All in all, ground-water is to be looked upon as a most important geological agent. When it is remembered that a very large part of all the water which falls on the surface of the earth, either in the form of rain or snow, sinks beneath the surface; that much of it sinks to a great depth; that much of it has a long underground course before it reappears at the surface; that it is everywhere and always active, either in subtracting from the rock through which it passes, in adding to it, in effecting the substitution of one mineral substance for another, or in bringing about new chemical combinations; and when it is remembered that this process has been going on for untold millions of years, it will be seen that the total result accomplished must be stupendous. The rock formations of the earth to the depths to which ground-water penetrates are to be looked upon as a sort of chemical laboratory through which waters are circulating in all directions, charged with all sorts of mineral substances. Some of the substances in solution are deposited beneath the surface, and some are brought to the surface where the waters issue. Much of the material brought to the surface in solution is carried to the sea and utilized by marine organisms in the making of shells. Without the mineral matter brought to the sea by springs and rivers, many shell-bearing animals of great importance, geologically, would perish. Biologically, therefore, as well as geologically, ground-water is of great importance. [Illustration: PLATE XVII. Fig. 1. HUNTERDON COUNTY, NEW JERSEY. U. S. Geol. Surv. Fig. 2. NEAR PIKEVILLE, TENNESSEE. U. S. Geol. Surv.] [Illustration: PLATE XVIII. Fig. 1. WASHINGTON. U. S. Geol. Surv. Fig. 2. CALIFORNIA. U. S. Geol. Surv.] [Illustration: +Fig.+ 210.—A “talus glacier,” in Silver Basin, near Silverton, Colo. (Cross, U. S. Geol. Surv.)] [Illustration: +Fig.+ 211.—A huge mass of rock settling into the Canyon of the Colorado. A result of gravity action. (Atwood.)] SPRINGS AND FLOWING WELLS. The term _spring_ is applied to any water which issues from beneath the surface with sufficient volume to cause a distinct current. If the water issues so slowly as to merely keep the surface moist, it is not called a spring, but seepage. The spring from which water issues with a strong current, especially if it be upward, is comparable to a flowing well, while the spring from which water issues with little force, and without upward movement, is comparable to the flow of water into a common well. Springs often issue from the sides of valleys (Fig. 212), the bottoms of which are below ground-water level. They are especially likely to issue at the surface of relatively impervious layers, and where the valley slopes cut joints, porous beds, or other structures which allow free flow of ground-water. [Illustration: +Fig.+ 212.—Diagram illustrating positions, _a_ and _b_, favorable for springs.]