Davis.] 56 [May 3 ? 



D. The Argument from Necessity. — The last argument for 

 glacial erosion is of small value. Carried to the extreme, it ex- 

 plains all the Alpine valleys by ice-action, water and weather 

 erosion being insufficient ; it ascribes the very disappearance of 

 the Alpine glaciers to their having worn down the mountains on 

 which they lay ; it explains the (apparent) small proportion 

 of high mountains in northern regions by their having been worn 

 away by the great " polar ice-cap " ; it proves the former exist- 

 ence of an enormous glacial sheet in North Carolina by the uni- 

 form height of many hill summits, the remnants of an old plain, 

 for how could this have been produced save by glacial erosion. 



W. C. Kerr explains the origin of some features in the Topography of 

 North Carolina (Amer. Journ. Sci., xxi, 1881, 216-219) by referring them 

 to an old glacial period. These features are the uniform heights to which 

 the hills rise in the Piedmont region, indicating the existence at that level 

 of an old plain since deeply cut by river action. Nothing could explain the 

 origin of such a plain "except the presence of a great glacier, which in some 

 ancient time had moved down the valley and left the surface nearly level." 

 " The reasonableness of this hypothesis will appear if it is considered 

 what would be the consequence of the movement of such a glacier over the 

 present surface for a few thousand years." Mr. Kerr notes that this evi- 

 dence of the " existence and action of glaciers is totally different from the 

 commonly recognized marks and results of glacial action," in which we 

 agree with him more fully than in the preceding quotations. 



And yet great prominence has been given to this argument 

 from necessity by the more pronounced glacialists. Ramsay says 

 (Geol. Soc. Journ. xvm, 1862, 193 ; see also J. Geikie, Great Ice 

 Age, 277), "If the Lake of Geneva do not lie in a synclinal 

 trough, in an area of subsidence, in a line of fracture, nor in an 

 area of mere aqueous erosion, we have only one other great 

 moulding agency left by which to modify the form of the ground, 

 namely that of ice." Put into another form, this reads, if the 

 phenomenon L cannot be explained by any one of four causes, it 

 must be by a fifth. Now this assumes so exact a knowledge of 

 L and of the four causes, that they can be. excluded ; it assumes 

 the sufficiency of the fifth cause and the absence of any other 

 alternatives. We may well ask whether all the so-called rock- 

 basins to which this argument has been applied are really quite 

 enclosed by solid rock ; secondly, have the four causes been 

 completely excluded from any share in the result; thirdly, are 



