BY H. B. GUPPY, M.B., SURGEON, R.N. 951 



fathom " line lying about three-fifths of a mile from the edge of 

 the reef and representing a general inclination of about 10°. 

 The gradual character of the submarine slope of this barrier-reef 

 is a feature on which I lay a particular stress : I have previously 

 referred, in the instance of the long line of barrier-reef on the 

 Bougainville side of the Straits, to the more rapid submarine 

 slope (15° to 20°) as indicated by the nearer approach to the 

 coast of the " hundred fathom " line. 



On the line of barrier-reef which incloses Choiseul Bay five 

 wooded islets have been formed. They are for the most part 

 formed of materials thrown up by the waves at the present sea 

 level ; but the presence in some of the larger islets of elevated 

 coral-rock in mass affords evidence of the whole line of reef 

 having been upheaved recently some six feet or more. An islet 

 of coral-limestone, which rises up in the midst of the lagoon- 

 channel to between 20 and 25 feet above the high-tide level, 

 affords testimony of a recent movement of upheaval to that extent. 

 To the northward this line of barrier-reef meets the coast at the 

 head of the bay where it joins the shore-reef ; to the southward, 

 it is continued as a sunken line of reef covered by five or six 

 fathoms of water with a channel thirty fathoms deep inside. 



The interior of the adjacent portion of Choiseul Island displays 

 long level ridges with intervening valleys running parallel to the 

 coast — a surface-contour resembling that of the interior of the 

 Shortland Island before described. An examination of the hills 

 near the coast has shown that the geological features are much the 

 same as those of the Shortland Island ; a soft calcareous deposit 

 containing pteropod-shells, foraminiferous tests, and other oi'ganic 

 remains, forms the bulk of these hills, being itself encrusted by 

 the coral-limestone. Here then, as in the Shortland Island, 

 barrier-reefs have been formed in a region which has been under- 

 going upheaval during a prolonged period : but in neither locality 

 was I able to find a clue to the problem of their formation until I 

 had taken a series of soundings off the outer edge of the Choiseul 

 Bay reef, a subject to which I will immediately refer. 



