304 GEOLOGY OF THE EUKEKxV DISTRICT. 



the mines rarely attain au average width of more than 15 to 20 feet across 

 the broadest expansions, although instances of much greater width occur in 

 the Phoenix. Decomposed rhyolite is recognized along the fissure in both 

 the Jackson and Ph(i>nix mines. It is intersected by the Jackson shaft 

 above the thu-d level, and the cross-cuts from the old Jackson shaft on both 

 the third and fourth levels expose the rhyolite body on the main fissure. 

 In the Phoenix, rhyolite is found on all the lower levels wherever they inter- 

 sect the fissure. Proceeding westward the fissiu'e narrows, but the rhyolite 

 may still be detected on the sixth level of the KK, although so thoroughly 

 altered as to have lost the distinctive characters of a lava. In the Rich- 

 mond mine no rhyoHtes nor rhyolitic clays are recognized, nor have they 

 been observed anywhere along the fissure to the northwest. The fissure 

 gradually narrows and finally dies out and the fault, is lost where the rocks 

 pass beneath Spring Valley, a short distance beyond the Albion mine. 



Transition products from unaltered lava to highly kaolinized rhyolite 

 are found along the fissure in every stage of decomposition. In the Jack- 

 son mine the rhyolite origin of much of the filling of the fissure is deter- 

 mined by the presence of mica flakes and quartz grains imbedded in blue 

 clay. These transition products grade off" into nearly pure clays holding- 

 grains of quartz still unaltered, whereas the feldspars and glass base have 

 undergone such complete kaolinization that the volcanic origin of much of 

 this material could not be made out but for its association with fresher 

 rock. In places the entire filling between the walls of the fissure, which 

 may be only a few inches in width, is composed of rhyolite clays, the 

 extreme product of the action of steam and solfataric fumes upon injected 

 volcanic rock. They have all the physical properties of and behave like 

 ordinary clays. Between the Phoenix and the Richmond occur bodies of 

 clay which are undoubtedly derived from the rli}^olite, with an admixtm-e 

 of more or less calcareous material. Such material abounds where the 

 fissure walls stand only a few inches apail, and a movement has pulverized 

 the limestone along the fixult plane, producing an admixture of rhyolitic 

 clay with comminuted siliceous limestone. In the Richmond the filling 

 of clay between the fault planes is derived solely from attrition of the 

 walls; at least, no rhyolite can be detected. It is possible that the crack 



