HOUTMAN ABROLHOS ISLANDS. 



177 



encircling reefs with talus formation would have resulted in greater depths 

 on the western side of the encircling reef. 



In this connection it is worthy of note that Darwin in an appendix to his 

 famous work, 'Coral Beefs,' p. 230, refers to the Abrolhos, about which it is 

 true he had very little information, in the following terms : — 



"Houtman's Abrolhos (lat. 28° S. on west coast) have lately been surveyed 



by Captain Wickham (as described in Naut. Mag. 18-41, p. 440) 



From the extreme irregularity of these reefs with their lagoons, and from 

 their position on a bank, the usual depth of which is only 30 fathoms, 

 I have not ventured to class them with atolls, and hence have left them 

 uncoloured."* 



The reefs of the Pelsart Atoll cannot be said to be arranged irregularly, 

 and the fact that the islands rise from a bank is no reason against terming 

 any of the groups an atoll, unless we argue that every atoll must be formed 

 according to Darwin's views. 



5. 1 consider that the Wallaby and Easter Groups represent stages in the 

 formation of such a type as the so-called Pelsart Group. Easter Croup, for 

 example, can be compared with an atoll having still a large island in the 

 centre of the lagoon. The Wallaby Group is least like an atoll and consists 

 of a group of islets, the remains of a large limestone mass, surrounded by 

 irregularly arranged reefs. North Island is still practically only an elevated 

 limestone topped with sand dunes and rising from a reef-flat which extends 

 seawards. Lagoon formation is only commencing on its eastern side, and 

 encircling reefs cannot be said to exist. It is possible that North Island will 

 develop along the lines indicated by the other groups. 



There are several interesting points of resemblance between the Wallaby, 

 Easter, and Pelsart Groups. In the first place, the large openings into the 

 lagoons are always found to the north. This is the only way into the Pelsart 

 Atoll. Both the other groups can be entered from the south, but careful 

 navigation of the channels is required. We find a long rim islet without a 

 break, in the Pelsart Group, and we have noticed that there is evidence of 

 confluence of the rim islets in the Easter Group to bring about a similar 

 condition. More primitive conditions prevail in the Wallaby Group and the 

 rim islets are not even regularly placed. It will be noticed that rim islets 

 are developing on the eastern, the leeward, side in each group. 



6. Judging from the presence of certain elements in the fauna of the 

 Wallaby Islands — the Wallabies, and the Amphibia — it is extremely probable 

 that land connection existed between the mainland of Australia and this part 

 of the Abrolhos Group at least. 



The foundation of the Abrolhos Groups is probably Tertiary limestone, but 

 of this we have no very direct evidence. It is worthy of note, that north of 



* Uncoloured on the map given by Darwin. 



