880 Proceedings of the Royal Society 
shallower parts of the lagoon, where they are covered by from 6 
inches to a foot of water at “ low water springs,” lie on their hinge- 
borders upon the sand, and not, as I have usually found them, in 
cavities in the coral rock. 
The Formation of Barrier-Reefs. — During my lengthened stay in 
these islands, I devoted considerable attention to the subject of the 
formation of barrier-reefs as well as to the growth of coral reefs 
in general. In the course of my investigations I took about 300 
soundings, ranging between 4 and 50 fathoms, off the weather sides 
of reefs, and I obtained in several instances the submarine profiles.* 
As typical of other reefs in this group, I will briefly describe the 
seaward slopes of the following reefs : — 
(a) Onua Reef (vide section 2). — This is a fragment of a barrier- 
reef on the south-east side of the Shortland Islands. For the first 
80 or 90 yards from the edge of the reef-flat, there is a gradual 
slope down to a depth of about 5 fathoms. There is then a rapid 
fall of about 10 fathoms; at the foot of this declivity there lies a 
narrow ledge, and beyond there is a moderate slope of 13° to the 
100 fathom line. Judging from my soundings, corals thrive down 
to depths of 15 fathoms, where the sand commences and covers the 
slopes below. This depth, it will be noted, corresponds with the 
base of the submarine declivity, a situation where, as indicated by 
the cross in the section, we should expect the sand to collect. 
(b) The Barrier-Reef of Ghoiseul Bay. — As shown in section 3 the 
submarine portion of this reef at first slopes gradually for the first 
70 or 80 yards from the edge of the reef-fiat to a depth of 4 or 5 
fathoms, when it plunges down by a steep declivity another 9 or 
10 fathoms, from the foot of which there is a rapid talus-like slope 
to a depth of about 20 fathoms from the surface. Beyond, there 
extends a broad ledge, covered by from 23 to 25 fathoms of water, 
which terminates in a slope of about 10° to a depth of 100 fathoms, 
which is the limit of the section. An inspection of this section 
would lead one to expect that an accumulation of sand and gravel, 
preventing the growth of reef-corals, would be found at the foot of the 
declivity, viz., in depths of from 15 to 20 fathoms, and that in the 
level region beyond, from the absence of such accumulations, there 
* In a short paper, published in the Annals and Magazine of Natural His- 
tory for June 1884, I have dealt more particularly with the soundings. 
