PAPERS READ. 



SUGGESTIONS AS TO THE MODE OF FORMATION OF 

 BARRIER REEFS IN BOUGAINVILLE STRAITS, 

 SOLOMON GROUP. 



By H. B. Guppy, M.B., Surgeon, R.N. 



[Plate LVIIL] 



A broken line of barrier-reef skirts the eastern extremity of 

 the large island of Bougainville at a distance of about fifteen 

 miles from the coast, and incloses a wide expanse of water, forty 

 to fifty fathoms deep, dotted by an archipelago of islands and 

 inlets, mostly of volcanic formation. This line of reef fringes the 

 edge of a sub-marine platform which may be described as the sab- 

 merged extension of the adjacent coast of Bougainville. On its 

 seaward side the slope of the reef descends rapidly beneath the sea 

 at an angle varying between 15° and 20 Q , the "hundred fathom " 

 line being removed to between one-quarter and one-third of a 

 mile from the outer edge. Reserving a general description of the 

 reefs of these Straits until the completion of the survey, I will at 

 present confine my remarks to a sub-group known as the Short- 

 land Islands, a collection of islands which have been upheaved 

 along the line of the barrier-reef at the south-west corner of the 

 submerged platform above alluded to. 



Viewed from seaward the Shortland Islands have a low-lying 

 level profile never probably attaining an elevation much in excess 

 of 400 feet above the sea. They consist of one main island named 

 Alu (alu), eight to ten miles in length, the coasts of which, more 

 especially those on the weather sides, are skirted by lines of 

 smaller islands and islets. Alu — the main island — is composed in 

 great part of a soft calcareous deposit containing numbers of the 

 shells of pteropods, foraminiferous tests, and other organic 



