202 THIRTEENTH LETTER ON GLACIERS. [1846. 



We are therefore relieved from the difficulty of accounting 

 for the cold which would be necessary to freeze the infiltrated 

 water which was at one time believed necessary to explain the 

 conversion of the nSve into proper ice. This would be liable 

 to most of the objections urged against the Dilatation Theory. 



(3.) ON THE FIRST APPEARANCE OF STONES (ERRATICS) ON THE 

 SURFACE OF GLACIERS. 



I shall conclude this letter with a few very partial observations 

 upon what 1 must consider as one of the phenomena of glaciers, 

 still obscure as to its explanation, although most familiar as a 

 fact, I mean their supposed tendency to reject impurities, and 

 the undoubted fact that stones are always found near or upon 

 the surface of the ice. It is strange that it should not have 

 occurred to every one who sought to explain the appearance of 

 stones on the surface by the ablation of the ice, that in order to 

 arrive there at all, the blocks must previously have been im- 

 bedded in the virgin ice, where popular belief, and, generally 

 speaking, more accurate observations also, give them no place. 

 Yet, in the thousands of crevasses which an active observer 

 passes and examines in a week, how few cases of imbedded 

 stones in these vast vertical sections are ever visible ? I might 

 almost ask whether they are ever seen, except in the neigh- 

 bourhood of the sides of a glacier, i.e., under the lateral mo- 

 raines ; and even there how rare ! 



The object of the present memorandum is, on the one hand, 

 to direct attention anew to the apparently perpetual paradox 

 which the glaciers present in this respect, and which, I am per- 

 suaded, offers something yet for careful observation, and, on the 

 other, to give what seems to me incontrovertible evidence, that 

 what I have so seldom seen myself must yet exist, and that 



(uot unlike, in general size and polish, to those of hypersthene in hypersthene rock), 

 shew a development of crystallizing force which must evidently be the effect of long 

 time, and probably of comparatively slow motion. The reflection of the sun or 

 moon-beam from these plates gives to the glaciers I have mentioned a dazzling effect, 

 which I have not observed on the glaciers of the Pennine Chain. [I have since 

 noticed a similar structure in the Nygaard Glacier in Norway.] 



