64 DISCOVERY REPORTS 



North America, Hungary, New Zealand, etc.). The hypersthene-andesite of Admirahy Bay (Table i b, 

 col. 9) closely accords, except for lower potash, with an average hypersthene-andesite computed by 

 the author from 114 analyses derived from the circum-Pacific region, including the East and West 

 Indies, and certain European fields (Sardinia, Hungary, Aegean Sea).i The quartz-gabbro (Table 16, 

 col. 10) agrees well with an average of 11 analyses of rocks so called taken from Washington's Tables 

 {op. cit. siipra).^ The tholeiitic basalts of the series (Table i^, cols. 11, 12, 14) are accordant with the 

 average Non-porphyritic Central Magma-type of MuU,^ and with as yet unpublished average analyses 

 of tholeiitic types from the Tertiary igneous region of Scotland. They also accord with the sparsely 

 developed basalts which are found in the great andesitic regions. 



The above-mentioned rocks are all over-saturated with silica (positive O) ; and in this respect the 

 under-saturated olivine-basalt (0=— 5-2) of the newly discovered Penguin Island volcano (King 

 George Island) stands quite apart from the rest. With M', 80-2, it is also the most basic lava type 

 from the South Shetland Islands so far analysed. Its closest analogue appears to be the olivine-basalt 

 or 'plateau-magma type' of the Tertiary igneous series in Scotland,^ although it is richer in alumina 

 and lime and poorer in the ferromagnesian oxides than that type, and is thus richer in plagioclase 

 feldspar and poorer in olivine. It is precisely in these chemical and mineral characters that the 

 comparatively rare basalts occurring in andesitic regions differ from the olivine-basalts which are the 

 most abundant and characteristic types of oceanic regions and of many mildly and richly alkaline 

 regions on the continents. Thus the olivine-basalt of Penguin Island preserves its relationship with 

 the associated andesites, notwithstanding its superficial similarity to the olivine-basalts of quite 

 different petrographical regions. 



CONCLUSIONS ON THE GEOLOGY OF THE SOUTH 

 SHETLAND ISLANDS 

 A synopsis of the geology of the Danco Land Coast (Graham Land), the Palmer Archipelago, and 

 the South Shetland Islands was given in my memoir of 192 1 ((i), p. 75). The following are relevant 

 excerpts from that summary : 



The oldest rocks in the region (excluding a possible basement of crystalline schists and gneisses) appear to be 

 a series of folded bluish slates and mudstones, with subordinate fine-grained sandstones and greywackes, and 

 abundant intercalations of coarse breccias made up principally of igneous fragments.. . .The igneous breccias. . .may 

 possibly be as much due to the rapid denudation of an earlier range of porphyry mountains under arid conditions, 

 as to explosive igneous action.. . . 



Because of the abundance and size of the plutonic masses the sedimentary series is only visible in small fragmentary 

 exposures on the Danco Land coast. It appears, however, to occur in great force on the islands of the Palmer 

 Archipelago, in which the igneous breccias are also especially prominent. The sedimentary series constitutes a large 

 part of the South Shetland Islands, especially King George Island. Blue mudstones are intercalated with the older 

 andesites around Admiralty Bay, and are intersected and metamorphosed by the intrusion of Noel Hill, in Marian 

 Cove. . . . 



The presumably Mesozoic mudstones are interbedded with an early series of andesite lavas in King George 

 Island, and possibly also in the other islands of the South Shetland group. The plutonic masses of Noel Hill and 

 Le Poing intersect and cause hornfelsing in both sediments and lavas.. . . 



The next event in the geological history of the region seems to have been the extrusion of a great series of later 

 andesites, which, in King George Island, are regarded by Mr Ferguson as being banked up against the older series 

 and interbedded mudstones to the north-west. An eruptive focus of this period is probably to be seen in Three 

 Brothers Hill, Potter's Cove, a columnar plug of typical fresh bandaite lava. . . . 



1 G. W. Tyrrell, 'The South Sandwich Islands. Report on Rock Specimens', Discovery Reports, iii, 1931, p. 195. 



2 G. W. Tyrrell, The Principles of Petrology, 1926, p. 120. 



3 'Tertiary and Post-Tertiary Geology of Mull', Mem. Geol. Surv., Scotland, 1924, p. 17. 

 * G. W. Tyrrell, 'The Geology of Arran ', Mem. Geol. Surv., Scotland, 1928, p. 121. 



