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, . Fig. 31.—Andesite of Camiguin. Section of plagioclase, nearly perpendicular to PjM. j^ crossed nicols. Fio. 32.—Andesite of Oamiguin. Section of plagioclase, Baveno, albite, and pericline twins. is crossed nicols. This (fig. 32) shows I, Baveno twin; II, twin crystals of albite; I and III, twincrystals of pericline ; I, IV, Baveno twin. Fig. 33 shows I and II Carlsbad twin and one of albite; the extinctions f
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, . Fig. 31.—Andesite of Camiguin. Section of plagioclase, nearly perpendicular to PjM. j^ crossed nicols. Fio. 32.—Andesite of Oamiguin. Section of plagioclase, Baveno, albite, and pericline twins. is crossed nicols. This (fig. 32) shows I, Baveno twin; II, twin crystals of albite; I and III, twincrystals of pericline ; I, IV, Baveno twin. Fig. 33 shows I and II Carlsbad twin and one of albite; the extinctions for I are35° on the average (32° to 38° for a, and 32° for b). The section then approaches theface a; for this individual. The individual II extinguishes at 10° for a and 6° for b;therefore the section for II approaches the face P. I prr Fig. 33.—Andesite of Camiguin. Section of plagioclase, albite and Carlsbad twin. ijg crossed Fig. 34.—Andesite of Camiguin. Section of plagioclase following M, pericline twin. sV crossed nicols. The section shown in fig. 34 is parallel to M, and shows a pericline twin. The 166 THE VOYAGE OF CHALLENGER. two individuals are joined in the plane of the rhombic section ; as the figure shows, thisplane is visibly inclined towards the face P in the same direction as the extinction,which is negative and of 39° for one individual, for the other 20°. The principal characters of the plagioclase in the rocks of Camiguin may be sum-marised as follows. The optical properties of this mineral, its structure, groupings, andtwinnings, indicate that it represents a plagioclastic mixture intermediate betweenoligoclase and labradorite. One of the most interesting features of this felspar is, thatthis mineral has crystallised in these rocks with numerous and very well-developedfaces ; the traces of M P T I x y can be seen in the sections. This abundance offaces is a somewhat rare occurr
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectscientificexpedition
