 A giant intrusion of magma happened in the Jurassic forming diabase, or dolerite which gives many of the Tasmanian mountains their characteristic appearance.Continental breakup happened in the Cretaceous and Cainozoic Periods, splitting off undersea plateaus, forming Bass Strait and ultimately breaking Tasmania away from Antarctica.In the Cainozoic, a couple of basins extended inland from Macquarie Harbour and the northern Midlands.