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, . und that the principal individual(that in which the veinules are imbedded) extinguishes at + 5°, the value of extinctionfor orthoclase on this face. The spindle-shaped veinules, on the contrary, have anextinction of the same sign, but much greater, the angle attaining 18°, the extinctionof albite. We may conclude that this fel-spar is orthoclase, including fine lamellae ofalbite (see fig. 12). This determinat
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, . und that the principal individual(that in which the veinules are imbedded) extinguishes at + 5°, the value of extinctionfor orthoclase on this face. The spindle-shaped veinules, on the contrary, have anextinction of the same sign, but much greater, the angle attaining 18°, the extinctionof albite. We may conclude that this fel-spar is orthoclase, including fine lamellae ofalbite (see fig. 12). This determination as micro-perthite is again confirmed by the fact that wehave never been able to detect in any of thefelspathic sections the intercrossed lamellae ofmicrocline. The innumerable gas enclosures withwhich the sections are riddled, giving a scorifiedappearance to the mineral, seem to characterisethis felspar, and perhaps to indicate the hightemperature to which it was exposed during itstransport by the molten lava. With the excep-tion of this the sections of felspar show only very slight traces of modification. Hornblende presents itself in irregular sectionsare very pleochroic :. Fig. 12.—Amphibolic granite. Section of orthoclasewith veinules of albite ranged in lines following theprismatic cleavage (microperthite). fe crossed nicols. They almost black. y > dark green. brownish yellow. 64 THE VOYAGE OF CHALLENGER. Were it not that there are certain sections showing the characteristic cleavages of horn-blende to guide us, we might hesitate in some cases to classify these green sections withthis species. Sometimes they may almost be mistaken for indented plates of mica ; inother cases, when they are not lamellated, they are more like a mineral of the clintonitegroup; but the cleavages are certainly those of hornblende. Quartz has crystallisedlast. The sections of this mineral are cracked in a remarkable way, each forming atrue breccia, the fragments of which are sur
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectscientificexpedition
