516 DESCRIPTIVE GEOLOGY. 



In some of the decomposed feldspars are colorless acicular crystals, which 

 may be muscovite. The hornblende is remarkable for presenting, as a pro- 

 duct of decomposition, black, opaque, angular grains like magnetite, which 

 do not, however, occur in the fresher hornblendes. This body of granite- 

 porphyry is immediately overlaid by calcareous shales. 



To the east of the valley is a broad, table-shaped ridge, made up of 

 flows of rhyolite, showing great variety in structure and composition. The 

 main flow is made up of a grayish-white rhyolite, which has a rough tra- 

 chytic feel. It consists of a very porous felsitic groundmass, in which can 

 be distinguished only a few scattered crystals of free quartz, and long, thin 

 sections of tabular crystals of feldspar, often showing a very beautiful twin 

 development. The groundmass, examined closely, has a porcelain-like 

 appearance. Under the microscope, it is seen to be a mixture of transpar- 

 ent polarizing particles and dull yellowish-gray bodies. Adjoining this 

 rock on the east is a porphyritic rhyolite of reddish-purple color, still of 

 somewhat porous texture, containing large crystals of quartz and sanidin, 

 the former of which show frequently perfect pyramidal points ; likewise no 

 hornblende or mica. Under the microscope, the groundmass is seen to con- 

 tain fibrous sphserulites, often as much as half a millimetre in diameter, 

 which sometimes have a feldspar crystal in the centre. It also contains 

 tridymite. To the eastward, these rhyolites pass into distinctly hyaline 

 varieties, or pearlites. The first is a grayish porphyritic-looking rock, con- 

 taining large grains of quartz the size of a pea, and very glassy sanidins, 

 together with considerable hornblende and mica, in a gray pearlitic, and 

 somewhat granular groundmass. Under the microscope, the groundmass 

 is seen to be made up of imperfectly dihexahedral quartz, sanidin, plagio- 

 clase, brown biotite, some yellowish-green augites, a few brown hornblendes 

 and magnetite, together with pale yellow microlites, which are doubtless 

 augitic. Both quartz and feldspars contain plentiful glass-inclusions. The 

 mass between these ingredients shows an interesting arrangement of micro- 

 lites, which appear to flow around the crystals, as will be seen in Volume 

 VI, Plate X, fig. 2. The most remarkable of these hyaline rhyolites forms 

 low rounded hills on the extreme eastern slopes, toward Surprise Valley. 



