Erratic Phenomena of Lake Superior. 105 



furrows are sinuous, acting more powerfully upon the soft 

 parts of the rocks or fissures already existing ; whilst gla- 

 ciers smooth and level uniformly the hardest parts equally 

 with the softest, and, like a hard file, rub to uniform continu- 

 ous surfaces the rocks upon which they move. 



But now let us return to our special subject, the erratics 

 of North America. 



The phenomena of drift are more complicated about Lake 

 Superior than I have seen them anywhere else ; for, besides 

 the general phenomena which occur everywhere, there are 

 some peculiarities noticed which are to be ascribed to the 

 lake as such, and which we do not find in places where no 

 large sheet of water has been brought into contact with the 

 erratic phenomena. In the first place, we notice about 

 Lake Superior an extensive tract of polished, grooveS and 

 scratched rocks, which present here the same uniform cha- 

 racter which they have everywhere. As there is so little 

 disposition, among so many otherwise intelligent geologists, 

 to perceive the facts as they are, whenever they bear upon 

 the question of drift, I cannot but repeat, what I have al- 

 ready mentioned more than once, but what I have observed 

 again here over a tract of some fifteen hundred miles, that 

 the rocks are everywhere smoothed, rounded, grooved and 

 furrowed in a uniform direction. The heterogeneous mate- 

 rials of which the rocks consist are cut to one continuous 

 uniform level, shewing plainly that no difference in the polish 

 and abrasion can be attributed to the greater or less resist- 

 ance on the part of the rocks, but that a continuous rush cut 

 down everything, adapting itself, however, to the general un- 

 dulations of the country, but nevertheless shewing, in this 

 close adaptation, a most remarkable continuity in its action. 



That the power which produced these phenomena moved 

 in the main from north to south, is distinctly shewn by the 

 form of the hills, which present abrupt slopes, rough and 

 sharp corners towards the south, while they are all smoothed 

 off towards the north. 



Indeed, here, as in Norway and Sweden, there is on all the 

 hills a lee-side and a strike-side. As has been observed in 

 Norway and Sweden, the polishing is very perfect in many 



