Antarctica has long been assumed to consist of a large Precambrian shield, flanked on the Pacific side by an orogenic belt of dominantly Mesozoic and Cenozoic age. Although shield and circum-Pacif:c elements are certainly present, it now appears certain also that a belt of Paleozoic geosynclinal sedimentation and orogeny lies between them. Between 10° and 145° East, tha coastal region of Antarctica exposes a complex assemblage of plutonic and metamorphic rocks, characterized by charnockites. Isotopic age determinations confirm the expected Precambrian age of this terrane. Mountains bounding East Antarctica against the Ross Sea and Ross Ice Shelf have long been called the “Great Antarctic horst,” although no horst exists and the broad structure of the range, as shown especially by the sedimentary rocks, is anticlinal in some places and consists of a series of tilted fault blocks in others. At least the part of these mountains west and south of the Ross Sea and Ross Ice Shelf probably follows the site of late Precambrian and early Cambrian miogeosynclinal sedimentation and later Cambrian(?) deformation, metamorphism, and plutonic intrusion, rather than being formed of old Precambrian crystalline rocks as has generally been assumed. Jsotopic age determinations, the distribution of erratics bearing Lower Cambrian pleosponges, and the petrologic continuity and distinctiveness of the granitic rocks in this mountain system all support this interpretation. The crystalline rocks in question are overlain by little-deformed Lower Devonian to Permian or Triassic sandstones and other sedimentary rocks, intruded by thick sills of diabase. The slates and low-grade metagraywackes west of the mouth of the Ross Sea, near Cape Adare, were probably metamorphosed regionally during Ordovician time, to judge by the available age determinations. The Palmer Peninsula is composed dominantly of Carboniferous( ?), Jurassic, and Cretaceous eugeosynclinal rocks, deformed and intruded by quartz diorites and other granitic rocks during Cretaceous or early Tertiary time. The rest of West Antarctica, between the Palmer Peninsula and the Ross and Weddell Seas, is of problematical affinities, as no direct data bear upon the ages of its metasedimentary rocks and granites. A number of features are suggestive that this region may be one dominated by Paleozoic sedimentation and orogeny, but further information is greatly needed. The new interpretation of Antarctic tectonics presented here is essentially that required by Du Toit’s reconstruction of southern hemisphere continents in a Paleozoic, pre-drift super-continent. The early Paleozoic granites of South Australia are strikingly similar to the probable Cambrian granites of Antarctica; and granites and rocks metamorphosed at about the same time form the basement of southernmost Africa.