In the lake region of the Valley of Mexico Nature and Man have combined to .produce radical changes in the character of the landscape. The giant elliptic bowl about forty-five miles wide by fifty-five miles long, which was formed during the elevation of the Mexican plateau, has been the stage of violent volcanic outbursts, followed by interludes of sedimentation. In geologically speaking recent times and, accordingly, since man made his appearance in the valley, sedimentation has been the dominating, if not sole factor. Owing to the fact that the Valley of Mexico has no natural outlet, the sediments brought down from the surrounding steep and high mountain ranges have accumulated in enormous quantities, forming a huge floor, over which the water from the mountain sides spreads in a shallow sheet. Since precipitation is often violent, and very irregularly distributed throughout the year, and since even evaporation under the strong tropical sun is too slow to prevent great quantities of water from remaining in the valley for a considerable time, the history of man in this region is a narrative of an endless struggle against the wet element. Even today the work is going on, although the tunnel outlet finished in 1900 has put a stop to inundations on a large scale. The colonial works have turned the lake bottom into an arid and salty desert, only partly covered by water.