111 Concluding Remarks on Basalts

The Geology and Petrography of Crater Lake National Park, 1902




At the time these rocks were studied no analysis of the andesitic types of basalts had been made, and the rocks were named in accordance with their structural features. In individual cases the resemblance of these andesitic basalts of type D could not well be distinguished from certain of the Crater Lake andesites. It was only by taking all the specimens collected from a particular volcanic center that the real differences could be made to appear. The surmise that these rocks are nearly if not quite the equivalent of the andesites is borne out by an analysis made later and found on page 161. This rock, No. 189, comes from the lava flow northwest of the Pumice Desert on the extreme northern edge of the area mapped. The analysis is not far different from the average analysis of the Crater Lake andesites. It is very closely similar to the analysis of the andesite No. 31, given on page 94.

From a comparison of these analyses it is evident that chemically there is no marked distinction between this andesitic basalt and some of the rocks that may be considered as true andesites.

If this individual rock sample could well be considered apart from the immediately surrounding rocks it should be designated as an andesite, but the preponderating evidence seems to point to the general rock mass from which this was taken being a basalt.

The features that distinguish these rocks from the andesites are, first, the abundance of olivine which, though very fluctuating, is a nearly constant ingredient, supplanting in whole or in part hypersthene; second, the frequent disappearance of the hypersthene; third, the relatively greater abundance of augite both among the phenocrysts and in the groundmass and, in the latter case, their greater size and less distinctly microlitic form; fourth, the comparative scarcity of a glassy base; fifth, the greater abundance of magnetite.

The basalts of Crater Lake of all types are, like the andesites and dacites, almost completely unaltered. With the exception of olivine, the ingredients are apparently as fresh as when first formed. The only forms of alteration in the olivine as far as noted are the partial serpentinization and the development of magnetite and hematite. The development of the homogeneous pseudomorphs after olivine, described by Iddings from somewhat similar rocks in the Eureka district, Nevada,a and named iddingsite by Lawson,b have not here been noticed.

aGeology of the Eureka district: Mon. U. S. Geol. Survey, Vol. XX, 1892, p. 387.bBull. Dept. Geol. Univ. California, vol. I, pp. 31-36.

The hypersthene-bearing basalts of the Eureka district, Nevada, which have been described by Mr. Iddings in the monograph just above quoted, appear from Mr. Iddings’s description to be closely related to the basalts of Crater Lake, more especially, however, to the andesitic basalts, except that the Eureka district basalts are much less feldspathic. Not only is the general character of the groundmass the same, but also the pyroxenes and the fluctuating olivine that comes in with the disappearance of hypersthene. Furthermore may be noted the relationship of the generally older hypersthene to the younger augite and the parallel growths of augite around hypersthene phenocrysts, which is an almost universal occurrence in the Crater Lake basalts of all types. Comparison may also be made with the quartz-bearing basalt of Cinder Cone, north of Lassen Peak, in northern California, described by Mr. Diller in a paper on A Late Volcanic Eruption in Northern California and its Peculiar Lava.a With the exception of quartz, the resemblance is quite close. More especially may be mentioned the magnetite rims around the olivine crystals and the plagioclase phenocrysts with their frequently corroded forms and glass inclusions, as well as the development of the monoclinic and orthorhombic pyroxenes.

aBull. U. S. Geol. Survey No. 79, 1891.
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