ENDLicH.] ACIDIC VOLCANIC ERUPTIVES PORPH. TRACHYTE. 237 



SIERRA EL LATE. 



The El Late Mountains are located in the extreme southern corner of 

 Colorado. Mr. Holmes visited and examined them during 1875. His 

 report thereon is thorough and replete with interest.* Essentially the 

 genesis and present structure of the group is the same as that of the 

 La Platas. Ute Peak is the highest mountain of the El Late. Mr. 

 Holmes furnishes a very lucid explanation of the primary distribution of 

 the porphyritic trachyte. He assumes an ejection through a narrow 

 fissure or tube in the hard, underlying sedimentary beds. Upon reach- 

 ing the softer, more easily yielding Cretaceous shales, the volcanic ma- 

 terial found an opportunity to expand laterally. An arching of the 

 shale strata was formed, and during the passage of the lava these were 

 broken into innumerable fragments. Being inclosed in the hot, viscous 

 trachyte, they were, in part assimilated thereby, in part very thoroughly 

 metamorphosed. A notable fact is, that fragments of no other rock 

 than of shales is found in the trachyte. This tends to show that the 

 underlying strata were either very little broken, or, if broken, the frag- 

 ments have been so thoroughly altered as to enter into the composition 

 of the trachyte. This latter view seems probable from the fact, that 

 Mr. Holmes observed small crystals of quartz in the rock from Hermano 

 Peaks. At that point the rock is composed of a bluish-gray micro- 

 crystalline paste, with large crystals of white oligoclase. Crystals of 

 sa-nidite occur sparingly and are very minute. Acicular crystals of a 

 green amphibolite are dispersed throughout the entire mass. 



A number of dikes are in connection with the group, some of them 

 extending for considerable distance. Viewed as a whole, the mountain 

 group does not appear so much as the result of one massive outpouring 

 of volcanic material, but as a distention, on a large scale, of sedimentary 

 beds by the intrusion of trachytic masses. The type is expressed in this 

 mountain group as well as in the La Platas, and is one that may be regarded 

 as a standard for eruptions of porphyritic trachyte. Distinct in all 

 minor details from other ejections of volcanics, they present a class that 

 cannot be mistaken if once the genesis and structure is recognized. It 

 seems probable that eventually groups of mountains that now are not 

 fully understood will be referred to this class. 



SAN MIGUEL GROUPS. 



Mount Wilson Growp. — The highest of all isolated trachytic groups is 

 that containing Mount Wilson. Eising from a base of about 8,000 feet, 

 this peak reaches an altitude of 14,280 feet.t During the season of 

 1874 1 visited the locality, after having seen the enormous outflows of 

 volcanic material to the east and northeast. This group stands per- 

 fectly isolated at present, but I am inclined to assume a former connec- 

 tion to the eastward. So far as could be determined, a portion of the 

 base of the mountains is formed by trachyte No. 3. Porphyritic tra- 

 chyte has broken through this, however, and occupies the most central 

 l)ositions. We have in this instance a more complete type of eruption 

 than is generally observed among the rocks of this class. This fact may 

 account for the superior height of the main peak. Dikes occur in con- 

 nection with the main mass, and the characteristic dome-shaped curv- 

 ing may be noticed. It seems, however, as if the force projecting the 

 volcanic material upward had been one so severe that ruptures imme- 

 diately took place sufficiently large to allow the passage of enormous 

 amounts of the lava. 



* Rep. U. S. Geol. Surv., 1875, p. 272. tRep. S. U. Geol. Surv., l«74,p. 207. 



