J. G. Goodchild—On Drift. 501 



must have been going on every now and then during the formation 

 of the beds ; and the agent which jorodueed the crumpling could not 

 have accomplished it by an endlong thrust, or more of the underlying 

 beds would have participated in the disturbance. The principal faces 

 of oblique lamination in the clays very frequently, perhaps nearly 

 always, conform in direction to the outer slope of the mound in 

 which they occur, and usually, also, to the slopes of the rock over 

 which the drift is moulded ; where the till has been denuded, the 

 surface of the overlying sand and gravel usually follows the outline 

 of the till over which it lies, whence we may conclude that the drift- 

 forming agent was detained over one particular spot until the whole 

 of the drift, till, sand and gravel, loam, and laminated clay, had 

 accumulated. 



Taking another line of inquiry, we find that nests and vertical 

 pipes of sand and gravel frequently extend upwards through a con- 

 siderable thickness of till, in such a way as to show that the building 

 up of the surrounding till went on simultaneously with the deposition 

 of the fine sand and gravel : this again seems to point to the deten- 

 tion, over the same spot, of the agent which produced both resiilts. 

 Evidence of the same kind seems to be afi'orded by the occasional 

 presence of well-glaciated and quite unrolled rock fragments in the 

 midst of thick deposits of sand and gravel ; the amount of rolling 

 which was sufficient to efface the striae from most of the stones could 

 not fail to have acted alike on all ; — there must have been something 

 from which a continual supply of these scratched stones and angular 

 blocks was being derived, while the other stones were being rolled 

 into shingle. Such instances, doubtless, are rare in proportion to 

 the distance of the mass of gravel from the head of the physical 

 basin in which it occurs ; but a careful search in almost any sand 

 and gravel mound will usually bring one or more such instances to 

 light. 



Eeasoning from these data last winter, I was led to the conclusion 

 that the only agent by which we could satisfactorily account for 

 these A'aried, and, in part, seemingly unrelated phenomena, was the 

 great ice-sheet itself, throughout which the component materials of 

 the various forms of drift were in some way or other dispersed. 



It seems strange that so many geologists who have written about 

 drift should have assumed that the stones and mud were confined to 

 the bottom of the ice-sheet, and that the body of the ice was nearly 

 or quite free from detritus. Yet mention of even considerable 

 quantities of rock fragments imbedded in icebergs is frequently 

 made by voyagers, and all writers seem agreed that stones and 

 mud do occasionally occur in the bodies of the Swiss and other 

 glaciers. Had no mention been made of these included stones, their 

 presence in the ice-sheet might have been inferred from the now 

 undoubted recurrence in the ice of crossing currents, the probability 

 of the existence of which was long since pointed out by Professor 

 Eamsay. 



The evidence for the existence of these crossing currents may be 

 stated as follows : — 



