MICROSCOPIC CHARACTEK AND MINERAL CONSTITUENTS 181 



Garnet occurs onl}^ sporadical!}^ and is not idiomorphic, but appears 

 in reaction-rim fashion between the magnetite and feldspar, sending out 

 tongues into the latter mineral. 



Of the pja-oxenes, both hypersthene and augite are present, the latter 

 usually predominating. Parallel growths of the two frequently occur, 

 often of repeated fine lamellse, the contact faces being as usual. Other 

 growths appear also, and sometimes one mineral is found wholly in- 

 cluded in the other, each appearing in that condition. Their boundaries 

 are never idiomorphic, but always irregular and more or less rounded. 



The h3'persthene is quite typical, except that the prismatic cleavage 

 is more pronounced than the pinacoidal. The augite recalls diallage in 

 some respects, but the pinacoidal cleavage is absent or but poorl}^ 

 developed. The color is green, with a very slight pleochroism in the 

 thicker slides, fi having a yellowish tinge. The ordinary color is very 

 similar to the green of the hypersthene. Besides the prismatic cleavages 

 there is a well developed parting parallel to the base. The extinction 

 angle is between 45 and 50 degrees. 



In the rock immediately under discussion hornblende is found in 

 very slight quantity, though elsewhere present in considerable amount. 

 Like the pyroxene, it appears in irregular grains, though sometimes there 

 is an approximation to idiomorphic outlines. The absorption and ple- 

 ochroism are very strong, a being pale yellowish, tl deep brown, and 

 C dark green, with 6>c>a. The 6 color is very like the brown of the 

 biotite, which also occurs sparsel}^ in the more hornblendic rocks. 



The felds])ar is almost wholly microperthite. A few grains of plagio- 

 clase always appear and invariably extinguish nearly parallel to the 

 trace of the albite twinning lamella?. The greatest departure from 

 parallelism in any of the slides was 7 degrees. In no other case does it 

 reach 1 degrees. These fragments must therefore be referred to oligo- 

 clase. The chemical analj^sis shows that the plagioclase present must, 

 as a whole, be considerably more acid than normal oligoclase and nearer 

 to albite. 



The felds])ars are veiy fresh. They contain a small amount of cloudy, 

 dust-like inclusions, and also include the zircon, apatite, titanite, and 

 small augites, these latter being exce}>tional and idiomorphic. They also 

 include small, idionxprphic or else rounded quartzes, as determined by 

 Becke's method. 



Orthoclase is onl}^ present as a constituent of the microperthite. 

 There is no indication of zonal structure — that is, microperthite with 

 oligoclase cores — as in the similar rock described by Smyth from Diana.* 



*C. H. Smyth, Jr.: This Bull., vol. G, pp. 271-274. 



