484 0. W. Tyrrell — Petrography of South Georgia. 



In section, the most typical rock (C 10)^ shows a thin network of 

 slender striated felspars, with nearly straight extinction, interspersed 

 with a few microphenocrysts which have a well-marked multiple 

 twinning, and the extinction and refractive index of almost pure 

 albite. The felspars of the groundmass are appreciably less sodic, 

 and belong to albite-oligoclase. The interstices of the felspar 

 network are filled with chalcedonic silica, chlorite (mostly in 

 amygdales), epidote, a little quartz, very minute felspar microlites, 

 and a dark- green indeterminate material. The chlorite amygdales 

 are often lined with a thin layer of cryptocrystalline silica, and are 

 further banded with concentric layers of slightly different varieties 

 of chlorite, or chlorite mingled with grains and rods of epidote. 

 There is not a trace of the original ferro-magnesian or iron-ore 

 minerals. They have all been replaced by chlorite and epidote, 

 with, no doubt, cryptocrystalline silica as a bye-product of the 

 reaction. Thin veins of quartz, chlorite, and epidote occur in the 

 rock. This is a fairly typical spilite. 



Another specimen (C 11) shows the felspars still more thinly 

 dispersed in a dense groundmass, consisting mainly of a greyish- 

 green indeterminate material, with chlorite and a few felspar 

 microlites, still showing traces of its original texture. There are 

 numerous amygdales of uniform bright-green chlorite. The micro- 

 phenocrysts of albite tend to segregate into small groups; and 

 mingled with them are a few crystals of untwinned felspar, mottled 

 in polarized light, which appear to be soda-orthoclase. In one part 

 •of the slide rounded areas of granular epidote become common. 



A somewhat different type of spilite is represented by other 

 specimens. In the mass they are dark-grey, compact rocks, with 

 numerous spherical cavities filled with radiating needles, of green 

 epidote. In thin section they show numerous small laths of albite- 

 oligoclase, with a few stouter microphenocrysts, in a dense 

 groundmass consisting of minute felspar microlites, abundant grains 

 of ragged skeletal iron-ores, and a pale yellowish-green fibrous 

 chlorite which no doubt represents original pyroxene. Large 

 ^uhedral crystals of magnetite are scattered sparingly over the 

 section. Large rounded patches or amygdales of epidote and chlorite 

 also occur, the epidote forming masses of radiating crystals, with 

 rosettes of chlorite filling the remaining interspaces. Sometimes, 

 however, chlorite fills the amygdale to the almost complete exclusion 

 of epidote, and shows a remarkable violet polarization tint. Epidote 

 and chlorite occur only sparingly in the groundmass of these rocks. 

 They are to be regarded as falling midway between the typical 

 spilites and the more mafic varieties described below. 



Two of the rocks (C 18, C 19) are richer in mafic constituents than 

 those described above. The hand-specimens are compact, dark-grey 

 rocks, becoming grey-green upon weathering, and showing numerous 

 veins and impregnations of white quartz, pyrites, and magnetite. 

 The thin sections exhibit a dense, closely-woven mesh of minute, 

 diverse, unorientated felspar laths (albite-oligoclase), with abundant 

 small skeletal grains of iron -ore, chlorite, and an indeterminate grey 



^ The numbers within brackets refer to those of the specimens preserved in 

 the Hunterian Museum, University of Glasgow. 



