Science for beginners . rse graveland sand slopes down hill, the ground-water passes much morerapidly through it than it does through the layers of fine clayabove and below the gravel. But this would scarcely be called 398 GROUND-WATER AND GROUND-AIR an underground stream; it certainly is not what most peoplemean by that term. The term underground stream may properly be appliedonly to those comparatively rare cases where the water hasdissolved portions of the rock, washing the dissolved portionentirely away and leaving an open channel through which thewater flows (Fig. 235). But such condition
Science for beginners . rse graveland sand slopes down hill, the ground-water passes much morerapidly through it than it does through the layers of fine clayabove and below the gravel. But this would scarcely be called 398 GROUND-WATER AND GROUND-AIR an underground stream; it certainly is not what most peoplemean by that term. The term underground stream may properly be appliedonly to those comparatively rare cases where the water hasdissolved portions of the rock, washing the dissolved portionentirely away and leaving an open channel through which thewater flows (Fig. 235). But such conditions are so rarelyfound that the underground stream is not of any importancewhen considering the source of water supply for wells. 475. A Vein of Water.—If to strike a vein of waterdoes not mean the tapping of an underground stream, whatdoes it mean? In meeting this question we shall also bemeeting the question, What is the real difference between agood well with plenty of water and a poor well yielding butlittle water ?. Fig. 236.—A vein of water. In Art. 456 we saw that the relation of the water table to theland surface has been determined by noting the height ofwater in wells. We have also seen that the water in a shallowwell will stand at exactly the height of the water table, pro-vided that no water is used from the well. Why should usingwater from the well affect the height of the water in thewell? If the well is sufficiently deep so that its bottom isbelow the level of the water table, why should not the wellcontain a sufficient supply of water at all times ? The answerto these questions is this: Although the well does reach be-low the water table we cannot expect much of a flow till wereach a vein of water. But what is a vein of water? RELATION OF GROUND-WATER TO WELLS 399 It is certain that it is rarely, almost never, an undergroundstream. When a vein of water is struck, it is always found in a layerof sand and gravel, or some other open, porous soil; it isalways a la
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