Vol. 4] Lawson. — The Robinson Mining District. 



321 



chalcoeite and pyrite disseminated through them. The pyrite 

 also occurs in nests and veinlets. 



A thin section shows only quartz in allotriomorphic granular 

 aggregate with numerous irregular grains of opaque sulphides. 

 In the quartz are abundant liquid inclusions with bubbles and 

 numerous small ragged nearly opaque solid inclusions are scat- 

 tered through it. 



Another sample of the porphyry taken from near the bottom 

 of shaft No. 2, of the same mine, is a light gray rock with vague 

 and uncertain traces of porphyritic structure. Rather large 

 stringers of quartz, large nests of bronzy mica and stringers and 

 disseminated grains of chalcoeite and pyrite are common in the 

 rock. 



In thin section the rock is seen to be an allotriomorphic gran- 

 ular aggregate of quartz and biotite, with a small shear zone 

 running through it. The biotite is evidently secondary with the 

 quartz and the sulphides. 



A specimen of the porphyry from the Josephine claim ex- 

 hibits a characteristic phase resulting from extreme silicifica- 

 tion. Microscopically it is a mass of honeycombed quartz with 

 patches of light yellow ochre. Under the microscope the rock 

 is composed only of quartz in an aggregate of fairly uniform 

 sized anhedrons, with patches of yellow ochre. 



Chemical Composition. — It is evident that a rock varying so 

 much in degree of alteration as the porphyry is difficult to char- 

 acterize chemically. Scarcely any two of the specimens described 

 above as representative facies of the formation would yield the 

 same results on analysis ; and the results in every case would 

 differ greatly from the original unaltered rock. The best that 

 can be done in such a case, where the chief object is to deter- 

 mine the character of the fresh rock, is to select the least altered 

 material available and subject it to analysis in the hope that, 

 thereby, some light may be thrown upon the nature of the altera- 

 tions which have affected it. This has been done. A sample of 

 the rock from a tunnel at the upper end of Lane Valley, on the 

 south side, was analyzed by Mr. Herbert Ross with the following 

 results : 



