Proceedings of the United States National Museum . Fig. 25.—Stilbite ofthe usual habitshowing pyrami-dal faces. 74 PROCEEDINGS OF THE NATIONAL MUSEUM. VOL. small inclined extinction, up to 5°, on either side of the twinningplane. The crystals of stilbite described above have the habit of epides-mine while the analysis gives the composition of stilbite. The crys-tals are zoned somewhat, the zones differing in refractive index butseeming to cover the very small range between the refractive indicesof stilbite and those of epidesmine. When the crys-tals are oriented similarly, the optical dir
Proceedings of the United States National Museum . Fig. 25.—Stilbite ofthe usual habitshowing pyrami-dal faces. 74 PROCEEDINGS OF THE NATIONAL MUSEUM. VOL. small inclined extinction, up to 5°, on either side of the twinningplane. The crystals of stilbite described above have the habit of epides-mine while the analysis gives the composition of stilbite. The crys-tals are zoned somewhat, the zones differing in refractive index butseeming to cover the very small range between the refractive indicesof stilbite and those of epidesmine. When the crys-tals are oriented similarly, the optical directions coin-cide exactly with those of epidesmine. As regardsthe internal twinning structure, numerous crystalsfrom various specimens from the Goose Creek quarrywere carefully mounted in balsam, lying on the(010) face and carefully examined in comparisonwith each other and with fine little stilbites fromthe Faroes. Although otherwise similar with oneanother the Goose Creek stilbites varied in degreeof visibility of the twinning. In these crystals theextinction ranged from 3°, when the twinning couldbe discerned, down to 0° when, of necessity, thetwin
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Keywords: ., bookauthorun, bookcentury1800, bookdecade1870, booksubjectscience
