486 G. W. Tyrrell — Petrography of South Georgia. 



progressive epidotization of certain obscure igneous rocks was shown 

 to occur, resulting, in its final stage, in the production of a rock 

 composed mainly of epidote and quartz.^ The same characteristic 

 reactions are exemplified in the series of rocks now under description, 

 with the addition that veins of epidotic material are now seen to be 

 common. These are especially abundant in the spilites. Their 

 average thickness is about 1 inch, but they may swell out to a width 

 of 3 inches. They consist of a hard, dense, pale-yellow epidosite, 

 containing irregular patches and segregations of greenish-black 

 chlorite, and associated with a compact flint-hard, grey-green 

 material. In thin section the epidosite appears extremely turbid, 

 and is only translucent on the thinnest edges of sections, where it is 

 seen to consist of an almost cryptocrystalline aggregate of epidote 

 and silica. The grey-green flinty material is even denser. It is 

 very feebly birefringent, and is irresolvable even under a ^in. 

 objective. It carries tiny patches of epidote and quartz. The 

 epidosite encloses irregularly-shaped areas which have a narrow 

 border of granular epidote, or of epidote and quartz, with the 

 remainder of the space filled with radiate masses of chlorite in which 

 particles of leucoxene are enclosed. Smaller areas are filled with 

 epidote and quartz, or with epidote alone, suggesting that the order 

 of deposition has been first epidote, then quartz, and finally chlorite. 

 The boundaries of the veins against the enclosing spilite are generally 

 marked by a slight segregation of iron-ores within the rock. 



Three specimens of massive epidosite occur in the collection 

 (C 9, C 13, C24), all from Larsen Harbour. They are hard, dense, 

 splintery rocks of yellowish-green colour, which, in thin section, 

 show an intimate granular admixture of epidote and quartz. As 

 noted in the previous paper, the epidote grows euhedrally into the 

 quartz wherever the latter mineral forms plates large enough for 

 the relation to be observed. Chlorite, and pyrites in euhedral 

 crystals, occur in varying amounts. 



Quartz veins also occur al5undantly in these rocks. In one 

 specimen (C 185) the rock (spilite) has been veined in all directions, 

 leaving sharply angular fragments of country rock entirely surrounded 

 by quartz. These fragments are highly silicified, as is also the rock 

 adjacent to the sides of the veins. The quartz is very finely 

 granular, except in some later veinules. Intermingled with the 

 qnartz are euhedral crystals of epidote and pyrites, with flakes of 

 chlorite, and irregular masses of magnetite (strongly attracted by the 

 bar magnet). In another specimen (C 25) the quartz is much 

 coarser in grain, and in addition to the above-mentioned minerals 

 also carries irregular patches of a translucent, reddish, optically 

 isotropic ore-mineral, which is probably chromite. In C 18« there 

 are curious, spherical, amygdale-like areas of very fine-grained quartz, 

 which enclose sectors in which the quartz shows a concentric and 

 radiate structure, as shown by the appearance of a black cross 

 between crossed nicols. Other similar areas carry large euhedral 

 crystals of pyrites. Still another shows a narrow border of minutely 



1 Geol. Mag., Dec. VI, Vol. Ill, pp. 439-40, 1916. 



