Springs are the natural outlets for the underground water. But, you ask, why should this water have any outlets, and what makes it rise to the surface? Let us suppose that a flat layer of some impervious rock, like clay, underlies another layer of a porous material, like sand. The rain which falls on the surface of the ground, and sinks through the upper bed, will be arrested by the lower one, and made either to gather there, or find its escape along the surface of that lower bed. If a hollow or valley should have its bottom below the level of the line along which the water flows, springs will gush out along the sides of the valley. The line of escape may be either the junction between two different kinds of rock, or some of the numerous joints already referred to. Whatever it be, the water can not help flowing onward and downward, as long as there is any passage along which it can find its way; and the rocks underneath are so full of cracks, that it has no difficulty in doing so. But it must happen that a great deal of the underground water descends far below the level of the valleys, and even below the level of the sea. And yet, though it should descend for several miles, it comes at last to the surface again. To realize clearly how this takes place, let us follow a particular drop of water from the time when it sinks into the earth as rain, to the time when, after a long journey up and down in the bowels of the earth, it once more reaches the surface. It soaks through the soil together with other drops, and joins some feeble trickle, or some more ample flow of water, which works its way through crevices and tunnels of the rocks. It sinks in this way to perhaps a depth of several thousand feet, until it reaches some rock through which it can not readily make further way. Unable to work its way downward, the pent-up water must try to find escape in some other direction. By the pressure from above it is driven through other cracks and passages, winding up and down until at last it comes to the surface again. It breaks out there as a gushing spring.