OMBE MOUNTAINS. 499 



Although quite friable, many of the beds split up readily into thin plates, 

 with an almost iissile structure. Under the microscope, the rock shows its 

 eruptive origin, and its glassy, pumice-like nature. 



The volcanic outbursts connected with the Ombe Mountains neither 

 cover an extensive area, nor have they apparently had much effect upon the 

 present structural position of the older Palaeozoic beds. They are found, 

 for the most part, at the extreme northern end of the mountains, where the 

 Carboniferous limestones plunge down beneath the plains, and would other- 

 wise have formed the termination of the Ombe upheaval. At this point, 

 both rhyolites and basalts occur, the former preceding the latter, and being 

 in part concealed beneath the heavy basic rocks. 



A noteworthy feature of these rocks, taken together, is found in the 

 exceedingly glassy nature of the rhyolites, showing almost the extreme 

 form of glass base, in which at the same time well-defined crystals of feld- 

 spar are abundant; while the basalts, which in Nevada are frequently 

 characterized by a half-glassy base, are here found made up largely of a 

 fine but distinctly crystalline groundmass. The main occurrence of rhyo- 

 lite forms a rounded outlying hill, or butte, with steep slopes, rising some 

 300 feet above the valley, but falling away gradually to the eastward with 

 an angle of 2°, and presenting much the appearance of fortifications or 

 embankments. Lithologically, it more closely resembles many of the out- 

 bursts of the Nevada Basin than of the Plateau, and belongs, as mentioned, 

 to the glassy and half-glassy varieties. A specimen in the collection from 

 this hill is an exceedingly striking rock, made up of two kinds of glass, 

 mainly, however, of a reddish-yellow or amber-colored one, with white 

 sanidins and translucent quartz scattered through it. Imbedded in the 

 sanidin are macroscopical inclusions of the same amber-colored glass, 

 which makes up the base. In the report of Professor Zirkel, the micro- 

 scopical structure of this glassy base is described at some length, and a 

 thin section of the rock will be found illustrated in Plate 10, fig. 1, of his 

 report. Another rhyolite from near the same locality shows much the 

 same general character, but is lighter in color, and the glass appears drawn 

 out in narrow bands and streaks. 



The main mass of the basalt lies to the westward of the rhyolite, and 



