GLACIO-AQUEOUS ACTION IN NORTH AMERICA. 



175 



away southeasterly. At least, I have foimd no exception to this 

 general fact ; and this abrasion on the northern slopes of moun- 

 tains must, therefore, have been produced by some body moving 

 up the slopes in a southeasterly direction. This surely will not 

 be imputed to mere currents of water, pushing along detritus ; 

 because in this way it must have been forced around, not over, 

 the hills. Could immense icebergs have been stranded on the 

 northern slope of the hill, and afterwards, by the force of currents, 

 have been driven over the summit ? Or would it be necessary to 

 suppose, that after the stranding, the water must have risen so as 

 to lift up the iceberg ? Or would a vast sheet of ice, lying upon 

 the earth's surface, by mere expansion, without the presence of 

 water, have been able to produce the smoothing and furrowing 

 in question ? I shall not here attempt to decide between these 

 'hypotheses ; but I think we may decide that ice was concerned 

 in the process. 



It has been stated as a general fact, that the strise are remarkably 

 parallel. But on some surfaces we find two sets of these striae, 

 which make a small angle, say between fifteen and twenty degrees 

 with each other; although in each set the lines are parallel to one 

 another. And usually, one set is much fainter than the other. 

 Some striking cases of this sort have been given by Professor 

 Locke in his Report on the Geology of Ohio. 



Such cases as the above, teach us that the force producing 

 strisB, was exerted on the same spot more than once, and in a 

 different direction. Such would be the precise effect of forcing 

 successive masses of ice over the same surface : such is the effect 

 produced at the present day by advancing glaciers in the Alps ; 

 as we learn by the lucid descriptions of Agassiz. I have never 

 seen a third set of striae on the same surface : not, as I suppose, 

 because the force producing them was not applied three times in 

 the same spot, but because the earlier striae would soon be oblit- 

 erated. For one cannot observe how deeply the surface has often 

 been eroded by this agency, without being satisfied that the work 

 has required hundreds, nay thousands of repetitions in the pro- 

 ducing cause. 



The smoothing and furrowing which have now been described, 



