SHAG ROCKS 9i 



A third quartz-vein rock consists of sheared quartz with films and foHa of almost colourless, 

 isotropic chlorite. 



There is no evidence of the nature of the rocks penetrated by these veins. While there appears to 

 be secondary quartz in the greenstones, there are no sharply defined veins. However, from their 

 mineral composition and associations, it is likely that the quartz veins cut rocks of metamorphic type. 



CHEMICAL COMPOSITION OF THE GREENSTONE 



A composite sample from three of the least altered greenstones was analysed with the result shown 

 in Table 7, col. i. This analysis has a characteristically basaltic pattern with its comparatively high 

 lime and alumina which, in the rock itself, is accounted for by the abundance of epidote and tremolite, 

 and in comparable basahs, by richness in lime-plagioclase. The analysis is, for example, much like 

 that of the Porphyritic Central Basalt type of Mull (Table 7, col. A), and like the basalt of the South 

 Shetland Islands (Table 7, col. B). The latter, however, has a much higher k ratio than the Shag 



Rocks greenstone. 



Table 7 



I. Tremolite-epidote-greenstone, stones dredged near the Shag Rocks, 130 miles west of South Georgia. Anal. 

 F. Herdsman. rr, ■ , n 



A. Porphyritic basalt (Porphyritic Central Type), Mull. Anal. E. G. Radley. Quoted from 'The Tertiary and Post- 

 Tertiaiy Geology of Mull', Mem. Geol. Surv., Scotland, 1924, p. 24. 



B. Olivine-basalt (Recent), Penguin Island, King George Island, South Shetlands. Anal. F. Herdsman. See this 



Memoir, p. 59. • at ■ 



C. Chlorite-actinolite-clinozoisite-albite-schist, Clarence Island. Anal. E. Kluver. See this Memoir, p. 87. 



The West Antarctic rock to which the Shag Rocks greenstone shows most resemblance is the schist 

 from Clarence Island (Table 7, col. C). There is obviously a close mineralogical similarity, and the 

 chemical analyses have the same pattern, although SiO^ is lower and (Fe, Mg)0 higher, in the 

 Clarence Island rock. The latter, however, is of more advanced metamorphic grade than the greenstone 

 of the Shag Rocks. From Tierra del Fuego, Kranck {op. cit. supra, pp. 43, 47, 54, no) has described 

 several ophiolitic greenstones, greenstone-schists, prasinites, etc., containing chlorite, epidote, 

 actinolite, sphene, leucoxene, and albite, but the only analysis given of these rocks (cited in Table 6, 

 col. A) does not accord very closely with that of the Shag Rocks greenstone. 



The chemical affinities of this rock clearly accord with those of a common type of basalt, and it may 



