Proceedings of the United States National Museum . btained fromtwomuscovite plates superposed at rightangles to each other, while the end sectorsgive still more complicated interferencefigurs8, suggesting 3 mica plates at 60° to each other. The acutebisectrix of allof the intergrown crystal units is perpendicular to thetable. The simplest explanation which will fit these several peculiaritiesis that the crystals are made up, as before, of an underlying homo-geneous crystal which, however, is not of uniform thickness but thick-ens in all directions from the center. Thinning would produce thesam
Proceedings of the United States National Museum . btained fromtwomuscovite plates superposed at rightangles to each other, while the end sectorsgive still more complicated interferencefigurs8, suggesting 3 mica plates at 60° to each other. The acutebisectrix of allof the intergrown crystal units is perpendicular to thetable. The simplest explanation which will fit these several peculiaritiesis that the crystals are made up, as before, of an underlying homo-geneous crystal which, however, is not of uniform thickness but thick-ens in all directions from the center. Thinning would produce thesame result but the thickening is actually noticeable when the prehn-ite crystals are examined. This simple tapering crystal is overlainby layers having the arrangement of the sector pattern, figure c and c there is one overlying plate with optical directions atright angles to those of the fundamental crystal beneath. In a —aand b — h three layers, the two upper being oriented at 90° to eachother and 45° to the underlying crystal. /\\^.. Fig. 20.—Datolite of first generation showing acute habit. .ABT. 2. PETROLOGY AT GOOSE CREEK SHANNON. 67 A small vein in the south end of the quarry varied from 1 to 2 and was filled with granular fine green porous prehnite. Thecavities are lined with brilliant little crystals which are elongated onthe a axis and have the crystal habit shown in the orthographic andclinographic drawings of figure 12. The prism faces gave goodmeasurements indicating the prismatic angle to be 80° 59. Thefront pinacoid also gave good signals butthe side pinacoid (010) is etched dull. Thebase is horizontally striated as crossed nicols these crystals gavethe pattern shown in figure 19. Althoughat first glance appearing more complicated,this is found to be only a variation of thestructure shown in figure 18, and is capableof the same interpretation. These interpretations are applicable onlyto the prehnites here described and this
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Keywords: ., bookauthorun, bookcentury1800, bookdecade1870, booksubjectscience
