THE ORIGIN AND CLASSIFICATION OF ORE DEPOSITS. 201 
breaking down by their own weight, have formed masses of debris, cemented 
together by the ore, which has thus acquired its peculiar brecciated character. 
From the description of gash-veins given above, it will be seen that they have 
much in common with the pockets and chambers previously described ; but there 
is this important difference, that the ore filling the gashes and irregular chambers 
of the lead-bearing limestones is indigenous, having been derived from the leach- 
ing of the adjacent rock, while in the chamber-mines of the West the ore is ex- 
otic, having been brought up through fissur.s from a remote source below; so 
that, while in physical characters the western gold and silver-bearing ore-cham- 
bers resemble gash-veins, they are really but appendages to true fissure-veins, and 
only occur in a country that has been much broken by subterranean forces. 
Segregated veins are confined to metamorphic rocks, are conformable with 
their bedding, and are limited in extent both laterally and vertically. ‘Their ore- 
bodies form lenticular masses of greater or less dimensions, of which the materi- 
al is chiefly quartz, which has segregated (that is, separated) from the surround- 
ing rock. ‘The quartz-veins so abundant in the gneisses and schists of Canada, 
New England and the Alleghany belt are all examples of this class of ore-de- 
posits. The most important constituent of segregated veins is gold, which here 
seems to have been mechanically dispersed throughout sedimentary rocks, and to 
have been concentrated with the quartz in the process of metamorphism to which 
they have been subjected. With the gold we always find iron pyrites, sometimes 
chalco-pyrite, and the latter occasionally in sufficient quantity to be worth work- 
ing. From these remarks it may be inferred that segregated veins have no deep- 
seated origin, are less continuous in depth and laterally than fissure-veins, and 
therefore constitute a less permanent foundation for mining enterprises. It may 
be said, however, that some of them are of enormous dimensions, and that they 
not unfrequently occur in succession, or so approximate that they’are equivalent to 
a continuous mineral deposit. 
Lissure-veins occupy crevices which have been formed by subterranean forces 
and have been filled from a foreign source. They traverse indiscriminately all 
kinds of rock, and are without definite limits laterally or vertically. They have 
as characteristic features smooth, striated, sometimes polished, walls (slickensides) 
clay gouges or selvages on one or both sides, and a banded or ribboned structure 
throughout. The veinstone is usually quartz, and the constituents include the 
ores of all the metals. The mode of formation of fissure-veins is apparently this: 
In the regions where the earth’s crust is broken up in the adjustment of the cold 
and hard exterior to the cooling and shrinking nucleus, cracks are formed, often 
miles in extent, along which the rocks suffer displacement, sliding on each other 
to form what are known as “ faults.”” As the planes of these faults are more or 
less undulated, with displacement the bearing is upon the projecting bosses of 
each side. Between these, open fissures are left of greater or less dimensions. 
These reach down to a heated zone, and form the conduits through which ther- 
mal waters flow to the surface. Such waters coming in different localities 
