310 (;k()1,()(iv of tuk kfueka jmsthkt. 



Kaoiinization of Rhyoiite.— A careful studv ot' tlu' transition products of 

 kaoliiiization of the rliyolite shows how complete the decomposition has 

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

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

 derburg- and on the sunnnit of Hamburg Ridge above the Dunderburg and 

 Hamburg mines wherever the rhyolites ofi'er good exposm-es on the surface. 

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

 tlioroiighly whitened rhyoiite and the dark limestone there has been con- 

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

 instance of the alteration of the rhyoiite is foimd where the drainage chan- 

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

 a deep passage through the Hambm'g limestone ridge. It is seen in the 

 limestone bluff on the south side where an exploi'ing tunnel was run into 

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

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

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

 except that the quartz grains of rh}'olite still remain unacted upon with 

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

 kaolinized rhvolite without the })resence, 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 actiou wliicli accoiujjanied the filling 

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

 thi'oughout 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 ]3rol)ably held by all geologists who have exam- 

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



The enormous amount of oxidized ])roducts 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 ])yrites 

 found in a perfect!}- fresh state scattered throughout the ore bodies near 

 the sm-face as well as at great depths. These fragments are frequentl}- sur- 

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



