ROCK SPECIMENS 



"95 



to andesites. From Saunders Island he described an " olivine-free basalt ", consisting of 



plagioclase (with Anjg-es), an almost colourless pyroxene of the enstatite-augite series, 



and magnetite. An analysis of this rock, which is reproduced in Table III, i, was given. 



Hypersthene-andesites are common lavas in the Andes and in the South Shetland 



Islands. rr> u i ttt 



Table III 



Analyses of Andesites, etc. 



1. "Olivine-free basalt", Saunders Island, South Sandwich Islands. Anal. N. Sahlbom. Quoted from 



O. Backstrom, Bull. Geol. Inst. Upsala, xill, p. 173 (1915). 



2. Hypersthene-andesite lava, Thule Island, South Sandwich Islands. Anal. F. Herdsman, A.R.S.M. 



A. Hypersthene-andesite lava, Admiralty Bay, King George Island, South Shetland Islands. Anal. Lassieur. 



Quoted from H. S. Washington, "Chemical Analyses of Igneous Rocks", Prof. Paper 99, U.S. Geol. 

 Si4rv. p. 419 (1917). 



B. Hypersthene-andesite lava, Alt Bumey, Patagonian Andes. Anal. G. Nyblom. Quoted from P. D. Quensel, 



Bull. Geol. Inst. Upsala, xi, p. 107 (191 1). 



C. Average hypersthene-andesite, computed from 114 analyses of rocks called hypersthene-andesite and 



pyroxene-andesite (containing hypersthene) from Recent and Kainozoic volcanoes and lava fields of the 

 circum-Pacific region (including the East and West Indies), and certain European fields (Sardinia, 

 Hungary, Aegean Sea). 



A chemical analysis of the hypersthene-andesite of Thule Island by F. Herdsman is 

 displayed in Table III, 2. It is there compared with the analysis of the "olivine-free 

 basalt" of Saunders Island (i), the hypersthene-andesite of Admiralty Bay, King 

 George Island, South Shetland Islands (A), and the hypersthene-andesite of Mt 

 Burney (B), one of the southernmost volcanoes of the Andes. As a standard of com- 

 parison a new average analysis of hypersthene-andesite has been computed from 114 

 analyses found in Washington's compilation and more recent literature (C). This 

 average hypersthene-andesite differs remarkably little from the average which was 

 computed some years ago on the basis of 71 analyses, including, however, some pre- 

 Kainozoic examples.^ 



1 G. W. Tyrrell, " Some Tertiary Dykes of the Clyde Area," Geol. Mag. (6), iv, p. 310 (1917). 



