898 
Proceedings of the Poyal Society 
Other macroscopic Foraminifera, such as Operculina complanata, 
Alveolina boscii, &c., occur, both alive and dead, amongst the sand 
and gravel. To fragments of Nulliporce the Foraminifer Polytrema 
priniaceum is sometimes attached. Small unattached corals of the 
genus Heteropsammia came up frequently in the sand and gravel, 
from depths between 20 and 40 fathoms off reefs. 
The composition of this deposit on the outer slopes of reefs and 
the depths to which it extends are points of importance. Its 
materials would form a white compact limestone, such as I have 
described in my paper on the calcareous formations (p. 574)* as 
the commonest type of the so-called coral limestones in the Solomon 
Group. A rock of this description might be formed at any depth 
between 20 and 100 fathoms in places where the submarine slope 
is steep; but where the slope is moderately rapid, i.e ., less than 20°, 
as it usually is, these deposits will be probably restricted to depths 
considerably within the 100 fathom line. 
I should here observe that these remarks apply only to the deposits 
forming off coral reefs. On coasts, such as those of St Christoval, 
and in harbours, such as Treasury Harbour, where numerous streams 
carry down a quantity of sediment into the sea, this material is mixed 
in varying proportions with the calcareous sand and mud. 
Of the nature of the deposits forming off the coasts of these 
islands in depths beyond 100 fathoms, I have been able to judge 
from the character of the soundings made by Lieutenant Oldham in 
H.M.S. “Lark” between the north coast of St Christoval and the 
adjacent islands of Ugi and the Three Sisters. There appears to be 
a maximum depth here of rather over 400 fathoms. In depths of 
100 to 200 fathoms the nature of the bottom varies according to the 
locality. Thus, off the St Christoval coast, where the sediment 
brought down by the numerous large streams is in great part derived 
from volcanic rocks, the deposits forming in these depths consists of 
a dark calcareous mud sometimes foraminiferous and mixed with 
coarser volcanic detritus; while off the coasts of the small islands 
of Ugi and the Three Sisters, the bottom between these depths of 
100 and 200 fathoms is formed for the most part of calcareous 
sand mingled with macroscopic f oraminif era. f The material 
* Trans. Edin. Roy. Soc., vol. xxxii. part 3. 
t From a depth of 100 fathoms there came up on one occasion dead coral 
fragments in addition to the sand. 
