ENDLTCH.] GEOLOGY SAN LUIS DISTRICT, SECTION A. 319 



needed in petrography. Proi)ylite, andesite, trachyte, and rhyolite 

 are very closely related in lithological character as well as in the 

 mode of occurrence, &c., and almost invariably are found in close 

 proximity to each other. It seems impossible to trace, while in the 

 field, the boundaries of each one of these orders, although propylite 

 perhaps may be more readily separated than the rest, and I shall, there- 

 fore, speak in this report of trachorheites, which term comprises the 

 four above-mentioned rocks. Propylite does not occur in our district, 

 so the term will indicate only andesite, trachyte, and rhyolite, the 

 definition of which is accepted as Eichthofen gives it. 



The patch of formations belonging to this group occurring imme- 

 diately south of Pike's Peak presents as its most prominent feature 

 lighter colors than ordinarily, varying in color from light red and gray 

 to yellowish brown and bluish gray. Several very regularly-shaped 

 cones indicate the points of outflow^ in this portion ; Mount Pizgah 

 being one of the most prominent among them. Layers of more or 

 less thickness have overflowed tlie underlying granite, forming high 

 plateaus, with quite frequently steep edges. Farther west we find a 

 second large mass of trachorheites, upon which stations 69 and 70 are 

 located. Andesite forms the main bulk of this portion, accompanied 

 by a series of breccias and tuffs. Eichthofen says, {ibid., p. 25,) "Ande- 

 sitic mountains are characterized by monotony in scenery. They form 

 continuous ranges, which are often of considerable elevation and extent, 

 but exhibit gentle outlines in their summits as well as in their slopes. 

 Breccias only, which accompany the solid rock ordinarily in vast quan- 

 tities, cause local interruptions of the monotony by their more rugged 

 forms. They appear in castle-shaped rocks on the crests of andesitic 

 mountains, and form high walls, naked and steep along their slopes." 

 Long and narrow plateaus run out a considerable distance, falling off 

 steeply on several sides, usually on those farthest distant from the point 

 that supplied them with volcanic material. 



Frequently a stratified structure is noticeable, giving the bluffs a 

 very unique terrace-shaped form. It would be of importance to estab- 

 lish the mineralogical and chemical nature of these terraces relatively. 

 At 'some points, strata of white ashy material, that probably did not 

 reach its place of deposition in a liquid state, occur among the trachytes 

 and rhyolites, and can be traced for a considerable distance. As far as 

 could be determined, andesite forms the heaviest masses of hills and 

 mountains, but immense quantities of tuff and breccia, the latter 

 appearing sometimes as if it might have been redeposited by water, 

 cover the greater part of the andesite, and allow it to outcrop only at 

 comparatively few places. True trachytes are by no means wanting. 

 Station 69 is located on sanidin-trachyte, having a maroon-colored 

 cryptocrystalline paste, containing a large number of sanidite and oligo- 

 clase crystals, with here and there a crystal of bronze-colored mica and 

 of a hornblende prism. The tuff frequently forms cliffs of 100 to 150 

 feet in height, thereby imparting a very characteristic appearance to 

 the country. The trachorheites farther west scarcely vary in character. 

 Andesite seems to be the oldest rock among them, and forms the bulk 

 of almost all the higher peaks. Station 72 is located on a bluish-gray 

 andesite, with microcrystalline paste, with small crystals of oligoclase 

 and some of andesite. Narrow needles of hornblende are scattered 

 throughout, and a small quantity of black mica is found. Upon being 

 pulverized, magnetite can be extracted from the pow^ler. Hot vesicular 

 breccia, that has been covered again by a subsequent flow, and baked 

 toajaspery hardness, occurs throughout^ changing sometimes with a 



