It has been the fashion for prospectors to dilate upon the fact that they have located "true fissure veins." This expression, formerly on the tongues of most mining men in districts possessing veins at all, is now obsolete and hence should be placed in the discard. There can be no such thing as an "untrue" vein nor an "untrue" fissure. Neither can there be any vein without a fissure. Therefore, if there is any vein, it must be a real or true vein. Accordingly, the verbiage is to be discouraged. The intention of a miner, in using this pet phrase, has been to convey the impression that his vein extended downward, indefinitely; there having arisen a notion that some veins are rather superficial and liable to "peter out" at slight depths, while others—the kind he invariably has located—persist both in size and value to extreme depths.
There are districts in which are found short fissures, generally confined to certain horizons in sedimentary rocks, such as the limestones of the great Mississippi Valley, from which are mined lead and zinc ores. These are called "gash veins." These are always readily recognized and there is not the slightest excuse for confusing them with the fissures which are common to other kinds of rock formations.
A bed or blanket vein is the term applied to any nearly flat deposit conforming to the bedding. Such a body of ore must be in a sedimentary series of rocks. Coal bodies are all of this type. Many bodies of iron ore are also of this type.
A chimney is an ore body which has not the tabular form of a vein but is rudely elliptical in outline, horizontally, and with a very considerable vertical extent. A stock is a similar body but it is of still greater irregularity of boundary.
These bodies are usually the filling of extinct volcanoes or geysers, and therefore they are presumed to extend to very great depths. The diamond mines of Kimberly, Africa, are of this type and the ore is a sort of hardened geyserite or mud in which are enclosed the precious gems. In Custer County, Colorado, the ore body of the Bassick Mine is a conglomerate of rounded boulders of all sizes cemented together, somewhat like concrete, by the materials which really carry the values. This mass occupies an ancient volcanic neck or throat of a geyser, probably the latter. The main portion of the Cripple Creek District is the crater of a great prehistoric volcano. It might be called a great chimney, but custom seems to limit the use of the word chimney to a smaller body such as might be included in a single mining property.
A mass is a deposit whose irregularity of shape is so great that it cannot be recognized as belonging to any of the types already mentioned. Masses conform to no rules as to shape or size. They are usually the result of a chemical dissolving of the original barren rocks with a simultaneous or subsequent substitution of valuable materials. There are many instances of ores that have been deposited, molecule by molecule, replacing equal volumes of the previous rock, much upon the order of the petrifaction of wood. Again, there are immense masses which are believed to have accumulated in caves already dissolved out of the containing rocks.
While recent geological study of the districts in which such ore bodies abound have disclosed numerous facts about their occurrence, there still remains much conjecture concerning their origins, and we may still believe that they do not conform to any rules as to regularity or size. The ore bodies of Leadville are of this type, and they may be described by the homely similes that they are as like and as unlike, and their occurrences are about as regular, as potatoes in a hill. The potato-tops give the farmer a suggestion as to where to dig. So, also, do certain geological relations guide the miner. And yet a shaft may be sunk hundreds of feet down among masses and not happen to penetrate a single one.
There are numerous recognized types of ore body not enumerated here; but it is sufficient for the average layman in mining matters to understand these few distinct types and to believe that all other types are rarities, and are, as a general thing, but intermediate forms of those defined.
Shaft No. 3, Tamarack Mining Company, Calumet, Michigan.