Vol. 4] 



Law son. — The Robinson. Mining District. 



343 



course of the kaolinization of the orthoclase would react upon 

 the calcium bicarbonate. Potassium bicarbonate would be 

 formed and calcium silicate precipitated, thus : 



K 2 Si0 3 + CaH 2 ( CO, ) ^CaSiOa + 2KHC0 3 . 



The calcium silicate thus formed would in turn be decomposed 

 by carbonic acid, with the precipitation of insoluble silica, thus: 



CaSi0 3 +H 2 C0 3 =CaH 2 ( C0 3 ) , + H 2 + Si0 2 . 



These reactions were verified experimentally for the writer 

 by Mr. Herbert Ross, and while it is not insisted that they repre- 

 sent the detail of what must be a complex series of chemical 

 reactions, yet they indicate with great probability the source of 

 the silica which is so abundantly found in the form of quartz 

 blout on the periphery of the porphyry mass, and the general 

 mode of its derivation from the porphyry. It accounts also for 

 the secondary silica in the body of the porphyry. Moreover, 

 since lime silicate is formed in the process above outlined, it may 

 have been locally precipitated, in combination with iron, in such 

 situations, on the periphery of the porphyry, as to escape decom- 

 position by excess of carbonic acid and so give rise to the garnet 

 ro'ck found sporadically at the contact with the limestone and 

 to some extent also in the midst of the quartz blout. Further, 

 since the copper ores of the district are intimately associated 

 with the porphyry, except for certain unimportant occurrences 

 on the periphery of the monzonite which probably have an anal- 

 ogous history, there seems little escape from the conclusion that 

 the copper was originally minutely disseminated through the 

 porphyry in its unaltered conditon ; and that the same ascending 

 waters which robbed the porphyry of part of its silica to form 

 the quartz blout also leached it of its copper, depositing it as 

 chalcopyrite in the blout, whence by oxidation and the agency 

 of descending waters it was carried down into the porphyry 

 again and deposited as chalcocite, the permeation of the rock 

 by such descending waters being greatly facilitated by the rup- 

 turing and collapse of the mass due to the kaolinization. 



