ENDUCH.] GEOLOGY SAN LUIS DISTRICT, SECTION B. 331 



obliterated by the slight dip occurring' tliere. As a rule, it may be 

 Doted that wherever the Cretaceous rocks are in close proximity to 

 the granite westward, the Tertiary blufis will also be relatively close, 

 and* in the reverse case their position will be regulated by that of the 

 Cretaceous. At the southeastern end of section b it comes up very 

 close to the granite, and probably at some points farther south rests 

 immediately upon it. Near Dobson's Eanch several bluffs yielded fos- 

 sils that indicated their Tertiary age, belonging probably to the late 

 Eocene period. From there eastward, the blufis cease to be numer- 

 ous as soon as some distance from the mountaius is reached, arid the 

 gentle slopes stretch over the plains. As Avell as the Cretaceous, no 

 Tertiary Hood seems ever to have reached far into the interior of section 

 6, but to have remained east of it throughout ; south of it, however, 

 coming in more closely upon the higher lands. Besides archcean and 

 sedimentary, section b contains a limited quantity of volcanic rocks. 



Trachorheites. — Opposite station 4, to the north, a portion of the 

 trachytic material has crossed the Arkansas, extending into section b. 

 Mostly it is trachyte and tufi" of a light-gray to yellowish or brownish 

 color, flowing to the south and southeast. As stated, when speaking of 

 the Arkansas sandstone, a number of bluffs composed of it are capped 

 by the tuff, appearing on the maps as islands. To the west of station 

 4 several small isolated hills are composed of trachyte, that probably 

 at one time were in connection with the main mass north of the river. 

 On Texas Creek, about a mile west of station 8, volcanic activity was 

 manifested by trachytic rocks on a small scale, and thereby a number 

 of small hills have been formed, flanked by ashy material. Granite 

 surrounds this little patch of trachyte on every side, which may be 

 regarded as the sentinel of a larger, horizontally, quite extended eruption 

 and flow of trachorheites. From station 15 onward, in southeasterly 

 direction, we find a strip of these rocks about thirty-five miles in length, 

 and on average from two to four miles in width, bordered on either side 

 by granite. Station 83 is located at the southeastern extremity of sec- 

 tion b, on Mount Greenhorn, having an elevation of 12,341 feet, which 

 is composed of a compact andesite. Although of comparatively rare 

 occurrence, this mountain seems to present one of the volcanic ande- 

 sitic eruptions. The rock weathers in slabs from one-half an inch to 

 more than a foot in thickness, the thinner ones of which give a sub- 

 metallic sound when struck. A more massive eruption of the same ma- 

 terial continues to the northwest, beyond station 84. Proceeding in the 

 same direction, the character of the eruptive rocks is found to become 

 more trachytic, and, toward the lower hills bordering the range, decid- 

 edly rhyolitic. Usually the color of the rock is a light-gray, with a 

 pinkish tinge, representing the type in which the Kosita mines are 

 located, becoming almost white and very compact to the north. In 

 accordance with the general character of rhyolite, the foot-hills show 

 gentle outlines, are of rounded form, gradually sloping off toward the 

 vf^Uey, and are accompanied in the higher lands by the well-known 

 grassy or swampy plateaus. West of station 15 the lithological char- 

 acter of these rhyolites changes somewhat, as they become very com- 

 pact, show scarcely any crystallization of minerals contained in the 

 paste, and present a more or less stratified aiJi)earance. White ash sets 

 in, loose in texture, and of a feldspathic composition. Besides the 

 trachorheites, we have basalt in section b. A little southwest of the 

 warm spring, near Canyon City, two small hills occur, composed of a 

 black basalt, weathering in rough outlines. To the south of these, at 

 station 12, at an elevation of 7,026 feet, a cone of basalt is found, which 

 is very hard, of black color, and contains numerous inclosures of olivine. , 



