SOUTH SHETLAND ISLANDS 



6i 



The Deception Island series. It is difficult to match the rocks of the Deception Island series with 

 those of other andesitic fields. Very occasionally one finds soda-rich andesites as, for example, in the 

 Andean petrographic region, and in that of western North America ; but the more normal andesitic 

 types are overwhelmingly predominant in these regions. As a suite the Deception Island rocks are 

 almost unique. The only other series which approaches them in richness in soda is that of the Santorin 

 volcano in the Aegean Sea, as has already been pointed out by Barth and Holmsen. But even among 

 the Santorin analyses only two are closely comparable to the ' santorinite ' of Deception Island. In 

 Table 2, col. B, the closest Santorin analogue of the Deception Island santorinite (Table 2, col. A) 



Table 2. Deception Island ' santorinite ' and comparable analyses 



A. Average santorinite, Deception Island (Table i a). 



B. Hyalodacite, east lava flow, August 1925, Fouque Kaimeni, Santorin, Aegean Sea. Quoted from H. S. Washington, 

 'Santorin Eruption of 1925', Bull. Geol. Soc. Atner. xxxvn, 1926, p. 378. 



C. ' Santorinite', average of eleven analyses of the Recent lavas of Santorin volcano, Aegean Sea. 



D. Biotite-andesite, Inca-loma, Cotopaxi, Ecuador. A Young, Hochgeb. Republik Ecuador, 11, 1904, p. 256. Quoted from 

 Washington's Tables [op. cit. supra), p. 154. 



E. Keratophyre, Trevennen, St Goran, Cornwall. Quoted from Cliem. Anal. Ign. Rocks, etc. Geol. Surv. Gt. Brit. 

 1931, p. 85. 



is tabulated. It agrees closely with the Deception Island analysis except for silica, which is 3 per cent 

 lower. The von Wolff parameters also show concordance except for 0. Even Santorin is not a very 

 close analogue for the Deception Island volcano, as is shown by the average of eleven accordant 

 analyses of the lavas of that volcano (Table 2, col. C). The Deception Island rock is distinctly richer 

 in soda and silica, and poorer in potash than that of Santorin. 



Among Andean andesites the biotite-andesite of Inca-loma, Cotopaxi (Table 2, col. D) provides 

 a close comparison with the santorinite of Deception Island. Further, some rocks of the keratophyre- 

 spilite association are chemically similar to those of the Deception Island series, as is shown by an 

 analysis of a Cornish keratophyre (Table 2, col. E) ; but the k ratio of this rock is notably smaller, and 

 the nak ratio higher, than those of the Deception Island rock (see also Table 4). 



