820 THE JOURNAL OF GEOLOGY. 



hydroxide has separated. Its inclusions are not abundant. A 

 few tiny grains of plagioclase, a rounded grain of quartz and an 

 occasional mass of leucoxene or flake of pyroxene, besides small 

 irregular grains of magnetite are the only ones noticed. 



Here and there, between the olivines, is a plate of pink 

 augite, which is slightly pleochroic and which in most of its 

 characteristics approaches the diallage of the normal gabbro, 

 except that it never contains the gabbroitic inclusions. Its out- 

 lines are irregular and its contours are moulded by those of the 

 olivine, so that it is undoubtedly younger than this component. 

 Locally the diallage has acquired a fibrous structure due to alter- 

 ation into a very pale yellow or colorless mineral supposed to 

 be tremolite (or actinolite). 



Most of the latter substance forms groups of fibres, whose 

 relation to the augite can be inferred only from the fact that they 

 occur between the olivine grains, with the contacts between the 

 two minerals quite sharp. The olivine has undergone little, if 

 any, decomposition, so that the fibres must be due to the altera- 

 tion of the pyroxene, if they are secondary rather than primary 

 in origin. The little groups or bundles of these fibres are very 

 compact except at their ends, where they are frayed out into 

 single fibres whose extinction is sometimes parallel to their long 

 axes, and sometimes is slightly inclined to them. Their double 

 refraction is strong and their polarization colors are brilliant. 

 The axis of least elasticity is nearly parallel to the longitudinal 

 direction of the needles, which are therefore negative, if the 

 orientation of the optical axes is as it is in normal hornblende. 

 Two cleavages are discernible, one parallel to the long axes of the 

 needles and one (a parting) transverse to them. Cross sections 

 of the bundles were not seen, so that it is impossible to say 

 positively that the mineral is a hornblende, but its similarity to 

 certain fibres in other rocks, 1 that are certainly hornblende, is 

 so strong that there can be little question that these are horn- 

 blende as well. 



'Cf. description of actinolite in actinolite-magnetite schists, Amer. Jour. Sci., 

 XLVL, 1893, p. I76- 



