G. W. Tyrrell — Petrography of South Georgia. 485 



material probably representing original pyroxene. There are also 

 rare microphenocrj'sts of oligoclase, and occasional small patches of 

 quartz, epidote, and chlorite. The quartz is undoubtedly secondary, 

 and introduced at the same time as the quartz veins by which tlie 

 rocks are penetrated. These rocks may be regarded as basic types of 

 spilite intermediate between that rock and mugearite. 



(2) Soda-felsite. — Two non-spherulitic felsites (C 14, C 15), which 

 differ in no essentials from those described in a pi'evious paper as 

 quartz-felsite,' occur in the collection. The phenocrysts of quartz, 

 albite, and orthoclase are perhaps less abundant, the groundmass 

 finer-grained and more abundantly epidotized than in the formerly 

 described specimens. These rocks greatly resemble the soda-felsite 

 or soda-granite-porphyry of Porthallow Cove, Cornwall, which is 

 also associated with a spilitic series.* 



With the soda-felsites may be described a quartz- trachyte (C 20) 

 from the same locality (Larsen Harbour). In thin section this rock 

 shows numerous irregular areas of turbid, mottled, untwinned 

 alkali-felspar, together with a few, elongated, simply twinned laths 

 of sanidine, and irregular areas of quartz, in a groundmass consisting 

 of minute, fluidally arranged laths of albite-oligoclase, a little 

 orthoclase, and abundant interstitial quartz. The sanidine laths and 

 large quartz areas are frequently invested by an irregular, 

 discontinuous zone of cloudy alkali-felspar, which envelops the 

 fluidal laths of the groundmass. While many of the constituent 

 minerals are quite fresh and undecomposed, the rock is impregnated 

 Avith irregular areas of epidote and particles of pyrites. This rock is 

 clearly related to the soda-felsites, but differs in being less quartzose 

 and in possessing a trachytic groundmass. 



(3) Greenstone {Alhite-doleritef). — In hand-specimens these are 

 fine-grained "greenstones" (C5, C 6, C 7), all from Larsen Harbour, 

 penetrated by quartzose veins, and impregnated with pyrites and 

 quartz. One rock (C 6) shows numerous amygdales filled with 

 greenish-black chlorite. In thin section the principal minerals 

 shown are albite-oligoclase in thin laths, enclosed ophitically in 

 masses of chlorite which doubtless represent original pyroxene. 

 The felspar laths are often cloudy and mottled on account of minute 

 inclusions of quartz, epidote, and chlorite. Ilmenite in process of 

 alteration to leucoxene is an abundant constituent, and there is a 

 considerable quantity of interstitial secondary quartz and epidote. 

 A prominent feature in one of the rocks (C 6) is the occurrence of 

 numerous, large, rounded vesicles filled with rosettes of yellow-green 

 chlorite which shows in great perfection the characteristic ultra- 

 blue polarization tints. C 5 is of finer grain than C 6, and is devoid of 

 amygdales. C 7 has abundant secondary quartz and pyrites. These 

 rocks are much decomposed greenstones, but enough is left of their 

 original minerals and texture to establish the fact that they belong 

 to the spilitic series, and were probably oligoclase or albite- 

 dolerites similar to the types tliat accompany spilites in other regions. 



(4) Epidosite and other Vein Rocks. — In the previous paper 



^ Geol. Mag., Dec. VI, Vol. Ill, pp. 438-9, 1916. 



^ Geology of the Lizard and Meneage (Mem. Geol. Surv.), 1912, p. 186 j 

 see pi. xii, fig. 6. 



