﻿Vol. 66.] DEDOLOMITIZATION IN THE MAEBLE OF PORT SHEPSTONE. 511 



amount of plagioclase. The perthifcic structure is rather peculiar : 

 even in ordinary light the intergrown felspars can be distinguished 

 by a slight difference in turbidity ; between crossed nicols the 

 intergrowth is marked by the different orientation of the alter- 

 nating streaks, a wavy appearance being a conspicuous feature. 



The ferromagnesian mineral is of a dark emerald-green, and 

 shows distinct pleochroism ; it has, however, the characteristic 

 rectangular cleavage of a pyroxene, and its maximum extinction- 

 angle is about 38°. There can be little doubt that it is an aegirine- 

 augite. 



From this composition it is evident that the rock belongs to the 

 alkali-granites, being either a soda-granite or a soda-aplite. 1 



Fig. 2. — Diagram of the granite inclusion and its reaction-zones 

 {tf.jig.3,p.512). 



A = Granite inclusion ; B = Dark-mica zone ; 

 = Light-mica zone ; D= Ophicalcite zone ; 

 E = Normal dolomite. 



(b) The dark-mica-olivine zone. — This zone varies in 

 width from 2 to 5| inches, and averages about 4 inches. It owes 

 its dark colour partly to the presence of a dark-brown mica, 

 arranged in peripheral layers in alternation with white carbonate ; 

 and partly to a dark band consisting mainly of olivine, but with 

 some spinel, both of these minerals having the coal-black appear- 

 ance already mentioned. A section cut from a portion of this 



1 A mechanical separation made, with the aid of the electromagnet, by 

 Mr. T. Crook, in the laboratory of the Imperial Institute, yielded 4*6 per cent, 

 of the green augite, 02 per cent, of magnetite, and 02 per cent, of sphene, the 

 remaining 95 per cent, being felspar and quartz. 



