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PROCEEDINGS OF THE NATIONAL MUSEUM 



VOL. 63 



their mass and scattered over the surface. In the large lithophysae 

 or in the more lithoidal rock the cristobalite is replaced by crystalline 

 aggregates of platy triclymite, and here the rodlike feldspar gives 

 way to rather coarse crystal forms. Where the feldspar crystals are 

 large enough to be determined crystallographically, two types are 

 recognizable. One with the normal prismatic habit has the forms 

 (001), (110), (101), and (100) (Hg. la), and another with an un- 

 usual platy habit, is tabular parallel to (001) and has the form (100) 

 absent and (101) reduced to a narrow face or entirely missing (fig. 

 lb). The first type is normal in that the plane of the optic axis is 



perpendicular to (010) and has a 

 dispersion of p greater than v. The 

 second type, which forms beautiful 

 glassy crystals in the lithoidite, is 

 usually much larger than the first, 

 is less abundant, and in most cases 

 the plane of the optic axis is the 

 normal one for feldspar, but at times 

 the sanidine orientation is encoun- 

 tered. 2V is small, v is greater 

 than p, a=1.518, ;8rrrl..525, 7=1.527. 

 These indices are near those of 

 orthoclase but the analysis given by 

 Id dings shows the crystals to be a 

 sodium-bearing orthoclase with a 

 ratio of Na : K of almost 1:1. The 

 crystals show a small extinction 

 angle with the edgecb wdien lying 

 upon the base. They are eminently 

 suitable for optical work and evi- 

 dence of microperthitic intergrowth, 

 although carefully sought for, could 

 not be found, unless an orthopina- 

 coidal blue chatoyancy is evidence 

 for heterogeneity. 

 Tridipnite. — Triclymite is very common, not only in the cavities 

 and lithophysae but also in the groundmass of the lithoidite. Prac- 

 tically all of the quartz shown in the norm of the rh^'olite appears 

 in the form of tridymite in the mode of its crystalline phase. 

 Large crystals are never found, 0.5 millimeter being perhaps the 

 average size. In the larger lithophysae it is present as simple 

 hexagonal plates and can be readily distinguished from the feldspar, 

 which it often resembles, by its decidedly lower index of refraction. 

 Tridymite plates in parallel position are common, but twins are rare 

 if not entirely absent. This is rather remarkable wdien the rarity 



Fig. 1. — Two habits of feldspar 



KUOM THE LITHOPHYSAE OF OB- 

 SIDIAN Cliff 



