Prof. J. W. Gregory — Geological Relations of S. Georgia. 61 



Igneous Rocks. — Apart from the crystal tuffs described above, 

 only two specimens of igneous rocks are found in the collection. 

 One is a whitish, veined, altered rock, intrusive in the black shales 

 of the Lower Division of the Cumberland Bay Series. This appears 

 to be a coarse diabase or ophitic gabbro. In thin section it shows 

 large plates of comparatively fresh colourless augite, penetrated by 

 laths of decomposed felspar. There is also highly altered interstitial 

 felspathic material with leucoxenic patches representing iron-ores. 

 The other specimen was obtained from the moraine at the head of 

 Moraine Fiord. This is a quai-tz-monzonite-porphyry with pheno- 

 crysts of oligoclase-andesine, orthoclase, and hornblende in a micro- 

 crystalline quartzo-felspathic ground-mass. 



3. The Geological Relations of South Georgia. 

 By J. W. Gregory, D.Sc, F.E.S. 



THE special interest of the island of South Georgia depends on the 

 evidence it promises as to the geological history of that part of 

 the Southern Ocean which lies south of the South Atlantic. According 

 to the well-known views of Professor Suess, South Georgia is on 

 a continuation of the mountain line of the Andes, which at the 

 southern end of South America bends eastward along the northern 

 margin of Drake's Sea and continues 30° to the east, where it turns 

 southward; it completes a great horseshoe-shaped course by passing 

 through South Georgia and returning westward through the South 

 Orkneys to Grahamlaud. 



The geology of South Georgia has remained but little known. The 

 first important contribution was made by Thiirach, a member of the 

 German South Polar astronomical expedition of 1882-3. According 

 to Thiirach * the island consists of a series of metamorphic rocks 

 ranging from graniilar gneiss to clay-slate, and of beds of diabase 

 tuff. The granular gneiss was only found as a shore pebble, but 

 the inference that it came from the island was natural. The main 

 rock series collected by Thiirach includes a ' phyllite-gneiss ' con- 

 taining tourmaline, apatite, iron-ores, and andalusite ; the prevalent 

 rocks are phyllites and clay-slates, with which are interbedded some 

 granular limestones, calc-phyllites, and quartzites. According to 

 Thiirach, there is a complete passage from tlie phyllitic gneiss to 

 the slates,^ and his illustrations^ show that the rocks are very 

 crushed and crumpled. The abundant diabase and tuifs recorded by 

 Thiirach suggested that South Georgia had been an active volcanic 

 centre, and Gunnar Andersson's identification of some of the igneous 

 rocks as porphyrite (i.e. altered andesite) was in favour of South 

 Georgia having been part of the Andean line. Thiirach obtained 

 no evidence as to the age of the rocks, and the first fossil from 

 the island was collected by the Swedish expedition. Dr. Otto 



^ H. Thiiracli, " Geognostische Beschreibung der Insel Siid-Georgien " : 

 Internat. Polarforschung, 1882-3. Die deut. Exped., vol. ii, No. 7, pp. 109-66, 

 1890. 



- Ibid., p. 131. ^ Ibid., pp. 154, 157, 158. 



