Antarctica has long been considered to consist simply of a large Precambrian shield, flanked on one side by the circum-Pacific belt of Mesozoic and Cenozoic orogeny. The great amount of geologic information obtained during recent years indicates that between shield and circum-Pacific belt are belts of Paleozoic orogeny.

The continent is probably crossed by a late Precambrian and Early Cambrian miogeosyncline whose contents were metamorphosed and intruded by batholiths during the Middle (?) Cambrian. Erratics bearing Lower Cambrian pleosponges, like those characterizing the Adelaide geosyncline of South Australia, have been found in four places; K/Ar dates suggest crystallization about 500 million years ago; and the distinctive granites are remarkably similar to those of Middle Cambrian age in South Australia. This orogen crosses the Antarctic coast between 145° and 160° E., and flanks the Ross Sea and Ross Ice Shelf, and possibly Filchner Shelf and Weddell Sea, as a continuous system of high mountains. These mountains have long been termed the "Great Antarctic horst," thought to be of old P ecambrian rocks; the horst concept was based on a misinterpretation of structures near McMurdo Sound, where the broad structure of the present range is anticlinal. At least in the 1,100-mile segment from Terra Nova Bay to the Horlick Mountains, the crystalline rocks are metasedimentary rocks whose structures are subparallel with the mountain system, and distinctive granitic rocks of a composite batholith. Rocks of the orogen are overlain by little-deformed Devonian to Triassic sedimentary strata.

Northeastern Victoria Land, west of the Ross Sea, is composed of metasedimentary rocks, striking generally west-northwestward, whose metamorphism and intrusion by granites occurred about 350-400 million years ago. On strike across the Ross Sea, and trending toward the Weddell Sea at least to the region of 90° W., 82° S., are metasedimentary rocks intruded by distinctive granites which contain relatively large trace amounts of tin. These rocks are undated, but are unlike the Mezozoic crystallines of Palmer Peninsula; a middle Paleozoic age is likely. It may be significant that Devonian granites of eastern Australia are also relatively rich in tin.

The coast between 35° and 165° E. is characterized by charnockites, granulites, and gneisses, in part polymetamorphic, and by varied younger crystalline rocks. The rocks resemble those of the Precambrian K/Ar ages. As has long been recognized, this is part of a Precambrian shield, which may extend farther toward the Weddell Sea, and which presumably extends far toward the South Pole also.

The Palmer Peninsula belongs to the circum-Pacific orogen, as has long been known, and the Thurston Peninsula also may belong to it.

This revised pattern of Antarctic tectonics is essentially that required by Du Toit's reassembly of the southern hemisphere continents before post-Paleozoic continental drift.

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