In the last issue of these notes, the evidences that a mountain did exist where Crater Lake now lies, were discussed.

What has become of Mt. Mazama ?

It may be claimed that no mountain existed and that the crater as it was now lies before us. But, other than the evidence that a mountain once existed is the fact that the walls of the caldera show cut edges of lava and are not coated or covered with a material which might have issued from this opening.

Two possibilities then remain: first, that a tremendous explosion removed the mass of the peak, or second, that the volcano fell in upon itself.

That terrific explosions did occur in the history of Mt. Mazama is evidenced by the pumice material found about the rim and the outlying region. However, as Diller points out in his paper "Geology of Crater Lake National Park" (Professional Paper No.3, Series B) this material is different from the material of the mass of the mountain judging that mass from the lava of the rim. Furthermore, the quantity thus removed would total approximately 17 cubic miles, which, if laid down three miles from the rim would give a layer a thousand feet thick. No such mass of material is present.

The remaining possibility is that the peak fell in upon itself. Had it fallen in as a solid mountain the displacement of the material at the base would have split the base completely assunder. The rim however, is not split; it is entire throughout its circumference. That the summit should fall into the neck of lava is possible, but offers no solution, since the amount of material would be just as great, although more of it now molten. That the lava of the volcano should escape through a radial vent, or a great fissure upon the lower slopes, thus draining the center remains as a final possible explanation.

Whether the entire column of lava escaped from one vent, or appeared at one point on the surface may be open to question. If the original mountain was equal to Shasta in height the column of lava totaled ten thousand feet, since the lake bottom is 4,200 feet above sea level. Furthermore, the material of the solid mountain must be accounted for, as having fallen into the resulting crater, when the support of the original lava was withdrawn. This material would then also have been fused and withdrawn.

A difficulty in this theory lies in the fact that the lava which can be definitely claimed as having issued from Mt. Mazama has not been determined. The level of the final withdrawal could not be above the 4,200 foot contour since this is the level to which it receded in the caldera.

That the entire mass could have drained into some great subterranean cavity is also possible but rather dodges the issue.

Whatever the explanation, a caldera has resulted nearly three thousand feet deep from the lowest point on the rim to the bottom, and this in turn filled with a lake, two thousand feet deep at its deepest sounding.

The possible origin of the lake itself will be discussed in a future issue of these notes.