THE PETROLOGY OF SOUTH GEORGIA. 825 



and these rocks are coarser, affording transitions towards the arenaceous end of the 

 series. In other types the quartz, grains diminish in quantity, and the rocks pass 

 into siliceous shales or slates. There is frequently a banding due to the alternation 

 of more gritty and more shaly types. 



The coarser conglomeratic rocks of this series are probably in part those described 

 by Nordenskjold as " porphyroids," but he undoubtedly includes in this group some 

 types of sheared tuffs. Many of the rocks certainly contain large augen of quartz 

 and felspar in a dense, sheared, siliceous matrix ; but all transitions between these 

 and unquestionably clastic rocks can be traced. Moreover, some of the conglomeratic 

 types contain pebbles of fine schistose grit, shale, and chert, as well as crystals of 

 quartz and felspar. I do not regard these rocks as of igneous origin, but merely as 

 products of the coarser conglomeratic phase of the argillaceo-arenaceous deposition 

 in South Georgia, although they have probably been derived from the waste of an 

 igneous area. Some rocks from New Fortune Bay, however, are devoid of rock- 

 pebbles, and contain quartz and felspar augen in a dense, sheared, quartzose matrix. 

 These may possibly be porphyroids ; but further evidence, such as a passage into an 

 undoubted igneous type, is desirable before this identification can be accepted. 



The first effects of kataclastic deformation on these rocks is the alignment of 

 the minute constituents of the groundmass, especially the scales of sericite, in the 

 direction of shearing. Presently the larger quartz and felspar grains undergo the 

 same alignment, and show good undulose extinction. Faint wisps of carbonaceous 

 material wind round the grains and begin to outline the later development of an 

 augen texture. A further stage is the rupture of some of the quartz and felspar 

 grains, the latter along cleavage planes, into two or more pieces. The quartz breaks 

 into irregular pieces which become separated and intermingled with the cement ; the 

 felspar fragments, however, frequently remain in close proximity to one another, and 

 show good overthrust faulting in the direction of shearing. The final stage is marked 

 by intense shearing, resulting in the production of great fissility, and a fine augen 

 texture. The laminae of the rock, outlined by dark carbonaceous matter, wind round 

 numerous ovoid or eye-shaped remnants of quartz and felspar. New mineral forma- 

 tion is not at all conspicuous. Films, streaks, and folia of sericitic mica have been 

 produced at the expense of the felspathic material in the cement ; but the larger 

 felspar augen have suffered little or no mineral change save, perhaps, on their borders. 

 These rocks are to be regarded as phyllitic grits, and are probably the same as those 

 described by Thurach as " phyllit-gneiss." The unsheared types described above 

 are probably to be correlated with his " quartzit-schiefer." 



To judge from Mr Ferguson's specimens and his descriptions of the rocks, the 

 pure argillaceous types are subordinate in quantity to the more arenaceous. They 

 consist of black shales, mudstones, slates, and phyllites. The chief difference between 

 the shales and mudstones lies in the almost complete absence of lamination in the 

 latter ; the mudstones are also frequently hard and cherty. In thin section the shales 



