Report on the scientific results of the voyage of during the years 1873-76 : under the command of Captain George SNares, , Captain Frank Turle Thomson, . larger than those of the amphibolic andesite just mentioned. Hornblende 152 THE VOYAGE OF CHALLENGER. plays only a subordinate part, being substituted by augite. The microliths in theglassy base are not so numerous, but of the same species as in the preceding rocks. Numerous and well - defined plagioclase sections are full of vitreous of them show simultaneously the twinnings of pericli
Report on the scientific results of the voyage of during the years 1873-76 : under the command of Captain George SNares, , Captain Frank Turle Thomson, . larger than those of the amphibolic andesite just mentioned. Hornblende 152 THE VOYAGE OF CHALLENGER. plays only a subordinate part, being substituted by augite. The microliths in theglassy base are not so numerous, but of the same species as in the preceding rocks. Numerous and well - defined plagioclase sections are full of vitreous of them show simultaneously the twinnings of pericline and of Baveno;that of albite is subordinate. The two series of polysynthetic lamellae, whichcorrespond in the principal individuals, cross at an angle of about 90°. The albiticstriae extinguish at 30°, a fact which indicates that we have to do with a mixtureapproximating to labradorite. When the sections present the lamellae of periclineclearly defined, the extinctions for the latter are a little smaller than for the principalindividual, being about 27° for the lamellae in question and 30° to 31° for thepolysynthetic lamellae twinned following the albite law (see fig. 26).. Fig. 26.—Augite-andesite of Kandavu. Section of twinned plagioclase. I. II. ... Pericline twin. I. II. ... do. (I. II.) (I. II.) Baveno twin. III. I. . Pericline twin. IV. V. . Twinned with I. and II. having the face P common. The augite presents its usual characters in augitic andesites. It is sometimestwinned polysynthetically; in other cases the sections show a fibrous structure causingthem to resemble diallage. Augite contains felspar and magnetite as inclusions. Horn-blende, which has a very small part to play in this rock, is represented by sectionsoften twinned, with worn angles and surrounded by magnetite. The plcochroism ofthis hornblende is— yellow-brown. /3brownish yellow. > pale yellow. REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 153 XII.—THE VOLCANO OF GOONONG API (BANDA ISLANDS
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectscientificexpedition
