40 DISCOVERY REPORTS 



Dr Tyrrell's main conclusions are as follows : 



Two dredgings were made from 'Discovery IT in the neighbourhood of the Shag Rocks in 

 November 1930. Of the nineteen specimens obtained fifteen are described as tremolite-epidote- 

 greenstone or greenstone-schist. This is an important find, as it can be paralleled both with rocks 

 from Clarence Island and with specimens from Tierra del Fuego. 



Fresh material has been obtained in the South Sandwich Islands both in situ at Saunders Island 

 and from dredgings elsewhere in the group. These rocks are all volcanic in origin and of Recent age. 

 The new material, along with earlier collections, shows that the South Sandwich rocks have more in 

 common with rocks from the Antilles of North America than with any specimens so far known from 

 the Andes. Dr Tyrrell considers that the South Sandwich Islands probably lie on a ridge parallel 

 to, but east of, the main Scotia Arc. 



Elephant Island and Clarence Island and others east of the main South Shetland Islands not 

 only lie at some distance from the South Shetlands proper but also differ from them geologically. 

 A greenstone-greywacke-mudstone association is present, such as is formed in the geosynclinal stage 

 of a mountain-building cycle and is affected as would be likely by low-grade metamorphism. 

 Assemblages of this character are found not only in the Elephant and Clarence Group but also in the 

 South Orkneys. They are paralleled near Ushuaia in Tierra del Fuego, and a somewhat similar 

 assemblage occurs in South Georgia. Dr Tyrrell considers that these types may also be expected to 

 form the at present unknown rock basement of Graham Land. 



There are extensive collections from the South Shetlands which modify earlier conclusions. The 

 occurrence of sediments of presumed Mesozoic age on certain of the islands has apparently been 

 over-emphasized, and one should now regard the South Shetlands as of preponderatingly volcanic 

 origin, made up either of lavas, mainly andesites, dacites and rhyolites, or of their associated tuffs, 

 breccias and agglomerates. Plutonic rocks may, however, be commoner than so far supposed. There 

 were two lava periods, and the intrusive rocks, such as the diorite on King George Island, are regarded 

 as the underground equivalents of the later period. The Recent volcanoes along Bransfield Strait are 

 still younger than either of the above lava periods, and it is even probable that Deception Island and 

 Bridgeman Island have been active in historic times. The chemical characters of the Deception Island 

 lavas indicate a soda-rich andesite, not readily paralleled in the Andes. Elsewhere the andesites and 

 basalts are of normal circum-Pacific, that is to say undoubted Andean, type. 



Finally, a fifth section deals with some specimens from Graham Land. These are less numerous as 

 a collection, but they include a quartz-porphyry formation at Adelaide Island of the same nature as 

 the rocks of a belt 400 km. in length already known from Patagonia. 



No new rocks are to hand either from the South Orkneys or from South Georgia. Both localities 

 are now well known. The importance of the new material lies in the nature of the rocks themselves, 

 and Dr Tyrrell, in these five papers, has provided petrographic arguments for what was up till now not 

 more than a matter of inference. The petrographic evidence is more or less complete. To settle the 

 actual line of the Arc, however, requires that the bottom contours should be better known. Soundings 

 over a wide area are much to be desired, and will decide whether there is a single arc or a series 

 of concentric curves. Meantime one can safely say that Suess's, Andersson's and Nordenskjold's 

 arguments no longer relate merely to a possibility, and that Suess's vision of the Pacific structure 

 advancing into the Atlantic must now be regarded as firmly established. 



