BY H. I. JENSEN, D.SC. 171 



The Break-up of Gondwana Land. — Faulting. 



The coasts of Western Australia, India, Arabia, and 

 East Africa are unmistakably faulted coast lines [Atlantic 

 type, see 12]. 



Recent faulting is still enlarging the Indian Ocean. 

 Madagascar is a horst, and the Mozambique channel is a 

 Senkungsfeldt, the fault which separated the former from 

 Africa having taken place in Pleistocene times. 



The faulting in progress at the present time in the 

 Rift Valley of Africa — a region of great modern earth- 

 quakes—is a later instalment of the same process of 

 disruption. 



Gondwana Land existed from the earlj^ Palseozoic 

 (Cambrian) to the Per mo- Carboniferous as a compact 

 landmass. In the Carboniferous, subsidence areas of the 

 nature of trough-faults had already commenced, and over 

 some of them passed transgressive arms of the sea : thus 

 not only in Africa are almost horizontal Carboniferous 

 marine sediments met with, but in West Australia a long 

 strip, occupying mainly the west coast of modern W.A., 

 was submerged in the Carboniferous, and during part of the 

 Permo-Carboniferous, and again at intervals in the Mesozoic. 

 The land connection between Australia, Africa and 

 Antarctica began to break down during these periods, though 

 island bridges probably remained until the end of the 

 Mesozoic, these accounting for the African affinities of 

 the West Australia flora. 



This disruption was, no doubt, due to secular con- 

 traction. The interior of the old Continent became 

 unstable, and was downthrown as soon as the cooling of 

 the deeper portions of the earth's crust commenced to make 

 them contract. Portions of the upper crust became 

 separated from the cooling, lower portions by the forma- 

 tion of shrinkage cavities (maculse), some of which were 

 not reached by igneous invasions from the zone of flowage. 

 Thus the upper shell become unsupported, and fell in. 

 The rim of the old Continent was largely strengthened 

 by igneous injections, and thus escaped the general 

 collapse. In this way the old complex of Western 

 Australia, and that of Western Tasmania consist very 

 largely of igneous materials. 



