Vol. 4] 



Lawson 



. — The Robinson Mining District. 



323 



caleite. The surplus silica, of such plagioclase over and above 

 that fixed in the kaolin has probably also been removed. The 

 pyrite has doubtless resulted from the reaction of sulphur bear- 

 ing solutions upon the iron of the biotite which is now bleached 

 white, thus accounting' for the absence of oxides of iron in the 

 analysis of the rock. 



In view of this interpretation it seems pretty certain that 

 structure of this facies of the rock is much better preserved 

 the original rock had the characters of a rhyolite. The original 

 than its mineralogieal character and seems to have been holo- 

 crystalline. This, however, may be due to its rather deep seated 

 situation and more superficial portions may quite possibly have 

 been hypocrystalline. 



Resume. — Reviewing now the petrographical descriptions of 

 the porphyry from various localities, it would appear that we 

 have a gradation in the degree of alteration to which the mass 

 as a whole has been subjected, ranging from that exemplified 

 in the specimen analyzed to cases where every trace of the orig- 

 inal minerals has been removed and nothing has been left but 

 an aggregate of cellular secondary quartz constituting a phase 

 of the quartz blout. This extreme end product is, however, 

 probably quite superficial since in all underground openings in 

 the porphyry there is an abundance of kaolin in the rock with 

 notable quantities of residual feldspars. It probably represents 

 a result of the weathering of the kaolinized and silieified mass 

 as it became exposed to the direct attack and downward leaching 

 of meteoric waters above the ground water level. Setting aside 

 this extreme facies of the altered porphyry as a superficial phe- 

 nomenon, we may safely characterize the porphyry in general 

 as a very much kaolinized, silieified, carbonated and pyritized 

 rhyolite. 



Porphyry at the Chainman Mine. — Near the Chainman Mine 

 mill there is exposed, in a road cutting, a mass of porphyry which 

 is quite different in its general physical characters from the 

 main body of the copper-bearing porphyry above described. It 

 is very probably, though not certainly, a distinct intrusion from 

 the general body of the porphyry. It is, however, mapped with 

 the latter. The rock is quite fresh and shows little or none of 



