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PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. 6G. 



small inclined extinction, up to 5°, on either side of the twinning 

 plane. 



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 but 

 seeming to cover the very small range between the refractive indices 

 of stilbite and those of epidesmine. When the crys- 

 tals are oriented similarly, the optical directions coin- 

 cide exactly with those of epidesmine. As regards 

 the internal twinning structure, numerous crystals 

 from various specimens from the Goose Creek quarry 

 were carefully mounted in balsam, lying on the 

 (010) face and carefully examined in comparison 

 with each other and with fine little stilbites from 

 the Faroes. Although otherwise similar with one 

 another the Goose Creek stilbites varied in degree 

 of visibility of the twinning. In these crystals the 

 extinction ranged from 3°, when the twinning could 

 be discerned, down to 0° when, of necessity, the 

 twinning ceased to exist. There is some " aggre- 

 gate efifect" in the optical behavior, since some crys- 

 tals which when measured against the edges gave 

 essentially parallel extinction, yielded when crushed, 

 fragments showing extinction inclined up to 5°, meas- 

 ured from the (100) cleavage. 



I am inclined to the belief that '•epidesmine" has 

 no right to be considered a distinct species, the mate- 

 rial described under that name being merely a variety 

 of stilbite in which the angle of extinction has varied 

 through 5° to zero. The crystallography, cleavages, 

 and optical directions of the two coincide when the 

 elongation is made vertical and the most perfect cleav- 

 age is made b (010). Epidesmine, then, may be regarded as ortho- 

 rhombic stilbite which does not show an anomalous small inclined 

 extinction. The same conclusion has been reached from studying 

 similar crystals from Idaho. Stilbite is best regarded, crystallo- 

 graphically, as orthorhombic, the optical anomalies being disregarded 

 as mere anomalies. In some minerals the dissociation of the crys- 

 tallography from the optical structure could not be tolerated, but 

 in this case it seems permissible, especially when the cases of some 

 other zeolites are compared. 



In the great pile of blasted-down rock in the central portion of 

 the quarry in August, 1923, some of the larger blocks showed sur- 

 faces of cracks, sometimes totaling 4 square meters in area, coated 

 with drusy stilbite crystals, somewhat stained by ocherous limonite. 



Fig. 26.— Laumon- 

 tite showing com- 

 mon habit, the 

 unit peism "with 

 the negative 



DOME e (101). 



