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



319 



crysts. The feldspar of the ground mass has a distinctly lower 

 refractive index than the balsam and this taken with its non- 

 twinned character indicates that it is orthoclase. 



Across the road from the last locality, a few hundred feet 

 distant, a shaft has been sunk into the porphyry at the base of 

 a vertical limestone bluff. Here the rock is of a light gray color 

 and strongly porphyritic. A blue-gray dense matrix holds abun- 

 dant dull white phenocrysts of feldspar ranging up to 4 mm. 

 in length, besides occasional much larger phenocrysts of ortho- 

 clase in Carlsbad twins up to 15 mm. in length. Throughout 

 the rock is disseminated much pyrite which is locally bunched 

 in nests and veins, and in portions of it there are large crystals 

 of fluorite the cleavage faces of which range up to 25 or 30 mm. 

 in breadth. The pyrite does not occur in the phenocrysts but 

 occurs freely in the fluorspar. The rock effervesces with acid. 



In thin section this rock shows remnants of a holocrystal- 

 line feldspathic ground mass, much replaced by calcite, with 

 grains of quartz scattered through it. In this are cloudy feld- 

 spar phenocrysts wholly altered, shreds of white mica of consid- 

 erable size and small scattered grains of pyrite. The larger 

 phenocrysts of orthoclase are fresh and show Carlsbad twinning. 

 The white mica is probably a bleached biotite. Some portions 

 of the rock are so charged with sulphides as to constitute ore. 

 A specimen of such ore shows numerous parallel and obliquely 

 intersecting seams of chalcocite and pyrite alternating with thin 

 sheets of rock more thoroughly kaolinized than the rest. The 

 seams vary from 1 to 2 mm. in thickness. A little gypsum is 

 plastered on the surface of the specimen. 



At the first fork of the road east of the Ruth Mine, the rock 

 has a bleached white appearance with occasional yellow iron 

 stains. The porphyritic structure is very pronounced. The 

 phenocrysts are of two kinds: (1) Remains of sharply idio- 

 morphic feldspars of a dull white color and ranging usually 

 from 2 to 5 mm. in length, though sometimes attaining 10 mm. 

 (2) Lustrous white blades — evidently pseudomorphs of some fer- 

 romagnesian mineral from which the iron has been leached. 

 These phenocrysts are imbedded in a matrix which under the 

 lens appears to consist almost altogether of silvery white scales 



