a matrix of quartz. At the same time
the wall rocks were extensively mineralized and altered; the fissure
veins grade off into the wall rock, and in fact the larger part of the
ore is simply altered granite with disseminated sulphides. The solutions
which deposited the ores are inferred to have been hot from the nature
of the wall-rock alterations, from the presence of hot-water minerals
like fluorite, cassiterite, and others, and from the general association
of the ores in time and place with the porphyry intrusions. The
solutions are believed to have originated from the porphyry and possibly
from other intrusives.

In the Butte district, and in the great majority of copper sulphide vein
ores throughout the world, secondary concentration by surface waters has
played a considerable part in developing ores of commercial value. Near
the surface the copper is leached out and carried down by waters
containing various solvents, particularly sulphuric acid from the
oxidation of pyrite. A leached zone is formed containing the ordinary
products of rock weathering,--rusty quartz and clay, sometimes black
with manganese oxides. A small part of the copper remains in this zone
as oxides, carbonates, and silicates. Below the oxidized and leached
zone there is evidence of deposition of a large amount of secondary
copper sulphide in the form of chalcocite. This is supposed to have been
formed by the leaching of copper from above as soluble copper sulphate,
and its precipitation below by iron and other sulphide minerals which
the solutions meet on their downward course--a reaction which has been
demonstrated experimentally. It was formerly supposed that most of the
chalcocite was of this origin; but as chalcocite is found in important
amounts with enargite and chalcopyrite to great depths (now 3,500 feet),
where the veins are still rich and strong, it begins to appear that much
of the chalcocite is of primary origin.

The fissures along which the Butte ores occur are in three main sets,
which in order of age strike roughly east-west, northwest-southeast, and
northeast-southwest. Two-thirds of the ore is in the first set, about 30
per cent in the second, and the remainder in the third. The
mineralization of the several vein systems cannot be discriminated, and
it is thought that it was accomplished as a more or less continuous and
progressive process. There is some evidence, also, that the fracturing
in the several fracture systems was