142 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Below and somewhat to the east of the central pocket (not shown in 

 the figure) and apparently continuous with the aegirite-microcline rock 

 surrounding the pocket, is a considerable mass of a fine-grained, light 

 greenish-white rock of quite unusual character. The microscope re- 

 solves it into : quartz, microcline, and albite, a little microperthite, 

 accessory zircon, ilmenite, calcite, fluorite, rarely parisite and probably 

 the rare-earth silicate, beckelite. The albite forms perthitic inter- 

 growths with the microcline only to a small extent, but occurs abun- 

 dantly in the form of slender, simply twinned crystals elongated 

 parallel to the edge 001 AOIO. In width they rarely exceed 0.06 mm., 

 while the length varies from 4 to 20 times the width. These lie 

 clustered around the microcline grains or between them and the quartz, 

 often parallel to the margin and again projecting at random into them. 

 They also occur quite thickly scattered through the microcline crystals, 

 less commonly in the quartz. Their relation to the aegirite crystals is 

 like that of plagioclase to augite in the diabases. Where the albite 

 crystals lie together in parallel or sub-parallel position, their side lines 

 are straight, but where they cross one another, or where they touch or 

 penetrate microcline, quartz, or aegirite, the boundaries are sinuous and 

 curiously irregular. The aegirite also includes, or is indented by the 

 microcline. 



The zircon is abundant for an accessory and lies mostly in the quartz. 

 It exhibits much the same features as the zircon in the fine-grained 

 portions of the main zone. The same is true of the calcite and ilmenite 

 and the same alteration products are associated with them. 



Associated with the zircon in the quartz is a small amount of a min- 

 eral whose properties are as follows : — color, pale yellow to almost 

 colorless ; habit, usually in rounded grains, but occasionally in sharp 

 octahedra ; cleavage, occasional, as prominent cracks, thought to be 

 cubical ; index of refraction, very high ; isotropic. Unless this be 

 some new species, its properties, as enumerated, point to its being the 

 rare mineral, beckelite, a silicate of calcium and the cerium earths, dis- 

 covered and described by J. Morozewiez from Balka Wali-Tarama, 

 where it occurred in a dike-like apophysis of mariaupolite. The 

 apophysis consisted of a sugar-grained groundmass of albite con- 

 taining phenocrysts of nephelite and magnetite. (See Rosenbusch, 

 Mikroscopische Physiographie, 1905 Ed., p. 395.) The occurrence of 

 the parisite, also a calcium, cerium-earth mineral, in the pegmatite 

 perhaps supports the identification of this mineral as beckelite. 



An estimate of the percentages of the main constituents by the 

 Rosival method, in this case only approximate, gave: quartz 26.1 per 

 cent, microcline 22.5 per cent, albite 35.0 per cent, aegirite 14.2 



