in the Solomon Islands. 465 
accumulation of detritus would be found in depths of from 15 
to 20 fathoms at the foot of the declivity, and that in the more 
level region beyond, from the absence of such accumulations, 
there would be more favourable conditions for the growth of 
coral. My soundings afford evidence that such is the dispo- 
sition of the detritus on the outer slope of this reef. Hight 
out of twelve casts in depths between 15 and 20 fathoms 
brought up sand or gravel on the arming; whilst out of 
eleven casts in depths between 23 and 40 fathoms, seven 
showed a perfectly clean indentation, in two of which the 
nature of the coral was shown. ‘There would thus appear in 
this reef to be a belt of sand and gravel separating the zone 
of coral into two parts, and situated in those depths in which 
corals are stated to cease to flourish. I was on the point 
of concluding that I had found in this band of sand and gravel 
the lower limit of the corals; but a subsequent extension of 
my line of soundings seaward prevented my falling into this 
error. 
The question then arises, whether in other reefs there may 
not be a belt of detritus dividing the coral-zone into two por- 
tions. In reefs of which the submarine slope is rapid, or which 
are exposed to the whole strength of the trade-swell on the 
weather side of the island, it would be a difficult matter to 
ascertain the existence of such a belt, as in the former case 
an accumulation of detritus of small horizontal extent would 
reach downwards to a great depth; whilst the violence of the 
surf in the second instance would distribute the sand and 
gravel over a considerable area. In the somewhat gradual 
slope of the reef of Choiseul Bay there are more suitable con- 
ditions for the formation of such a belt of detritus*. 
My observations on the outer slopes of reefs in this group 
point to the conclusion that the corals grew in greatest 
profusion in depths under 10 fathoms. Beyond this depth 
the sloping surface of the reef usually presented the character 
of extensive tracts of bare coral-rock, studded here and there 
with bosses of living massive corals, and marked at intervals 
by small patches of calcareous sand and gravel. In some 
isolated reefs, sunken some 6 to 8 fathoms below the surface, 
and only indicated to the navigator by the “ rips”’ produced 
by the powerful tidal currents of these regions, I was much 
surprised by their comparatively barren aspect. Lark Shoal, 
an isolated reef exposed to the full force of the trade-swell in 
the eastern portion of the Solomon group, is covered by from 
* Itis worthy of note that in the cases of the soundings off the reefs of 
Santa Anna and Onua Islet, the lower limit of the coral-zone corresponds 
with the base of the declivities shown in the sections. 
