56 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



RELATIONS AND COMPOSITION OK BASALT OF SIEBERT MOUNTAIN. 



Particulars concerning the age of these two occurrences can not be given, 

 but the basalt on Siebert Mountain has been more carefully studied than that 

 north of Ararat Mountain. The white tuffs which make up the bulk of the 

 mountain are overlain by a breccia of yellow pumice containing fragments of 

 scoriaceous basalt. This breccia probably rests unconformably on the tuffs, which 

 are tilted, and is overlain by a flow of vesicular basalt 40 or 50 feet thick. This 

 flow extends southwest of the mountain, beyond the limits of the area mapped. 

 Basalt inclusions occur also in the Brougher dacites (see p. 45). 



Under the microscope this basalt shows small porphyritic crystals in a fine 

 holocrystalline groundnrass consisting chiefly of feldspar and augite. The porphy 

 ritic crystals are predominating pale -green augite, brown hornblende partly or 

 wholly altered to iron oxide by magmatic reactions, and feldspar. 



AGE. 



This basalt overlies the tuffs unconformably, so it must have been erupted 

 subsequent to the tilting. It and the tuff are intruded by the neck of dacite 

 which outcrops all over the summit, and which by its resistance to erosion has 

 created the mountain. On the east side of the mountain a fault has displaced 

 the basalt flow and the tuff, but has not affected the dacite (Pis. X, XI). 



CHEMICAL COMPOSITION OF LAVAS.' 



For the purpose of comparison the analyses of the fresh rocks of the district 

 have been assembled in the accompanying table. To represent the earlier andesite, 

 since no fresh specimen is available, an ideal type of hornblende-mica-andesite 

 (p. 217) has been substituted, practically identical with the analyses of the least 

 altered earlier andesite except as to the amount of silica. The knowledge obtained 

 by these analyses, though valuable, is only fragmentary, and more investigation 

 would certainly show a greater variation. 



TRANSITIONS IN SILICA CONTENT. 



The analyses have been arranged according to their silica content, which shows 

 the following differences: Between the basalt and the later andesite about 3 per 

 cent; between the earlier and the later andesites approximately 6 per cent; between 

 the earlier andesite and the least siliceous dacite about 6 per cent; and between 

 this dacite and the siliceous rhyolite about 5 per cent. This transition of silica 

 content is, then, fairly equable, but considering the analyses as a whole there is 

 a marked break between 4 and 5 that is, between the andesite-basalts on the one 

 hand and the dacite-rhyolites on the othe.r. The same break is shown, even 

 more plainly, in the iron and magnesia content, and, to a less degree, in the lime 

 percentage. 



