93 Interstitial Basalts

The Geology and Petrography of Crater Lake National Park, 1902




This type of basalt is represented by four specimens (152, 153, 154, 155) taken from the hill that rises 500 to 600 feet above the level of the plain of Diamond Lake, in the extreme northwest corner of the Crater Lake area; also by two specimens from the lava flow at the east base of Red Cone, about 1-1/2 miles northwest of Llao Rock on the crater rim (156, 157); also by four specimens taken from different places in the basaltic area west of Anna Creek in the extreme southwestern corner of the region mapped (158, 159, 160, 161).

These rocks are all rather light colored, of a grayish to brownish tint, and contain occasionally a few cavities, but are never really scoriaceous. The grains are remarkably uniform and are rather dense, but not extremely so. Under the lens they appear holocrystalline and disclose occasionally greenish to reddish olivine grains and likewise greenish grains of hypersthene. Both olivine and hypersthene have a distinctly resinous luster.

An absolutely holocrystalline structure can only occasionally be made out under the microscope (156, 152), but in all cases a glass base plays a very subordinate role. The structure is dominated by the feldspars, which occur in mostly very distinct lath-shaped forms, while the augite is largely confined to the more or less angular spaces between the feldspar laths, after the manner of a mesostasis. This structure may be termed interstitial (Rosenbusch’s “intersertale struktur”). The rocks in question, however, are too decidedly feldspathic to give typically interstitial structures. In one or two cases they assume a nearly hypidiomorphic (156) and in others a porphyritic structure. The porphyritic development, however, is caused by the occurrence of phenocrysts of olivine and hypersthene, and rarely of augite or plagioclase.

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