90 Secretions with Basalt-like Structure

The Geology and Petrography of Crater Lake National Park, 1902

 PART II.

HYPERSTHENE-DACITES.

DACITIC EJECTAMENTA.

SECRETIONS WITH BASALT-LIKE STRUCTURE.

In addition to the above-described secretions there remains a specimen collected in the bottom of the Sun Creek Canyon and already mentioned on page 123, that can not be classified with any of these varieties. The specimen in question (123) has a grayish color and is rather fine grained, but is distinctly granular and porous. In thin section the structure appears at first glance to be distinctly basaltic, with dark-brown glass in the angular interstices formed by the intersections of rather short, thick-set plagioclase laths. Both hypersthene and augite abound and occur in irregular grains not dissimilar to the basaltic pyroxenes, also in long, slender, prismatic forms. In addition to these minerals, hornblende in dark reddish-brown grains and prisms is quite abundant, and magnetite in occasional black grains. Occasionally a plagioclase or hypersthene crystal assumes somewhat larger form and appears in the role of a phenocryst. What appears to be dark-brown glass inclosing slender augite microlites and filling the interstices between the plagioclase laths is seen in polarized light to be small, spherulitic crystallizations. In spite of the general structure this rock is quite different from any of the Crater Lake basalts. Its association with dacites and the presence of abundant hornblende, as well as the spherulitic base, lead to the conclusion that this is in reality a secretion in a dacitic magma.

An inclusion in a dacite dike below Llao Rock, described on page 122, is considered to be a similar occurrence.

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