6o 



DISCOVERY REPORTS 



iron ore and apatite. The symbol nak represents the percentage of alkah-feldspar in total feldspar, 

 and k the percentage of potash feldspar in total alkali-feldspar. Thus : 



salicCNa^O-KaO) 



nak- 



100, 



X I GO. 



salic(Na20.K20.CaO) 

 , _ salic K2O 

 salic (Na^OTK^O) 



The geographical arrangement of the analyses in Tables i a and i b shows at once that there is a 

 considerable difference between the Deception Island series at the southern end of the South Shetland 

 archipelago, and that of King George Island and Bridge- 

 man Island at its northern end. The Deception Island 

 series is characterized throughout (except no. 8) by 

 comparative richness in alkalis as against lime, as shown 

 by the high nak ratios. Moreover, in the alkalis, soda is 

 extraordinarily high in relation to potash, as is shown by 

 the low k ratios. The members of this series show regular 

 chemical variations throughout, again with the exception 

 of no. 8, which stands apart in several particulars. This 

 rock is described by Gourdon as ' doleritic basalt '. It is 

 stated to occur as ' blocks ' (.'' in agglomerate or tuff), and 

 is not found in situ} As its analysis agrees fairly closely 

 with those of the Recent basalts of King George Island 

 (no. 13) and Bridgeman Island (no. 14), it is possible that 

 the rock represents a fragment torn from a foundation 

 of Recent basalts through which the Deception Island 

 volcano, of quite different constitution, has burst. It will 

 be so regarded in the present investigation. 



The serial relations of the Deception Island series are 

 shown in the variation diagram (Fig. 8). The silica per- 

 centages, and the values for F' and M' , were tried as 

 abscissae against which the other constituents were plotted. 

 It was found that F' gave the smoothest curves. In all 

 cases analysis no. 7 (Gourdon 's ' labradorite ') was some- 

 what discrepant from the others. The curves show the 

 same general trends as for other andesitic series. The 

 distinguishing feature of the diagram, however, is the 

 height of the NagO curve and its distance from the K,0 

 curve. 



The Deception Island rocks may thus be regarded as an andesitic series of quite abnormal sodic 

 composition (Barth and Holmsen, (4), p. 13). On the other hand, the King George Island and 

 Bridgeman Island suite, together with the block of doleritic basalt (no. 8) from Deception Island, 

 constitutes a quite normal series of pyroxene-andesites ranging to olivine-basalt, with accompanying 

 plutonic types, and belongs to the great circum-Pacific petrographic region of which the characteristic 

 lava type is hypersthene-augite-andesite. 



1 E. Gourdon, ' Sur la constitution mineralogique des Shetland du Sud (lie Deception)'. C.R. Acad. ScL, Paris, CLViii, 

 1914, pp. 583-6. 



Fig. 8. 



