70 SURFACE GEOLOGY. 



considerable height, is better explained by icebergs than by glaciers. In some in- 

 stances the grinding body must have been forced upward, above the general sur- 

 face, which is also striated hundreds if not thousands of feet, as on Mt. Monadnoc 

 and the White Mountains. Now a glacier, descending as a whole in every known 

 instance, is able to force portions of its mass over obstacles a few feet only in height. 

 But here we must suppose one not on a slope, but moving over a level surface 

 for hundreds of miles, to be able to crowd large portions of its mass hundreds of 

 feet over opposing mountains. If we could suppose a huge iceberg, suspended in 

 an ocean rising above the mountains, to impinge against its top, with an immense 

 momentum, it might force a portion of its mass over the top ; especially if at the 

 same time the mountain were sinking ; though perhaps this descent would be too 

 slow to meet the case. 



2. Iceberg action explains better than that of glaciers, that sorting of materials 

 and of laminations, which we sometimes find in the drift. I know it is customary 

 to speak of drift, (I mean the lowest and coarsest variety,) as a mass mingled in 

 perfect confusion. But I have rarely seen a section in it, of very considerable 

 extent, in which I could not discover some marks of the action of water in the 

 parallel arrangement and separation of the materials into finer and coarser. 1 

 have often been struck with this evidence of a tumultuous and quiet action in close 

 juxtaposition; and we know that not unfrequently the aqueous action appears to 

 have predominated. But if huge icebergs tore off and accumulated the detritus, 

 we might expect that the currents which bore them onward would, to some extent, 

 separate and arrange the materials, especially where masses of ice were stranded; 

 and that sometimes the icebergs would be absent altogether. Glaciers, however, 

 have no such powder, save that the stream which usually issues from them, will 

 cause some alluvial accumulations in the valley below the terminal moraines, but 

 not in the midst of the moraines. 



3. The facts concerning the dispersion of boulders can be more satisfactorily 

 explained by icebergs than by glaciers. It appears that the work of scattering 

 these boulders continued till after the time when a large part of the beaches and 

 terraces were formed, for they are scattered over the surface of these sandy deposits. 

 (See Mr. Desor's account of the Drift of the Lake Superior land District, in Foster 

 and Whitney's Report, p. 190.) Now glaciers could not have done this; for they 

 would have ploughed a track through the stratified deposits of sand and clay 

 beneath, if they had transported these boulders ; and so would such icebergs as 

 I have supposed might have produced the drift below the terraces and beaches. 

 But such icebergs as now traverse the Atlantic might have carried boulders 

 over the beaches and terraces and dropped them from time to time, as we now 

 find them scattered over the western prairies. By the same agency, also, we 

 can explain the intermixture of coarse angular blocks in any of the beach and 

 terrace deposits. 



4. The supposition that a glacier once existed on this continent, wide enough to 

 reach from Newfoundland to the Rocky Mountains, is the grand difficulty in the 

 way of the glacier theory. All known glaciers occur in valleys, not many miles 



