24 Transactions of the South African Philosophical Society. 



The Island of South Georgia, separated from South America by 

 1,200 miles of deep ocean, is made of slates and crystalline schists. 

 It is, therefore, said to be on the same continental ridge as the 

 Falkland Islands ; but as the soundings indicate that this sub- 

 marine ridge is continued out towards the Sandwich Islands, it 

 is highly probable that these also stand on a continental base.* 



I have already referred to the Eockall Island, and have classed it 

 as belonging to the continental shelf because it lies near the sub- 

 merged plateau, that is to say, it can be regarded as an island off the 

 submerged mainland. All the same, the channel separating it 

 from the plateau round the British Isles runs to depths of 1,300 and 

 1,500 fathoms. Eockall Island lies 240 miles distant from the Irish 

 coast, 290 miles away from the nearest point of Scotland, and 

 170 miles from St. Kilda ; it is situated in lat. 57° 36' N., long. 13° 

 42' W. It is about 250 feet in circumference at its base and 

 about 70 feet in height. At a radius of 2J miles from the rock the 

 depths are from 40 to 70 fathoms, but within this area two other 

 small rocks rise nearly to the surface of the sea. "Haslewood 

 Kock " is a small half-tide, detached rock, lh cables from Eockall, 

 and " Helen's Eeef " is If miles, and has about six feet of water 

 upon it at low tide. Minute as it is in size, Eockall is exposed 

 to the full swell of the Atlantic, which rises and falls here more 

 than twelve feet in the calmest w T eather. The Eev. W. S. Green, of 

 the Irish Fisheries Board, visited the spot in 1896, in SS. Granuaile, 

 and got within 20 yards of the rocks. He describes the general 

 character of the mass in the following words : — " The east face 

 seemed like a great slab of grey granitoid rock with rectangular joints 

 broken off at the north, so as to show the square edge of another 

 slab, and this was in turn broken off, showing the face of a 

 third. This granitoid mass rests on a rock showing a kind of 

 bedding or jointing, dipping about east and at an angle of 30° or so." 

 Three specimens of rocks from this place were submitted to Prof. 

 Judd for determination, one collected in 1810 by Captain Basil 

 Hall in H.M.S. Endymion, the others in 1862 by the officers of 

 H.M.S. Porcupine. In each case the specimens were obtained by 

 an active sailor, with a line attached to him, springing from a boat 

 on to the rock, and, when he had secured fragments of it, throwing 

 himself into the sea to be towed back to the boat. 



Two of the specimens were distinctly granitic in structure, the 

 quartz, felspar, and augite of which they are made being allotrio- 

 morphic, the other was more of the type of a dyke rock. Apatite 



* Thurach, " Geognistische Beschreibung der Insel Sud Georgien, Ergeb. d. 

 Deutsch," Polar Expedition, Allg. Theil, II. p. 7. 



