274 A UGITE-A NDESITE. 



breccias are seen, and in Owhee Bluffs pink and red angular frag- 

 ments are embedded in the thin lava streams like pieces of inlaid 

 wood. At New Pass, in the Desatoya Mountains, the rhyolites are 

 not less than 1000 feet thick. The breccias in this pass are white 

 and green below, and pink and red above. The sanidine, found in a 

 porphyritic rhyolite on the west, has a play of colour like labradorite, 

 and sanidine with this property is also found in the Pahute and 

 Augusta Mountains. It is noticed that the soda in this sanidine is 

 almost equal to the potash. At Antimony Canon the rhyolites are 

 6000 feet thick, and throughout the region of the Augusta and Fish 

 Creek Mountains the whole country is covered with rhyolite to the 

 thickness of from 2000 to 7000 feet, and this exposure is nearly 100 

 miles long, by from 12 to 20 miles wide ; but the character of the rock 

 seems to have changed a little with each successive outpouring. On 

 the eastern base of Pahute range, the rhyolite is a minutely microf elsitic 

 rock, approaching the texture of porcelain, and has few minerals that 

 can be recognised by the naked eye. In Eayless Canon, in the 

 southern end of the Montezuma range, is a ridge of rhyolite a mile 

 long, and 300 to 400 feet high, made up of well-developed prismatic 

 columns, varying from an inch to two feet in diameter. In almost 

 every district the colour is very variable, and frequently passes through 

 such tints as white, pale green, pale lilac, bright indian red, deep 

 purple, and olive brown. Its laminated structure is often marked by 

 bands of colour as fine as the leaves of a book, though it is as fre- 

 quently structureless ; and in mineral composition it presents as wide a 

 range as any known rock. Tertiary rhyolite is never covered by any 

 lava except basalt. 



Augite Andesite. 



Augite andesite is essentially a combination of augite with plagio- 

 clase, and is free from olivine. This absence of olivine is held by 

 Eosenbusch to similarly characterise diabase, and to distinguish that 

 rock from basalt and melaphyre. These rocks further differ from 

 basalts in frequently containing hornblende and biotite, and the quan- 

 tity of augite is less than in basalts. The ground mass may be crystal- 

 lised or glassy, and occasionally shows fluidal structure. The char- 

 acteristic crystals in it are oligoclase and augite. 



The percentage of silica ranges from 55 to 60. Occasionally there 

 is a little quartz, which rarely occurs in basalt as an original element. 

 The silica more frequently is used up in the formation of sanidine, 

 which sometimes may make up as much as one-half of the felspar, 

 though the quantity is usually subordinate. Magnetite and apatite 

 are always found, and in a few localities tridymite occurs. The chief 

 products of decomposition in augite andesites, are chlorite, iron oxide, 

 calcite, opal, and chalcedony. 



The plagioclase is found in rod-shaped crystals, but the larger 

 forms become twins. The felspar is often more or less replaced, and 

 is converted into opal in the augite andesites of Hungary. The 





