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, . ite. Olivine appears in sharply defined sections. The decomposition of this mineral issomewhat remarkable, as it changes into hematite with the simultaneous developmentof trichites. Such an altered crystal with the curved and parallel lines of the trichitesinvading the mineral is shown in fig. 10. Some-times the little olivines of the ground-mass havea quite pronounced prismatic form, which makesit difficult
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, . ite. Olivine appears in sharply defined sections. The decomposition of this mineral issomewhat remarkable, as it changes into hematite with the simultaneous developmentof trichites. Such an altered crystal with the curved and parallel lines of the trichitesinvading the mineral is shown in fig. 10. Some-times the little olivines of the ground-mass havea quite pronounced prismatic form, which makesit difficult to distinguish them from microlithicaugite. The augitic microliths are colourless likeolivine, but the sections of the latter are edgedwith a zone of limonite which serves to distinguishthe species. There is little to say about thelarge crystals of augite, which appear less oftenin these specimens than is usual in basalts, thecommonest form in this case being microlithic. These rocks have often a vesicular appearance when in thin slices, although theyseem perfectly massive when looked at with a lens. The vacuoles are generally due to (PHTS. CHEM. CHALL. EXP.—PART VII.—1889.) 8. Fig. 10.—Basalt of Bed Hill. Section of olivinedecomposed into hematite and filled with crossed nicols. 58 THE VOYAGE OF CHALLENGER. the disappearance of sections of peridotite, which crumble and are swept away duringthe polishing. The basalts of the island are often scoriaceous; a basaltic lava fromBiding School which we have examined is particularly so : it is a reddish scoriae, veryalveolar and rough, and containing heterogeneous half-fused fragments. The microscope shows a ground-mass of a very fine grain and pitted with and plagioclase appear in microporphyritic sections; the former predominatesand is often fragmentary, although when the crystals are very small they are sharplydefined. Augite is found in the form of microliths in the ground-mass, which contai
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectscientificexpedition
