950 FORMATION OF BARRIER REEFS IN BOUGAINVILLE STRAITS, 



remains, and overlaid by a crust of coral-limestone which probably 

 rarely exceeds a hundred feet in thickness : whilst the lines of 

 islands, which skirt its coasts, are in reality elevated lines of 

 barrier-reefs formed of the coral-rock. The most interesting 

 feature, however, of the geology of this small group of islands — a 

 subject to which I can only briefly refer in connection with the 

 subject of this paper — is the occurrence of volcanic formations in 

 the north-west corner of the main island of " Alu," the volcanic 

 portion passing into the calcareous region of the island without 

 any indication shewn in the profile and surface-contour of such a 

 change of formation. 



My examination of the Shortland Islands has led me to the 

 conclusion that they have been formed during a movement of 

 elevation by the advancement of successive lines of barrier-reefs 

 in a prevailing south-eastern direction from the north-west corner 

 of the main island of " Alu," where the volcanic formations 

 occur. The ancient lines of barrier-reefs are still preserved in the 

 interior of this Island by ridges of coral-limestone, which usually 

 have a constant trend at right angles to the prevailing trade-wind 

 The more recent lines of barrier-reefs, which have also experienced 

 elevation, are repi*esented by the broken lines of islands and 

 inlets, some of which rise over a hundred feet above the sea, that 

 skirt the weather coasts of "Alu." The accompanying diagram, 

 which represents a section drawn N.W. to S.E., may make the 

 foregoing remarks more clear. I have purposely drawn it on an 

 exaggerated scale, since on the true scale the more characteristic 

 features could not be delineated. 



On the opposite side of Bougainville Straits, a broken line of 

 barrier-reef skirts the western extremity of Choiseul Island 

 inclosing a lagoon-channel known as Choiseul Bay, which has a 

 breadth varying between half and three-quarters of a mile, and a 

 depth in the deeper parts between thirteen and eighteen fathoms, 

 where a sheltered anchorage is obtained. (Vide plan of this 

 locality.) The submarine slope on the outer side of the barrier- 

 reef has a more gradual descent than that which prevails among 

 other reefs of this character in the Solomon Group, the " hundred 



