186 Proceedings of the Boyal Physical Society. 



But it may at least be said that these two varieties of 

 structure are in a singular degree complementary of one 

 another. Both of them, each in its own way, tells the story 

 of the building-up of the boulder-clay slowly and under 

 pressure. 



It seems certain that the pressure must have been that of 

 a vast and wide-spreading mass — a mass closely investing 

 and slow-moving and heavily dragging. I had meant to 

 enter in some detail into the theories of glaciation as bear- 

 ing on these structures; but it seems almost unnecessary. 

 I have tried to conceive of thousands of boulders dotted 

 through the ooze of a silty sea-bottom over hundreds of square 

 miles, each awaiting the chance scrape of a passing berg, and 

 eight out of every ten (in many open situations), though objects 

 so small and so loose, fated to receive a heavy and persistent 

 striation on one cheek ; but in the case of boulders 6 and 9 

 inches long it seems physically impossible. I have also tried 

 to imagine long hollows trailed out by dragging icebergs into 

 which the semi-fluid clay of the sea-bottom should run, some- 

 what as the wind runs along with a train, but it gives to the 

 prevalence of fluxion-structure an exceedingly lame and 

 partial explanation. Nor is loose coast ice, which glaciates 

 horizontally in zones, nor the frame of the ice-foot, which 

 must glaciate radially from the centre of every island and 

 promontory, at all a more adequate explanation. 



But to the theory, if it must still be termed a mere theory, 

 of confluent glaciers, one turns as to a really competent agent. 

 For the fluxion-structure it accounts at once. It needs only 

 to be assumed that the dragging ice communicated something 

 of its own motion and structure to the clay over which it 

 passed. We know, if only from Tresca's experiments, that 

 ice assumes a fluxion-structure under a pressure that induces 

 movement. Erom his other experiments we are also made 

 aware that even solids as dense as lead and iron are made to 

 " undergo an internal motion of their parts " when they are 

 " placed between the jaws of a powerful compressing machine."^ 

 That under the jaws of a much more powerful compressing 

 machine boulder-clays should undergo this " motion of parts " 



1 A. Geikie, Text Book of Geology, p 313. 



