508 J. G. Goodchild—On Drift. 



sank to their ordinary levels, the deposition of material of lesser de- 

 grees of coarseness would again go on over the lately formed channels. 



Still farther from the watershed so much water would be flowing 

 between the ice and the rock in the low grounds that the finer parts 

 of the clay — ^in places which, farther up the valley, would be out of the 

 reach of the water — would be carried away, so that beds of loam only 

 would be left to represent the more clayey deposits at higher levels. 



It is obvious that the farther from their origin the stones were 

 rolled, the greater amount of attrition they would undergo, so that 

 stones which, when melted out of either the top or the bottom of the 

 ice, were either angular or well scratched, would soon be rolled into 

 shingle undistinguishable from ordinary river gravel ; and between 

 these extremes we should find every degree of water- wornness, and 

 thus get many stones from which the scratches would be only partially 

 effaced, or would retain what Prof. Eamsay terms " ghosts of scratches." 



When at its lowest ebb, the water of the sub-glacial streams would 

 permit of the formation of laminated clays in the minor tributary 

 channels. The thin sheets of water highly charged with mud would 

 deposit part of their load as a thin film of clay on the surface over 

 which they were flowing, and the long continuance of such action 

 would, in the end, give rise to deposits of considerable thickness. 

 The presence of "the ice overhead would help to scatter throughout 

 the mass the same proportion of stones and boulders as was being 

 dropped around in the other deposits accumulating near ; but the 

 proportion of clay to stones in the gutta-percha clays would be larger 

 by the quantity that was being abstracted from the beds from which 

 the clay-bearing streams took their rise. In the end, we should thus 

 get a deposit of clay of various degrees of fineness, in exceedingly 

 thin laminae, often of different colours, and occasionally exhibiting 

 false stratification up to high angles. We should be prepared to find 

 it varying from a clay in which stones are only occasionally found, 

 to a deposit in which scratched stones occurred in the normal 

 proportion, and which would thus pass insensibly into the character 

 of the ordinary stony clay around it ; — the difference in character 

 depending entirely upon the time any given thickness of each deposit 

 took to form. 



Much interest attaches to the origin of these clays now that a 

 human bone has been found beneath them in the Victoria Caves by 

 Mr. Tiddeman, to whom also geologists are indebted for calling 

 attention to the true nature and importance of these laminated gutta- 

 percha clays. 



It may reasonably be asked whether the enormous downward 

 pressure of the ice upon the underlying beds would not tend to 

 crush and contort them more or less, especially if, as is probable, the 

 ice-sheet occasionally settled down by small starts upon the half 

 consolidated beds which were accumulating between its lower parts 

 and the rock. 



To this it may be answered that abundant evidence of such crush- 

 ing of the drift exists, although the singularity of the appearances so 

 produced has led observers to refer the phenomena to other causes. 



