The principle geologic members in Crater Lake National Park are formed from igneous domal extrusions, flows, or ejecta, and from glacial redeposition of igneous materials. The dominant lavas of the southern high Cascades are olivine-bearing basaltic andesites. The pre-Mazama lavas are predominantly of these materials. The Mazama lavas are primarily hyperesthene andesites which Williams assigns to two classes, basaltic and dacitic (1942:130-147).
There are no known deposits of dense, fine-grained, homogeneous basalts, or obsidian, suitable for flaking stone artifacts anywhere within the Park or in adjacent areas. Experiments in the field indicate that the Mazama andesites are practically unsuitable for almost the entire range of stone working techniques. Fracture is indifferent and the extremely numerous and large crystalline inclusions makes the material unmanageable Williams (1942:137) states that the dacite forming Llao Rock “is mainly composed of black obsidian,” but while glassy, the spherulitic dacite has indifferent, uncontrollable fracture. The same observation is true for the glassy dacites of Grouse Hill and Mazama Rock, and for the bands of “obsidian bordering the dacite dikes (Williams 1942:143) A specimen collected from the obsidian border” of the dacite dike beneath Llao Rock (Williams 1942: Plate 7, Fig, 2) contains numerous inclusions, the largest of which is about 1 1/2 cubic centimeters in volume.
Intense solfataric action converted some of the lavas to “masses of milky opal and kaolin,” and ‘”here and there chalcedony occurs” (Williams 1942:136) but these forms rarely occur beyond the caldera . Artifacts can be flaked from the opal material, as shown by the specimen, CLNP 858 in the Park collection. The opal masses are generally very small, occurring as inclusions in the parent rock, and no deposits large enough to be mined are known (Hans Nelson, Seasonal Ranger-Geologist, personal commication).
The obsidian artifacts were obviously imported. Obsidian is obtainable in any quantity from vast deposits to the east which are to be found extendng from Massy Buttes, east of Burns, to Glass Mountain, in northern California east of Lassen Peak. In the-Umpqua drainage to the west, fine-grained quantites, called jasper by rock collectors, were mined by local Indian tribes, and nodules of obsidian are found in gravel bars of the North Umpqua at Steamboat (H. L. Lilligren, Umpqua National Forest Service, personal communication).