ENDLicH.] ACIDIC VOLCANIC ERUPTIVES — TRACHYTE. 217 



and have, in turn, been covered by the enormous masses of lava coming 

 from the southwest. 



One of the most typical occurrences is found on White Earth Creek.* 

 The tuffs are of white, gray, and yellowish colors, disintegrating very 

 readily. It is, perhaps, softer here than at most other localities, and 

 for this reason does not exhibit the usual picturesque products of ero- 

 sion. Farther northward, in Dr. Peale's district, the tuffs either do not 

 continue or they are covered by younger members of the trachorheitic 

 division. Inasmuch as granite is exposed along the Gunnison, north of 

 White Earth Creek, the former view is probably the correct one. The 

 extent of the tuffs on the White Earth could not be determined, as the 

 trachytes above them obscured everything. 



Two localities along the Rio Grande show a good development of the 

 White Earth group. At the junction of Lost Trail Creek with the Rio 

 Grande, we find a highly typical exposure.t Bluffs nearly 900 feet 

 high show the outward characteristics of variegated marls. Upon ex- 

 amination they prove to consist of trachytic tuff". A striking feature is 

 the marked, apparent stratification. This, however, is not due to con- 

 secutive strata in the proper sense of the word, but is owing to accu- 

 mulations of coloring material within certain horizontal zones. Mag- 

 netite is dispersed throughout the entire mass of the tuffs, and it is, 

 primarily, this that produces a number of colors. Different stages of 

 oxidation manifest themselves by different colors. Thus are produced 

 various shades of yellow, orange, brown, and green. Westward the 

 tuffs extend beyond Pole Creek, but are mostly hidden from sight by 

 trachyte. On Pole Creek they once more crop out, and it may be ob- 

 served that they are harder, more compact, than farther east. 



Another locality where the tuffs appear is near Crooked Creek, at the 

 head of Antelope Park. They differ in nothing from the others, and 

 are of but small horizontal extent. Although the characteristics and 

 appearance of this group within restricted areas are very constant, 

 their occurrence is practically an accidental one, and they are not 

 posessed of the same value in systematic classification as the younger 

 members of the Trachorheite group. 



An interesting occurrence was found north of Saguache Creek, near 

 Rock Cliff Dairy. Between two andesitic hills a valley was in part 

 filled with tufis4 Only a slight accumulation was found, resting upon 

 the andesites directly ; the remainder seemed to overlie portions of a 

 local Tertiary fresh-water deposit. Through this tuff" a dike of por- 

 phyritic obsidian had found its way to the surface.§ The black obsidian, 

 15 feet in thickness, contrasts admirably with the bright yellow, green- 

 ish, and pink colors of the tuff", l^umerous small fragments of the 

 tuff" are inclosed by the obsidian, and they are in this case thoroughly 

 baked. In appearance they closely resemble jasper. For several feet 

 on either side of the dike has this baking influence extended itself, 

 until the heat of the molten obsidian was no longer able to melt and 

 change the feldspathic tuffs. Small concretions of cloudy agate and 

 semiopal occur at the borders of the dike. ' It seems uncalled for that 

 siliceous concretions should occur so frequently near the junctions of 

 two different volcanic rocks, neither of which contains free quartz else- 

 where. From the constant form in which the quartz occurs — that of 

 jasper and semiopals mostly, less frequently of chalcedony and agate — 

 I am led to the inference that the silica may have been brought forth 

 in a hydrated condition, together with the younger erupted material. 



' * Kep. U. S. Geol. Surv., 1874, p. 202. t Rep. U. S. Geol. Surv., 1873, p. 344. 



t Rep. U. S. Geol. Surv., 1874, p. 201. $ Ibidem, p. 345. 



