146 MR, E. HULL ON THE VESTIGES OF EXTINCT, ETC. 



ing these phenomena than was supposed by the founders 

 of the glacial theory. It is indeed an almost unsolved 

 problem, how we are, in all cases, to distinguish the 

 effects of icebergs charged with stones scraping along the 

 sides and bottoms of the channels through which they 

 float, from the effects of subserial glaciers. If of large size, 

 and impelled by prevalent winds or currents in one gene- 

 ral direction, they would produce polished, grooved and 

 rounded surfaces on the rocks with which they would come 

 in contact, and leave behind blocks and debris strewn so as 

 to resemble the matter of moraines. At the same time, 

 there are several classes of objects which could only have 

 been produced by subserial glaciers, and others which bear 

 the unmistakeable impress of aqueous deposition. 



I shall now only repeat, that the great object to be 

 accomplished is the production of maps showing the di- 

 rection of the strise, the position of the moraines, and the 

 limits of the drift, amongst the highlands of Britain. 



