THE GLACIAL HYPOTHESIS 315 



mining, and felt that, while the greater part of the work was accom- 

 plished before the continent had emerged very considerably from the 

 waters, nevertheless, the work of erosion went on for some time after 

 emergence began. 



It was in this connection that was made the first suggestion, so far 

 as I am aware, of a possible recurrence of glacial periods, as fully 

 elaborated later by Chamberlin. Eeferring to the occurrence of two 

 series of stria?, the direction of which did not coincide, and the possible 

 existence of still a third series, he wrote : ' Perhaps there were two 

 periods of glaciers, one before, and one subsequent to the drift/ 



The facts concerning the dispersion of boulders Hitchcock thought 

 could be more satisfactorily explained by icebergs than glaciers, since 

 the transportation and scattering continued until after the time when 

 a large part of the beaches and terraces were formed. Glaciers, he 

 thought, would have plowed tracks through stratified deposits. Ice- 

 bergs such as now traverse the Atlantic might carry boulders over the 

 beaches and terraces and drop them from time to time, forming thus 

 the intermixture of coarse angular blocks and beach and terrace ma- 

 terial, as we now find it. 



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 glacial theory. 



The writings of the Eogers brothers are singularly lacking in more 

 than casual references to the drift, though, in one case, at least (that 

 relating to the Eichmond boulder train) they advanced some theories 

 which were extraordinary, to say the least. In the, for its time, mag- 

 nificent publication of the first geological survey report of Pennsyl- 

 vania one would naturally look for an extension of the views of Pro- 

 fessor H. D. Eogers, but such, nevertheless, are not found. The fact 

 that he considered, if not fully comprehended, Agassiz's views is shown 

 only by a brief paragraph in which he described and figured drift strise 

 seen on an exposed surface of umbral sandstone on the south side of 

 the Wyoming Valley. These he described as ' pointing up the slope 

 toward the southwest, as if produced by fragmentary debris violently 

 propelled against the slope of the moimtain wall of the valley from the 

 south.' The presence of such ascending striae, both here and elsewhere, 

 effectually refuted, according to his conception of it, the glacial theory 

 of their origin. 



Like Hitchcock, he failed to conceive of other than local mountain 

 glaciers of the Swiss type, and he gave the following, even then anti- 

 quated, matter for a general discussion of the distribution of the drift 

 and the various phenomena accompanying it. Of the earlier drift, it 

 should be noted, he offered no explanation whatever, other than that 

 implied in a reference to a period of repose ' which separated the con- 

 vulsed epochs of the earlier general and later local drift.' 



