mining, and felt that, while the greater part of the work was accomplished 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. Referring 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. Icebergs 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 material, as we now find it.
The writings of the Rogers brothers are singularly lacking in more than casual references to the drift, though, in one case, at least (that relating to the Richmond boulder train) they advanced some theories which were extraordinary, to say the least. In the, for its time, magnificent publication of the first geological survey report of Pennsylvania one would naturally look for an extension of the views of Professor H. D. Rogers, 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 striæ 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 mountain 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 antiquated, 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 convulsed epochs of the earlier general and later local drift.'
