106 ME. C. E. TILLEY OX THE aEAXITE-GXEISSES [yoL IxxyII, 



AVith regard to the individual minerals, the monoclinic pyroxene 

 occurs usually in pale greenish grains, not sensibly pleochroic. 

 It is characterized by a high extinction-angle. In addition to the 

 prismatic (110) cleavages, a parting parallel to (100) is commonlY 

 developed (diallage type). Occasionally, a second lamellation 

 parallel to the base (001) is developed (salite type). Twinning 

 on (100) may be sometimes noted. As inclusions, there mav be 

 present a dendritic type of iron-ore. 



Hypersthene is a constant constituent of these rocks. It is 

 readily distinguished from the monoclinic pyroxene by the straight 

 extinction, low birefringence, and remarkable pleochroism in green 

 and pink tints. The pleochroic scheme is : 



X = pink. Y = light yellow, Z = green. 



It bears, therefore, a distinct resemblance to the highly-ferri- 

 ferous member of the enstatite-liA'persthene series, amblystegite. 

 The original amblystegite described by C. vom Eath, contained 

 2-5 '6 per cent, of ferrous oxide. J. P. Iddings - records a hyper- 

 sthene from Mount Shasta containing 22 per cent, of ferrous oxide 

 with but feeble pleochroism ; and Y. Groldschmidt ~ records the 

 same mineral in the contact-zone of the Christiania essexite with a 

 ferrous-oxide content of 22 "28 per cent. This mineral also shows 

 no strong pleochroism, and that characteristic is doubtless one of 

 the factors which induces him to note that hypersthene in other 

 contact-rocks has quite probably been mistaken for andalusite. 



It would appear, then, that the exceptionally strong pleochroism 

 of hypersthene does not become apparent until the percentage of 

 ferrous oxide reaches or exceeds 25 per cent., unless it is that an 

 abnormal quantity of titanic oxide can produce a like effect with 

 much lower iron contents, and the important influence of a 

 titanium content in the monoclinic pyroxene is suggestive in this- 

 connexion. 



The hypersthene, as noted above, is occasionally found in parallel 

 intergrowth with a monoclinic pyroxene. Compared Y'ith augite, 

 the hypersthene is particularly prone to alteration and decom- 

 position, and in many cases it is represented only by pseudomorphs. 

 The nature of these pseudomorphs calls for some remark. The 

 commonest type of alteration appears to be a serpentinous product, 

 optically negative and pleochroic in yellow and green tints with a, 

 double refraction approaching that of quartz. In addition, how- 

 ever, there appears in some examples a chloritic product. These 

 alterations are often accompanied by a separation of iron oxide in 

 the form of haematite or limonite, representing a further degrada- 

 tion of what were probably original magnetite granules : — 

 3MgFe (SiOa),^ 2H,0 + = H,Mg3Si,0, + 4SiO,4-re30,. 



Hornblende is a fairly-constant constituent of the pyroxene- 

 bearing types, and is often indeed the dominant constituent, repre- 



1 ' Eock-Minerals ■ 1911, p. 304. 



- • Die Kontaktmetamorpliose im Kristianiagebiet ' Yidensk. Selsk. Skrif- 

 ter. 1911. Xo. 1. pp. 321-22. 



