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XXV.— The Petrology of South Georgia. By G. W. Tyrrell, A.R.C.Sc, F.G.S., 

 Lecturer in Mineralogy and Petrology, University of Glasgow. 



(MS. received March 2, 1914. Read July 6, 1914. Issued separately May 20, 1915.) 



[Plate XCIV.] 



Introduction. 



The first petrographic description of the rocks of South Georgia was given by 

 H. Thurach in 1890, as part of the results of the German Scientific Expedition to 

 that area of 1 882-83. # He described the rocks as clay-slates interstratified with 

 phyllite and phyllite-gneiss, with quartz-phyllite, calc-phyllite, and quartzite-slate. 

 He also mentions a great development of interbedded schalsteine or diabase tuffs, and 

 describes a rolled pebble of " korniger gneiss." His phyllite-gneiss is said to contain 

 tourmaline, apatite, and andalusite. He emphasises the great crushing and disturb- 

 ance the rocks have undergone. 



0. Nordenskjold later described the South Georgian rocks obtained by the Swedish 

 South Polar Expedition of 190 1-3. t He mentions the great development of phyllitic 

 slates, but would prefer to assign Thurach's " phyllite-gneiss " to the sheared 

 porphyries known as porphyroid. These rocks are said to contain andalusite. 

 Thurach' s schalsteine are described as tuffs, consisting of fragments of trachytoid 

 and pilotaxitic " porphyrites." 



On the south-east coast of the island Fr. Heim, a member of the German 

 Antarctic Expedition of 1911 led by Lieutenant Filchner, found a series of basic 

 eruptive rocks, including diabase and melaphyre, as well as blocks of plutonic rocks 

 of dioritic and granitic habit. He describes the whole north coast, except Royal Bay 

 and part of Cumberland Bay, as built of dark grey and bluish-grey schists and tuffs. J 



The present paper gives the results of an investigation of about 110 rock 

 specimens collected by Mr David Ferguson in 1911-12, mostly from the northern 

 coast of South Georgia. The majority of these rocks are of sedimentary origin, but 

 there is a large series of tuffs, and two specimens of igneous rocks. 



I. The Sedimentary Series. 

 The sedimentary rocks form a series varying from pure argillaceous types, such 

 as black shales and slates, to coarse, angular, quartzose grits, with the greater number 

 of specimens intermediate between the argillaceous and arenaceous extremes. These 



* " Geognostische Beschreibung der Insel Siid-Georgien," Intern. Polarforschung, 1882-S : Die deutsche Exped., 

 vol. ii, 1890, pp. 109-166. 



+ " Petr. Unters. a. d. Westantarctischen Geb.," Bull. Geol. Inst. Upsala, vol. vi, 1900, pp. 234-46. 

 + "Geologische Beobachtungen uber Sud-Georgien," Zeitsch. Ges. Erdkunde, Berlin, April 1912. 

 TRANS. ROY. SOC. EDIN., VOL. L, PART IV (NO. 25). 117 - 



