782 DESCRIPTIVE GEOLOGY, 



ish-brown color, looking in the mass like a fine-grained compact sandstone, 

 but disclosing, under closer examination, plentiful small crystals of glassy 

 sanidin and quartz ; the other is a porous, rough-feeling rock, of light mauve 

 color, showing white opaque feldspars and some included fragments of 

 other rhyolites. Under the microscope, the quartz and feldspars of the 

 latter are seen to have glass-inclusions, and the groundmass to be a net- 

 work of yellowish strings radially and concentrically arranged. In the 

 canon below these rhyolites, erosion has disclosed a bod}^ of blue limestone 

 and shales, which, on grounds of general probability, but without palaeon- 

 tological or distinct stratigraphical evidence, have been referred to the 

 Jurassic formation. The extreme eastern foot-hills at the mouth of the 

 canon are formed of a dark slate-colored rhyolite, whose weathered surface 

 resembles that of a basic rock, but which in fracture shows the peculiar 

 banded structure of rhyolites in wavy lines through the mass, in which can 

 also be distinguished decomposed feldspars and a little free quartz. 



On the main crest, just north of Pah-keah Peak, occurs a lavender-col- 

 ored porphyri tic rhyolite, of rough trachytic texture, and rich in small plates 

 of black biotite, besides which can be detected only a few white feldspars, 

 both glassy and opaque. The number of crystalline ingredients is not 

 augmented by microscopic examination. The groundmass shows no fibrous 

 structure, but only an undeveloped granular mass. 



The central and highest peak of these mountains, just to the north- 

 east of Pah-keah Peak, is of granite, to the north of which the next point is 

 formed of a rhyolite, which is the heaviest, most compact rhyolite of the 

 range, and has almost the appearance of an older porphyry. It is of a 

 greenish-yellow color, and contains small but distinct crystals of quartz, 

 feldspar, and mica in a compact felsitic groundmass, through which run 

 occasional thin, reddish, vein-like seams. Under the microscope, the 

 groundmass is seen to be formed of a network of peculiar linear aggre- 

 gations of grains, between which is a concentric, radially fibrous structure 

 like sphserulites. Glass-inclusions are found in both quartz and feldspar.' A 

 .section of this rock, giving its peculiar structure, as shown under polarized 

 light, is depicted in Vol. VI, Plate VII, fig. 2. Associated with this is a 

 rhyolitic breccia, formed of angular fragments of greenish lithoidal rhyolites, 



