232 EEPORT UNITED STATES GEOLOGICAL SURVEY. 

 PORPHYRITIC TRACHYTE. 



By far greater interest than in any of the preceding groups of erup- 

 tive rocks is centred in the group of porphyritic trachytes. This terra 

 has been applied by the geologists of this survey since 1873. As the 

 knowledge of the special character and of the general correlations ac- 

 cumulated, the appellation was restricted somewhat. At present we 

 comprise under the name of porphyritic trachytes eruptions that (1) are 

 isolated as to their topographical character and geognostic position ; 

 (2) show all the typical characteristics of trachyte, with such additional 

 details as may specifically separate them therefrom. The separation 

 I hat has been made from the beginning has been fully sustained by 

 evidence subsequently collected. Such evidence is furnished not only 

 within Colorado, but also in adjacent Territories. In a very ablepaper* 

 Dr. A. C. Peale has set forth and discussed the leading features of this 

 group. His personal examinations of a number of the localities involved 

 admirably fitted him to complete the task. 



MIDDLE PARK. 



Mr. Marvinet mentions the occurrence of porphyritic trachyte at 

 Park View Mountain. It occurs there in the form of dikes, analogous 

 to other localities. It appears that the dikes are not confined to Park 

 View alone, but spread out from it and assert their influence wherever 

 they go A small map accompanying the report of Mr. Marvine shows 

 a quadricircular radial arrangement of the dikes. He says : " These 

 dikes vary from 5 to 30 feet in thickness, some being apparently over 

 five miles in length, and extending across the country like huge, 

 broken walls. Where several intersect or occur near one another, their 

 combined resistance to erosion has formed a hill, every spur of which 

 contains a dike." These latter are well defined, and are composed of 

 very characteristic rock. A greenish, micro-crystalline paste contains 

 numerous opaque white crystals of oligoclase. Large simple and twin 

 crystals of orthoclase occur throughout the mass, associated with thin 

 transparent laraiuse of oligoclase. Small six sided crystals of chlorite 

 are Ibund in the paste. 



In speaking of dikes found on Williams River, Mr. Marvine | has cor- 

 rectly interpreted the method of action and the importance of the por- 

 phyrite trachytes from a geological point of view. He says : " The in- 

 trusive masses, instead of breaking across the strata here, 



followed along their planes of bedding, and forcing apart and upward 

 the strata between which they wedged themselves, caused them to in- 

 cline." We have here, therefore, the same action that has 



taken place in the Spanish Peaks, La Plata and Henry Mountains, 

 though not carried out to its fullest extent. The result has been, as ob- 

 served by Mr. Marvine, the intrusion or interleaving of sedimentary 

 beds with eruptive material. Evidently the force exerted was not suf- 

 ficiently great to cause an extensive rupture, and the trachytes remained 

 hidden from sight to a great extent. The recognition of this fact and 

 of the ultimate result, in spite of difficult surroundings, is one that re- 

 flects great credit upon the work of our former co-laboring geologist. 



MOUNT RICHARD OWEN. 

 Station 32, of 3874, made by Dr. Peale and Mr. Gannett, is known as 



* Bull. U. S. Geol. Surv., No. S, vol. iii, 1«77. t Rep. U. S. Geol. Sarv., 1873, p. 174. 

 i Rep. U. S. Geol. Surv., 1873, p. 186. 



