878 
Proceedings of the Royal Society 
Corals only appear to thrive at the margin of this plateau, where 
they have produced the lines of barrier-reefs and submerged shoals 
already alluded to. Narrow shore-reefs fringe the coasts of the larger 
islands, but in the interior waters of the plateau corals do not 
flourish, and reefs are almost absent altogether from the long 
stretches of dark sandy beaches that form the adjacent coasts of 
Bougainville. 
On the opposite side of this strait a broken line of barrier-reef 
skirts the western extremity of Choiseul, enclosing a lagoon-channel 
known as Choiseul Bay, 13 to 18 fathoms in depth, and from J to 
f of a mile in width, where a sheltered anchorage is afforded. On 
this line of barrier-reef five wooded islets have been formed. They 
are made, for the most part, 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 recently upheaved some 6 feet or more. An islet 
of coral limestone, which rises up in the midst of the lagoon- 
channel between 20 and 25 feet above the high tide level, affords 
testimony of previous upheaval; and the hills near the coast, 
composed as they are of foraminiferous and pteropod muds incrusted 
by coral limestone, have been antecedently upheaved. Here, then, 
as in the Shortland Islands, a barrier-reef has been formed in a 
region which has been undergoing elevation during a prolonged 
period I should have added that this barrier-reef of Choiseul 
Bay meets the coast at the head of the bay where it joins the shore- 
reef : to the southward it is continued as a submerged line of reef 
covered by 5 or 6 fathoms of water with a channel, 30 fathoms 
deep, inside it. 
Before concluding my remarks on this strait I will refer to Oima 
Atoll {vide plan), a singular atoll of small size which lies in the northern 
entrance about lj miles within the edge of the submerged plateau. 
It measures nearly 2 miles in its longest diameter and rises from 
depths of 40 to 50 fathoms with a submarine slope varying between 
12 and 26 degrees. This atoll has been formed around a group of 
rocky islets which are composed of hornblende — andesite and range 
in height from 20 to 100 feet above the sea. The variation of the 
texture of the rock in different islets, and other facts, show that 
there were four volcanic necks here represented. This atoll encloses 
