Again the region approached sea-level and accumulated, above its new limestones, other beds of sands. New river systems formed and brought other accumulations from distant highlands. It was then a low swampy plain of enormous size, whose northern limits reached Montana, and which touched what now is Kansas on its east. Upon the borders of its swamps, in Cretaceous times, lived gigantic reptiles, the Dinosaurs and their ungainly companions whose bones are found to-day in several places. For the last time the region sank and a shallow sea swept from the Gulf of Mexico to the Arctic Ocean. Again new limestones formed, and as the surface very slowly rose for the last time at the close of the Cretaceous Period many new deposits were added to the scenic exhibit of to-day. Meantime other startling changes were making which extended over a lapse of time which human mind cannot grasp. Responding to increasing pressures from below, the continent was folding from north to south. The miracle of the making of the Rockies was enacting. During all of Tertiary times earth movements of tremendous energy rocked and folded the crust and hastened change. The modern Sierra rose upon the eroded ruins of its predecessor, again shutting off the moisture-laden western winds and turning the southwest again into a desert. One of the mountain-building impulses spread eastward from the Sierra to the Wasatch Mountains, but Nature's project for this vast granite-cored tableland never was realized, for continually its central sections caved and fell. And so it happened that the eastern edge of the Sierra and the western edge of the Wasatch Mountains became the precipitous edges, thousands of feet high, of a mountain-studded desert which to-day is called the Great Basin. It includes southeastern Oregon, nearly all of Nevada, the western half of Utah, and a large area in the south of California, besides parts of Idaho and Wyoming. It is 880 miles north and south and 572 miles wide. Its elevation is five thousand feet, more or less, and its area more than two hundred thousand square miles. This enormous bowl contained no outlet to the sea, and the rivers which flowed into it from all its mountainous borders created a prehistoric lake with an area of fifty-four thousand square miles which was named Lake Bonneville after the army officer whose adventures in 1833 were narrated by Washington Irving; but it was Fremont who first clearly described it. Lake Bonneville has evaporated and disappeared, but in its place are many salty lakes, the greatest of which is Great Salt Lake in Utah. Attenuated rivers still flow into the Great Basin, but are lost in their sands. The greatest of these, the Mohave River, is a hundred miles long, but is not often seen because it hides its waters chiefly under the surface sands. Lake Bonneville's prehistoric beaches exist to-day. Transcontinental passengers by rail cross its ancient bed, but few know it.