158 Proceedings of the Royal Physical Society, 



structureless." ^ " Save with some rare exceptions," says Mr 

 Archibald Geikie, "its boulders are not arranged in any deter- 

 minate manner." ^ " Tjrpical till," says Professor Green, " is 

 a tough, dense clay . . . stuck as full as it can hold of 

 stones of all sizes, which are not arranged in any order, but 

 look as if they had been forcibly rammed in anyhow." ^ 



Good till greatly resembles one of these concretes in 

 which stones and shingle of many shapes and sizes have been 

 thrust into a paste of soft asphalt. N'ow, in a pavement of 

 this kind, we might expect, in favourable circumstances, to 

 find certain structures. If the pavement had been repaired 

 or patched by simply placing a new concrete here and there 

 upon the top of the old, we should find inside it groups of 

 stones with their upper surfaces worn down by the traffic, or 

 perhaps grooved, after the manner of some of our street 

 causeways, by the passage of wheels. Again, if the asphalt 

 had been placed upon a slope, and in a too pasty condition, 

 it might have made of itself a sort of miniature glacier — 

 such as Sir William Thomson, I am informed, occasionally 

 exhibits to his class at Glasgow^ And having been in 

 motion, we might expect to find a rude sort of fluxion- 

 structure developed, comparable to that which is found 

 among certain igneous rocks. 



Both these structures are characteristic of the boulder- 

 clay in a degree hitherto not much noticed ; and it is to the 

 facts respecting them that I devote this paper. They will 

 be found to bear shrewdly upon its origin. 



I. — Glaciation of the Uppee Suefaces of Bouldees in 

 situ. — Pavement-Bouldees. 



The earliest observations upon the glaciation of boulders 

 in situ in the till of which I am aware were recorded by 

 Charles Maclaren in the Scotsman in 1828,* as having been 



^ Phys. Geol. and Geog. of Ireland, p. 79. 



2 On the Glacial Drift of Scotland (Trans. Geol. Soc, Glasgow, vol. i., 

 part 2, p. 41). 



3 Geology for Students, 2d ed., p. 263. 



4 In a leading article upon Sir James Hall's celebrated paper "On the 

 Revolutions of the Earth's Surface." See Maclaren 's Geology of Fife and the 

 Lothians, 2d edit., p. 290. 



