EKDUcn.i ACIDIC VOLCANIC EEUPTIVES TEACHYTE EHYOLITE. 229 



Near Mount Wilson a slender, trachy tic monolith attracted our atten- 

 tion.* Upon a steep ba?e we find an obelisk-shaped rock, 290 feet in 

 height. The tendency to columnar structure of the rocks composing 

 trachyte No. 3 has been noted above. It is owing to this tbat the rock 

 " Lizard's Head " has been able to form itself. It is but the remnant of 

 an extended flow, all other portions of which have disappeared under 

 the destructive hands of erosion and decomposition. 



EHYOLITE. 



With rhyolite I close the main group of trachorheites. Although 

 lithologically readily distinguished from the preceding members, it does 

 not offer points sufficiently characteristic generally to admit of separa- 

 tion in hurried field-work. It may be stated that, as a rule, rhyolite 

 is intimately associated with the older trachorheites. Exceptions, how- 

 ever, are not wanting. Dependent upon its associations, the character 

 of tbe rock is such as to be classed among either massive, intrusive, or 

 anarhactic eruptives. In the first instance we find it together with 

 trachyte; in the second it usually accompanies older volcanic rocks, and 

 m the third it may be observed at many points, without being in any 

 direct connection with other formations of similar genesis. 



At no point in Colorado were very extensive masses of rhyolite ob- 

 served. It occurs together with the trachorheites of the Front range, 

 in the Elk Mountains, in South Park, and in the Uncompahgre Mount- 

 ains. Frequently the connection with trachyte, both as regards posi- 

 tion and petrograpbic character, is so intimate that the question may 

 arise, whether or not we have belbre us simply a changed trachyte in- 

 stead of a specitically distinct rock species. 



Perhaps the most distinguishing leature of rhyolite, as compared 

 with other volcanics, may bo found in the presence of free silica. This 

 occurs, when visible, in the shape of small fragments, grains, or crys- 

 tals. Not unfrequently, however, it appears in none of these forms, and 

 then only the microscope or analysis can reveal the species of the rock. 

 In general habitat the resemblance of rhyolite to the various trachytes 

 is so great that unless Iree, visible silica be present, mistakes can readily 

 occur. 



Throughout Colorado we find rhyolite. At no place does it show 

 itself in large masses, but appears usually of subordinate importance 

 in connection with other eruptives. In order to present a general idea 

 as to the form in which it is most frequently met with, I shall discuss, 

 first, the massive, then intrusive, and finally anarhactic eruptions of 

 this rock. All of these are of great interest, and are important at the 

 regions where they occur. Intimately associated with other groups and 

 formations, the rhyolites comprise a prominent chapter in the geologi- 

 cal history of the State. 



Massive. — Flows of rhyolite, rather limited in extent, may be noticed 

 near the Front range, south of Black Mountains.t It there forms low 

 bluffs, skirting the base of the mountain. These are superimposed, in 

 part, on trachyte and tuffs ; in part, on Carboniferous strata. South 

 of the Arkansas River, west of Wet Mountain Valley, is the Eosita 

 mining district. There again we find rhyolite. Its horizontal extent 

 is not considerable. At that locality the rhyolite rests directly on 

 granite. Ore-bearing veins traverse both these rocks.| No change can 

 be noticed in the characteristics of the lodes upon leaving the granite 



* Rep. U. S. Geol. Surv., 1874, p. 207. tEep. U. S. Geol. Surv., 1873, p. SCO. 



lEep. U. S. Geol. Surv., 1873, p. 331. 



