abstracts: geology 581 



been recorded from this region up to 1908, but in that year a bonanza 

 shoot of gold quartz was discovered in the National mine, which within 

 four years yielded about $4,000,000. This shoot is, in fact, one of the 

 most remarkable and interesting bodies of high-grade ore discovered 

 in the West. 



The district is located in the Santa Rosa Mountains, a narrow range 

 rising abruptly out of the desert. The northern part of this range in 

 which the camp of National is located consists of flows of basalt, latite, 

 rhyolite, and trachyte and some rhyolite dikes. These are of Tertiary, 

 probably Miocene, age. The flows dip at gentle angles to the east or 

 northeast. 



The mineral deposits are steeply-dipping, narrow fissure veins which 

 are distinctly later than any rock in the district. The National vein is 

 the only one that has been extensively developed. Most of the veins 

 are essentially silver veins of verj^^ moderate tenor and contain in a 

 drusy, fine-grained quartz gangue, small amounts of pyrite, sphalerite, 

 galena, chalcopyrite, arsenopyrite, and stibnite, the latter being the 

 most characteristic mineral of the deposits. At least one deposit car- 

 ries cinnabar. Native gold is abundant onty in the unique rich shoot 

 of the National vein, where it occurs as electrum carrying about 50 

 per cent of silver. The National ore shoot was encountered 40 feet 

 below the surface and has been followed for about 800 feet down the 

 dip of the vein; the stope reaches 250 feet in length. It appears to be 

 of the same age as the leaner parts of the same vein and the other veins 

 of the region. Much of the ore was remarkably rich, the first-class 

 ore during one period averaging about $25 a pound, and the second- 

 class ore $4000 a ton. 



The National and other veins of the district are believed to have 

 been deposited at slight depths by ascending waters during an epoch of 

 hot-spring action following the eruption of the rhyolitic rocks. The 

 fine-grained quartz, scarcity of pyrite, constant presence of stibnite, 

 and occasional occurrences of cinnabar, point decidedly to deposition 

 near the surface. The propylitic alteration of the country rocks points 

 to the same conclusion. 



Subsequent to the principal mineralization there has been in the 

 National mine a deposition of secondary marcasitc, pyrargj'-rite and 

 stibnite along joints and fissures below the water level. This deposi- 

 tion was accompanied by solution of quartz and the development of 

 irregular cavities. There are no placers in the region. E. S. B. 



