Geology . was abovewater, the formation is less commonly sandstone. Clay or shale ishere more abundant, and beds of coal of workable thickness give someclue to the physical conditions which prevailed, at least locally. The Dakota formation has commonly been regarded as a lacustrineformation, deposited during an epoch of crustal oscillation during whichthe depth of the basin increased. The necessity for postulating numer-ous oscillations and nice adjustments is largely removed, if the forma-tion be regarded as the joint product of subaerial and fluviatile depo- 1 Dawson, Bull. Geol. Soc. Am., V
Geology . was abovewater, the formation is less commonly sandstone. Clay or shale ishere more abundant, and beds of coal of workable thickness give someclue to the physical conditions which prevailed, at least locally. The Dakota formation has commonly been regarded as a lacustrineformation, deposited during an epoch of crustal oscillation during whichthe depth of the basin increased. The necessity for postulating numer-ous oscillations and nice adjustments is largely removed, if the forma-tion be regarded as the joint product of subaerial and fluviatile depo- 1 Dawson, Bull. Geol. Soc. Am., Vol. XII, p. 77. 2 Ibid., p. 78. lit; GEOLOGY. sit ion, for deposits of this class furnish (heir own adjustments. The presence i^ l>inl (racks in the Dakota of Kansas1 and the preserva-tion o( some 500 species of plant fossils, mostly the leaves of angio-sperms, at various points and in conditions which forbid much trans-portation, imply the prevalence of subaerial conditions to a notableextent at Fig. 392.—A Dakota hog back. The rock at the left is the Red beds; the ridgenear the center is occasioned by the outcrop of the resistant Dakota Boulder, Colo. (Lees.) The thickness of the formation is, on the whole, rather uniform,averaging perhaps 200 or 300 feet, though greater thicknesses are the south (Texas), the Dakota formation rests on the Comancheansystem unconformably. Farther north it is often in apparent con-formity with the Comanchean, though it often, as in the Wasatchand Uinta Mountains, rests on older formations. 1 Williston, Univ. of Kans. Geol. Surv., Vol. IV, p. 50. 2Darton, 19th Ann. Rept. U. S. Geol. Surv., Pt. IV, and Knight, Bull. 45, Station. THE CRETACEOUS PERIOD. 147
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