How many times, since the Tertiary epoch, the Salton Sink has been alternately emptied and refilled, we have no means of knowing; but the instability of the conditions that now determine the course of the Colorado below Yuma seem to indicate that, at intervals of four or five hundred years for many millenniums, the river, like a great liquid pendulum, swung back and forth across its delta, now emptying into the Gulf on the Arizona side, and then discharging into the Sink on the California side. Every time the lake was deprived of the river water it dried up, and every time the Sink was revisited by the river it again became a lake. That the Colorado must have returned to this basin many times, and flowed into it for long periods, is indicated by the fact that after the Sink was separated from the Gulf of California, the river carried into it something like seventeen cubic miles of silt.[2] Artesian well borings at Holtville show that the sedimentary deposits in that part of the Imperial Valley are now more than 1000 feet in depth. For three centuries or more--from 1540 to 1902--the Salton Sink was a hot, arid desert. Melchior Diaz, a Spanish explorer in the service of Cortes, reached the edge of it in the fall of 1540, and the Spanish captain Juan Bautista de Anza crossed it two hundred and thirty four years later; but neither of them saw anything like a lake. The only evidence that the Colorado River ran into the Sink, at any time between 1540 and 1905, is furnished by the so-called Rocque map, now in the British Museum, which was compiled from all the sources of information that were in existence in 1762. This map shows a considerable body of water in the Salton Sink, with the Colorado River flowing into it; but no written record in support of the map has ever been found, and the probability is that the water was nothing more than a comparatively small lake, or lagoon, fed by the Colorado in time of flood. Overflow water in considerable quantities often reached the basin when the river happened to be more than bank full; but the main current of the Colorado continued to flow into the Gulf, and the flood water in the Sink soon evaporated.