258 EEPORT UNITED STATES GEOLOGICAL SURVEY. 



View Mountain,* he applies that name to the rocks comx)osing a num- 

 ber of dikes. Since that time it has been used when speaking of that 

 peculiar type of eruptives whose existence was first demonstrated in 

 Colorado. The term is by no means a new one, but the " porphyritic 

 trachyte " of former geologists has been superseded by liparite.t This 

 name, therefore, may be applied to the group under consideration, and, 

 inasmuch as it is inverted from the usual trachyte-porphyry, may be 

 claimed a,s a good, specific term. 



In Europe no erupted mountains have been found, so far as I am 

 aware, analogous to those formed by the group of porphyritic trachytes 

 in our own country. They have been studied very carefully and suc- 

 cessfully in Colorado by various members of the United States Geolog- 

 ical and Geographical Survey. Since 1873 their structure and import- 

 ance in the geological history of the regions has been recognized. As 

 the survey progressed, one mOuntain-group after the other was visited, 

 until to-day we are enabled to present a thorough analysis of distribu- 

 tion and structure. There is scarcely any one group within the limits 

 of Colorado that affords so inviting and remunerative a subject of study. 

 It has been found that the same class of mountains — referable to this 

 group — that occurs in Colorado extends west and south. The Sierra La 

 Sal and Sierra Abajo in Utah and the Sierra Carriso in Arizona furnish 

 evidence of this. Mr. Gilbert has studied, within the past few years, 

 the Henry Mountains. From his brief account| it appears that ttie 

 occurrences there are in strict conformity with those we have observed 

 in Colorado. He says : " The strata arch over the crest in a complete 



dome"; "the top of the dome has cracked open, and tapering 



fissures have run out to the flank, and they have been filled with molten 

 rock, which has congealed and formed dikes." "So the mount- 

 ain (Mount Ellsworth) is a dome or bubble of sedimentary rocks with an 

 eruptive core, with a system of radial dikes, and with a system of dikes 

 interlaced with the strata. It is a mountain of uplifted strata, distended 

 and permeated by eruptive rock." 



From a descrii)tion of the Chisos Mountains, given by C. 0. Parry,§ 

 it may be inferred that they belong to the same class of eruptives. 

 Wherever they have been observed their character is so definite that, 

 once recognized, they can never be mistaken for any others. No doubt 

 subsequent explorations of New Mexico, Arizona, and Utah will develop 

 the fact that the horizontal distribution of this interesting group is not 

 so circumscribed as it now appears to be. 



Khyolite is the remaining member of this class of rocks. Among all 

 of them this one has, perhaps, most frequently been the recipient of 

 misappellatious. Its mineralogical constitution brings it into close con- 

 nection with the older porphyries. What has been described as quartz- 

 porphyr from Transylvania is mostly rhyolite. Beudant has termed it 

 porhyre trachytique. Dacite is another name applied to the Hungarian 

 occurrences. Ehyolite is the name proposed by Richthofen ; nevadite 

 is one of his rhyolitic varieties. Although sufficiently well represented in 

 Colorado, it finds its greatest development farther west and south. Ee- 

 garding its age there is no serious disagreement in the observations 

 made. It is younger than trachyte, older than basalt. The case pro- 

 duced by Mr. Marvine, where, at Truxtoii Springs, Arizona, basalt was 



*Rep. U. S. Geol. Surv., 1873, p. 174. 



t Compare Cotta, Geologie der Gegenwart. Leipzig, 1872, p. 54. 

 t Rep. on Geol. of the eastern portion of the Uinta Mountains, by J. W. Powell, 1876, 

 p. 20. 



§ Rep. U. S. and Mex. Bound. Surv., W. H. Emory, 1857, vol. i, part ii, p. 56. 



