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, . The cleavage is not well marked, doubtless because the slices cut off thissomewhat scoriaceous rock are not so thin as those obtained by polishing a morecompact mass. Magnetite, presentiug no noteworthy peculiarity, is also an essentialconstituent of this andesite. Some other specimens, which must also be classed with the andesites, resemble thatjust described very closely in their microscopic characters, onl
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, . The cleavage is not well marked, doubtless because the slices cut off thissomewhat scoriaceous rock are not so thin as those obtained by polishing a morecompact mass. Magnetite, presentiug no noteworthy peculiarity, is also an essentialconstituent of this andesite. Some other specimens, which must also be classed with the andesites, resemble thatjust described very closely in their microscopic characters, only the ground-mass isdarker, more iridescent, and less vesicular. There are some minor differences also REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 159 which must be referred to in detail. Fig. 29 shows a section of plagioclase withhemitropic lamellae, following the albite law. These belong to two principal individuals,which mutually penetrate each other, and present, each in its turn, a larger developmentin the different parts of the section. The two principal individual crystals, whichsometimes form the groundwork and sometimes the lamellae, are twinned in the followingmanner:—. Fig. 29.— Andesite of Ternate. Section of plagioclases, albite, pericline, and Carlsbad twin. I. II. . Albite twin. I. 1/ . . Pericline do. II. II. ... do. do. III. (I. II.) . Carlsbad do. Cleavage parallel to the face P is noticeable in both individuals. This is shown inthe figure by lines sensibly perpendicular to the albitic lamellae. Extinction takes placeat 33° to 34° from on the trace of M. The polysynthetic lamellae following the periclinelaw (I. IF), extinguishing at 27°, meet at an angle corresponding exactly to the trace ofPP, which is clearly indicated at the lower jtart of the figure. The third individual (III),joined to the preceding group in the plane M, must be considered as forming a Carlsbadtwin with (I. II); in fact, this individual gives an asymmetric extinction at 20°. The a
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectscientificexpedition
