In place of this chapter a volume could be written, and then the subject would not be exhausted. The object of the present work is not a scientific treatise, or even a guide for engineers. Such ambitions are far beyond our objects which are simply to give the facts which will bear on the advisability of accepting a mining proposition, and some little information touching on the relation of mining to geology will be desirable. There are many forms of ore deposits, and of these large deposits moderately rich are more favorable than small but immensely rich formations. An assay tells very little of what may be expected from a mine. Suppose a mine ran fifty thousand dollars to the ton, as sometimes has been found; it sounds attractive, but usually such deposits are so very small that they scarcely pay for working. If this fifty thousand dollar ore were taken from a seam, or pay streak, less than an inch wide, the mine would not amount to much, because great tunnels or expensive shafts would have to be opened to get at the ore; and when one did get at it there would not be enough to compensate for the expense, and thus we can see that the important question to consider is not how rich but how abundant an ore may be. A mine having hundreds of thousands of tons of ore easily worked, and worth ten dollars per ton would be a mighty fortune, where a mine containing but a few tons of fifty thousand dollar ore might not give any returns beyond the expense of opening the property, and, perhaps, even not so much as this; for the expense of opening tunnels, and sinking shafts might easily amount to more than the value of the ore obtained.

The question, therefore, in first considering a mine is not how rich the ore may be but how much there may be of it. A big mine is the best though rarely yielding the richest ore.

Of the different forms of ore deposits we have most commonly vein formations, and in almost every prospectus the claim is made that the deposits occupy true fissure veins, and when such claims are presented it would be just that the prospective investor should ask the promoters to prove it. Fissure veins are rare, and in their largest meanings indicate fissure or cracks in the earth's crust caused by some great seismic action, and extending to the depths of the interior earth far beyond the point to which man's efforts can follow them. Cracks in the rocks, though they may be extensive, are not fissures in the earth's crust, and should not be spoken of as fissure veins; yet everywhere we find the claim made, true fissure veins, but of these there are few well defined examples in all the world; and for a long time it was claimed that in America true fissure veins were not to be found at all, but this is not the fact, and several have been reported on by eminent authorities.

A fissure in the earth's crust, and cracks in the rocks, these two are commonly claimed as fissure veins, and is not a crack a fissure? Certainly it is, yet a fissure vein is not a crack in the rocks, but a former great opening in the earth's crust which has become filled up with mineral bearing ores. A fissure vein must be large, the sides or walls well defined, it will extend for some distance, and pass through two or more different rock formations, but if the ore deposit is found occupying a vein in one rock formation only it is usually a crack or local fissure in that formation. It may be large and of importance, but it can never be so large as a fissure in the crust of the earth passing through all rock formations down to unknown depth and should not be called a fissure vein.