SOUTH SHETLAND ISLANDS 53 



A visit was also made to the bluff on the south-west side of Neptune's Bellows (Fig. 7). The lower 

 part consists of conspicuous red cliffs, but higher up there are outcrops of the yellowish agglomerate 

 characteristic of the other side of the channel. The main range of hills in this locality appeared to be 

 composed of ' cindery lava or scoria ' with reddish black tints. It appears to be the weathered surfaces 

 of this rock which impart the striking red colour to the lower cliffs. Three rock specimens were 

 collected from this locality, and a few from localities north of Whaler's Bay (Anchorage?). 



The petrography of Deception Island has been dealt with by the writer ((i), pp. 67, 71), who 

 described olivine-basalt and basaltic tuffs, ^ and hyalo-dacite (ungaite). Dr H. H. Thomas ((2), 

 pp. 81-5) described ophitic olivine-dolerite, various types of andesite and their tuffs (mostly glassy), 

 and soda-trachyte (oligoclase-trachyte). He also noted the presence of tridymite and iron-olivine 

 (fayalite) in some of the more acid types, and of anorthite in the hyalo-andesites. Barth and Holmsen 

 ((4), pp. 8-17) described andesine-basalt and a vesicular, glassy 'pillow-lava', both of which they 

 regarded as of bandaitic composition, a view which is borne out by their chemical analyses. Further- 

 more, they gave a full description of a rock which seems to be identical with my oligoclase-dacite 

 and Thomas's oligoclase-trachyte. Barth and Holmsen find the closest analogues of this rock in the 

 products of the Santorin volcano in the Aegean Sea, and as it contains 17 per cent of tridymite they 

 call it tridymite-santorinite. 



From the study of Mr Ferguson's original specimens on which I based my first account of the 

 rocks of Deception Island, of Dr Mackintosh's new material, and of the above literature, it seems clear 

 that four main types of rock have been erupted from the Deception Island volcano, namely, olivine- 

 basalts or dolerites (of which there are no analyses), lavas of bandaitic composition, hyalo-andesites 

 of more acid type, and finally, the trachytic type which has been variously called oligoclase-dacite, 

 oligoclase-trachyte, and tridymite-santorinite. Eight analyses of Deception Island rocks have been 

 published (p. 58) from which it seems clear that they form a perfectly gradual series varying from 

 basic to acid, all of which (except the olivine-basalts) are highly sodic and relatively poor in potash ; 

 and are mineralogically characterized by the presence of calcic feldspars, orthorhombic and monoclinic 

 pyroxenes, and, in the more acid types, by fayalite, tridymite, and sodic feldspars. 



The following account of the petrography of Deception Island is based on the study of the specimens 

 collected by Dr Mackintosh, and on the re-study of the material collected by Mr Ferguson ((i), 

 pp. 58 et seq.). 



Olivine-basalt. Only two rocks, both from the Ferguson collection, belong to this type. One is 

 described in the following terms ((i), p. 67): 'A beautifully fresh rock showing more or less rounded 

 olivine phenocrysts in a ground-mass of good fluidal texture, which consists of elongated microlites 

 of labradorite with subordinate granules of augite and magnetite.' The texture can be described more 

 exactly as fluxional intergranular. A few of the augite crystals are of slightly larger dimensions and 

 more euhedral than the granules of the ground-mass, and can be regarded as micro-phenocrysts. 

 The rock has a close resemblance to the Dalmeny type of the Scottish Carboniferous basalts. Its 

 occurrence is as a pebble in a tuff or agglomerate. 



The other olivine-basalt is flow-banded in the hand specimen, but its ground-mass is not so con- 

 spicuously fluxional as the above. The ground-mass is of coarse intergranular type and consists of 

 laths of andesine, with granules of pale augite and magnetite. Numerous phenocrysts and glomero- 

 porphyritic aggregates of fresh olivine and brown augite, together with smaller and much less numerous 

 feldspar crystals (labradorite) are embedded in the ground-mass. This rock has aflinities with the 

 Craiglockhart and Dunsapie types of the Scottish Carboniferous basalts. 



Basaltic andesites of bandaitic type. These rocks differ from the basalts described above in not 



1 These are now regarded as andesitic tuffs. 



