It is probable that this vast blue-bottomed caldron was once covered with a volcanic peak. This vanished volcano is named Mount Mazama. The geological story is that the upper half of the peak collapsed. There was volcanic violence. But it did not, like Mount Rainier and Mount Baker, explosively blow its summit to pieces. A mile or more of the upper half simply collapsed and dropped into the crater. Had an explosion hurled the enormous fragments of the top afar, they must have been found scattered about. But only small fragments of pumice have been discovered. [Illustration: CRATER LAKE AND WIZARD ISLAND] This collapse appears to have been preceded by a rupture of the base, allowing the lava to escape. This lava had filled the crater and supported its walls, and the collapse followed its removal. The upper part of this peak that apparently dropped into the crater must have been six thousand or more feet high, with a basal diameter of about six miles. Its bulk was equal to, or greater than, the whole of Mount Washington, the highest peak in New England. An early impression that this lake crater gave me was that it had been formed by breaking off an enormous conical and hollow volcanic peak which was inverted and jammed, small end downward, into the earth. This caldron remains. It is now a jagged, gigantic central opening in the deep surrounding lava-beds. These exhibit the former fiery flooding activity of Mazama. The volcano was active at intervals in the glacial period. This is shown in the glaciated rock-surfaces of the rim that are covered with layers of pumice and rhyolite. The lake is encircled by about twenty miles of precipitous walls that rise from five hundred to two thousand feet above the surface of the water. The lake-level is 6177 feet. The surface fluctuates a few feet each year. The water is deep, much of it from twelve hundred to nineteen hundred feet. In a few places it is less than three hundred feet deep, with near-by surroundings several hundred feet deeper. Are these shallow spots above the tops of other volcanic cones or lava-masses? The lava-beds in the surrounding outer slopes of the crater overlie one another at an angle that indicates that the lava was poured to them from a central point above. Extend the slopes upward from the rim on the angle of the slopes below, and the outline of the former peak is restored. This would make a peak about the size of Mount Shasta. At the altitude of the crater rim, about eight thousand feet, the diameter is about six miles, the same as that of Mount Shasta at the same altitude. As both peaks are composed of like kinds of lava, we may safely assume that Mount Mazama before it collapsed was about the size and height of Mount Shasta (14,380 feet).