of Edinburgh , Session 1885-86. 
883 
away of the detritus towards the further edge of this ledge where 
some of the armings showed clean impressions. 
(d) Oima Atoll. — On the other slopes of this atoll, for the first 70 
or 80 yards from the edge of the reef-flat, there is a gradual descent 
down to a depth of 4 fathoms. Beyond this the reef slopes 
rapidly away at an angle varying between 12 and 26 degrees on 
different sides of the atoll, to depths of from 45 to 50 fathoms, the 
general level of the surrounding bottom. There appears to be no 
sudden break or declivity on the seaward slope of the reef, such as 
came under my notice in the cases of most other reefs. The corals 
flourish down to depths of 25 fathoms. The slopes beyond are for 
the most part covered with sand and gravel. 
I will now sum up the principal characters of the seaward slopes 
of reefs in this group, as exemplified by those I have just described, 
and it should be observed that, although these remarks apply to 
reefs of any of the three classes, I have more particularly in view 
at present the reefs belonging to the barrier class. For the first 
70 or 80 yards from the edge of the reef-flat there is usually a 
gradual slope to a depth of from 4 to 5 fathoms. This gentle slope, 
which is largely bare of living coral ( vide page 858), is traversed by 
prolongations of the fissures or channels that cross the outer 
portion of the reef-flat between the tide levels. These fissures may 
sometimes be traced by means of the white sand that collects in 
them, to a distance of 100 yards from the edge of the reef-flat. I 
have observed them on the seaward slopes of some reefs to be 
placed at regular intervals of from 25 to 30 feet. They vary in 
width between 2 and 5 feet. In the depths less than 5 fathoms 
the corals prefer the sides of these fissures, and are not, as I have 
already stated, in any quantity on the intervening spaces. 
Beyond this gentle slope, which terminates as observed above, 
in depths of from 4 to 5 fathoms, there is generally a rapid descent 
to a depth varying between 12 and 18 fathoms, which is usually 
sufficiently steep to be pronounced a declivity. In some reefs, 
however, as at Port Mary, this declivity may be situated at a 
somewhat greater depth. It is, however, on the face of this g. pre- 
cipitous slope that the corals flourish. This is, in fact, the growing 
edge of the reef.* The sand and gravel, produced by the action of 
* I learned from a pearl diver in these islands that he had frequently 
noticed, when diving on the outer sides of the reefs, that at the base of a 
