SEAWARD SLOPES. 315 



The reef edge and the area immediately outside it are extremely difficult and dangerous 

 to examine on account of the rollei-s, which, always breaking to some extent even in dead 

 calm weather, are followed by a strong outrush of water. The surface of the rock in this 

 position is fairly smooth, being generally covered with incrusting Lithotharnnion or with great 

 colonies of a yellow Zoanthid Actinian. On those masses and buttresses, that are exposed 

 in the backwash preceding each breaker, the upper surface of the rock is generally pitted, 

 a little green weed often growing in its hollows. The sides of the masses commonly overhang, 

 and are usually covered with LiUiotJiamnion. Down to 5 fathoms the corals are of stunted 

 growth and are only found in such hollows as are more or less protected from the rushes 

 of water, which follow each breaker. The bottoms of the channels between the ridges usually 

 appear white in colour, as if silted with small pebbles or coarse sand. In addition large 

 coral masses may occasionally be distinguished. The filling up then of the fissures between 

 the buttresses and masses would naturally to some extent follow their upward growth. 



The dominant organisms in the formation of the masses of rock between the surface 

 and 5 fathoms are undoubtedly those of the genus Lithothamnion. Unfortunately we know 

 little or nothing about the mode or rate of growth of these plants, but the rate can scarcely 

 be nearly as rapid as that of corals. The latter organisms may then perhaps be of far 

 greater importance than the number or bulk of their living colonies in this area would at 

 first sight lead anyone to suppose. That the reef is growing up admits of no doubt. In 

 the few fragments of rock, I obtained between 1 and 5 fathoms, the species of Lithothamnion 

 show a definite, though perchance somewhat irregular, layered growth. Boring organisms, etc. 

 in them are rare, and neither they nor the corals of the area show any rotting of any 

 sort. In Fiji I saw a few masses of rock broken off the edges of reefs in hurricanes and 

 hurled back on to the reef flats, i.e. true negroheads. In the Maldives on the other hand, 

 such negroheads do not exist on those reefs which fringe the open ocean. Moreover I never 

 saw any sign whatever of the breaking off of definite masses of rock, either large or small, 

 to seaward of any ordinary reef, nor for at least a dozen yards within its edge'. The absence, 

 too, from the seaward shores of islands of fragments of coral or recent rock except such 

 as grow or form on the reef flats proves of how little importance in the Maldives the de- 

 structive action, even of heavy gales, is on the edges of the reefs. 



The next zone, 5 to 10 fathoms, shows a transition to the third, 10 to 1.5 fathoms. 

 In the latter buttresses and masses are scarcely recognisable as such. No overhanging rocks 

 with definite charmels or fissures between are found. There is, nevertheless, a tendency for 

 lines of low elevations — in section transverse to the slope appearing as rounded undulations 

 — to run out at right angles to the reef behind, but even these are not always very distinct. 

 Occasionally hollows, 2 fathoms or more deep, with sandy bottoms are seen, generally con- 

 siderably overhung by growing corals. Such pools are, however, rare on those sides of the 

 reefs, which are fully exposed to the ocean, but the whole of this area is sometimes 

 represented by a succession of them. This is notably the condition, where the slope is 

 protected from the full force of the ocean, such as against channels between atolls and 

 against the central basin of the gi'oup, the Great Maldive Sea. 



1 In this connection I do not refer to tUe breakinfj off of surface, but by far the greater part is undoubtedly carried out 



branches of corals, etc. by the undercurrent on the reef slope, over the edye of the steep, thus extending the whole reef 



•which follows each breaker (see p. 24). Any loose material further and further seaward, 

 may assist to fill in the hollows and crevices on the reef 



