638 DESCRIPTIVE GEOLOGY. 



good deal of hornblende and biotite. But for the absence of titanite it 

 would closely resemble the granites of the Sierra Nevada. 



The porphyry, which also shows but a small outcrop in the same canon, 

 is a dark-greenish rock, containing white crystals of orthoclase feldspar and 

 of hornblende and quartz porphyritically imbedded. Under the microscope, 

 the quartz is seen to abound in fluid-inclusions, and to contain but very 

 few glass-inclusions. The groundmass is mostly crystalline, and made up 

 of feldspar and quartz-grains. It contains also a li-ttle amorphous base. 

 The feldspars are remarkable for including distinct prisms of apatite, which 

 is a rare occurrence. 



The diorite of Ravens wood Peak is generally a fine greenish-gray rock, 

 among whose uniformly fine crystals of plagioclase and hornblende occur 

 large, tabular masses of triclinic feldspar, rendered more or le^s impure by 

 included hornblende. The rock contains, moreover, some quartz, biotite and 

 a little apatite, and is frequently impregnated with iron pyrites. In contact 

 with the porphyry, the rock is considerably decomposed, the feldspars be- 

 ing more or less kaolinized, and the hornblendes being of a light-green 

 color, while the biotite is more abundant. 



The structure of the sedimentary rocks exposed in the canons to the 

 south of Ravenswood Peak is much obscured by the accumulations of debris 

 and flows of rhyolite. The western bodies exposed near the summit of the 

 ridge have a strike about north 20° east, and dip steeply to the westward. 

 They consist of greenish clay slates and white and blue quartzites, the 

 upper member being a dark, nearly black, cherty quartzite. In the canon 

 immediately under Ravenswood Peak, these rocks, adjoining the granite, 

 dip steeply to the eastward, and near the mouth of the canon are overlaid by 

 a body of 500 to 600 feet of blue limestones. The same blue limestones are 

 observed on the north spur of Ravenswood Peak, having a strike a little 

 east of north, and dipping also to the eastward at a high angle, underlaid 

 by similar quartzites. The limestones are considerably metamorphosed, 

 and in some instances almost marbleized. In their upper portion is a body 

 of reddish sandstone, while near the base are the same black, cherty quartz- 

 ites observed to the south of Ravenswood Peak. No fossils were found in 

 any of these limestones, nor is their lithological character very typical. 



