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APPENDIX II. 
an old crater cavity, and a smaller one in a reef on the east 
side, where also there is an elevated barrier reef. Within 
a mile or two to the south are a couple of submerged coral 
patches with level summits covered by from 5 to 10 fathoms 
of water, and in each case measuring within the latter 
contour line about half a mile in length. They both rise 
on all sides from water in which casts of 100 fathoms did 
not reach the bottom. 
East of Bougainville Island, m the strait between it and 
Choiseul Island, is a submarine plateau, about 15 miles in 
width, extending from the former, and covered by from 30 
to 50 fathoms of water. This, at its outer edge, termin- 
ates abruptly in a steep slope of from 15° to 25°, ‘ which is 
sharply delineated on the charts, by the 100 fathom line, 
and descends to considerable depths.’ There is a hole 80 
fathoms deep in its generally level surface, towards the 
middle of the strait, and another, not bottomed by 100 
fathoms of line off Cyprian Bridge Island. A narrow neck 
rather over 2 miles wide links this plateau to a smaller 
one prolonging Choiseul Island. ‘ Broken lines of barrier 
reef (sometimes elevated) and elongated coral shoals, 
covered by 4 to 10 fathoms of water, which may be re- 
garded as incipient barrier reefs, mark the edge of the Bou- 
gainville plateau, within a few hundred yards of the 100 
fathom line.’ The western extremity of Choiseul is skirted 
by a broken line of barrier reef, which encloses a lagoon- 
like channel, and supports islets on which coral rock indicates 
an elevation of at least a few feet ; and there is an island 
in the lagoon which bears similar testimony, while ‘ the 
hills on the coast, composed as they are of foraminiferous 
and pteropod mud encrusted by coral limestones, have been 
antecedently uplieaved.’ Oima atoll, about 2 miles long, 
has been built up above a group of islets composed of 
hornblende-andesite, each probably indicating four separate 
volcanic necks. This atoll rises from depths of 40 to 50 
