Proceedings of the United States National Museum . ofthe other faces are planeand brilliant, giving excel-lent reflections. In the late summer, 1923numerous specimens of thispostprehnite datolite werefound, mostly granular butsome showing free specimen handed tome by the owner of thequarry in August, 1923,had been laid aside by thequarry foreman, and whatpart of the quarry it came from was not known. This had a relativelylarge cavity filled with datolite surrounded by an earlier crust of prehn-ite. The datolite crystals of this specimen are the most flattenedcrystals of this mineral
Proceedings of the United States National Museum . ofthe other faces are planeand brilliant, giving excel-lent reflections. In the late summer, 1923numerous specimens of thispostprehnite datolite werefound, mostly granular butsome showing free specimen handed tome by the owner of thequarry in August, 1923,had been laid aside by thequarry foreman, and whatpart of the quarry it came from was not known. This had a relativelylarge cavity filled with datolite surrounded by an earlier crust of prehn-ite. The datolite crystals of this specimen are the most flattenedcrystals of this mineral which have come under my notice. The largestof these may reach a breadth of 15 mm. with a thickness of only 1 are imperfectly developed and it was found impossible to orientthem except by placing them on the flat side and examining themoptically. The emergence of an optic axis nearly perpendicular tothe flat face identified it as the dome x (102), and this, together withthe position of the optic axial plane, served to orient them. It was. Fig. 22.—Datolite of second generation showing pro-nounced TABULAR development PARALLEL TO X (102). ART. 2. PETROLOGY AT GOOSE CREEK SHANNON. 71 found most advantageous to measure these in the Goldschmidt posi-tion. The faces, except x (102), are small, and those of a (100),m (110), and c (Tl2) are etched to complete dullness. The tabularface X (102) and n (HI) give fair reflections and m^ (Oil) givesexcellent signals. The broad face of x (102) is marked by triangularelevated spots with apices pointing toward a (100) and bases upwardtoward the position of c (001). The habit of the crystals is indicatedin the drawing, figure 22, although in this position the tabular habitof the crystals is not so obvious. CHABAZITE.
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Keywords: ., bookauthorun, bookcentury1800, bookdecade1870, booksubjectscience
