Geology . lel, see Kings Report, Vol. I,already cited. For the latest attempt at correlating the several lake formations,see Dall, 18th Ann. Rept., U. S. Geol. Surv., Pt. II. See also J. H. Smith, , Vol. VIII, pp. 444-471. 2 Wilder has recently called into question the separability of the Fort Union and theLaramie, in western North Dakota. Jour, of Geol., Vol. XII, p. 290. 3 For criteria for distinguishing lacustrine and subaerial formations, see Davis,Science, N. S., Vol. VI, p. 619, 1897, and Proc. Am. Acad. Arts and Sci., Vol. XXXV,p. 345, 1900. 4 Little Belt Mountain, Mont., Folio
Geology . lel, see Kings Report, Vol. I,already cited. For the latest attempt at correlating the several lake formations,see Dall, 18th Ann. Rept., U. S. Geol. Surv., Pt. II. See also J. H. Smith, , Vol. VIII, pp. 444-471. 2 Wilder has recently called into question the separability of the Fort Union and theLaramie, in western North Dakota. Jour, of Geol., Vol. XII, p. 290. 3 For criteria for distinguishing lacustrine and subaerial formations, see Davis,Science, N. S., Vol. VI, p. 619, 1897, and Proc. Am. Acad. Arts and Sci., Vol. XXXV,p. 345, 1900. 4 Little Belt Mountain, Mont., Folio, U. S. Geol. Surv. 206 GEOLOGY, Nn ibaum (N 11111 Uuohara ? t c oJ9 — . Poison Canyon \ Laramie \ PP Trinidad Pierre shale •< = Apishapa -j Timpus J Carlisle jGreenhorn Graneros { Dakota J Morrison (Co- -manchean (?)) Badito (Carbo- i niferous (?)) \ Schist andgranite Fig. 422. — Columnor section of theformations at theeast foot of theRocky Mountains,Colo. (Hills, Geol. Surv.). THE EOCENE PERIOD 207 begun by the time the Arapahoe and Livingston formations weredeposited. To the Early Eocene, the Telluride (or San Miguel *) and PoisonCanyon2 formations (Fig. 422) of Colorado are commonly assigned,although their equivalence to the Arapahoe of the Denver basin hasbeen suggested. Locally, the Cretaceous had suffered as much as7000 feet of erosion subsequent to the post-Laramie uplift beforethe deposition of the Telluride formation;3 but great as this is, it doesnot exceed the post-Laramie erosion which is thought to have pre-ceded the deposition of the Arapahoe formation (p. 158). The Tellu-ride formation is conglomeratic, and has a maximum thicknessof about 1000 feet, while the Poison Canyon formation, of sandstoneand conglomerate, is said to attain a thickness of 2500 feet. Theassignment of these formations to the Eocene is based on stratigraphy,for neither has yielded distinctive fossils. While both formationshave been described as lacustrine, it is no
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