[Footnote F: See an elaborate discussion of the subject in its new phases by Chamberlin and Leverett, in the American Journal of Science, vol. xlvii, pp. 247-283.] Mr. Leverett also presented an important paper before the Geological Society of America at its meeting at Madison, Wis., in August, 1893, adducing evidence which, he thinks, goes to prove that the post-glacial erosion in the earlier drift in the region of Rock River, Ill., was seven or eight times as much as that in the later drift farther north; while Mr. Oscar H. Hershey arrives at nearly the same conclusions from a study of the buried channels in northwestern Illinois.[G] But even if these estimates are approximately correct--which is by no means certain--they only prove the length of the Glacial period, and not necessarily its discontinuity. [Footnote G: American Geologist, vol. xii, p. 314f. Other important evidence to a similar effect is given by Mr. Leverett, in an article on The Glacial Succession in Ohio, Journal of Geology, vol. i, pp. 129-146.] At the same time it should be said that these investigations in western Pennsylvania somewhat modify a portion of the discussion in the present volume concerning the effects of the Cincinnati ice-dam. It now appears that the full extent of the gravel terraces of glacial origin in the Alleghany River had not before been fully appreciated, since they are nearly continuous on the two-hundred-foot rock shelf, and are often as much as eighty feet thick. It seems probable, therefore, that the Alleghany and upper Ohio gorge was filled with glacial gravel to a depth of about two hundred and fifty or three hundred feet, as far down at least as Wheeling, W. Va. If this was the case, it would obviate the necessity of bringing in the Cincinnati ice-dam (as set forth in pages 212-216) to account directly for all the phenomena in that region, except as this obstruction at Cincinnati would greatly facilitate the silting up of the gorge. The simple accumulation of glacial gravel in the Alleghany gorge would of itself dam up the Monongahela at Pittsburg, so as to produce the results detailed by Professor White on page 215.[H] [Footnote H: For a full discussion of these topics, see paper by Professor B. C. Jillson, Transactions of the Academy of Science and Art of Pittsburg, December 8, 1893; G. F. Wright, American Journal of Science, vol. xlvii, pp. 161-187; especially pp. 177, 178; The Popular Science Monthly, vol. xlv, pp. 184-198.]