ig6 



DISCOVERY REPORTS 



With the exception of the average analysis the analyses in Table III are arranged in 

 order of increasing silica. With that increase there are concomitant decreases in ferrous 

 iron, magnesia, and lime, increases in potash and soda, but with practically constant 

 alumina, showing that the chemical variations within this group of andesites arise 

 chiefly from variations in the relative proportions of the felsic and mafic groups of 

 minerals. The lavas of the South Sandwich Islands are clearly among the more basic 

 types of andesite, but are distinguishable from basalts by their richness in feldspars, the 

 frequent presence of orthorhombic pyroxenes, and their comparatively high degree of 

 supersaturation with silica (see the norms. Table IV, i and 2). The Saunders Island 

 rock, however, is very near the borderline of the andesites and basalts. The South 

 Shetland and Patagonian lavas are much closer to the average hypersthene-andesite, as 

 is well shown by the norms (Table IV). A notable chemical feature of the hypersthene- 

 andesites of the region under consideration is their poverty in potash as compared with 

 the average hypersthene-andesite. 



Table IV 



Norms of Analyses in Table III. 



The norms are arranged in the same order as the analyses in Table III. The symbols of the rocks in 

 the American Quantitative Classification are as follows : 



The mineralogical variations corresponding to the above-described chemical 

 variations are shown from left to right in the table of norms (Table IV) by the increasing 

 free silica, increasing orthoclase, increasing ratio of albite to anorthite, with a concomi- 

 tant decrease in the amount of pyroxenes. The norms also illustrate the curious fact, 

 which was also pointed out in a former paper (Tyrrell, op. cit. supra, p. 311), that 

 andesites which show no visible quartz may nevertheless contain up to 15 per cent, of 

 free silica. They must therefore be derived from magmas which, under plutonic 

 conditions, and with a little differentiation in the direction of further separation of 

 mafic minerals, would give rise to such rocks as tonalite and quartz-diorite, namely, the 



