Washington — Plauenal Monzonose (Syenite). 131 



Plauenal Monzonose. 



Megascopic characters. — General color, light pinkish-brown ; 

 phanerocrystalline and apparently noncrystalline, medium 

 grain, 1 to 5 mm , granular equant fabric, inclining to trachytic ; 

 composed of dominant feldspar, of a brownish-pink color, mostly 

 in tables about 5 mm wide and l mm thick, without twinning 

 lamellae, which show a subparallel arrangement ; subordinate 

 hornblende in greenish black, equant anhedra, 1 to 3 mm in 

 diameter ; a few grains of colorless quartz and rare, small, 

 brownish titanites. 



Microscopic characters. — Holocrystalline, automorphic gran- 

 ular. Minerals present: alkali-feldspar, hornblende, quartz, 

 plagioclase, magnetite, titanite, apatite. 



Soda-orthoclase. — About 65 per cent ; in stout subhedral 

 plates, tabular parallel to h (010), Carlsbad twinning common ; 

 evidently a soda-orthoclase, with moire appearance and micro- 

 perthitic structure very common, but no microcline. 



Hornhlende. — About IT per cent ; in stout, subhedral pris- 

 moids or irregular anhedra, often twinned on a (100) ; color 

 olive-green, pleochroic, c and b olive-green, a greenish yellow, 

 c = ft > a ; some individuals have a narrow, irregular border 

 of bluish green hornblende ; magnetite a common inclusion, 

 titanite and apatite less so, and orthoclase rare. 



Quartz. — About 10 per cent ; xenomorphic, in irregular 

 anhedra, interstitial between the other constituents, especially 

 the feldspar and hornblendes ; occasional undulatory extinction. 



Oligoclase. — (Ab 4 An t ). About 3 per cent ; stout subhedral 

 tables, twinning lamellae according to the albite law very nar- 

 row and numerous. 



Magnetite. — About 2 per cent ; small, subhedral to anhedral 

 grains. 



Titanite. — About 2 per cent ; automorphic, giving the 

 usual lozenge sections ; pale brown, slightly pleochroic. 



Apatite. — About 1 per cent ; automorphic, stout prisms ; 

 clear. 



Chemical composition. — The chemical composition is shown 

 in the table, the earlier analyses by Zirkel and Griffith (quoted 

 from Brogger) being given in columns I and II, the complete 

 analysis of my specimen in III, that of Cross's specimen in 

 IY, followed by the alkali determinations of the other speci- 

 mens, and the final average with the molecular ratios in YII 

 and Till. 



The two older analyses are very closely alike, a fact to 

 which Brogger has called attention. They are, however, 

 both unsatisfactory according to modern standards, the sum- 

 mation of I being unwarrantedly high,* and both being incom- 

 * The sum would be still higher were the ferric oxide reckoned. 



