We must now go a step further on the way of subterranean water, and trace its action in the depths below the plane of ordinary caves, which, as we have noted, do not extend below the level of the main streams of the cavern district. The first group of facts to be attended to is that exhibited by artesian wells. These occur where rocks have been folded down into a basinlike form. It often happens that in such a basin the rocks of which it is composed are some of them porous, and others impervious to water, and that the porous layers outcrop on the high margins of the depression and have water-tight layers over them. These conditions can be well represented by supposing that we have two saucers, one within the other, with an intervening layer of sand which is full of water. If now we bore an opening in the bottom of the uppermost saucer, we readily conceive that the water will flow up through it. In Nature we often find these basins with the equivalent of the sandy layer in the model just described rising hundreds of feet above the valley, so that the artesian well, so named from the village of Artois, near Paris, where the first opening of this nature was made, may yield a stream which will mount upward, especially where piped, to a great height. At many places in the world it is possible by such wells to obtain a large supply of tolerably pure water, but in general it is found to contain too large a supply of dissolved mineral matter or sulphuretted gases to be satisfactory for domestic purposes. It may be well to note the fact that the greater part of the so-called artesian wells, or borings which deliver water to a height above the surface, are not true artesian sources, in that they do not send up the water by the action of gravitation, but under the influence of gaseous pressure.
Where, as in the case of upturned porous beds, the crevice water penetrates far below the earth's surface or the open-air streams which drain the water away, the fluid acquires a considerable increase of temperature, on the average about one degree Fahrenheit for each eighty feet of descent. It may, indeed, become so heated that if it were at the earth's surface it would not only burst into steam with a vast explosive energy, but would actually shine in the manner of heated solids. As the temperature of water rises, and as the pressure on it increases, it acquires a solvent power, and takes in rocky matter in a measure unapproached at the earth's surface. At the depth of ten miles water beginning as inert rain would acquire the properties which we are accustomed to associate with strong acids. Passing downward through fissures or porous strata in the manner indicated in the diagram, the water would take up, by virtue of its heat and the gases it contained, a share of many mineral substances which we commonly regard as insoluble. Gold and even platinum—the latter a material which resists all acids at ordinary temperatures—enters into the solution. If now the water thus charged with mineral stores finds in the depths a shorter way to the surface than that which it descended, which may well happen by way of a deep rift in the rocks, it will in its ascent reverse the process which it followed on going down. It will deposit the several minerals in the order of their solubilities—that is, the last to be taken in will be the first to be crystallized on the walls of the fissure through which the upflow is taking place. The result will be the formation of a vein belonging to the variety known as fissure veins.
Fig. 14.—Diagram of vein. The different shadings show the variations in the nature of the deposits.
A vein deposit such as we are considering may, though rarely, be composed of a single mineral. Most commonly we find the deposit arranged in a banded form in the manner indicated in the figure (see diagram 14). Sometimes one material will abound in the lower portions of the fissure and another in its higher parts, a feature which is accounted for by the progressive cooling and relinquishment of pressure to which the water is subjected on its way to the surface. With each decrement of those properties some particular substance goes out of the fluid, which may in the end emerge in the form of a warm or hot spring, the water of which contains but little mineral matter. Where, however, the temperature is high, some part of the deposit, even a little gold, may be laid down just about the spring in the deposits known as sinter, which are often formed at such places.
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.
The passage between the work done by the deeply penetrating surface water and that due to the fluid intimately blended with the rock built into the mass at the time of its formation is obscure. We are, however, quite sure that at great depths beneath the earth the construction water acts alone not only in making veins, but in bringing about many other momentous changes. At a great depth this water becomes intensely heated, and therefore tends to move in any direction where a chance fissure or other accident may lessen the pressure. Creeping through the rocks, and moving from zones of one temperature to another, these waters bring about in the fine interstices chemical changes which lead to great alterations in the constitution of the rock material. It is probably in part to these slow driftings of rock water that beds originally made up of small, shapeless fragments, such as compose clay slates, sandstones, and limestones, may in time be altered into crystalline rocks, where there is no longer a trace of the original bits, all the matter having been taken to pieces by the process of dissolving, and reformed in the regular crystalline order. In many cases we may note how a crystal after being made has been in part dissolved away and replaced by another mineral. In fact, many of our rocks appear to have been again and again made over by the slow-drifting waters, each particular state in their construction being due to some peculiarity of temperature or of mineral contents which the fluid held. These metamorphic phenomena, though important, are obscure, and their elucidation demands some knowledge of petrographic science, that branch of geology which considers the principles of rock formation. They will therefore not be further considered in this work.