230 EEPOKT UNITED STATES GEOLOGICAL SUKVEY. 



and entering the younger volcanic rocks. An analogous case occurs in 

 South Park, where gold is found in some of the rhyolites. Mr. Marvine 

 found a number of rhyolite outcrops in his district for 1874. They are 

 all of small extent only, but typical. 



In the Uncompahgre group this rock occurs. It occupies, usually, a 

 position superimposed upon the trachytes. Bluffs and small hills are 

 formed by it near the edges of the main mass of volcanics. More ex- 

 posures are found on the northern side of the Gunnison Eiver than 

 south of it. At that locality the rhyolite overlies trachytic breccia.* 

 Within the main mass of the Uncompahgre Mountains a very interest- 

 ing occurrence of rhyolite was observed.} A very typical rhyolite was 

 found on a tributary of Henssen's Creek, near Station 10. At the base 

 of a deep, narrow valley heavy masses of basalt protruded through the 

 surrounding trachorheites. Numerous caves and tunnels, the proofs 

 of gaseous expansion, were found in the basalt. Overlying it was rhyo- 

 lite. This latter rock was thinly bedded; the stratification-lines were 

 well marked. At first sight it appeared probable that the rhyolite Lad 

 been ejected subsequent to the protrusion of the basalt. This would 

 not be in accordance with the adopted and usually verified succession 

 of the groups. Examination, however, showed that the rhyolitic strata 

 dipped toward the valley at an angle of 60° to 70'^, and we had before 

 us but a detached fragment of a more extensive rhyolitic flow farther 

 west. By the basalt this fragment had been carried up to its present 

 position, being too small to offer sufficient resistance to disruption. 

 At various times the relative age of rhyolite and basalt have been called 

 into question. Perhaps the most important observation leading thereto 

 was made by Mr. Marvine.| He found, at Truxton Springs, Ariz., rhyo- 

 lite overlying and interbedded with basalt. It is to be regretted that it 

 was impossible for him to find the sources whence these lavas came. 

 Probably a rhyolitic eruption may have taken place at some locality 

 long after the activity ejecting that rock had generally ceased, or an 

 eruption of basalt may have occurred before the main masses made 

 tbeir appearance. In view of the large amount of testimony which 

 favors the greater age of rhyolite, an isolated or even several isolated 

 cases of such a kind can have no influence upon the classificacory ar- 

 rangement. Basaltic rocks speak for a new era of volcanic activity from 

 their very lithological and chemical character, from their mode of occur- 

 rence, and from the intimate relations they bear to erupted material of 

 the present day. In contradistinction thereto, the trachorheites com- 

 prise a series that is well defined within its own limits, characteristically 

 separated from others, and one that brings to its close a period of the 

 most enormous volcanic action. 



Intrusive. — At two localities, mainly, were intrusions of rhyolitic 

 material observed, in the mountains of South Park and in the Elk 

 Mountains. At Horseshoe Mountains Dr. Peale has found § volcanic 

 rocks " interstratified" with sedimentary strata. The volcanics ap- 

 pear to be a rhyolitic trach.y te. By their passage between the separated 

 sedimentary beds they have very thoroughly metamorphosed the latter. 

 Sandstones are changed into quartzites, and limestones partly into 

 marble. Some of the volcanic interstrata show very excellent columnar 

 structure there, which is due to the rate of cooling and partly to the 

 pressure exercised by superincumbent beds. At the Silver Heels 



*Eep; U. S. Geol. Surv., 1874, p. 170. 



tEep. U. S. Geol. Snrv., 1874, p. 197. 



tEspl. and Snrv, West 100th Mer., vol. iii, p. 202. 



§ Eep. U. S. Geol. Surv., 1873, p. 234. 



