PEALE ON ERUPTIVE MOUNTAINS IN COLORADO. 561 



ure. It is an island of trachyte in a sedimentary sea. The surrounding 

 foniiatious belong- to the Lower Cretaceous, Jurassic, and Triassic, with 

 perhaps two small outcrops of the Upper Carboniferous. There is a 

 central nucleus of trachyte, from which the sedimeutaries dip away to 

 a limited extent, and then flatten out, i. e., become horizontal On these 

 sedimeutaries, and dippiug with them, is a bed of trachyte of similar 

 character to that of the central nucleus. In the uplift, great fragments 

 of the stratified beds have been caught and are held as iu a vise. In 

 most of the areas already described, the trachyte has been iu the Creta- 

 ceous shales. In the Carriso, erosion has removed the shales, and we 

 have au opportunity of seeing what occurs in the uuderlying strata. 



The trachyte resting on the sedimeutaries lies on the Lower Creta- 

 ceous, and probably represents a portion of a mass that was pushed Irom 

 tae nucleus to the sides aud forced between the strata. By erosion, the 

 connection with the central mass has been broken, as iu a siuiilar case 

 mentioned when describiug the Snow Mass area of the Elk Mountains. 

 Whether the original flow ever reached the surface, it is of course 

 impossible to say, on account of the enormous amount of subsequent 

 erosion. 



In reviewing the descriptions given above, it will be noticed that, 

 although the areas differ somewhat, they still possess points of general 

 resemblance, and that iu the Elk Mouutains aud adjacent groups we 

 can find exauiples of the structure seeu in the other groups. 



It is a question iu the case of the areas in Blue liiver Valley, at Sil- 

 verheels, and iu the Park Kange, whether the intrusive layers noted 

 between the sedimeutaries came through fissures, and theuce followed 

 the planes of stratification, or were intruded laterally frotu some central 

 mass, as is the case in the sout^ii western or San Juan groups and at 

 various places in the Elk Mouutains. Mr. Marviue believed the former 

 in regard to the Blue River area, and notes places where the strata were 

 broken across to the extent of several feet. On the west side of Blue 

 River is a hill of trachyte, which is the central mass from which these 

 intrusive sheets may have come, if we take the other view. At Silver- 

 lieels and iu the Park Range, we have not, so far as kuown, any such 

 centre. The following are the principal points of resemblance between 

 the various areas : — 



1. Their isolation. On glancing at a map containing all the areas 

 colored, tiie first thing noticed is their isolation. They seem to be scat- 

 tered about without any system. This isolation is distinctly recognized 

 in the Spanish Peaks, in the JMiddle Park, West Elk, and San .luan 

 areas, and in the Sierra La Sal and Sierra Abajo. Iu the case of the 

 Elk Mountains and the Mount Guyot area, the i)roximity to the great 

 mountain ranges is a])t at first to disguise the fact of tlieir isohition, 

 which, however, is evident on a closer study. Beyond the mountains, 

 however, they stand like islands rising high above the surrounding 

 sedimeutaries. 



2. The areas all present abnndant evidence of their eruptive char- 



