by the passage over them of Sharp Detrital Matter, 117 



or, as it was then considered, or at least called, diluvial 

 matter — over them. 



D'Halloy (1831), in his Elements, states, that M. Brong- 

 niart had observed portions of the primordial rocks (granites, 

 &c.) where the surface had been polished and grooved in the 

 direction of the trainees of debris, as if it had been worn by 

 the passage over it of the blocks composing them These 

 trainees in Scania and Smalande form longitudinal hills, 

 which the Swedes call as, or ose ; our Irish escars. M. 

 D'Halloy classifies, from the observations of M. de Beau- 

 mont, the drift or diluvium thus : — " It apparently occurs 

 in three different forms. In the bottoms of valleys it ap- 

 pears as terraces, separated by a longitudinal depression, in 

 which, like a secondary valley, is the ordinary watercourse ; 

 in plains it is spread out in vast horizontal 'nappes,' or sheets; 

 and in mountains and hills it occurs as erratic blocks. It 

 cannot be said that it exhibits a true stratification, though its 

 masses often exhibit a partial parallelism. M. de Beaumont 

 considers it generally in unconformable stratification with the 

 strata which it covers ; and, it may be added, that excepting 

 where secondary valleys have been worn in it, the tendency 

 of drift has been to fill up hollows and inequalities, and to 

 reduce the surface of the earth to horizontality. 



It will be observed that the proximate cause of the polish- 

 ing and grooving rocks was naturally sought in the passage 

 of drift over them, and that to explain the cause of the move- 

 ment of that drift was the real difficulty. Fluviatile, marine, 

 and diluvial currents have all, in turn, been called into action ; 

 and had our learned member, Mr Mallet, given us a clear and 

 satisfactory explanation of a new mode in which such large 

 masses of drift might have been put into motion and spread 

 over the surface of the earth, he would, indeed, have deserved 

 our thanks. His reference to the movement of landslips 

 does not appear to me such an explanation, as it involves a 

 generalization similar in character, and certainly more de- 

 fective than that of Agassiz, in respect to glaciers. 



It was, indeed, principally to supply the defective cause of 

 the motion of drift that Agassiz endeavoured to generalize 

 the phenomena of glaciers, so as to make them consistent 



