876 
Proceedings of the Royal Society 
elevation of an intervening coral reef. The northern part, in which 
no traces of volcanic activity came under my observation, is entirely 
surrounded by fringing-reefs, with the exception of the north point, 
where the land terminates in a bold precipice. Off the east coast 
there is a very steep submarine slope to the 100 fathom line of 
from 30 to 35 degrees. In the midst of the reef on the west side 
there is a large hole, 18 fathoms in depth and about 150 yards 
across, which may mark an old crater cavity. In the southern half 
of the island, where the subterranean fires are not yet extinct and 
where many fumaroles exist, no reefs occur on the west and south 
coasts. On the east side, however, there is an elevated barrier-reef, 
which attains a height of rather under 200 feet, and is known as 
Simbo Islet. Off the west side of the low connecting neck, which 
is formed by an elevated coral reef, there is a short barrier-reef that 
by its protection forms the harbour. Just within its inner margin, 
this reef encloses a remarkable hole, 14 fathoms deep, which 
resembles the one on the north-west coast of the island. 
Two shoals or submerged coral patches lie 1 and 2 miles to the 
southward of the island, rising on all sides from deep water where 
casts of 100 fathoms did not reach the bottom. They have level 
summits covered by from 5 to 10 fathoms of water; and as 
delineated by the 10 fathom line, they measure, each of them, about 
half a mile in length. In their characters they are counterparts of 
Lark Shoal, which is described on p. 865, and, as I hold, they have 
probably reached the upward limit of their growth. 
Bougainville Strait . — This strait, which previous to the survey 
by H.M.S. “Lark” was little known, separates the two large 
islands of Bougainville and Choiseul, and is about 25 miles across. 
On examining the chart of the strait,* it will be observed that a 
broad submarine plateau, about 15 miles in width and covered by 
from 30 to 50 fathoms of water, forms the submerged extension of 
the east extremity of Bougainville Island. At its outer edge this 
plateau terminates abruptly in a steep slope of from 15 to 25 
degrees, which is sharply delineated on the chart by the 100 fathom 
line, and descends to considerable depths. Its surface is uniformly 
level, but a deep hole occurs towards the middle of the strait 
* I am greatly indebted to the Hydrographer, Captain W. J. L. Wharton, 
R. N. , for an early copy of the chart of Bougainville Strait. 
