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XII. On the poiver of Icebergs to make rectilinear, uniformly - 

 directed Grooves across a Submarine Undulatory Stirface. By 

 C. Darwin, Esq., Vice-Pres. R.S., F.G.S.'*^ 



HAVING been induced to believe, with many geologists, 

 that certain continuously scored and polished surfaces of 

 rock were due to icebergs, and not to glaciers, I have neverthe- 

 less always felt much difficulty in understanding how long, rec- 

 tilinear scratches, running in one given direction across an un- 

 dulatory surface, could have been thus formed. Others have 

 felt this same difficulty, and it has been advanced as an insuper- 

 able difficulty by the opponents of iceberg action. The following 

 considerations, though possessing little or no novelty, have in 

 my own case removed the difficulty. But first, to give one in- 

 stance of such scratches, I may quote a passage from Agassizf, 

 who, in describing the state of the surface near Lake Superior, 

 says, " nothing is more striking in this respect than the val- 

 leys or depressions of the soil running E. and W., where we 

 see the scratches crossing such undulations at right angles, 

 descending along the southern gentle slope of a hill, traversing 

 the flat bottom below, and rising up the next hill south in un- 

 broken continuity.^' He proceeds to state that the scratches 

 run up even steep northern slopes, though the southern faces of 

 the hills are generally rugged. A glacier driven straight for- 

 wards over its unequal bed would perfectly account for these 

 facts ; but not so, at first appearance, floating ice, whether that 

 of coast-ice or of icebergs. For such masses being borne along 

 on the level ocean, would, when driven on shore or against a 

 submarine hill, be deflected, as it might be thought, from their 

 course, and mark the rocks horizontally or nearly so, — some 

 allowance being made for the rise and fall of the tide. And 

 although during either the submergence or emergence of the 

 land, the whole surface of a mountain might become thus marked, 

 yet the successive scores at each level would all be nearly hori- 

 zontal. No doubt short inclined grooves might be formed 

 by masses of ice being driven by gales up the beach ; but as 

 sea-shores run in every possible direction, it is obvious that 

 such grooves could follow no uniform course, nor could they be 

 of any considerable length ; hence grooves thus made would not 

 be comparable with those now under discussion. 



The plasticity of glaciers, as shown by the manner in which 

 they immediately expand after passing through gorges, and in 

 which they mould themselves to every sinuosity and prominence 

 in their beds, is now, thanks to the labours of a few eminent 

 men, familiarly known to every geologist. It is asserted by 



* Communicated by the Author. 



t Lake Superior, its Physical Character, &c., by L. Agassiz, p. 406. 



