ART. 16 ZEOLITES FEOM OEEGON HEWETT, SHANNON", GONYEE 7 



The specimen is contained in a piece of greenish, rather coarsely 

 crystalline basalt which appears fresh and unaltered, and contains 

 a few sparsely scattered vesicles of nearly spherical form. The 

 smaller of these are empty except the thin lining referred to diaban- 

 tite. The largest cavity, however, nearly 2 centimeters across, con- 

 tains a thicker layer of diabantite, and a filling of tabular crystals 

 of the white zeolite. The crystals are transparent, colorless, and 

 are as much as 1 millimeter thick by 8 millimeters broad. Though 

 the centers are glassy and transparent there is a narrow^ border, 

 visible where the crystals are broken, [across] which parallels the 

 basal faces and is whiter, less transparent and has a cross-fibrous 

 silky appearance. 



When the crystals are crushed and examined in poAvder under the 

 microscope, grains broken across the plates show the border to have 

 a cross-striated appearance and higher birefringence than the cores, 

 although the border is continuous with the central part and uniform 

 with it in extinction. Many grains show basal cleavage and a striped 

 appearance suggesting the polysynthetic twinning in a plagioclase. 

 Although the optical properties are variable and confusing, a ma- 

 jority of the grains seem to be optically positive ( + ) with 2V small, 

 marked dispersion r<v, and the indices of refraction a= 1.489, 

 ^=1.491, 7=1.494, all ±0.002. 



A tabular crystal selected from the specimen and embedded in an 

 oil of index 1.490 appeared homogeneous and dark between crossed 

 nicols. It was uniaxial and negative with the optic axis perpendicu- 

 lar to the basal face. The index ct= 1.493 ±0.002. These features 

 agree with the optical properties of levynite. 



Although the uniaxial character, if coinciding with the external 

 form would indicate a hexagonal or tetragonal mineral — and the 

 specimen looks like the tabular forms of the minerals of the chaba- 

 zite gi'oup — this symmetry could not be confirmed. The crystals 

 are not suited for crystallographic study. A single very unsatis- 

 factory crystal which gave a few approximate measurements on the 

 goniometer had faces of what appeared to be two pyramids with rho 

 (p) angles of 45° and 58° approximately and with phi (<^) angles 

 45° apart. These appeared hemimorphic and twinned on the 

 basal plane. Edingtonite is the only zeolite belonging to such a 

 symmetry class and the mineral is certainly not edingtonite. 



If collectors visit the locality and make extensive collections, suf- 

 ficient material of this character will probably be found to permit a 

 complete investigation. Until then its identity must remain in doubt. 



Calcite is widespread but is neither abundant nor of unusual 

 character or quality in the Oregon collection. It occurs as rhom- 



