“It is often said by vein-miners who have worked at great depths, that the jointed structure of rocks fades away and disappears in the deeper parts of the mines. This, however, is probably a case of the rule ‘de non apparentibus et non existentibus eadem est ratio.’ All the joints near the surface are more or less acted on by the weather. The deeper-seated rocks may be just as much traversed by joints, but they are merely mathematical planes of division, the faces of the blocks adhering as closely as if they did not exist till the weather makes them apparent, or some force tears the blocks asunder.”