After a time it was found that this region, covering about two hundred and twenty-five thousand square miles, not only was extremely dry, but had no outlet to the ocean. A rim of higher land all about made of it so perfect a basin that it became known as the Great Basin. None of the water that falls upon the surface of this basin ever reaches the ocean through surface streams. Some of it soaks into the rocks, but the greater part is evaporated into the dry air. We have already learned something about the way in which the ridges and hollows of the earth's surface are made. We have learned of the wrinkling of the crust, of the formation of fissures, and of the erosive work of running water. The interesting features of the Great Basin are mainly the result of two causes: the sinking of a portion of the earth's surface, and the lack of rainfall. Long ago the Wasatch Range of eastern Utah and the Sierra Nevadas of California formed parts of a vast elevated plateau. Then there came a time when the forces holding up the plateau were relaxed, and as the weight of the plateau pressed it down, the solid rocks broke into huge fragments. Some of the blocks thus made sank and formed valleys; others were tilted or pushed up and formed mountains. Thus the north and south mountain ranges and valleys of the Great Basin were born. We must understand, then, that the Great Basin is not a simple depression with higher land all about. The breaking up of the surface produced many basins, large and small. Some of these basins are six thousand feet above the level of the sea, others are much lower, and one has been dropped below the level of the sea, so that if it were not for barriers the water would flow in. Some of the basins are rimmed all about by steep mountains, others are so broad and flat that it is difficult to tell that they really are basins. Many of the valleys are so connected with one another that if a heavy rainfall should ever occur drainage systems would be quickly established. The Great Basin now appears like the skeleton of a dried-up world; but if the climate should change and become like that of the Mississippi Valley, the surface of the desert would undergo a wondrous transformation. The hundreds of basins, if fed by streams from the surrounding mountains, would then become lakes. The highest, overflowing, would empty into a lower, and this in turn into a still lower basin, until the water had accumulated in vast inland seas. These seas, overflowing the rim of the Great Basin at its lowest points, would send rivers hastening away to the ocean. [Illustration: Map of the Great Basin showing the location and extent of the ancient lake beds.] What a region of lakes this would be for a time! Then they would begin to disappear, for lakes are short-lived as compared with mountains. Some would be filled with clay and gravel brought by the streams. Others would be drained by a cutting down of their outlets.