ENDLicn.] GEOLOGY SAN LUIS DISTRICT, SECTION C. 345 



that could scarcely be distinguished from the andesite of station 56, 

 more than eighty miles distant. Variations occur within certain limits, 

 and are repeated at almost every place of occurrence in a greater or 

 less degree, but the specific character remains true to itself. In the 

 southern portion of section c andesite seems to form the highest peaks, 

 "vrell marked by their sharp, bold outlines, and their dark color, unusual 

 in a country of trachytes and tuffs, while these latter compose the lower 

 ridges and high plateaus. Station 34, west of Coochetopa Creek, is located 

 on a sanidite trachyte ; a brownish paste includes numerous crystals of 

 sanidite, and also some of hornblende, black and bronze-colored mica. 

 From that station north and eastward the tuffs, rhyolites, and trachy- 

 tic ashes become more numerous again than they were in the more 

 southerly portions, thereby changing somewhat the orographic features 

 of the country, although it cannot be denied that this change may be 

 partially produced by underlying sedimentary beds. The tuffs have 

 overflown the Cretaceous beds east of the Coochetopa, and were proba- 

 bly cooled under water, whereby their character is changed to a consid- 

 erable extent, as they present all possible varieties and differences in 

 composition and texture. Mostty, however, they are of light color vesic- 

 ular, containing crystals of sanidite, mica, and, if rhyolitic, free quartz. 

 At stations 26 and 27 we find a very interesting locality, showing a 

 comparatively large variety of volcanic rocks, and at the same time 

 the only deposit of sedimentary material that was observed in this great 

 lava region. Station 26 is located on a phonolitic andesite of a dark 

 bluish-gray color, slightly vesicular at places, containing small needles 

 of hornblende and small crystals of black mica and sanidite in a comj^act, 

 microcrystalliue paste. On the west side of the summit, the andesite shows 

 columnar structure, each column being separated at a right angle to its 

 longitudinal axis into numerous thin plates. Between stations 26 and 

 27 a low granitic hill makes its appearance, showing the andesitic cover 

 for some distance. This granite is thoroughly altered by the effects of 

 heat, which does not seem to have been quite sufficient, however, to bake 

 it. Upon this granite, («,) the Tertiary beds {d) are deposited, of which 

 mention has been made above, covered on their western side by the 

 rhyolitic ash lying upon the eastern slope of station 27. From white to 

 yellowish pink and greenish, this ash presents itself as an agglomeration 

 of feldspathic ingredients, mixed with small crystals of sanidite and 

 mica, grains of quartz, fragments of andesite, and of obsidian also occur, 

 altogether giving it the appearance of redeposited material. It is 

 quite possible that it was deposited there at a time when Tertiary waters 

 were still in that little basin, and that they owe a portion or all of the 

 quartz-sand they contain to the fact of having taken it up from the 

 lake-deposit. Not very far from the summit of the hill, the tuff becomes 

 more compact, the fragments it contains are larger, its color a dark 

 brown, forming a ijrominent bluff' on that slope of the hill. About 25 

 feet from the upper edge of that bluff", a horizontal dike or vein of ob- 

 sidian* (c) occurs, 15 feet thick, and extending across horizontally for 

 nearly 100 feet, until it is lost under the fallen debris. At the point 

 where our sketch is taken, the obsidian shows spheroidal concretions, 

 the largest one nearly 10 feet in diameter, the center of it forming a 

 solid, round ball, with the obsidian nearest to it separating from the 

 rest in concentrically spheroid scales. The obsidian contains small 

 crystals of sanidite — therefore belongs to the porphyritic variety — and en- 

 velopes numerous small fragments of the tuff". A curious fact is that the 

 tuff above the obsidian-dike is baked so thoroughly that it has become 

 jaspery in texture, while that below remains unchanged. Quartz is con- 



