Prof. Hull found[54] on the northern flanks of the Pendle Range, Lancashire, the Permian beds resting on the denuded edges of the Millstone Grit, and these were again observed resting on the Upper Coal-measures south of the Wigan coal-field. Now from the known thickness of the Carboniferous series in this part of Lancashire he was enabled to calculate approximately the quantity of Carboniferous strata which must have been carried away between the period of the Millstone Grit and the deposition of the Permian beds, and found that it actually amounted to no less than 9,900 feet. He also found in the Vale of Clitheroe, and at the base of the Pendle Range, that the Coal-measures, the whole of the Millstone Grit, the Yoredale series, and part of the Carboniferous Limestone, amounting in all to nearly 20,000 feet, had been swept away—an amount of denudation which, as Prof. Hull remarks, cannot fail to impress us with some idea of the prodigious lapse of time necessary for its accomplishment. [Footnote 41: _Geology of Canada_, 1863, p. 61.] [Footnote 42: Safford’s _Geology of Tennessee_, p. 309.] [Footnote 43: Lyell’s _Student’s Manual_, chap. xxiii.] [Footnote 44: _Geological Studies_, by Prof. A. Winchell, p. 165.] [Footnote 45: _Geological Studies_, pp. 93, 163.] [Footnote 46: Powell’s _Geology of the Uinta Mountains_.] [Footnote 47: _Geological Exploration of the Fortieth Parallel_, vol. ii. p. 456.] [Footnote 48: _Geological Studies_, p. 92; see also Dutton’s _Tertiary History of the Cañon District_.] [Footnote 49: _Tertiary History of the Cañon District_, pp. 20, 113; _Second Annual Report, U.S. Geol. Survey_, p. 125.] [Footnote 50: Powell’s _Geology of Uinta Mountains_.] [Footnote 51: _Geological Exploration of the Fortieth Parallel_, vol. i. p. 745.] [Footnote 52: _Memoir_ to Sheet 32, Geol. Survey of Scotland.] [Footnote 53: _Denudation of South Wales._ _Memoirs of Geol. Survey_, vol. i.] [Footnote 54: _Quart. Journ. Geol. Soc._ vol. xxiv. p. 323.]