824 MR G. W. TYRRELL ON 



rocks may be regarded as arenaceous shales or slaty grits. They are generally fine- 

 grained, dark in colour, resembling greywackes, and have suffered various grades of 

 metamorphism, chiefly kataclastic, with but little new mineral formation. Shales 

 have been converted into slates, with much crumpling of the lamination ; whilst the 

 coarser grits have been sheared, with the formation, in some cases, of an " augen " 

 texture. There are, however, certain mudstones and fine-grained argillaceous grits 

 which have not been subjected to any great deformation, and only show an imperfectly 

 developed cleavage. 



In the mass this assemblage of sedimentary rocks has a great resemblance to the 

 older Palaeozoic greywackes, grits, and shales of the Southern Uplands of Scotland, a 

 resemblance heightened by the occurrence of radiolaria in both series. Many of the 

 more highly metamorphosed types could be paralleled amongst the schistose grits and 

 slates of the Highland Border in Scotland. 



For descriptive purposes the sedimentary rocks of South Georgia may be divided 

 into arenaceous, argillaceous, and calcareous groups ; but, as stated above, there are 

 all gradations between the first two. Moreover, all the rocks have undergone some 

 pressure-deformation ; in some, however, the shearing is of much more advanced 

 character than in others, and a little new mineral formation has resulted. A sub- 

 division may therefore be effected on this basis. 



By far the commonest sedimentary type in South Georgia is a rock which is 

 intermediate between arenaceous and argillaceous, and may be described as a gritty 

 slate or a slaty grit. In its typical development it is a compact, fine-grained, dark 

 rock, with varying degrees of fissility. Whilst highly quartzose, it contains a con- 

 siderable quantity of felspars and other minerals, and might be described as a fine- 

 grained greywacke. Microscopically it consists of a minutely crystalline quartz paste, 

 with much filmy sericitic mica, which is occasionally pale green in colour. In this 

 are embedded numerous angular grains of quartz, with much less numerous felspars, 

 amongst which orthoclase, microcline, and oligoclase have been recognised. Other 

 minerals seen in small quantity are epidote, chlorite, magnetite, ilmenite with borders 

 of granular leucoxene, and sphene. Rock chips are rare ; but fragments of chert, 

 shale, and recrystallised schistose grit have been recognised. 



Some of the quartz grains have a broad corona in which the minute constituents 

 of the groundmass are arranged radially around their peripheries. These grains are 

 noticeably more rounded than others ; but in detail they have a crenulate margin 

 owing to an interlocking of the groundmass constituents with their borders. These 

 appearances are probably due to solution of the grain under pressure, and the 

 deposition of the dissolved material in its near vicinity. This process seems to go 

 on with a progressive recrystallisation of the quartz-sericite paste, producing a coarser 

 texture. With this type of metamorphism there has usually been no serious shearing. 



There is not much variation in the proportions of the various constituents. The 

 quartz grains occasionally become abundant relative to the argillaceous groundmass, 



