174 Proceedings of the Royal PMjsical Society. 



large trough, in which a ^potpourri of boulder-clay was stirred 

 together, ready to spread out eastwards and southwards.^ 



(3.) The value of the surface-glaciation of boulders in situ 

 as a distinguishing mark of the true till, or of the deposits that 

 are to be correllated with it, chiefly remains for the future to 

 show. To illustrate it fully is beyond my power ; and even 

 to illustrate it partially would lead me further into local 

 details than is at present desirable. I may say in a word, 

 however, that it has already gone far to resolve some ques- 

 tions of correlation that were long, to me at least, extremely 

 doubtful. 



(4.) It is interesting to find that boulder-glaciation in situ 

 by glaciers is not by any means a mere hypothesis. In front 

 of the glacier of the Argentiere, says Professor Bonney, " there 

 is an extensive area now covered with boulders, which within 

 the last few years has been abandoned by the ice. . . . 

 Many of the smaller blocks on this area, now almost concealed 

 by rubbish scattered from the retreating glacier, are smoothed 

 and striated as if by passing ice. . . . Here and there 

 are large, prominent, protogine blocks, several of which dis- 

 tinctly show, by the striatious on their sides and surface, that 

 the glacier has flowed over them. Three lie near together ; 

 their tops are polished and striated and littered over with 

 moraine; they do not form part of a lateral or terminal 

 moraine, but are in an open plain. Striation, stoss and lee 

 seite — everything is just as it should be had the glacier flowed 

 over them, and each has a tail of moraine. The largest was 

 12 by 7 by 5 yards." ^ 



Mr Bonney continues — and his remarks are an excellent 

 comment on the details of boulder-glaciation with which I have 

 troubled the reader, — " The above observations tend to show 

 that ice is a far more plastic substance than some physicists 

 would allow. ... It occasionally flows after the manner 

 of water, over and round an obstacle, instead of sweeping it 

 away, and appears to pass over debris as a river does over a 

 gravelly bed." 



^ The part, liowever, which ice-movements took in mixing up this boulder- 

 clay preparation is not yet quite certain. 



2 Notes on Glaciers, Geol. Mag., 1876, p. 198. 



