112 REPORT UNITED STATES GEOLOGICAL SURVEY. 



tion. This will, of necessity, result in producing large areas covered by 

 the same material, and whereas classification of isolated groups might 

 more readily be accomplished, it becomes difficult when the various 

 members under consideration have been able to commingle. 



Andesite occurs at various localit es, in rather a subordinate position. 

 Its relative position to trachyte is maintained, but its appearance is only 

 local. 



Trachyte is the group that claims our attention more particularly. 

 In 1874 I subdivided it into four numbers, mainly for the purpose of 

 facilitating description. Subsequent examinations have shown that 

 the divisions hold good over a very large extent of countrjs and I have 

 retained them. A very interesting conglomerate occurs in Ko. 3, which 

 shows that a temporary cessation of volcanic activity took place at 

 that period. As perhaps the most typical development of this group, 

 may be regarded the tJncompahgre Mountains. There the trachytes 

 occur in a thickness amounting to 7,000 feet. The four subdivisions 

 can be readily distinguished, not only by their position, but also by 

 their lithological and orographic features. From the Uncompahgre 

 region this series of trachytes extends southward until upward of 8,000 

 square miles are covered by it. Occurring, as it does, over so large 

 an area, the regularity of its members is necessarily a surprise. Local 

 features are certainly not wanting that produce slight variations from 

 the " standard," but they are of little consequence. In the highest 

 member of the trachytes (No. 4) the metalliferous veins of the San 

 Juan region occur. They show the same characteristics and same be- 

 havior that lodes of other formations exhibit. An analogous case, com- 

 paring more particularly the lodes of the lake district, is found in 

 Transylvania. 



Ehyolite, the youngest member of the division, occurs quite frequently 

 in Colorado. It may be found superimposed upon the trachyte, or it 

 may occur as vsome independent outflow. In the great group of tracho- 

 rheites, in Southern Colorado, it is almost invariably superimposed. 

 Its raineralogical constitution does not vary from that observed in 

 rhyolites elsewhere. Some doubt has been felt as to its age relative to 

 basalt. From what I have observed 1 should unhesitatingly pronounce 

 it to be older. In Colorado the two are rarely found in contact, and 

 then generally in more or less abnormal positions. 



Beside the extended area mentioned above, this division covers many 

 miles as more or less isolated groups, generally deposited upon meta- 

 morphic rocks, less frequently on sedimentary. 



Porphyritic Trachytes. — Distinct in their main features, though allied 

 in many respects, from the preceding division, are the porphyritic 

 trachytes. Br. Peale has published a very excellent article upon this 

 subject,* which presents views that I fully indorse. 



While the trachytes usually form extended flows, and, in conse- 

 quence, long unbroken benches and plateaus, the rocks of this division 

 are characterized by their isolated position and by their bold, abrupt 

 appearance. Mineralogically, too, they differ, and chronologically are 

 younger than the trachorheites. It is possible that the period of their 

 eruption may have been very near that of the rhyolite, inasmuch as the 

 lithological constitution is analogous; and they not unfrequently have 

 found their way through beds of the same age as those usually penetrated 

 by rhyolite. Be this as it may, they are totally distinct, nevertheless. 

 Many of the prominent isolated mountains or mountain-groups of Colo- 

 rado and adjacent regions owe their formation to this rock. Spanish 



~~ * Bull. U. S. Geol. Survey, No. 3, vol. iii. 



