348 R. Pumpelly—Paragenesis of Copper 
rocks more or less impregnated with metallic copper, seems to 
show a diversity of origin for the sulphur and arsenic on the one 
hand, and the copper on the other. It does not seem unreason- 
able to suppose the copper to have entered the vein-fissure from 
the gute rocks in solution, as carbonate, sulphate or sili- 
cate, and to have been then precipitated by sulphureted hydro- 
the Mendota vein traverses syenite, cuprite must have been 
formed by the oxydation of chalcocite or of native copper, and 
the oxide must have been subsequently decomposed by sulphu- 
reted hydrogen. ; 
The Huronian formation, which probably underlies all this 
region, contains in its upper members large amounts of carbon- 
aceous matter in the form of graphite; the gases may have orig- 
inated in a reduction of sulphates and arseniates by the carbon 
of these beds. 
pear cavernous, 10 per cent or more of the substance being 
This is the BOR Pei of this porphyry in the freshest 
pebbles. 
I have before me a pebble 4 inches in diameter, broke 
through the middle. It was the same variety of porphyty a 
have just described—the same brown matrix, with the sam 
grain : 
often enclosing crystals of triclinic 
. fiss 
the surfaces of the fissures are covered with a soft light 
