Rev. T. G. Bonney — Notes on Glaciers. 199 



grotto. I copy tlie following from my note-book as tlie result of my 

 observations : (a). The ice, when in contact with the rock, was 

 remarkably clean (as was the rock), and but few stones were frozen 

 into it. The glacier adapted itself very remarkably to the ine- 

 qualities of the rock (a gneiss, not protogine), actually fitting itself 

 into hollows some 2 feet deep by 3 or 4 wide. (h). The ice, though 

 thus filling up the larger inequalities, does not quite fit into the 

 smaller. The unworn space at the base of a lee seite cliff depends on 

 several things, but speaking generally it rarely exceeds the height of 

 that cliff, and is narrower on steep than on gentle slopes, (c). The 

 ice appears to enter these cavities, not by fracture and regelation, 

 but by change of form, as in the case of a plastic body, for it is 

 remarkably clear and solid-looking. (d). Looking into the ex- 

 quisitely blue mass of the glacier, I saw here and there a little 

 pocket of mud imbedded, and a few stones; but I think 27 cubic 

 inches of rock to a cubic yard of ice would be much abpve the average 

 proportion, (e). There were no signs whatever of the glacier being 

 able to break off or root up blocks of the subjacent schistose rock ; 

 it seemed to simply wear away prominences. 



It may be well to warn persons unaccustomed to glaciers that, in 

 examining their flanks and ends, there is often danger from falling 

 ice and stones. In doing the above work, I always acted under the 

 advice of an experienced guide, and know the habits of glaciers pretty 

 well myself ; yet, in the afternoon of the day when these observa- 

 tions were made on this Argentiere glacier, many tons of ice fell 

 either on or very near to a place where we had been working. 



To conclude. The above observations tend to show that ice is a 

 far more plastic substance (be the cause of the plasticity what it 

 may) than some physicists would allow, and this has been con- 

 firmed by the remarkable experiments of Herr Pfaff, an account of 

 which (Nature, vol. xii. p. 316) was almost the first thing to meet 

 my eye on my return to England. They further indicate that this 

 very plasticity, which in some respects is favourable to, in others 

 militates against, its erosive power ; as 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. I am thus confirmed in my view that '• Ground Moraines," in 

 the ordinary sense of the word, are very exceptional, and that the 

 explanation commonly given of Till {e.g., Great Ice Age, p. 87) is 

 incorrect. Further, I am confirmed in regarding a glacier as an 

 agent of abrasion rather than of erosion, and though I admit that on 

 the whole it seems sometimes the simpler plan to attribute to the 

 action of glaciers certain tarns and even small lakes, yet when I 

 come to study them in detail I am confronted with so many indi- 

 cations of the inability of glaciers to excavate, except under pecu- 

 liarly favourable circumstances, that I feel convinced great caution 

 must be exercised in referring to their action a rock basin of even 

 moderate dimensions. 



