88 DISCOVERY REPORTS 



of the spilitic suite. These geosynclinal greywackes are rich in fragments of intermediate, basic and 

 ultrabasic igneous rocks and their minerals, especially spilites and their associates.^ 



Spilitic lavas are of submarine or at least subaqueous origin. The greywackes formed of their debris 

 may be regarded as due to disintegration by submarine eruptions aided to some extent by subaqueous 

 gliding (Bailey),^ which distribute an enormous amount of ' greenstone ' debris, mingled with sand 

 and mud, far and wide over the oceanic regions affected. In its descent through the water this material 

 would become sorted with regard to grain size and would form graded sediments ranging from 

 greywacke to mudstone. This view would explain the frequent passage of greywackes to siltstones 

 and mudstones on the one hand, and into tuffs on the other. Furthermore, limy material lying on the 

 sea floor, and also the radiolarian cherts and impure limestones which are often associated with spilitic 

 lavas, would be incorporated in these sediments. Moreover, spilitic lavas and their tuffs are very 

 frequently saturated with carbonate of lime, which would reappear as calcite in the greywackes 

 resulting from their disintegration. 



Towards the deeper parts of the oceans these sediments would merge gradually into the blue 

 carbonaceous and ferruginous muds appropriate to this locus ; and towards the coasts they would pass 

 into the terrigenous sands and muds of the continental shelves. 



The greenstone-greywacke-mudstone association is generally formed during the geosynclinal stage 

 of the orogenic cycle, and is therefore commonly affected by the low-grade metamorphism which 

 ensues when the later orogenic movements take place. Slates, phyllites, and quartz-sericite-schists 

 are thus formed from the mudstones and siltstones; fine-grained quartzites and qviartz-schists from 

 cherts and other siliceous rocks; schistose grits, quartz-chlorite-albite-schists, and greenstones such 

 as those found in the ' Green Beds ' of the Scottish Highlands, from the greywackes and greywacke- 

 tuffs; epidiorites, greenstones, chlorite-schists, hornblende-schists, amphibolites, etc., with epidote, 

 zoisite, garnet, and other accessory minerals, from the basic igneous rocks and their tuffs. Glauco- 

 phane-bearing schists may be formed from the soda-rich varieties of these rocks, or from greywackes 

 composed of their debris. 



It is precisely an assemblage of this character which is encountered in the Elephant and Clarence 

 Group and the South Orkneys. South Georgia, too, is composed of greywackes and greywacke-tuffs 

 with slates and phyllites, and an occurrence of spilitic rocks is found at the eastern end of the 

 island. Such an assemblage may also form the basement of Graham Land and the adjacent archipelagos. 

 Above all, it is represented in Tierra del Fuego by the rocks of the Yahgan or Mt. Buckland formations, and 

 by some of the Central Schists of that region. Since radiolarian cherts are abundantly developed here, 

 it is probable that the whole assemblage belongs to the geosynclinal greenstone-greywacke-mudstone 

 association discussed above. It is difficult to read Kranck's descriptions of the petrography of these 

 rocks {op. cit. supra) and not to recognize that in West Antarctica we are dealing with exactly similar 

 groups of sedimentary and metamorphic rocks. The bearing of these considerations in favour of the 

 theory of the tectonic connexion between South America and West Antarctica put forward by 

 H. Arctowski, O. Nordenskjold, and E. Suess, is obvious.^ 



1 There are, of course, types of greywacke due to the waste of areas of miscellaneous rocks, including slates, basic igneous 

 rocks, etc. These may be styled continental greywackes, and are strictly equivalent to arkoses, which are derived from the 

 waste of a granitic or gneissic terrain. 



^ G. W. Tyrrell, 'Greenstones and Greywackes', C.R. Reunion Internat. pour I'etude du Precambrien et des vieilles chaines, 

 Finland, 193 1, pp. 24-6. E. B. Bailey, 'New Light on Sedimentation and Tectonics', Geol. Mag. lxvii, 1930, pp. 77-92. 

 The writer does not accept Bailey's view that greywackes are merely 'muddy sandstones'. 



^ For recent discussions of this problem see G. W. Tyrrell, 'Petrography and Geology of South Georgia', 'Quest' Ex- 

 pedition Report {Brit. Mus. Nat. Hist.), 1930, pp. 51-4; and H. F. P. Herdman, 'Report on Soundings taken during the 

 Discovery Investigations, 1926-32', Discovery Reports, vi, 1932, pp. 214-19. 



