The phenomena of fissure veins are in point. When a fault takes place, and one rock mass is slidden past another to which it had been joined it is usually the case that the opposed surfaces no longer fit together as they did before the movement, and interspaces are left. These become filled, at first by water, and afterward by minerals deposited from the water, and the mineral masses thus deposited are called fissure veins. But the preservation of the interspaces depends upon the rigidity of the rocks which inclose them; and it frequently happens that where a system of rocks is traversed by a fault, the harder will keep somewhat apart and maintain a fissure, while the softer will be crushed together without an interspace. If the mineral vein which forms in such a fissure is afterward explored in mining, it is found to be traceable and continuous so far as it is walled by the hard rock upon both sides, but when the hard is replaced by the soft in one or both walls, the vein is either reduced to a mere fillet or disappears completely. If the fault extends to a great depth, it will finally reach a region where the hardest rocks which it separates are coerced by so great a pressure that they cannot hold themselves asunder, but are forced together before the fissure can be filled by mineral deposits. Thus there is a definable inferior limit to the region of vein formation; and even while it is impossible to assign a downward limit to the fault which made place for a vein, it may be possible to assign a downward limit to the vein itself. Accordant with this view is the absence of fissure veins from the Henry Mountains. Displacement and thermal disturbance are usually regarded as the conditions of mineral concentration; and here were displacement and lavic intrusion coincident in time and place. The heat which metamorphosed great bodies of shale and sandstone was surely competent to excite the currents and reactions which concentrate minerals in veins; but the displacements did not open fissures, and the heated water could circulate only through the pores of rocks. Fissure veins were impossible, and the sluggish currents which were engendered in continuous rock masses did not effect a great change in the distribution of minerals. THE CONDITIONS OF ROCK FLEXURE.