310 GEOLOGY OF THE EUREKA DISTRICT. 



Kaoiinization of Rhyoiite. A careful study of the transition products of 

 kaolinization of the rhyolite shows how coinplete the decomposition has 

 been along the Ruby Hill fissure west of the Jackson fault. Equally com- 

 plete and impressive are the evidences of similar kaolinization in the Dun- 

 derburg and on the summit of Hamburg Ridge above the Dunderburg and 

 Hamburg mines wherever the rhyolites offer good exposures on the surface. 

 Along the line of contact on the summit of the ridge between the 

 thoroughly whitened rhyolite and the dark limestone there has been con- 

 siderable prospecting for ores, but without success. Perhaps the best 

 instance of the alteration of the rhyolite is found where the drainage chan- 

 nel of New York Canyon in coming down from Prospect Ridge has worn 

 a deep passage through the Hamburg limestone ridge. It is seen in the 

 limestone bluff on the south side where an exploring tunnel was run into 

 the hill following the contact between the nearly vertical rhyolite dike and 

 the inclosing limestone. There is exposed here a fine example of com- 

 pletely kaolinized rhyolite possessing all the properties of an ordinary clay, 

 except that the quartz grains of rhyolite still remain unacted upon with 

 here and there a little unaltered sanidin. This is an instance of thoroughly 

 kaolinized rhyolite without the presence, so far as known, of any ore body 

 as far as the tunnel was run. Finding no indication of ore, the tunnel had 

 been abandoned after running a long way into the hill along the contact 

 of the two formations. 



Ores Deposited as Sulphides. Solfataric action which accompanied the filling 

 of the intricate net-work of openings in the limestone may have continued 

 throughout a long period of time, the mineral matter accumulating slowly. 

 That the ores were originally deposited as sulphides there seems no good 

 reason to doubt, an opinion probably held by all geologists who have exam- 

 ined the district and who believe that the ores came from below. 



The enormous amount of oxidized products indicates that the original 

 ore was mainly galena and pyrites. Evidence that such was the case on 

 Ruby Hill is shown by the discovery of fragments of galena and pyrites 

 found in a perfectly fresh state scattered throughout the ore bodies near 

 the surface as well as at great depths. These fragments are frequently sur- 

 rounded by partially oxidized material showing a nucleus or kernel of still 



