_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.
