MOtTTMAN ABROLHOS ISLANDS. 179 



illustration o£ this erosion is left on the reef-flat to the west of West 

 "Wallaby Island and is depicted (Plate 14. fig. 7). It consists of a limestone 

 mass which overhangs on all sides, and agrees entirely in structure with the 

 rock of the adjacent island a few hundred feet away. A few more years 

 and it also will have gone. 



Erosion has done much more in the Easter Group, wdiere the original 

 central mass is now represented by Hat Island. The lagoon is deep, 

 particularly on the eastern side, and there are several islets on the encircling 

 reef to the east, of which Wooded Island is an excellent example. These 

 islands consist for the greater part of heaped coral fragments, loose or 

 cemented into a conglomerate, and very frequently lagoonlets are present 

 on them. 



In the Pelsart Group the central mass is now represented by only small 

 islets, and they are few in number. The outer reef is much more regular 

 than in the other groups, and is practically unbroken on two sides. 



So far, we have emphasized the action of the sea in producing the 

 lagoons. We must add that in some places the reefs are progressing 

 seawards on the outward margins by coral growth, whilst the area of the 

 lagoons is increasing. 



Small reefs occur irregularly in all the lagoons. In some places they 

 represent portions of the original limestone mass; in others, as for example 

 near the Easter Passage and at the N.E. end of the Pelsart lagoon, they 

 have grown up from the floor of the lagoon. 



Thus in the formation of the lagoons at the Abrolhos Islands the greatest 

 factor has been that of erosion and solution. Ooral growth has modified 

 this action in places, and the heaping of coral blocks by waves and currents 

 must be also taken into consideration. Subsidence has played practically no 

 part in lagoon formation. 



As to the future, the East and West Wallaby Islands and their adjoining 

 fragments will be probably still further reduced. Rat Island in a similar 

 manner will be further eroded, and the Easter Group become still more like 

 a typical atoll. The depth of the Pelsart lagoon at the extreme southern 

 end will probably decrease owing to deposition of material washed over the 

 encircling reef. 



APPENDIX. 



In the concluding section immediately preceding f his, a short summary of 

 the observations on the coral formations of the Abrolhos Islands has been 

 given. To this we may add a short summary of other daia. 



1. The Percy Sladen Trust Expeditions to (he Abrolhos have been arranged 

 for the purpose of obtaining information regarding the formation of the 

 islands, the conditions permitting of coral-reef formations in this southern 



