222 REPORT UNITED STATES GEOLOaiCAL SURVEY. 



tingnished, even from long distances. Adding the more or less hori- 

 zontal arrangement of its beds, we find the conglomerates to be one 

 of the most conspicuous groups of the entire volcanic series. 



Trachyte No. 3.* — This subdivision I have separated into two groups, 

 No. 3 lower and upper. The distinction was made not only on account of 

 lithologieal, but also for orographic reasons. No. 3 may be regarded 

 as the most widely extended of the trachytic series. Wherever the Eiver 

 group, by virtue of the dip of the flows, has disappeared from sight, 

 there we find No. 3 in the lower portion^ of the region. Forming promi- 

 nent, characteristic bluffs, with steep faces, it may at once be distin- 

 guished from the preceding number by its darker color. Offset by the 

 conglomerates, wherever they appear, it sharply marks its own horizon. 

 In the interior of the main volcanic area it is generally found high up, 

 as the lower positions are monopolized by the older groups. 



No. 3 lower is recognizable by its physical appearance. Almost al- 

 ways it forms steep slopes. When found in the valleys it incloses them 

 with precipitous walls. A tendency to columnar structure adds greatly 

 to this feature. Lithologically it is also distinct. When freshly broken 

 the specimens show a reddish-brown color, but are dark-brown upon 

 weathering. Toward the top of No. 3 lower some lilac-colored beds set 

 in, but they are of inferior thickness. Numerous small crystals of sani- 

 dlte occur in the brown paste. This is sometimes compact, sometimes 

 slightly vesicular. Mica (biotite) is found sparingly. It is black when 

 fresh, splendent bronze-color after exposure. When forming the slopes 

 of mountains the rocks of this group generally weather in slabs or an- 

 gular fragments. Cases of reheating and baking may be observed com- 

 paratively rarely only. For this lower group a thickness of 800 to 1,000 

 feet was accepted, wherever fully developed. 



No. 3 upper is lighter colored throughout. The paste is a grayish-red 

 or brown and lilac or pink. Less sanidite is found in this group than in 

 the lower one, but more mica. Hornblende crystals occur as accessory 

 mineral. Toward the top of the group a series of beds occur that I 

 have characterized as "a feldspathic matrix without any segregated 

 minerals."t In color they contrast strongly with the underlying group. 

 They are either white, light yellow, gray, greenish, or pink. About 

 400 feet may be regarded as the maximum thickness of this series. 

 Although found at a number of places, their occurrence is not constant 

 throughout. It may be that they owe their existence to local outflows, 

 or perhaps they were primarily distributed over but a small area. The 

 connection has been obliterated by erosion and other causes. Within 

 these beds pitch-stones, mostly porphyritic, sometimes occur. A local 

 conglomerate occurs near the top of No. 3 in the vicinity of the Eio 

 Grande Pyramid, both west and north of it. It stands in no connection 

 with the main conglomerate, but closely resembles it in its essential 

 features. Two hundred feet may be regarded as its maximum thickness. 

 For No. 3 upper we can assume a total thickness of 1,000 to 1,500 feet. 

 Variations in thickness frequently take place, rarely involving more, 

 however, than a few hundreds of feet. More disturbances of the orig- 

 inal position of the beds have affected this group than either of the 

 preceding. From the characteristic position occupied by the members 

 of this group near the edges of the main volcanic area, and from the 

 striking appearance the strata there presents, I term it the "Bluff 

 group." 



In case the volcanic beds are alternately hard and soft, we will ob- 



* Report U. S. Geol. Surv. , 1874, p 196. t Rep. U. S. Geol. Surv., 1874, p. 200. 



