ENDLicH.] ACIDIC VOLCANIC ERUPTIVES RHYOLITE. 231 



Mountains, and several adjacent localities, Dr. Peale has observed the 

 same phenomena. Dr. Hayden* speaks of the same occurrences, and 

 refers to them as intrusions. 



One of the most striking examples is that shown at Gothic Mountain. 

 Dr. Hayden t gives an elaborate account of it. Ehyolitic rocks intrude 

 into the Cretaceous shales, forming prominent, steep bluffs in tbe ordi- 

 . narily gentle slopes near the base of the peak. Several beds of this 

 rhyolite may be traced, one above the other, following the direction of 

 the sedimentary strata. On Eock Creek Mr. Holmes noticed a very 

 excellent illustration of intrusion.^ A prominent mountain is there 

 formed by a "rhyolitic" rock. Upon investigation it was found that 

 the Cretaceous sbales at the base were in normal position while they 

 were dipping from the hill on one of its slopes. Further examination 

 proved " that this upturned portion had been separated from the rest 

 and forced upward by a wedge-like mass of intrusive rock, which be- 

 longed to the central mass of the group." 



From these few examples it will be seen that true cases of intrusion 

 may be found more frequently in rhyolitic areas than in those occupied 

 by older trachorheites. It seems that some of the rocks at the local- 

 ities just quoted more closely resemble trachyte than rhyolite, but the 

 majority appear to belong to the latter. Intrusion occurs also in con- 

 nection with the smaller dikes, but in that case is usually of but limited 

 extent. 



Anarliactic. — Perhaps the most prominent form in which rhyolite occurs 

 in Colorado is in the character of dikes. In connection, generally, with 

 one of the main centres of eruption, the dikes occupy definite positions 

 relative thereto. As a rule they are found near the edges of the main 

 mass, at places that were most favorably situated for disruption. Some- 

 times the general arrangement of the dikes is a radial one, and, again, 

 they may be more or less parallel. It is evident that any upheaval, such 

 as may have accompanied at least a portion of the volcanic eruption, 

 will mostly result in local rupture of the upheaved masses. Fissures 

 and cracks thus formed .will be injected with the liquid or plastic vol- 

 canic material, thus forming dikes. Any grouping of fissures in definite 

 order will be iproductive of dike-systems. Among all the erupted rocks 

 of Colorado none offer so much material for study of this class of ejected 

 material as the porphyritic trachytes. A more extended discussion, 

 therefore, upon the various features exhibited and the factors involved 

 will be deferred until treating of these rocks. 



At the head of " Oh Be Joyful" Creek Dr. Peale § mentions a dike as 

 setting through the sandstones. He describes the rock composing it as 

 having a " very compact, fine-textured, dark greenish matrix, in which are 

 a few small crystals of feldspar." Particles of free quartz also occur in it. 

 In the vicinity of the Elk Mountains and in the Horseshoe group these 

 dikes most frequently occur. Near Horseshoe Mountain Mr. Holmes 

 sketched several of the intrusive volcanics,|| which Dr. Peale describes 

 as being very highly siliceous. I class them among the rhyolites, 

 mainly on account of their association, and agree with Dr. Peale iu 

 his assumption that the high percentage of silica may be due to the fact 

 of their intrusion among the sandstones. 



Besides these two regions, rhyolitic dikes are of rare occurrence. 

 Ehyolite, at best, occupies but a subordinate position in Colorado, if 

 compared with other volcanics, and the comparative paucity of its 

 various occurrences is therefore readily understood. 



* Rep. U. S. Geol. Surv., 1873, p. 44. ^ Rep. U. S. Geol. Surv., 1874, p. 165. 



t Rep. U. S. Geol. Surv., 1874, p. 55. || Rep. U. S. Geol. Surv., 1873, p. 233. 



X Rep. U. S. Geol. Surv., 1874, p. 64. 



