The Banda arcs are considered to represent the unstable northern "Pacific" margin of the main Australian continental mass. Intensely tectonized rocks show marked geosynclinal characteristics, with volcanic rocks, turbidites, and olistostromes indicating a long history of instability. The overthrust front is an intrasialic feature, comparable to the front of the Rocky Mountains. Posttectonic sequences are superimposed on the orogenic belts: intraarc deep-water carbonate rocks, marlstones, and tuffs reflect basinal downwarps with widespread volcanic activity and no terrigenous detritus, whereas the clastic "molasse" deposits filled the front-arc basins. By contrast, continental and restricted marine to open-marine clastic deposits and carbonate rocks characterize the stable western "Atlantic" margin of Australia. The eventual tectonic development of both the "Atlantic" and "Pacific" margins was present in an embryo stage at the time of the opening of the basins. Tectonic activity resulted from the northward drift of the Australian mass and consequent interaction between continental and oceanic crusts, with subduction of the latter. The volcanic arc is believed to be related to a recent dextral transcurrent fault. The original outer margins of Gondwana all comprised geosynclines, which eventually evolved into "Pacific" orogenic belts. Conversely, the intra-Gondwana margins, which resulted from intracontinental rifting, are of "Atlantic" type. The geosynclines are the preferential loci for the development of orogenic belts, as they evolved at those margins wh ch moved toward the oceans. The "plate-tectonic" theory is supported so far as rifting is concerned, whereas the mountain-building processes are considered to be in agreement with the "geosynclinal" theory. It is concluded that the continents are drifting away from each other without collision, as part of an expanding earth.