THE ORIGIN OF CORAL REEFS. 61 
as much as 10 degrees or 12 degrees, whilst the submarine 
ledge, when stripped of reefs and defined by the 100-fathom 
line, would possess a scarcely recognisable inclination, repre- 
sented by a fraction of a degree. It will be found, however, 
when we examine the contour of such an island as Vanikoro, 
that the distance of the barrier-reef from the coast may vary 
according to the slope of the land. Thus, on the west side of 
this island, the average angle of the land slope is 6 degrees, 
and the distance of the barrier-reef about 24 miles. On the 
north side the inclination of the land is between 11 degrees 
and 12 degrees, and the barrier-reef is rather over a mile 
distant. This is just what we should expect. ‘The more 
gradual the land-slope, the broader will be the submarine 
ledge, cut out in the course of ages by the action of the sea, 
and the more distant will be the barrier-reef that has grown 
up along its margin. This I believe to be the true explana- 
tion of the position of barrier-reefs. A submarine ledge is 
in the first place necessary ; and, since the sediment and mud 
in the shallower waters on the ledge repress the growth of 
corals, reefs will naturally spring up towards the margin of 
the ledge, where the water is clearer and where the depth is 
within that of the reef-coral zone. 
_.» Voleanic Peak. 
Soft Foraminiferous 
eRe ROE = Deposits. 
Coral Rock, 
IDEAL SECTION OF AN ISLAND IN THE SOLOMON GROUP. 
These ledges are sometimes of great antiquity. From the 
surface of the plateau inside the barrier-reet of Bougainville 
Island there rise up extinct volcanic cones, forming separate 
islands, a few hundred feet in height. Here lies also the 
large island of Fauro, composed of old volcanic formations. 
We notice the same in the instance of the great plateau inside 
the barrier-reef on the north side of the Fiji Group. A line 
of volcanic islands, varying between 700 and 1,800 feet in 
height, rises from the interior waters of the plateau. 
It should be remarked, in conclusion, that this explanation 
of the situation of barrier-reefs at the margins of submarine 
ledges is by no means a novel one. It was suggested in 
1831, by Messrs. Tyerman and Bennett, in the case of the 
reefs of the Society Islands.* 
* Voyage and Travels, vol. i. p. 215, 
