426 Mr Gardiner, The Coral Reefs of [Mar. 7, 



greater depths further and further seawards. Such a succession 

 was clearly visible off the south end of the " pocket," and in many 

 places to windward a gradual slope, such as the above, was 

 indicated by the waves breaking at considerable distances outside 

 the rim. The fissures (Fig. 2) vary greatly in breadth and shape ; 

 some are only a few inches broad while others are five or six 

 yards. Their depth varies up to 3 fathoms, the average being 

 at low tide about 1| fathoms where they run through the rim 

 of the reef. In all, the edges overhang the sides considerably, 

 and are very thin ; indeed, the fissures seemed as if they were 

 being enclosed by the edges growing out above and meeting one 

 another, while the channel below was being kept open by the 

 scour of the tide. In some the edges have absolutely met, and 

 the presence of the fissure is merely indicated by one or more 

 blowholes, out of which at low tide is driven a cloud of spray with 

 each in-rushing wave. Although I carefully examined a number 

 of these fissures after storms, I never saw any signs of their edges 

 being broken, and I think that there is no doubt that the Hat 

 surface of the reef extends by their means. The blowholes can be 

 found almost completely closed, and indeed every intermediate 

 stage, from what might be termed a boat channel to the solid 

 reef, formed in such a manner as I have suggested, can be seen 

 (Fig. 2). At the bottom of these fissures a few small boulders 

 often lie with a certain amount of sand around and between them. 

 Occasionally the sides are found to approach one another, and 

 even to have fused, cutting off from the fissures pools, which, when 

 the tide recedes, are left full of water. 



At the edges of these fissures the reef shows a somewhat 

 sharp rise of 1 — 1^ feet from the reef-flat (Fig. 2, a). It is here 

 much pitted on the surface and has a somewhat mammillated 

 appearance ; in the smaller pits Echini live in large numbers, and 

 crabs and other animals take refuge in them. Over the surface 

 there grow small green seaweeds a few inches high, and a small 

 slate-coloured species of Zoanthus. Both of these might have 

 to be carefully searched for, as generally nearly the whole is 

 covered by white, grey, brick-red and dark green coloured in- 

 crusting nullipores. The importance of these cannot be over- 

 estimated ; the lime they secrete is always very dense, and 

 apparently structureless. Its thickness, for a single plant is often 

 very considerable, and nearly the whole reef at the rim I consider 

 to have been formed by them. It is by means of the growth of 

 these nullipores that the fissures are bridged, and indeed, by the 

 use of the chisel, small spaces of a few inches in diameter can 

 be easily found completely surrounded by these organisms. The 

 reef-rock of the surface in this position, when broken into, shows 

 no trace of a coral origin, but simply consists of the lime formed 



