It should not be assumed, however, from the above statement that the location or foretelling of underground water is mostly hopeless from a scientific point of view. In most regions the kinds of rocks which would be pierced by wells can be more or less accurately foretold by careful studies of the rocks exposed at the surface. But foretelling the underground water is often much more uncertain. Where the geologic structure or arrangement of rocks in a region is fairly regular, as in the case of most sedimentary rocks, and a few scattering deep wells have been drilled, with records preserved, the geologist, by combining such data with his surface studies, can do much toward putting the facts regarding the underground waters of the region on a scientific basis. There are many such regions, an excellent case in point being Iowa, regarding which State the United States Geological Survey has published a report containing data by the use of which it is possible to foretell almost exactly what formations would be pierced by drilling from 1,000 to 3,000 feet or more, the thickness of each, which ones are water-bearing and, in many cases, even the character of the mineralization of the water for almost any part of the State. Such knowledge, through the years, is worth untold millions of dollars to the State. Where the rocks are igneous and rather uniformly dense, usually little or nothing can be accurately foretold about the underground water supplies, because in such rocks the water follows exceedingly variable and irregular cracks and fissures. In metamorphic rocks the difficulties are usually about as great. In limestone regions, with humid climate, much water travels in channels underground, but these are so exceedingly irregular that there is no way of locating them by surface studies. In humid climates it seldom happens, however, that a well does not reach at least a fair supply of water within a few thousand feet even in rock formations in which the water travels along irregular cracks and channels. Certain other important features of the geological work of underground water should be brought to the attention of the reader. One of these is its power to dissolve mineral substances of many kinds more or less rapidly. As already pointed out, limestone is especially susceptible to solution in water, both surface and underground.