188 Proceedi7igs of the Boyal Physical Society. 



of blast furnaces and of shot rubbish heaps in general, — that 

 they are, in short, irregular planes marking successive accre- 

 tions of till. If this be so, it follows that, where numbers of 

 these planes intersect the planes of forward movement in the 

 boulders, the fluxion-structure is the josmtZo-fiuxion-structure 

 I have referred to.^ 



Of the boulder-pavements on the Portobello shore, I have 

 no very novel explanation to offer. It seems likely that, as 

 my father suggested, they must mark some kind of contem- 

 poraneous erosion. The variety of their materials seems to 

 prove that they were first incorporated with the till in the 

 ordinary course of its formation, and assumed their peculiar 

 grouping subsequently. But whether the erosion was glacial 

 or aqueous it is difficult to say. I have seen straight ravines 

 on boulder-clay slopes lined with washed-out blocks that, if 

 passed over by a gigantic roller, would flatten out into trains 

 not unlike these pavements ; and, of course, in the case of a 

 glacier, the roller would be one that might both flatten and 

 erode. Nor need it be wondered at that the trains should 

 flatten out in places with pavement-like regularity. The 

 stones in any ordinary stream, if they have but flattened 



1 There is some reason to think that the structure in Fig. 10 is of this 

 kind. The shapes of the stones in the till, moreover, seem to prove that 

 some of them moved freely and independently. Boulders having a curved 

 sole appear to have rocked as they went ; boulders with a twist or slue 

 in them hirpled along more or less zig-zag. When a deposit like boulder- 

 clay is in question, however, it appears unsafe to draw sweeping deductions 

 from single facts. I find that Mr John Henderson, our well-known Edin- 

 burgh geologist, declares, respecting a large thin slab of sandstone, 7 

 or 8 feet in its diameters, and only about a foot thick, which he found 

 carried from its original bed half-a-mile on to felstone rocks near Bonally, 

 that it must have been floated by ice; "for," says Mr Henderson, "if it 

 had been picked up at the bottom of a glacier, and rubbed up one hill 

 and down another, to the spot where it is now lying, such a large thin slab 

 must have been broken all to pieces" (Trans. Geol. Soc, Edinb., vol. ii., p. 

 365). But are there no alternatives ? It may have fallen upon the ice during 

 a thawing ; or it may have worked up into the ice and scarcely touched the 

 rock at all ; or it may have been detached from a projecting out-crop ridge, 

 and so got inserted at once into the body of the ice ; or it may have lain in 

 drift a few feet from its parent bed, and in some slight change of ice-move- 

 ment been carried away with drift still round it. It is quite certain at least 

 that a few such facts or riddles will not upset the glacier theory. 



