"I was out in the dry part of South Dakota, one time," he said, "when some people, knowing that I was on the Survey, asked me to locate an artesian well site for them. That was a dry country, I reckon. Why, the little water that was there was so ashamed of itself that it tasted bad. Well, after I had studied the lay of the land for some time, I told them where to sink the well. It was an unlikely looking spot, I'll admit, but I knew there was water there if they would go down deep enough." "But how did you know," asked Roger. "Did you use a divining rod?" "I'm not a seventh son of a seventh son," said the older man with a laugh. "No, indeed, that sort of thing is done to-day by science, not by magic. You see, Roger, water will always be found in large quantities in porous rocks like sandstones, and none at all will be discovered in what are called impermeable rocks like shale and limestone." "Why not?" asked the boy, interrupting. "Because a porous rock is like a sponge, and will hold the water, and an impermeable rock isn't. So, you see, if a thick bed of shale is underlaid by a thick bed of sandstone, you are pretty sure of getting water if you drive a well through the shale." "But I don't see how that helps," interjected Roger; "it seems to me it would be as hard to tell that there was sandstone so far below ground as to tell that there was water there. You can't see through rock!" "No, my boy, but if you know the general make-up of the country, and how the rocks lie in the nearest mountains and in the ravines and so forth, you can tell. For example, if a river bed has been cut through the upper shale to the sandstone and through the sandstone to some other rock beneath, you are sure to find that sandstone under that shale everywhere, until you strike a place where geology will show that there has been some other change. In this particular case, the sandstone and the limestone appear in successive layers in the foothills of the Rockies, so that the water and snow from the mountains drains into the sandstone layer, which, being between two strata of harder rocks, can't sink any further down, but must force its way through the pores of that sandstone as far as the stratum runs. Of course things come up to complicate that, but such is the general plan. [Illustration: A LOFTY SPOUTER. Artesian Well at Woonsocket, South Dakota. Well throws a 3-in. stream to a height of 97 ft. _Photograph by U.S.G.S._] [Illustration: WATER ENOUGH FOR ALL. Artesian Well at Lynch, Nebraska. Flows more than 3,000 gallons a minute. _Photograph by U.S.G.S._]