138 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1902. [bull. 213. 
extensively, and seems to form by preference in the shale forming 
part of the Devonian system. Chalcocite and other sulphides are 
almost entirely absent. The zinc blende has been carried away as 
sulphate of zinc, which is frequently found in efflorescence on the 
walls of the tunnels. The magnetite and the garnet which originally 
formed a part of these deposits have also undergone decomposition, 
the resulting minerals being silica and limonite. 
The celebrated Longfellow mine is worked on one of these deposits 
occurring as, roughly speaking, a funnel-shaped mass in the Lower 
Silurian limestone, between two large porphyry dikes. Going farther 
west along the main porphyry contact, the Montezuma is encountered, 
and farther on the Detroit and the Manganese Blue mines. Both of 
the latter mines were worked on several tabular ore bodies, three 
or more in number, occurring in horizons varying from Silurian to 
the Lower Carboniferous. All of these deposits arc now largely 
exhausted. They contained a large quantity of very rich carbonate 
and oxide ore. The extent of these ore bodies was, however, much 
smaller than the large masses of chalcocite ore which now forms the 
main support of the camp. 
At Metcalf the Shannon mine contains several ore bodies of similar 
origin. A fragment of the Paleozoic series outcrops on Shannon Hill, 
and is cut b} r an extensive system of porphyry dikes, which in the 
lower part of the mountain join the main part of a large intrusive 
body of porphyry. In several horizons the limestones are greatly 
altered, the final product general^ being copper carbonates and 
limonite, with some quartz. In some places the ore bodies are less 
affected by oxidation, and their original character of garnet, epidote, 
magnetite, and sulphides may be plainly seen. 
Oxidation by surface waters, as at Shannon mine, also diffused 
much copper as chalcocite in some of the porphyry dikes, and the 
Metcalf mine on a lower spur of the same hill consists chiefly of a 
body of extremely decomposed porphyry containing chalcocite and 
carbonates. Very probably this copper has migrated into the decom- 
posing porphyry from bodies of contact-metamorphic rock at higher 
elevation, parts of which are probably now eroded. 
Fissure reins. — At many places in the district the copper deposits 
consist of fissure veins, cutting alike porphyry, granite, and sedi- 
mentary rocks. From the available evidence it would seem as if 
these veins had been formed a short time after the consolidation of 
porphyry. In lower levels the veins consist of pyrite, chalcopyrite, j 
and zinc blende, magnetite being conspicuously absent. At the sur- 
face many of the veins have been completely leached, and now show 
nothing but limonite and silicified porphyry. This rule is, however, 
not a general one, as, especially in porphyiy, oxidized ores are some- 
times found in the outcrops of the deposits. Between the leached 
croppings and the deep ores of pyrite and chalcopyrite is a more or 
