acted unequally upon 

 the beds in proportion to their relative powers of resistance to 

 mechanical erosion ; and also that in some cases it is almost certain 

 that a considerable thickness of rock must have been removed by 

 the ice in this way. The eroding agent followed the pre-glacial 

 configuration of the surface, removing much of the weathered part 

 of the rock, and replacing the notched and irregular weathered sur- 

 face, and the talus-covered slopes, by unbroken and sweeping lines 

 of scar, terraces of unweathered rock, and slopes coated with little 

 other superficial accumulations than the drift matter that had once 

 been dispersed throughout the entire thickness of the ice-sheet over 

 that particular spot. 



