Yuma to the rampart of the Cocopah Mountains at Black Butte. When the river had thus cut the Gulf of California in two, it happened to choose a course for itself on the southeastern side of the delta-plain that it had built up, and thereafter it discharged its waters into the lower Gulf, leaving what had been the upper Gulf isolated as a salt-water lake. Under the burning sun of that region about six feet of water evaporates every year, and in course of time the lake dried up, leaving the arid basin afterward known as the Salton Sink. This depression was about one hundred miles in length by thirty five in width. It then had a maximum depth of perhaps one thousand feet, and in the deeper parts its floor was covered with an incrustation of salt. [Illustration: The Ancient Gulf of California] How long this ancient sea-bottom remained dry cannot now be determined; but many thousands of years ago, probably in Middle Tertiary times the Colorado River, which had first cut off the basin from the ocean and thus allowed it to become waterless, proceeded to refill it. Running over a raised delta-plain of silt, which sloped both ways, the river could easily be diverted to either side, and in one of its prehistoric floods it capriciously changed its course, leaving the Gulf and pouring its waters into the dry basin of the Salton Sink. When it had refilled this basin, and transformed it into a great fresh-water lake, it broke through the silt dam, or levee, on the Cocopah Mountain side, and found a new outlet to the Gulf through what is now known as Hardy’s Colorado. For many years--possibly for centuries--the Salton Sink was a fresh-water lake, into which the Colorado poured 150,000,000 tons or more of silt every year. At last, suddenly or gradually, the river again changed its course, abandoning the Sink and cutting a channel to the Gulf through the eastern part of the delta plain. Then the Salton Sea again dried up, leaving a two-hundred-mile ellipse of fresh-water shells to mark its former level.