( 397 ) 



XIII. — Rocks from G-ough Island, South Atlantic (collected by the Scottish National 

 AntarcticExpedition, 1902-1904). By Robert Campbell, M.A., D.Sc, Lecturer 

 in Petrology in the University of Edinburgh. Communicated by Professor 

 James Geikie, D.C.L., LL.D., F.R.S. 



i(MS. received March 16, 1914. Read May 4, 1914. Issued separately October 3, 1914.) 



[Plate XXXVI.] 



Introduction. 



The specimens described in this paper were collected by Dr J. H. Harvey Pirie, 

 B.Sc, M.B., geologist to the Scottish National Antarctic Expedition, during a short visit 

 paid to Gough Island on the homeward voyage of the Scotia from the Antarctic seas 

 in 1904. The island is not often visited, and the only previous account of the rocks 

 is by Professor L. V. Pirsson,^ who described a series of beach pebbles collected by 

 the captain of a whaling vessel. Among those Professor Pirsson noted two varieties 

 of basalt, trachytic tuffs, and a tr achy tic obsidian carrying olivine, and shown by 

 chemical analysis to be of phonolitoid type. 



With the exception of a small fragment of limestone, the rocks in the Scotia 

 collection are all igneous. They were obtained in the neighbourhood of the usual 

 landing-place, the mouth of a small glen on the eastward side of the island. They 

 include soda trachytes, trachydolerites, basalts, an essexite, and tuffs. 



Lavas. 



The lavaform rocks fall readily into two well-marked groups — trachytes and 

 trachydolerites. The former are characterised by their abundant development of 

 anorthoclase felspar ; the latter are more basic types, in which olivine is always present, 

 and in which lime-soda felspars are accompanied by a varying amount of albite and 

 albite-oligoclase. 



A. Alkali Trachytes. 



The alkali trachytes comprise three distinct types : — (a) biotite trachyte, 

 (b) sodalite trachyte, and (c) segerine-augite trachyte. 



(a) Biotite trachyte [G. 5,t G. 12, G. 19]. — The rocks of this group were all 



* American Journal of Science, 1893, p. 380. 



+ The numbers refer to specimens in the museums of the Scottish Oceanographical Laboratory and the University 

 of Edinburgh. 



TRANS. ROY. SOC. EDIN., VOL. L. PART II. (NO. 13). 56 



