196 GEOLOGICAL SURVEY OF THE TERRITORIES. 



The group is very characteristic and sufficiently constant in its appear- 

 ance to be recognized ; must not, however, be mistaken for some of the 

 local accnmulations of tuffs that occur in higher trachytic beds. 



No. 2 does not show so many variations as the preceding number. 

 IMainly formiug either low plateaus, grassy or wooded, or appearing in 

 long, narrow ridges, it covers that section of country along the lower 

 tracts bordering upon the streams of the eastern part of the district. 

 Frequeutly caiions are cut through it, and then the walls are mostly 

 very steep. In color, it shows but little variation. When freshly 

 broken it has a i3ink tint, but npon exposure to atmospheric influences 

 becomes brown. Near the boixler of the volcanic area, it is the forma- 

 tion most frequently met with. Toward the upper strata (there are 

 only few in this group), bands and nodules of porphyritic pitchstone, 

 and of obsidian set in, running parallel to the stratification of the 

 trachyte. At times they are several feet thick, but rarely extend for 

 any distance. Between stations 21 and 22, however, one baud was found, 

 from iour to eight feet thick, that extended for several miles. Twelve 

 hundred feet may be considered the average thickness for these strata, 

 the most continuous and easily-traced ones of the entire series. The 

 rock generally contains a great many small crystals of sanidite ; crystals 

 of black mica are disj^ersed throughout the entire mass. At some 

 localities narrow prisms of hornblende occur, dark green to black in 

 color. Upon exposure the mica assumes a splendent bronze color. 



No. 3. — This group can readily be distinguished from No. 2 by the 

 darker colors shown in its lower member, and the lighter ones its 

 higher strata exhibit. In some localities plateaus are formed by No. 

 3, but more frequently it forms the highest blulis of narrow ridges. 

 It is divided into two subdivisions, the lower and the upper No. 3. 

 The former can be recognized by its dark color and its more precipitous 

 character, while in the upper, the colors are by far lighter and the steep 

 features less prominent, as it decomposes more readily and therefore 

 forms slopes rather than vertical bluffs. Lithologically there are dis- 

 tinctions also, as will be seen below. No. 3, lower, at times shows co- 

 lumnar structure, and on account of the dark color it assumes upon 

 weathering, can be mistaken for basalt. Brown is the prevailing color 

 of the lower members, while the upper ones are lilac, shading into gray 

 and reddish. For the latter the thickness may be estimated at 1,000 

 to 1,500 feet, when fully developed; for the former at 800 to 1,000 

 feet. This formation is well developed at a number of points, and 

 will be spoken of in more detail when treating of the localities where 

 it occurs. These two last numbers mainly give the country that ap- 

 pearance which has induced me to term it a " bluff country," a charac- 

 ter which extends from the 107th meridian to San Luis Valley, of course 

 with some interruptions caused by local upheavals or other disturbances. 



Numerous sanidite crystals occur in the sometimes compact, some- 

 times slightly vesicular, paste; more in No. 3, lower, than in the upper 

 subdivision. Mica is found more sparingly, black when fresh, bronze- 

 colored after having been exposed to trtmospheric influences for some 

 time. No. 3, upper, contains less sanidite and more mica, and may be 

 distinguished by the lighter color of its paste. Toward the upper por- 

 tion of this number a series of beds occurs, that have almost the character 

 of a paste without any segregated minerals. In color, they are usually 

 light, compact as a rule, rarely vesicular. 



No. 4 is by far more varied in its several members than any one of the 

 preceding series. It is to this group chiefly that the region designated 

 as "mountain country" belongs, and much of the wild, picturesque 



