ACAD^OFSCENCES] AMERICA. 67 



marls. Iddings's ('13) remarks on the presence of gabbro and granodiorite in Patagonia, and 

 Chrustchoff ('8S) obtained a pebble of peridotite from the shores of Magellan Straits. Pos- 

 sibly these were all connected with the Andean granodiorites forming the western portion of 

 the ranges. If so, some analogy might be drawn between the distribution of alkaline and sub- 

 alkine rocks in Patagonia and Montana (see Harker '09, p. 95). Quensel, however, while 

 drawing attention to the analogy with North America pointed out by Suess (IV, p. 485), inclines 

 to follow Daly's view in explaining the alkalinity of the eastern rocks by the absorption of the 

 associated calcareous marls. 



An interesting application of petrology to the study of tectonic conditions is suggested 

 by recent investigations in South Georgia. (Tyrrell, '14, '15, '16, '18, and Ferguson '15.) 

 Here there are a series of highly altered shales, sandstones, and gritty trachytic tuffs containing 

 radiolaria which, from the suggestion of obscure fossils, are possibly of Ordovician or Silurian 

 age. Less altered than these are a series of similar sediments with a greater abundance of 

 tuffs which appear to be of Mesozoic age. A very definite suite of spilitic rocks occur also 

 with albite-dolerites and soda-felsites. Thus the petrographical evidence seems to the 

 writer to oppose the suggestion that South Georgia was portion of an ancient continental land 

 (Gregory '15, Tyrrell '18), but favors instead the view that it is the folded remnant of what 

 was the offshore region of subsidence and sedimentation about such a continental mass. 



Very similar rocks occur in the South Orkneys, greywackes, slates, etc., which are of 

 Silurian age, and spilites (Tyrrell '15); and the islands of the west coast of Graham Land 

 again contain slate radiolarian jasper and tuffs. On the mainland are Jurassic sediments 

 invaded by the typical Andean series of plutonic rocks ranging from granite to gabbro, while 

 to the east are nearly horizontal Cretaceous and late sediments, comparable with those of 

 Patagonia. Thus there is a tectonic, and as Nordenskjold ('13) urges, a strong petrographic 

 affinity between Graham Land and the Andean chain. This has been supported recently by 

 Ferguson ('21) and Tyrrell ('21). e 



6 Attention may be directed to a recent study by Baeckstrom ('16) of the basalts of south Patagonia and the islands along the trend-line we 

 havetraced. These show a general association of alkaline and subalkaline types, except those in the South Sandwich Islands, which are entirely 

 subalkaline. 



