i7<3 PROF. W. J. DAKlN : EXPEDITION TO THE 



the Abrolhos Islands the sea is deeper at a similar distance from the coast. 

 In other words, the Abrolhos Islands rise from a bank connected with the 

 coast. We must start therefore from the stage when limestones formed 

 largely of corals, foraminifera, mollusca, and calcareous algEe were elevated 

 in this region and united to the mainland of Australia. In fact, the west 

 coast of Australia may be said to hare been brought much further west. 



This elevated limestone was weathered and eroded, and it is very likely 

 that 'the channels between the present groups of islands bear some relation to 

 the presence of small rivers, the Hutt River, the Bowes River, and the 

 Chapman River, opening on the adjacent coast. During this period the 

 Wallabies and Amphibia inhabited the extreme western region of the coast 

 now represented by the East and West Wallaby Islands. 



This period of elevation was followed by one of depression or subsidence, 

 and of this there is ample evidence on the present West Australian coast. 

 Jutson* states : " In the vicinity of Perth the coast-line was formerly farther 

 west, as indicated by the drowned valley of the Swan River." Further 

 examples occur in the south-west and also to the north. Thus the same 

 author states, " The North coast is the most broken in West Australia. Deep 

 sounds and bays run far into the land, and are manifestly the drowned 

 contiuuations of the adjacent rivers. This region has been deeply dissected, 

 probably to maturity, and then submerged. It thus affords excellent 

 illustrations of drowned valleys.''' 



This subsidence may be considered to have separated the Abrolhos 

 Islands from the mainland. It is also possible that a fault running approxi- 

 mately N. and S. occurs between the islands and the mainland, and has 

 helped to bring about separation, for there are several faults running parallel 

 with the coast in this region. In any case subsidence together with erosion 

 resulted in the separation of probably four land masses from the mainland, 

 the ancestors of the present four groups. 



We have now four islands of coral limestone, and we may assume that 

 either the altitude of the more northern ones was higher than that of the 

 others before subsidence, or else that the subsidence was not uniform and the 

 southern island was depressed most. This is to account for the much greater 

 altitude of the Wallaby Isles to-day. 



Around the original islands encircling reefs have developed, whilst at the 

 same time erosion has taken place and tended to remove them, lagoons taking 

 their place. Such erosion has produced least effect so far in the Wallaby and 

 North Island Groups, probably owing to their greater elevation to start 

 with. Evidence of this erosion is everywhere present. The East and West 

 Wallaby Islands have been separated, and a series of small islets to the 

 east of them are all remnants of the original central mass. An excellent 



* Jutson. "An Outline of the Physiography of Western Australia." Bull. 61. Geol. Surv. 

 West. Australia. Perth, 1914. 



