ELEPHANT AND CLARENCE GROUP 83 



but the conjecture may be hazarded that it is derived from a rock of the spihtic suite, perhaps a tuff. 

 It has a considerable resemblance to the slightly metamorphosed spilitic lavas of North Glen Sannox 

 (Arran).i 



Of the glaucophane rocks, one is a quartz-albite-epidote-chlorite-glaucophane-greenstone devoid 

 of schistosity ; the other is schistose and carries abundant calcite in addition to the above-mentioned 

 minerals. In both rocks quartz, albite, and calcite, form a coarse, even-grained mosaic, within and 

 between the grains of which the coloured minerals are developed. In the greenstone the latter are 

 interspersed among the colourless minerals, and are non-schistose ; in the schist the coloured minerals 

 occur as streams winding through the colourless matrix, or they form folia alternating with broad 

 bands consisting of quartz, albite, and calcite. 



The chlorite is of the deep green penninite variety with low birefringence and anomalous ' ultra- 

 blue ' interference colours ; it is associated with colourless to pale green muscovite. The epidote is of 

 the normal yellowish green variety and is associated with much leucoxenic material. Glaucophane 

 is abundant in both rocks. It has a striking pleochroism as follows: 



X=pale yellowish green, 

 F= violet, 

 Z= azure blue. 

 In the schist it appears to be altering to a greenish blue soda-amphibole devoid of the violet pleochroism, 

 and with a rather high extinction angle (up to 20 ). This may be the 'abnormal glaucophane' rich in 

 a lime molecule, which is mentioned by Winchell.'^ 



These rocks are probably due to the recrystallization of igneous rocks of the spilitic series under 

 dynamothermal metamorphism. The abundance of quartz and calcite, with a little muscovite, may 

 indicate that the original rocks were tuffaceous and mingled with normal sedimentary material. Very 

 similar rocks are mentioned by Harker as forming the prosinite type of the Alps.'' Kranck* has 

 described a glaucophane-garnet-schist from Bahia Pliischow in Tierra del Fuego. Its mineral 

 composition is: garnet, glaucophane, quartz, sericite, biotite, chlorite, calcite, apatite, magnetite. 

 This rock is interbedded with garnetiferous quartz-schists and belongs to the Yahgan or Mt Buckland 

 formation. Kranck regards it as due to the metamorphism of a carbonate-rich sandstone [greywacke .?] . 



THE GIBBS ISLAND GROUP 



This is a group of three small islands, O'Brien Island, Aspland Island and Gibbs Island (with 

 Narrow Island joined to it), lying about 20 miles south-south-west of Cape Lookout on Elephant 

 Island. Practically nothing was known of the geology of these islands until 1937 when a landing was 

 made on Gibbs Island by a party from the ' Discovery II '. D. Ferguson, however {op. cit. supra, p. 35), 

 was caught in a terrific gale and had to shelter for some time under the lee of Gibbs Island. He says: 

 ' The steamer was sufficiently near to show that the rocks were mainly stratified sediments. The rocks 

 on the west [south?] side of Gibbs Island are dark grey and banded, and dip about 40° W. A higher 

 horizon is represented by some uniformly and well bedded greyish-white rocks which dip about iS"" W. 

 They extend for about { mile, and look soft and friable in places. Aspland Island, 5 or 6 miles west 

 of Gibbs Island, is evidently formed of the same regularly bedded rocks, but they dip east.' 



A landing on Gibbs Island and Narrow Island was made by J. W. S. Marr on 2 November 1937, 

 and the following facts concerning the geology of the island have been culled from his report (un- 

 published MS.). 



1 G. W. Tyrrell, 'The Geology of Arran', Mem. Geol. Siirv., .Scotland, 1928, p. 26. 



2 A. N. Winchell, Elements of Optical Mineralogy, Part II, 3rd ed., 1933, p. 259. 

 ^ A. Harker, Metamorphism, 1932, p. 291. 



* E. H. Kranck, op. cit. supra, pp. 52-4. 



