In many cases the ore deposits are formed not only in the main channel of the fissure, but in all the crevices on either side of that way. In this manner, much as in the case of the growth of stalactitic matter between the blocks of stone in the roofs of a cavern, large fragments of rock, known as "horses," are often pushed out into the body of the vein. In some instances the growth of the vein appears to enlarge the fissure or place of the deposit as the accumulation goes on, the process being analogous to that by which a growing root widens the crevice into which it has penetrated. In other instances the fissure formed by the force has remained wide open, or at most has been but partly filled by the action of the water. It not infrequently happens that the ascending waters of hot springs entering limestones have excavated extensive caves far below the surface of the earth, these caverns being afterward in part filled by the ores of various metals. We can readily imagine that the water at one temperature would excavate the cavern, and long afterward, when at a lower heat, they might proceed to fill it in. At a yet later stage, when the surface of the country had worn down many thousands of feet below the original level, the mineral stores of the caverns may be brought near the surface of the earth. Some of the most important metalliferous deposits of the Cordilleras are found in this group of hot-water caverns. These caverns are essentially like those produced by cold water, with the exception of the temperature of the fluid which does the work and the opposite direction of the flow. In following crevice water which is free to obey the impulses of gravitation far down into the earth, we enter on a realm where the rock or construction water, that which was built into the stone at the time of its formation, is plentiful. Where these two groups of waters come in contact an admixture occurs, a certain portion of the rock water joining that in the crevices. Near the surface of the ground we commonly find that all the construction water has been washed out by this action. Yet if the rocks be compact, or if they have layers of a soft and clayey nature, we may find the construction water, even in very old deposits, remaining near the surface of the ground. Thus in the ancient Silurian beds of the Ohio Valley a boring carried a hundred feet below the level of the main rivers commonly discovers water which is clearly that laid down in the crevices of the material at the time when the rocks were formed in the sea. In all cases this water contains a certain amount of gases derived from the decomposition of various substances, but principally from the alteration of iron pyrite, which affords sulphuretted hydrogen. Thus the water is forced to the surface with considerable energy, and the well is often named artesian, though it flows by gas pressure on the principle of the soda-water fountain, and not by gravity, as in the case of true artesian wells.