276 NORTH AMERICAN AUGITE-ANDES1TE. 



augite, and a little hornblende, with some brown biotite. The augite 

 always predominates over the hornblende. Occasionally, as at the 

 mouth of Spring Canon, this andesite contains a little sanidine. In 

 Melrose Mountain biotite predominates, so that the rock might be 

 termed a mica andesite ; but Clarence King regards it as evidently 

 comparable to the rock of Spring Canon, with which it may have 

 been originally continuous. In Egyptian Canon this rock exhibits a 

 columnar structure ; but Clarence King describes the columns as 

 cylindroids rather than prisms. Sometimes considerable quantities 

 of apatite are seen under the microscope. It is difficult to draw a 

 distinction between this rock when free from hornblende, and basalts 

 which are free from olivine. It is also seen on the eastern side of the 

 Cortez range, north of Jacobsville, on the Reese river, and in many 

 other localities. The crystals vary much in size, and frequently con- 

 tain both in the augite and plagioclase yellow or brown glass inclusions. 

 The rock is commonly covered by rhyolite, but occasionally, as in the 

 Reese river, by basalt. The augite andesite of Augusta Canon is 

 overlaid by trachyte, and it is on this account that the American 

 geologists have grouped it rather with the andesites than with the 

 basalts. 



In the Fortieth Parallel District it occurs to the west of Basalt 

 Creek in Washoe, where it is a resinous brownish black rock with 

 ledge-shaped felspar crystals, and shows under the microscope a 

 brownish-yellow glass ground mass. It abounds in yellowish green 

 augite with colourless felspar microliths, and black grains of magnetite, 

 with larger crystals of augite, sanidine, and plagioclase. Like nearly 

 all the known rocks of the same class, it contains 58 per cent, of silica. 

 Other augite andesites occur in the hill to the west of Steamboat 

 Valley, Nevada. In a ravine north of the Truckee Road, by the 

 Truckee river, the augite is green, and the rock contains half-decom- 

 posed olivine with abundance of sanidine. Besides these exceptional 

 augite andesites, typical types of the rock occur in the Foot Hills south 

 of Wadsworth, near the Truckee river. The felspars are remarkable 

 for the quantity of their inclusions, which are chiefly kernels of ground 

 mass. Pale-yellow sections of augite occur with light-brown horn- 

 blende, surrounded by a narrow black border. Similar rocks are 

 found in Augusta Canon, Augusta Mountains, and similarly free from 

 olivine. Sometimes there is very little sanidine ; but at Susan Creek 

 Canon, Nevada, the augite is so full of glass inclusions, that upwards 

 of seven millions would occur in a cubic millimeter. Augite andesite 

 is found in the Foot Hills of the Cortez range, in Independence 

 Valley, Nevada, arid in Waggon Canon. In the rocks to the west 

 of White Rock, Cedar Mountain, the augite andesite contains some 

 brown biotite. In the eastern portion of the Fortieth Parallel Terri- 

 tory, these rocks have a pale grey glass base, while in the western 

 territory the glass base is brownish. 



Augite andesite occurs in the Palau Islands, at Kyneton in Vic- 

 toria, Australia, and in Java. 





