274 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



section. The nepheline and ldTicite phenocrysts are evidently somewhat 

 vicarious, for zones of the rock rich in one are poor in the other. 



Microscopically the rock is composed of the following minerals : 



Phenocrysts of nepheline and pseudo-leucite ; few crystals of augite 

 and occasional plates of biotite. 



Ground mass : aegirine-augite needles, orthoclase, nepheline as essen- 

 tials, with a little analcite, biotite, calcite, etc. 



The nepheline phenocrysts are glassy and fresh, bounded by base and 

 prism. The pseudo-leucites are composed of an aggregate of oi'thoclase, 

 partly in irregular grains, more often in radiating prisms, forming even 

 imperfect sphero-crystals, several of which may occupy one original 

 leucite. Between the orthoclase there is a comparatively small amount 

 of nepheline, usually in irregular patches, rarely in prisms, and this 

 nepheline lies between the orthoclase rays. The space in the centre of 

 the original leucite, especially that between the curving boundaries of 

 two opposite spheroids of orthoclase is filled with a clear isotropic sub- 

 stance with refractive index lower than the feldspar, traces of cubic 

 cleavage, gelatinizing with HC1, and determined as analcite. Into this 

 the square ends of the orthoclase rays sometimes project with a struc- 

 ture curiously analogous to that described by Iddings (" Obsidian 

 Cliff") in the obsidian spherulites, where rays of orthoclase project 

 into a quartz or tridymite paste. Associated with the analcite there 

 occurs an undetermined zeolite with equally low index, good double 

 cleavage and distinct though feeble birefringence. The very early period 

 of formation of the leucites is shown by their penetration by tongues of 

 the rock carrying nepheline and augite phenocrysts. 



The pyroxene phenocrysts are a deep green, slightly pleochroic, aegir- 

 ine-augite, with the direction of negative extinction about 38° to c'. 

 They sometimes have a centre of colorless or reddish augite and an 

 aegirine border. Small masses of purple fluorite are deposited within 

 or around them and they are frequently filled with secondary biotite. 

 Associated with them are a few melanite crystals, titanite masses, areas 

 of colorless fluorite and astrophyllite(l). 



The ground mass comprises a fine net work of slender pyroxene crystals 

 in a background of colorless minerals. These acicular pyroxenes are 

 pale to deep grass green in color, slightly pleochroic, parallel c' grass 

 green perpendicular to c' olive green; most of them extinguish nearly 

 parallel to c', others as high as 19° ; in either case this is the negative 

 optical direction and they would pass for aegirine or aegirine-augite. 

 The colorless background is resolved in polarized light into irregular 



