270 EEPORT UNITED STATES GEOLOGICAL SUEVEY. 



the more firmly I became convinced of the correctness of the hypothesis. 

 We have in Colorado mainly the two great groups of volcanic eruptives, 

 the trachorheitic and the basaltoid. Of these, I regard the former, to- 

 gether with the Elk Mountain eruptives, as remelted metaaiorphic gran- 

 ite, and the latter as the product of a series of hydrothermal fusions. 

 In speaking of the trachytes of the Auvergne, Quenstedt says,* "The 

 trachytes of the Auvergne, protruding from the granites, appear to be 

 simply a granite altered by heat and Uevoid of its free quartz." Leopold 

 von Buch, who made many careful examinations of that region, says : + 

 " Granite is the original mass from which this lava has been formed." 



The reasons for assuming such an origin for the trachorheites are : 



(1.) Tliey occur mainly within granitic areas. Although spreading 

 in many directions over sedimentary beds, they start from metaraorphic 

 regions. 



(2.) The similarity of composition of trachorheites and of granite upon 

 ultimate analysis. 



(3.) The enclosures of foreign granitic masses within trachorheites in 

 various stages of fusion. 



(4.) The definite ratio exhibited by trachorheites; the older ones pre- 

 senting types that admit of ready fusion, the younger ones less so. 



Taking into consideration the main area of trachorheites, we have 

 been able to discover but one single point of outflow. This is located 

 not far from Uncompahgre Peak.| This is located so that, although no 

 metamorphics are exposed within the immediate vicinity, their existence 

 there, unincumbered by sedirqentary strata, is undoubted. 



An interesting feature is that of foreign enclosures. They are of such 

 a character that they cannot always be regarded as having existed on the 

 surface at the time of the eruption, and having been enveloped by thelava, 

 but as having been carried* upwards along with it, from a point perhaps 

 but very little removed from their sources. Dr. Loew has observed 

 similar conditions in New Mexico. He says:§ "Here the rock {rhyoUte) 

 exhibits a close relation to the granite which it overlies, inasmuch as it 

 encloses semi-fused fragments of the latter. Moreover, we can trace 

 quite distinctly the eflects of various degrees of heat upon masses of 



feldspar It would appear that we here have a granite 



with partial transformation into a rhyolite On the 



San Francisco, seven miles above its mouth, masses of rhyolite occur 

 that contain through the whole particles of kaolin." Emmons mentions 

 an analogous occurrence.|| In the rhyolites of Washoe Mountains a 

 specimen was found showing quartz which contained " a number of 

 liquid-inclusions with mobile bubbles." Zirkel mentions that near this 

 fragment of quartz he found a piece of decomposed feldspar, and comes 

 to the conclusion that both minerals are "bodies foreign to the rhyolite." 



An examination of about two hundred specimens of metamorphic 

 granite from Colorado showed that none of them was without magnetite. 

 Some contained considerable quantities. 



Averages taken from, respectively, ten and thirteen analyses show 

 the percentages of silica and ferrous oxide to be as follows, viz : 



Trachyte. Ehyolite. 



Silica..-. - 60.87 74. 90 per cent. 



Ferrous oxide - 3.76 1.75 percent. 



* Epochen der Natur, 18t51, p. 162. 



t Mineralogische Briefo aus Auvergne, 1802. 



t Compare Eeps. U. S. Geol. Surv., 1874 and 1875. 



$ Rep. Expl. and Surv. West 100th Mer., vol. iii, 1875, p. 641. 



II Geol. Expl. 40th Parallel, vol. ii. 1877, p. 483. 



