If anywhere in the course of this stream a very deep well shaft is driven right down through the soil until it strikes the surface of this sloping layer of rock, then the water will rise in this shaft to the level of the highest point from which it is running.
[Illustration: ARTESIAN WELL BORINGS The sketch shows a wide section from northern Illinois to central Wisconsin, in which the cities have rejected the water supplies afforded by the rivers, choosing instead to bore down almost to hard rock to insure the purity of the supply.] If this highest point of the waterproof layer be many miles away, up in the hills above the surface of the ground where the well is dug, then the water will rise to the surface and sometimes even spout twenty, thirty, or fifty feet above it.

This forms what is known as a _gushing_, or _artesian_, well (from Artois, a province in France, in which such wells were first commonly used) and furnishes a very pure and valuable source of water supply.

If it rises only twenty, thirty, or fifty feet in the well-shaft, but keeps flowing in at a sufficient rate, then we get what is known as a "living," or _permanent_ well, and this also is a very valuable and pure source of water supply.
Springs.

Springs are formed on the same plan as the deep well, but with the difference that the waterproof layer on top of which the water is running either crops out on the surface again, lower down the mountain, or folds upon itself and comes up again to the surface some distance away from the mountain chain, out on the level.

This is why springs are usually found in or near mountainous or hilly regions.