492 DESCEIPTiVE GEOLOGY. 



Those adjoining the main body consist of a micro-crystalline ground- 

 mass, having the flesh-color of the orthoclase-feldspar, with scattered crystals 

 of dark-green hornblende and greenish- white plagioclase-feldspar porphyriti- 

 cally imbedded. The groundmass, as in all these rocks, is an entirely crys- 

 talline aggregation of quartz, feldspar, and hornblende, with no mica. In 

 the more eastern members, the rock has a more felsitic appearance, the 

 hornblendes are of lighter color, and more generally disseminated through 

 the groundmass, and, with its varying proportion, the color of the rock 

 varies from greenish- white to greenish-purple ; the extreme member has the 

 external appearance of a volcanic rock, breaking with a conchoidal fract- 

 ure, and having a dark, semi-vitreous, weathered surface. In all of these, 

 the crystalline ingredients predominate over the groundmass. 



On the eastern butte, the rocks have a macroscopically homogeneous 

 groundmass, of light color, with but few distinct crystals. Here the granite- 

 porphyries are of greenish-drab color, showing crystals of flesh-colored 

 feldspar, scattered hornblende prisms, and a few flakes of black mica. 



The felsite-porphyry of the main mass is a white homogeneous rock, 

 with a few scattered black specks of hornblende and mica. Under the 

 microscope, this latter rock resembles the Cornish elvan, the groundmass 

 being an entirely crystalline aggregation of quartz and feldspar. The quartz 

 contains glass-inclusions, which are not found in the neighboring granite- 

 porphyries, and which are a rare occurrence in a wholly crystalline rock. 

 The micas -are seen to contain apatite. 



In microscopical behavior, the granite-porphyries present the same gen- 

 eral features. The micro- crystalline groundmass is comparatively coarse, 

 showing rarely any spheerulitic tendency and no amorphous base. The 

 quartz contains only liquid-inclusions, which are sometimes very abundant 

 in the more granitic varieties, and consist, for the most part, of a satu- 

 rated solution of chloride of sodium, with cubic crystals, two or more 

 often appearing in the same cavity. Some of the larger quartzes also 

 contain "stone-cavities", i. e., amorphous inclusions of microfelsitically- 

 devitrified substance. The amount of hornblende is comparatively large ; 

 apatite and titanite are both of frequent occurrence. 



