Size and Depth of Ore Bodies As Determined From Outcrop
Where the ore body is harder than the surrounding rock, it stands out in conspicuous outcrops and is likely to show a narrowing below. Where it is softer than the surrounding rocks, and outcrops in a topographic depression, it is perhaps more likely to show widening below. These features are due to the general facts that, where the ore body is hard and resistant, the downward progress of erosion is likely to be arrested where the adjacent rocks occupy the larger part of the surface, that is, where the ore body is narrower. This principle is often vaguely recognized in the assumption that an exceptionally large outcrop of an ore vein may be "too good to last." Again, such a generalization must be applied to a specific case with much caution.
Attempts to forecast the depth of veins from their extent at the surface meet with only partial success. In a very general way great persistence horizontally suggests persistence in depth, on the ground that the section exposed on the surface is as likely to be a section of average dimensions as one along vertical lines.
Faith is the first article of the prospector's creed, and it is hard to shake his conviction that every ore outcrop must widen and improve below. As expressed by the French-Canadian prospector in the Cobalt district, the "vein calcite can't go up, she must go down." While the scientist may have grounds to doubt this reasoning, he is not often in a position to offer definite negative evidence.