568 DESCRIPTIVE GEOLOGY. 



Few localities afford a better opportunity for the study of the various 

 modifications in form, texture, and mineral composition, of trachytes than 

 the Wah-weah Range. There are represented here both the normal 

 sanidin-trachytes and hornblende-oligoclase-trachytes. The former, which 

 are generally the younger, are rough, porous, light-gray rocks, rich in 

 sanidin and brilliant flakes of biotite. In the latter, the groundmass is more 

 compact, more homogeneous, darker in color, and rich in triclinic feld- 

 spars, mostly oligoclase, associated with well-developed dark hornblendes. 

 Between these two types are seen every variety, from those rich in either 

 hornblende or mica to those in which both minerals would appear to opcupy 

 a very subordinate position. In one of the normal sanidin-trachytes. 

 Professor Zirkel detected, under the microscope, the somewhat rare occur- 

 rence of blue haliyne. Associated in the same rock were also found minute 

 forms of apatite. Quartz, which is a characteristic ingredient of rhyolite, 

 also occurs in some trachytes, and Professor Zirkel has pointed out that 

 the quartz of trachyte is almost always present, as a secreted mineral, in 

 large macroscopical grains, whil§ in rhyolites, in addition to the larger 

 masses, it occurs in minute grains disseminated through the groundmass. 

 Quartz-bearing trachytes are also well developed in the Wah-weah Range, 

 . and would appear to form no exception to the general rule, the rock possess- 

 ing all the habit of trachyte with the addition of large grains of clear, 

 colorless quartz. In the same region are also found augite-trachytes, the 

 groundmass of which presents a dark-gray, uniform character, with a 

 resinous, oily lustre, so marked in the augite-andesites. Scattered through 

 this dark groundmass are numerous light-colored feldspars, which give the 

 rock a decidedly porphyritic appearance. Brilliant striated plagioclase 

 crystals are abundant. Apatite may be detected under the microscope, 

 and the feldspars shown to contain some remarkable glass-inclusions. 



The rhyolites of the Wah-weah Range also play an important part in 

 the volcanic activity, and are found skirting the hills both at the extreme 

 northern and southern flanks, and also on the' summit of one of the higher 

 points. They all possess the same general habit, a bluish-gray microfelsitic 

 groundmass, characterized by large numbers of irregular black grains of 

 quartz and broken crystals of sanidin. In some localities, mica forms a 



