110 Roiinson — Tertiary Peneplain of the Plateau District. 



Southern Plateau District will be presented in order that the 

 problem under discussion may be given its proper setting. The 

 data for this have been taken from the reports of Gilbert.* 

 Dutton,f Davis,:}: and especially from the more detailed work 

 of Huntington and Groldthwait § in the Toquerville District of 

 Utah. Although the sequence of events is based principally 

 on observations in the southwestern part of the plateau, it is 

 believed to be equally applicable to the entire southern portion 

 of the plateau country, since the facts where best known, as at 

 the Mount Taylor Mesa and adjacent region in New Mexico, 

 fit into it without apparent discord. 



During early Eocene time, certainly, the southern plateau 

 country was a land area on which sediments were deposited 

 partly in lakes, partly probably as fluviatile deposits. It 

 is possible that the region was uplifted and eroded in the 

 latter part of the Eocene, since only strata of lower Eocene 

 age are found in the southern part of the High Plateaus of 

 Utah. The extensive volcanic activity present in the Basin 

 Range country of Nevada and Arizona, and the High Plateaus 

 of Utah, was absent, as there are no traces of volcanic plugs 

 or dikes, which can be referred to this period, to be seen any- 

 where in the southern portion of the plateau. At this time 

 the plateau had been differentiated but slightly, if at all, from 

 the Basin Range country, as is rather clearly indicated by the 

 character and distribution of the Eocene deposits, especially in 

 the High Plateaus of Utah, although it may be noted that 

 Lindgrenjl places the initial separation of the Sierra Nevadas 

 .and Wasatch Mountains from the Basin Range country in 

 middle Cretaceous time. 



In the latter part of the Eocene, or possibly in early 

 Miocene time, uplift occurred by monoclinal folding, which, 

 on the whole, appears to have raised the western part of the 

 plateau country above the eastern. In the southeastern section 

 this folding gave rise to an elevated tract of country, the axis 

 of which had a trend somewhat west of north, while a second 

 elevated region extended westward for an indefinite distance 

 from the Echo Cliff monocline situated on the east side of the 

 Colorado River. This tract appears to have had its principal 

 extension northward, and did not affect the extreme south- 



* Survey West of the 100th Meridian, vol. iii. 



f (a) Tertiary History of Grand Canvon District, IT. S. G. S. Mon. II, 1882. 

 (b) Mt. Taylor and the Zuni Plateau. IT.'S. G. S. 6th Ann. Kept. 1885. 



X («) An Excursion to the Grand Canyon of the Colorado, Bull. Mus. 

 Comp. Zool. Harvard Coll., vol. xxxviii, Geol. Ser. vol. vi, No. 4, 1901. 

 (b) An Excursion to tho Plateau Province of Utah and Arizona, Bull. Mus. 

 Comp. Zool. Harvard Coll., vol. xlii, Geol. Ser., vol. vi, No. 5. 1904. 

 • § The Hurricane Fault in the Toquerville District, Utah. Bull. Mus. Comp. 

 Zool. Harvard Coll. vol. xlii, Geol. Ser., vol. vi, No. 1, 1903. 



I The Age of the Auriferous Gravels of the Sierra Nevada. Jour. Geol., 

 vol. iv., pp. 894-896. 1896. 



