ENDLicH.J ACIDIC VOLCANIC EEUPTIVES TRACHYTE. 219 



On the higher portions of the mountainous region lying to the west- 

 ward of Pike's Peak we find trachytes that are referable to this group. 

 They are generally found superincumbent upon the andesites or tuffs. 

 Eegular stratification is shown, indicative of the regularity of the 

 flows. It may be observed that the upper portions of each flow or 

 stratum have been remelted or baked by succeeding flows of hot lava. 

 In color, mineralogical composition, and i)hysical features, these occur- 

 reuces agree very well with the characteristic of the western rocks be- 

 longing to the same group. Not only do we find them in the formerly 

 continuous series of volcauics of this region, but they may also be ob- 

 served in counection with smaller local massive outflows. East of Wet 

 Mountain Valley, toward the Greenhorn Range, a large portion of the 

 metamorphics are covered by such trachyte. Local outcrops occur at 

 various points along the Arkansas River, along the southern and 

 southwestern edge of South Park, and in the southern portions of the 

 Sa watch Range. 



In the region of the San Juan Mountains the River group is admira- 

 bly well-developed. It is in sight at all those points at which the 

 tuffs have been mentioned, and overlies them uniformly. Along the Rio 

 Grande we have occasion to study the River group most effectually. 

 Disruption of the volcanic strata has there produced a number of ver- 

 tical or very steep bluffs. On their faces we notice a very uniform ap- 

 Ijearance of the rock and the characteristic obsidianitic inclosures near 

 the top. Throughout the entire district IsTo. 2 can be traced, always oc- 

 cupying the same relative position. An astonishing regularity of the 

 flows can be observed, and it is possible to follow the same unbroken 

 stratum sometimes for nearly 50 miles. This feature indicates the 

 grand scale upon which the volcanic eruptions of the region occurred. 

 Southward, throughout the extension of the San Juan Mountains the 

 River group appears near the base of the mountains. It there totally 

 obscures any of the tuffs that may occur. Few of the streams within 

 the main mountain-mass cut down low enough to expose this trachyte, 

 and we therefore find it almost exclusively along the edge. Owing 

 to a constant, though gentle, easterly dip of the stratoid flows, No. U 

 no longer can be found on the eastern border of the range, but is con- 

 fined to the western. Its character is uniform throughout. 



Following the course of the Rio Grande, we find a full development 

 of the River group at Wagon-Wheel Gap. The total thickness there is 

 about 1,200 feet.* In the lower portions of No. 2 numerous inclosures 

 of jasper, chalcedony, and agate occur. Many fragments of these may 

 be found on the south side of the gap. Toward the northeastern and 

 northern border of the volcanic area these trachytes continue. They 

 form some of the low rounded bluffs along Saguache Creek, and, extend- 

 ing beyond the Los Pinos agency, are found fully developed again on 

 White Earth Creek. Wherever favorable opportunities were afforded, 

 plateaus are formed, in receding terraces, by the members of this and 

 the next higher group. North and west of my district Dr. Peale has 

 found the continuation of the River group in his district.! 



Although evidences of forcible disturbances are not wanting in this 

 group, they are not so numerous nor so characteristic as higher up. 

 Wagon-Wheel Gap affords one of the most striking examples of this 

 kind. There a hill, located transversely to the course of the Rio Grande, 

 has been rent asunder, and the river has found its way through it. 

 Columnoid structure of the upper portions of the trachyte may have 



* Eep. U. S. Geol. Sarv., 1875, p. 154. i Ibidem, p. 93. 



