So much of the Colorado Plateau region as lies within Colorado and Utah was covered during a geological age which it is convenient to call Cretaceous, by a sea, the waters of which appear to have become fresh toward the last. Then came elevation both general and differential. A great part of the sea bed became dry land, and the accumulated sediments together with many which underlay them were bent into great waves thousands of feet in altitude. The crests of the waves were subjected to erosion and truncated. Then came a second submergence which was purely lacustrine. In some way that has not been ascertained a lake basin was formed, and the region received a new system of sediments which it is convenient to call Tertiary, and which not merely filled the troughs between the great rock-waves but covered the truncated summits of the waves themselves. Then followed the desiccation of the basin by the cutting down of its rim where the water overflowed. The overflowing river as it deepened its channel and gradually lowered the lake, steadily extended its upper course to follow the receding shore; and finally when the basin was completely drained the river remained, its channel leading through what had been the deepest part of the Tertiary sea. That river is the Colorado. As portions of the lake bottom were successively drained they began at once to be eroded, and from that time to this there has been progressive degradation. The regions nearest to the central river were reduced most rapidly and have been completely stripped of their Tertiary strata, but broad areas of the latter remain at the west, and north, and east. (The reader will understand that this succinct history is shorn for the sake of clearness of all details and qualifications. There have been complicating eruptions and displacements or oscillations at every stage, and if the full story could be told, it would not be by a single paragraph nor by a single chapter.) When the Cretaceous strata were thrown into waves the site of the Henry Mountains remained in a trough, and it probably was not dried, but continued the scene of sedimentation while the crests of the surrounding rock-waves were worn away. Certainly it was not greatly eroded at that time; and when the Tertiary lake beds were thrown down it was favorably disposed for a heavy deposit. It is not extravagant to assume that four thousand feet of lake beds rested on the Masuk sandstone at the beginning of the final desiccation. In brief there may be distinguished— 1. The deposition of the Cretaceous. 2. The folding and erosion of the Cretaceous. 3. The deposition of the Tertiary. 4. The desiccation of the Tertiary lake basin. 5. The erosion which is still in progress. It is evident that the laccolites were not formed until the Cretaceous strata had been deposited; for their uplifts have bent and tilted all Cretaceous rocks up to and including the Masuk sandstone.