In the _Quarterly_ for March, 1880, a paper was published on "The Origin
and Classification of Ore Deposits," which treated, among other things,
of mineral veins. These were grouped in three categories, namely: 1.
Gash Veins; 2. Segregated Veins; 3. Fissure Veins; and were defined as
follows:
_Gash Veins_.--Ore deposits confined to a single bed or formation of
_limestone_, of which the joints, and sometimes planes of bedding,
enlarged by the solvent power of atmospheric water carrying carbonic
acid, and forming crevices, galleries, or caves, are lined or filled
with ore leached from the surrounding rock, e.g., the lead deposits of
the Upper Mississippi and Missouri.
_Segregated Veins_.--Sheets of quartzose matter, chiefly lenticular and
conforming to the bedding of the inclosing rocks, but sometimes filling
irregular fractures across such bedding, found only in metamorphic
rocks, limited in extent laterally and vertically, and consisting of
material indigenous to the strata in which they occur, separated in the
process of metamorphism, e.g., quartz ledges carrying gold, copper, iron
pyrites, etc., in the Alleghany Mountains, New England, Canada, etc.
_Fissure Veins_.--Sheets of metalliferous matter filling fissures caused
by subterranean force, usually in the planes of faults, and formed by
the deposit of various minerals brought from a lower level by water,
which under pressure and at a high temperature, having great solvent
power, had become loaded with matters leached from different rocks, and
deposited them in the channels of escape as the pressure and temperature
were reduced.
Since that article was written, a considerable portion of several years
has been spent by the writer continuing the observations upon which it
was based. During this time most of the mining centers of the Western
States and Territories, as well as some in Mexico and Canada, were
visited and studied with more or less care. Perhaps no other portion of
the earth's surface is so rich in mineral resources as that which has
been covered by these observations, and nowhere else is to be found as
great a variety of ore deposits, or those which illustrate as well their
mode of formation. This is so true that it maybe said without
exaggeration that no one can intelligently discuss the questions that
have been raised in regard to the origin and mode of formation of ore
bodies without transversing and studying the great mining belt of our
Western States and Territories.
The observations made by the writer during the past four years confirm
in all essentials the views set forth in the former article in the
_Quarterly_, and while a volume might be written describing the
phenomena exhibited by different mines and mining districts, the array
of facts thus presented would be, for the most part, simply a
re-enforcement of those already given.
The present article, which must necessarily be short, would hardly have
a _raison d'etre_ except that it affords an opportunity for an addition
which should be made to the classes of mineral veins heretofore
recognized in this country, and it seems called for by the recent
publication of theories on the origin of ore deposits which are
incompatible with those hitherto presented and now held by the writer,
and which, if allowed to pass unquestioned, might seem to be
unquestionable.