James Croll — Mechanics of Glaciers. 361 



in size as the ridge does in height. At the west end it may be 

 described as composed of Limestone pebbles, with here and there a 

 boulder, while at the east end it is built up almost entirely of large 

 limestone boulders and blocks, many containing from one to two cubic 

 feet and some more. Intermixed there are boulders from the size 

 of the closed hand and upwards. The larger blocks are sub-angular 

 and rounded, and no doubt get gradually worn down smaller and 

 rounder, until they become " boulders," by being moved about by 

 the sea; but some "on the other hand, being thrown over on to the 

 back of the bank, cannot be further affected by the waves. In this 

 ridge, as I have pointed out is the case with the Chesil Bank,^ the 

 stones follow the law of the bank itself the largest being collected to 

 form the highest part of the bank, which in both occurs where the 

 wave-action is most intense. 



In the present instance there is no doubt about the direction of the 

 gales being from the north-west, and here, as in the Chesil Bank, 

 the large stones are collected at the lee end of the ridge. It is also a 

 legitimate inference that they travel from the westward, as there are 

 block-making rocks on that side of the Bay. If they did not travel, 

 but simply moved up and down the slope of the beach, we should 

 find the smallest where the wave-action was greatest, as they would 

 be ground down in situ. This, in fact, is what takes place when 

 they arrive at the end of their journey ; but they are constantly 

 being renewed by other *' travellers," as the semi-rounded stones 

 show. Nevertheless, it is remarkable to find the stones so assorted ; 

 for gravel and pebbles seem to have no place among the larger 

 stones. These smaller objects most probably — like the mud derived 

 from the attrition — get sucked out by the retiring waves, and 

 travelling onwards are ground into powder, and finally end as mud 

 deposits. 



V. — Eemarks on Mr. Burns's Paper on the Mechanics of 



Glaciers. 

 By James Croll, LL.D., F.E.S. 



IFEAE I must express my ideas very indistinctly, seeing that I 

 am so frequently misunderstood by my critics. Quite recently 

 a well-known American astronomer, in reviewing my book, ''Climate 

 and Time," devotes nearly a dozen pages to the refuting of views 

 most of which I have never held. And in the July Number of the 

 Geological Magazine I am sorry to find a repetition of the same 

 thing. 



I feel certain, that had Mr. Burns been at the trouble to read my 

 more recent and complete exposition of the Molecular Theory of 

 Glacier Motion, given in Chapter xxxi. of " Climate and Time," he 

 would not have published his criticism. The following extract from 

 the chapter referred to will show how completely Mr. Burns has 

 mistaken the theory : 



" This is the form in which my explanation was tirst stated about half a dozen 



^ Geol. Mag. Vol. I. Dec. II. pp. 286, 287. 



