G. W. Tyrrell — Petrography of SoutJt Georgia. 487 



granular quartz, followed by a discontinuous zone of chlorite, with the 

 interior of the cavity filled by coarsely granular quartz. 



With these rocks may be described a fine, carnelian-red, jasper- 

 rock (C 26 — Larsen Harbour), which shows, in thin section, a 

 groundtnass of cryptocrystalline silica thickly impregnated by 

 haematite in peculiar globular, clubbed, or roughly radiate forms, 

 giving a texture which, after an obvious resemblance, may be called 

 amoeboid. The rock also carries large irregular masses of pyrites. 

 This rock may represent the siliceous material often found associated 

 with spilites, especially in the interstices between pillow-form masses. 



2. Sedimentary Rocks. 



The collection includes typical mudstones from King Haakon 

 Harbour (C 3, C 4), with only the beginnings of cleavage. These 

 rocks are penetrated by a great number of very thin, fine veins of 

 quartz. From Gold Harbour there comes a fine-grained, banded 

 greywacke (C 22), consisting of numerous thin alternations of 

 arenaceous and argillaceous material. The interest of this rock is 

 that it shows in a superlative degree the relative resistance of the 

 two types of material to differential movement. The arenaceous 

 bands show frilled and puckered folding; but in tlie adjacent 

 argillaceous bands each pucker is represented by a line of strain-slip. 

 As the argillaceous bands predominate, the rock, as a whole, splits 

 easily along the strain-slip cleavage. Some of the arenaceous bands 

 contain a few large crystals of quartz and albite, Avhich are deformed, 

 and in the case of the felspar sericitized, but which have nevertheless 

 formed nuclei of resistance, causing the folds to pass round them. 

 A similar rock has been described and figured in an earlier paper.' 



Other rocks from King Haakon Harbour (Cl, C 2) are sheared 

 crystal-tuffs with sporadic scapolitization, entirely similar to those 

 described in a former paper.^ They contain crystals of orthoclase 

 and albite, and fragments of shale, as augen in a sheared, almost 

 cryptocrystalline, groundmass, consisting apparently of sericitized 

 felspar and chalcedonic silica. The scapolite occurs in compact, 

 granular masses which appear to be pseudomorphs after rock- 

 fragments, but the character of the latter is completely obliterated. 

 The foliation is outlined by a wispy, greyish, argillaceous material 

 the character of which cannot be determined. A considerable 

 amount of pyrites has been introduced along the foliation planes. 



A specimen from Gold Harbour (C 23) appears to have been a tuff, 

 but has undergone much more advanced shearing than the rocks 

 from King Haakon Harbour. Only a few remnants of quartz and 

 felspar are left as augen, in a thoroughly foliated, granulitized paste 

 of quartz and felspar, with filmy sericitic mica. Similar rocks from 

 Gold Harbour were described in a former paper.' 



3. Conclusions. 

 The most striking new fact afforded by the study of this 

 collection of South Georgia rocks is the recognition of a spilitic 



' " Petrography of South Georgia " : Trans. Eoy. Soc. Edin., vol. 1, pt. iv, 

 p. 826, pi. xciv, fig. 1, 1915. " Ibid., pp. 827-30. 



* Geol. Mag., dec. VI, Vol. Ill, p. 436, 1916. 



