The third well was sunk 8 miles farther south, near the bank of Scott Creek, approximately 3/5 mile west of the Dalles-California highway. In the upper part of this well, the smoke-gray scoria and pumice were abnormally rich in small lithic fragments, to such an extent in fact that certain layers contained as much as 75 per cent of such ejecta. From 50 feet down to 75 feet, basic scoria was absent and lithic fragments were rare. In this range, the ejecta consisted almost entirely of coarse dacite pumice lightly mixed with crystals. From 75 feet to the bottom of the well at 100 feet, there were rapid alternations of fine, granular pumice and water-worn volcanic sand. Presumably, therefore, the base of the pumice-scoria flows may here be placed at 75 feet; the underlying deposits represent the products of older pumice eruptions mingled with river-borne sediment.
We are now in a position to make an estimate of the volume of the pumice-scoria flows. First, consider the area east of the park. From the foregoing well logs, and taking into consideration the probable pre-pumice form of the valleys that debouch from the park onto the plateau to the east, it may be calculated that approximately 4 to 4.5 cubic miles of material were laid down by the glowing avalanches. How much pumice escaped from the Klamath Marsh into the Williamson River and so was washed into Upper Klamath Lake cannot be determined.
Within the limits of the park, the volume of the pumice-scoria flows, prior to erosion, approximates 1 cubic mile. In the valley north of Timber Crater and in the depression bordering Diamond Lake and Lake Creek, the volume may be about 0.6 cubic mile. In the valley of the Rogue River down to the Union Creek junction, and in National, Copeland, Bybee, and Crater creeks west of the park boundary, the total volume is estimated to have been 0.75 cubic mile prior to erosion. Finally, between the Union Creek junction and the snout of the Rogue River flow near McLeod, the original volume must have been close to 0.3 cubic mile, whereas in the valley of Annie Creek south of the park boundary there may have been approximately 0.2 cubic mile of pumice. What can never be known is the amount of pumice that was washed into the Upper Klamath Lake below Fort Klamath, and the amount that was carried down the Rogue River below McLeod. It is likely, however, that this missing volume was not great.
The total volume of the pumice and scoria flows is therefore about 7 cubic miles. Almost certainly, it is not less than 6 nor more than 8 cubic miles. Of this volume, at most 20 per cent consists of old lithic detritus. In other words, of the 17 cubic miles that disappeared from the top of Mount Mazama, only a small fraction is to be found in the deposits of the glowing avalanches. The proportion of lithic detritus in the preceding pumice fall is even less. The significance of these figures in connection with the origin of the caldera needs no emphasis.
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