CHAPTER III. 



VOLCANIC ERUPTIVES. 



ACEDIC VOLCANIC ERUPTIVES. 



The class of acidic volcanic eruptives is the one by far most fully 

 represented in Colorado. To it belong the trachorheites and the 

 porphyritic trachytes. In using the comprehensive term of trachorheites, 

 I include therein all that uniform series of massive eruptives older than 

 the porphyritic trachytes and add to it the rhyolite. For the sake of 

 classificatory convenience the rhyolite will be enumerated di-rectly 

 after trachyte No. 4, thus leaving the question open of its relative age. 

 as compared with porphyritic trachyte. 



It has been mentioned above that propylite was not positively recog- 

 nized. This rock is the timazite of Transylvania, and, according to 

 Cotta, should be placed among the plutonic acidics. In case its exist- 

 ence should hereafter be established in Colorado, and it be found in 

 connection with the younger portions of the trachorheitic series, it 

 should be classed among the volcanic acidics. The development of 

 volcanic rocks in Transylvania is not so complete nor so typical as it is 

 in our Western Territories, and, therefore, as the oldest member of the 

 group, it could readily be regarded as a plutonic eruptive. In Colorado 

 more particularly, however, the correlation of groups is so well defined, 

 that if propylite should occur in connection therewith it could not be 

 separated from the acidic volcanic eruptives. "* 



TEACHOEHEITES. 



ANDESITE. 



As can readily be understood, the andesites, the oldest group, have 

 been subjected to many changes. Their position, with reference to 

 subsequent eruptions, aud their lithological constitution have rendered 

 them liable to alteration. Thus we find, at many places, a very com- 

 plete transition from typical andesite to typical trachyte. During 

 1873, while in the field, 1 was inclined to refer many of the eruptive 

 rocks to this group; but subsequent examinations showed me that its 

 occurrence, or rather its appearance upon the surface, is comparatively 

 limited. 



Perhaps the best exposures of andesite in Colorado may be found in 

 the Front Uange, west of Pike's Peak.* There the volcanics have found 

 their way to the surface directly through gneissic and granitic rocks. 

 Spreading from the central point, or along the lines of eruption, they 

 have produced plateau-like elevations, characteristic in every respect. 



Kichthofen says:t "Andesitic mountains are characterized by monot- 

 ony in scenery. They form continuous ranges, which are often of con- 

 siderable elevation and extent, but exhibit gentle outlines on their sum- 

 mits as well as on their slopes." This definition agrees closely with the 

 region above indicated. It remains to be stated that the ranges are not 

 formed by the andesite, strictly speaking, but summits and slopes of 

 certain ridges have obtained their character from that rock. Relative 



*Rep. U. S. Geol. Surv., 1873, p. 319. 

 t Mem. Cal. Ac. Sci., 1868, torn, i, part ii, p. 25. 



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