488 H. S. JEVONS, H. I. JENSEN, T. G. TAYLOR AND C. A. SUSSMILCH. 



anorthoclase, or microperthite. The plagioclase is nearly 

 idiomorphic, is of thick tabular habit extended parallel to 

 010, is irregularly twinned on the carlsbad and albitelaws, 

 and is zonal in composition. The centre part for about 

 one third of a diameter consists of an acid labradorite, as 

 shown by a maximum symmetrical extinction of rather less 

 than 30% and by its R.I. being decidedly above that of 

 balsam. Outwards it alters at first slowly in composition, 

 the remainder of the crystals being mainly andesine; but 

 in crystals whose growth continued until the end of the 

 period of consolidation we find that near the outside the 

 felspar becomes rapidly more acid in composition, until the 

 periphery is an alkali felspar with R.I. distinctly less than 

 balsam. The narrower series of albite twin lamellae thin 

 out towards the periphery, leaving the alkaline outer band 

 untwinned, except for some patchy secondary twinning. 



The other felspar referred to above, occurs in slightly 

 smaller individuals, tending also to euhedrism but more 

 nearly isometric in habit, and thus giving mainly short 

 rectangular sections. In many crystals there is no evidence 

 of primary twinning, but some are twinned on the carlsbad 

 law. Wherever edges of undecomposed ■ portions of the 

 crystals are in contact with balsam the R.I. is found to be 

 distinctly lower than that of balsam; and, if we may judge 

 by the strength of Becke's bright line in comparison with 

 the lines to which w T e are accustomed from pure albite and 

 orthoclase, the strength of this line would appear to be 

 intermediate between them, indicating for this felspar a 

 mean R.I. of about 1*530, Between crossed nicols under 

 a low power the crystals present a patchy appearance. 

 Under a high power the brighter patches, which have 

 slightly higher D.R., are seen to be finely twinned parallel 

 to the longer axis of the crystal section, and therefore 

 presumably on the albite law; whilst the less bright sur- 

 rounding patches are perfectly clear and free from twinning. 



