PEALE ON ERUPTIVE MOUNTAINS IN COLORADO. 557 



come more and more isolated. The priacipal mountain-masses close to 

 the main Elk Monntains are Crested Butte, (xothic Mountain, and 

 Cinnamon and TreawSurj'^ Mountains, from which they are separated 

 by Kock Creek and East River. 



Crested Butte and (xothic Mountain are both trachytic, and are prob- 

 ably connected by a dike. The eruptive material appears to have been 

 pushed up thronu'h a fissure, and spread iu sheet-like masses between 

 the Cretaceous layers, especially at (xOthic Mountain.* The latter is 

 named from the Gothic sfures iuto which its rocks have been eroded. 

 This is a common form of weatherino- in tliese isolated mountains. 



Cinnamon and Treasury Mountains are cotn{)()sed mainly of sedi- 

 mentary beds much metamorphosed. The former is of Cretaceous shales 

 intersected by dikes. Tre<isury Mountain is an oval-s!ia[)ed quaquaver- 

 sal, composed of PaUcozoic rocks, with Jurassic and Lower Cretaceous 

 reaching^ high up ou the arch, while Cretaceous shales are ranged 

 around and support the base. The entire series up to the Middle 

 Cretaceous is highly metamorphosed and intersected by numerous dikes. 



West and southwest of these groups are those described in the Report 

 for 1874 (pp. 163-168). In these the isolated character is beautifully 

 shown. They stand like islands in a sea. of Cretaceous rocks. The 

 latter are tipped against the eruptive nuclei to a greater or less extent 

 around the margins of the mountains. Only one of the groups (An- 

 thracite groupt) has the appearance of a range. The most western 

 group is about forty miles from the central portion of the Elk Mountains. 

 The rocks in these areas ditfer somewhat among themselves, although 

 there is a general resemblance in all to those of the Elk Mountains. In 

 Mount Marcellina, and in the large area adjacent on the north, it can- 

 not be distinguished from the Elk Mountain granite (?), and yet in some 

 places it grades into a porphyritic trachyte. In the more southern and 

 western areas, the trachytic (duira(;ter [)revails, especially so in the dikes 

 and intrusive sheets. Mount Owen is composed of highly metamor- 

 phosed shales an(l sandstones with intersecting dikes, closely resembling 

 Cinnamon Mountain. From the igneous masses, both dikes and intru- 

 sive sheets radiate, intersecting and separating the sedimentary strata. 

 The most western large area| seems to have had three centers of eleva- 

 tion, connected by narrow belts of igneous rock. Fragments of the 

 shales have been caught in the eruptive rock. West of this area, 

 between two low hills, is a finger like mass of trachyte rising several 

 hundred feet above the surrounding almost horizontal Cretaceous shales. 



SIERRA LA SAL. 



The Sierra La S.il is in latitude 38° 30', longitude 10!P 15', about V20 

 miles west of the Elk Monntains. Although composed of three groups, 



* See sections J aiulU on slieet oppo.-iit.o p. 72, lle^ovt Lluited States Geologic. il Surv^ey, 

 1874. 

 t Area A on innp opposite p. l(i(i, Report United States (reolojrical Survey, 1874. 

 t Area F ou map opposite [). IG13, Report, 1874. 



