of Edinburgh, Session 1885 - 86 . 
877 
(outside Masamasa Island), where a depth of 80 fathoms was found ; 
and another, in which 100 fathoms of line did not reach the bottom, 
lies off Cyprian Bridge Island. This plateau is connected with a 
similar hut very much smaller submerged extension of Choiseul 
Island by a narrow neck rather over 2 miles in width. The sub- 
marine contour of this strait is well shown by the 100 fathom line 
in the chart. It will he there seen that the two large islands of 
Bougainville and Choiseul are connected by a submerged ridge, and 
that an elevation of some 350 feet would join them together. 
Broken lines of barrier-reef (sometimes elevated) and elongated 
coral shoals covered by from 4 to 10 fathoms of water, which may 
he regarded as incipient harrier-reefs, mark the edge of the 
Bougainville plateau within a few hundred yards of the 100 fathom 
line. Thus, commencing to the west of the Shortland Islands, we 
find a broken line of harrier-reef. Following the edge of the 
plateau eastward, we come to the south side of the Shortland 
Islands, where elevated lines of barrier reefs, removed from 20 to 
100 feet and more above the sea, form the coasts.* Further east- 
ward, a broken line of harrier-reef follows the contour of the 
southern coasts of the island of Fauro where they approach the 
edge of the plateau, f Linear submerged patches of reef, covered 
by from 4 to 10 fathoms of water, prolong this line of barrier-reef 
to the eastward for a few miles, and subsequently follow the curve 
of the edge of the plateau as it sweeps northward to the mouth 
of the strait. Besides the islands already mentioned, numerous 
smaller islands and rocky islets of volcanic formation, which 
preserve in some instances their volcanic profile, rise out of the 
waters in the interior expanse of the plateau, and other larger 
islands of a similar character, such as Ovau, Oima, Masamasa, and 
Piedu, lie around the northern end of Fauro..' £ 
* In my paper on the recent Calcareous Formations, I have shown that Alu, 
the main island, has been formed by the upheaval of a succession of barrier- 
reefs that have grown outwards on a bottom of foraminiferous volcanic muds 
from a nucleus of volcanic rock. 
t I visited several portions of this line of reef. A detailed description is, 
however, unnecessary. 
J The island of Fauro is of volcanic formation. The rocks composing it are 
mostly andesites, together with altered dolerites, diorites, quartz -felsites, 
dacites, &c. Similar rocks are found in the other islands and islets of the 
straits. No traces of activity came under my notice in any of the volcanic 
islands that I visited. 
