Water Supply and Irrigation Papers of the United States Geological Survey . as about 11,000,000 acre feet per evaporation from a surface of the area of the old lake, underthe conditions that prevail here, has never been determined, but isundoubtedly high. If it is as high as 8 feet per annum it wouldnearly equal the average annual inflow from the Colorado; if it isbut 7 feet per annum the average inflow would exceed the evapora-tion by 2,000 second-feet, or somewhat less than 14 per cent of theinflow. In either event the waters of the lake would be markedlymore alkaline after a term
Water Supply and Irrigation Papers of the United States Geological Survey . as about 11,000,000 acre feet per evaporation from a surface of the area of the old lake, underthe conditions that prevail here, has never been determined, but isundoubtedly high. If it is as high as 8 feet per annum it wouldnearly equal the average annual inflow from the Colorado; if it isbut 7 feet per annum the average inflow would exceed the evapora-tion by 2,000 second-feet, or somewhat less than 14 per cent of theinflow. In either event the waters of the lake would be markedlymore alkaline after a term of years than those of the Colorado. Thecalcium carbonate incrustations (PI. Ill, A) on the rocky pointsabout the shores of the old lake are best explained by supposingthat the lake waters contained large quantities of this salt, so thatwherever they broke in spray and evaporated more rapidly thanusual the carbonate was deposited. This necessary excess of inflow aStearns, Robt. E. C, Remarks on fossil shells from the Colorado Desert: Am. Naturalist, vol. 13,pp. GEOLOGIC SKETCH. 19 over outflow at the period of maximum area of the lake, taken inconnection with the thick calcium carbonate incrustations on theshores, indicates distinctly hard water. It may be assumed thatother salts than calcium carbonate were also present in large amount,for the conditions that would lead to an abundance of the one saltwould also lead to an abundance of the others. The shells so thicklydistributed over the desert floor, however, are not salt-water forms,but are identical with those now found living in the springs andoccasional permanent streams about the desert borders. Many ofthese springs and streams are somewhat brackish, and the creaturesflourish in them. It seems probable, then, that the lake waters alsowere rather alkaline, perhaps even brackish, at the time when thelake attained its maximum area. The period at which this lake disappeared can not be preciselyfixed. The time units
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