File:Annual Reports of the Department of the Interior for the Fiscal Year Ended June 30, 1900-Twenty-First Annual Report of the United States Geological Society (1901) (14597881549).jpg

Identifier: annualreportsofd003unit (find matches)
Title: Annual Reports of the Department of the Interior for the Fiscal Year Ended June 30, 1900--Twenty-First Annual Report of the United States Geological Society
Year: 1901 (1900s)
Authors: United States Department of the Interior
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Contributing Library: Clemson University Libraries
Digitizing Sponsor: LYRASIS Members and Sloan Foundation

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- reason of the fact that erosion has not 3-et reached the resistantpurple limestone and Minnelusa sandstone, but is still at work on thesoft red l)eds, and accordingly produces a basin instead of a dome.The resemblance of this basin to a circus ring or race course is mostremarkable, and on this account it has been named Circus Flats. Lyingeast-southeast of Bear Butte—that is, in a similar direction, relative tothe greater mass, to that of the feeding reservoirs of Deadman orWhitewood laccolith—there is good reason for the assumption thatthis doming up of strata to the southeast indicates a subterranean lac-colithic swelling that marks the first effort of the Bear Butte magmaprior to the production of the fissure through which was forcedupward the mass which now makes the mountain.
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JAGGAR.) DEFORMATION BY USTTRUSION. 223 The deformation of the seuimenta at the western base of the moun-tain indicates a sharp synclinal bend between the east-dipping mem-bers of the Circus Flat series and the beds upturned by the Bear Butteirruption. The section here exposed is shown in PI. XXX. Imme-diately at the base of the mountain, dipping- ♦)(» to the westward, is adense gray quartzite overlain by red and Iniif sandstones and whitesandstone. Intercalated among- these sandstone beds are sills of rhyo-lite 30 and 50 feet thick, representing- that portion of the magmawhich breaks through the bends, as indicated in the original typesections in the Henry Mountains. The topographic expression of this section is shown in the severalprofiles (fig. 86) and in the sketches (Pis. XXXI, XXXII). Espe-cially striking is the drainage of Circus Flats, illustrating many fea-tures of stream capture, .and the development of subsequent drainage,in a fashion that makes this locality a type and

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