The discussion upon the subject of the continuity of the Glacial period was, however, somewhat elaborate (see pages 106-121, 311, 324, 332), and was presented with excessive respect for the authority of those who maintain the opposite view; all that was claimed (page 110) being that one might maintain the _unity_ or _continuity_ of the Glacial period "without forfeiting his right to the respect of his fellow-geologists." But it already appears that there was no need of this extreme modesty of statement. On the contrary, the vigorous discussion of the subject which has characterized the last two years reveals a decided reaction against the theory that there has been more than one Glacial epoch in Quaternary times; while there have been brought to light many most important if not conclusive facts in favour of the theory supported in the volume. In America the continuity of the Glacial period has been maintained during the past two years with important new evidence, among others by authorities of no less eminence and special experience in glacial investigations than Professor Dana,[A] Mr. Warren Upham,[B] and Professor Edward H. Williams, Jr.[C] Professor Williams's investigations on the attenuated border of the glacial deposits in the Lehigh, the most important upper tributary to the Delaware Valley, Pa., are of important significance, since the area which he so carefully studied lies wholly south of the terminal moraine of Lewis and Wright, and belongs to the portion of the older drift which Professors Chamberlin and Salisbury have been most positive in assigning to the first Glacial epoch, which they have maintained was separated from the second epoch by a length of time sufficient for the streams to erode rock gorges in the Delaware and Lehigh Rivers from two hundred to three hundred feet in depth.[D] But Professor Williams has found that the rock gorges of the Lehigh, and even of its southern tributaries, had been worn down approximately to the present depth of that of the Delaware before this earliest period of glaciation, and that the gorges were filled with the earliest glacial _débris_. [Footnote A: American Journal of Science, vol. xlvi, pp. 327, 330.] [Footnote B: American Journal of Science, vols, xlvi, pp. 114-121; xlvii, pp. 358-365; American Geologist, vols, x, pp. 339-362, especially pp. 361, 362; xiii, pp. 114, 278; Bulletin of the Geological Society of America, vol. v, pp. 71-86, 87-100.] [Footnote C: Bulletin of the Geological Society of America, vol. v, pp. 13-16, 281-296; American Journal of Science, vol. xlvii, pp. 33-36.] [Footnote D: See especially Chamberlin, in the American Journal of Science, vol. xlv, p. 192; Salisbury, in the American Geologist, vol. xi, p. 18.]