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



be taken as affording a very rougli idea of the composition 



of the pegmatites. 



Felspars. 



The plagioclase is mostly idiomorphic and thick tabular 

 in habit, all the larger crystals being zoned. Under the 

 microscope the interior is usually seen to be fairly clear 

 and glassy, and is proved by the R.I. being markedly higher 

 than balsam, and by the extinction angles in symmetrical 

 sections ranging up to 24°, to be an acid labradorite. These 

 labradorite centres must be nearly uniform in composition 

 throughout, for they show shadowy extinction only very 

 slightly. Perhaps at their edge they may be acid enough 

 to be called andesine. Then there is a sudden transition 

 to practically pure albite which has a mean R.I. very 

 slightly, though distinctly below that of balsam. These 

 outer zones of albite are turbid and brownish in colour from 

 kaolinisation, and twinning on the carlsbad and albite laws 

 is continued in them from the labradorite centres. Peri- 

 cline twinning occurs, though rarely. On measuring the 

 diameters across some of the short lath-shaped sections, 

 we found that the albite occupies about a quarter of the 

 diameter, the figures on three well formed crystals being 

 respectively 30, 25 and 27 per cent. Remembering that 

 the alkaline felspar forms the outer shell of the crystal, 

 calculation shows that at this ratio of diameter, each 

 completely formed crystal must consist of 60 per cent, of 

 alkali felspar, and 40 per cent, of the acid labradorite. 

 This proportion appears somewhat startling in comparison 

 with the results of the Rosiwal measurement, but the dis- 

 crepancy is easily accounted for by the fact that the 

 crystals never are perfectly developed, more than half the 

 space of the albite zone being occupied by interfering 

 crystals. 



The alteration of labradorite to analcite along cracks 

 and cleavage planes, producing veins and small patches of 



