PETROGRAPHY— EHY< )LITB. 495 



there being an occasional large piece. It contains microlitic needles ot 

 hornblende and but few cavities. 



Hornblende is quite plentiful in long, prismatic crystals and also as 

 minute needle-like microlites in great abundance. The crystals are of a 

 fine green color and dichroitic, but generally with ragged terminations, a 

 few of them only being sharply defined in form. 



The groundmass is crystalline, and consists of feldspar, associated with 

 quartz and hornblende needles, the latter being thickly crowded together. 

 The percentage of silica was determined to be 72.35, this large amount 

 being due to the quartz in the groundmass. 



The general behavior of the feldspar is much like that of granitic or 

 felsitic porphyry, particularly the impellucid border and the fibrous radiat- 

 ing aggregation of the large crystals, while again some appearances of the 

 groundmass are rhyolitic. Were it not for the geological occurrence given 

 as passing gradually into a trachytic rock, it would have been classed as 

 granite-porphyry. Unfortunately, no more exact observations could be 

 made as were intended, so that the rock must be provisionally named a 

 crystalline rhyolite of granitic character, until some future examination 

 shall confirm or change this determination. The rock [103], from the same 

 locality, presents no essentially different appearance from the preceding, 

 though it is somewhat fresher and the feldspar crystals more transparent. 

 Silica is 71.13 per cent. 



The rhyolite [109] from southeast of Terry Peak is of a light greenish- 

 gray color, with lustrous transparent crystals of sanidin plentifully scattered 

 through it. The rock is quite compact in texture, being hard and tough. 

 With the microscope, were observed sanidin, plagioclase, quartz and some 

 hornblende in a groundmass, which is an aggregation of very small needle- 

 like microlites. The large amount of silica present (75.09 per cent.) con- 

 firms the determination of the quartz in the thin section. 



The sanidin is abundant in fine, clear crystals, both large and small 

 polarizing brilliantly. Some are twinned according to the Carlsbad law, one 

 part of the twin with crossed nicols being- blue and the other colorless. They 

 also contain inclusions of glass, magnetite and small rectangular plagioclase 

 crystals, the latter showing very plainly, owing to their different color from 



