LAGOON FORMATION, 317 



The gradual passage of the outer slope of any reef into a fairly smooth platform points 

 to the fact that the foundations of the existing, encircling surface reefs of the atolls are 

 really laid at some depth below 25 fathoms. Many of the same genera of branching corals 

 (Madreporaria) — perhaps even the same species — occur at 30 fathoms as are found at 10 to 

 15 fathoms, but their form of growth is quite different. At the lesser depth the coralla 

 are dense and heavy, the branches relatively thick and massive, and the colonies of large 

 size compared with the sickly growths obtained at the greater depth. The genera of corals 

 found at 15 fathoms are also mainly those that occur on the reefs, but at 30 fathoms 

 additional genera make their appearance. Leaving genera of simple or of semi-simple species 

 out of account, Dendrophyllia, Goniopora and Alveopora all grow in great luxuriance. Our 

 swabs came up nearly every time thickly crowded with the branches of Dendrophyllia ramea, 

 and many masses and fragments of the others were secured. Of even greater importance 

 off Addu atoll was the Alcyonarian Heliopora coerulea, which was secured in each of six 

 dredgings from 25 to 45 fathoms. Its facies were staghorn, lamellate and almost massive, 

 and the quantity secured would show that large areas of the bottom must be covered by 

 its colonies. In two of the dredgings pieces of the Hydrocoralline Millepora were obtained, 

 and the same coral was also found incrusting a fragment of dead Anacropora. Ualimeda 

 is abundant, and Lithothamnion of incrusting or low branching facies also occurs. The position 

 of the latter, nevertheless, is largely taken by incrustations of white Polytrema, to the 

 importance of which Chapman' has lately drawn attention. Dead blocks of coral or other 

 rock are not infrequent, and are generally completely or in great part covered by this 

 organism. Of other animals, of little or no importance in building up reefs, it is only necessary 

 to remark that this area is essentially the home of Gorgonians and Crinoids, while Alcyona- 

 ceans are very common. No deposit of sand or small rubble, be it observed, is ever found 

 at these depths. 



The most striking character of the whole of the seaward slope to the commencement 

 of the ^eep, i.e. the reef platform, off the atolls of the Maldive group, is the almost complete 

 clothing of the bottom with organisms of various kinds. The bare rock is practically nowhere 

 exposed to the sea, so that the possibility of the latter eroding or dissolving away the rock 

 must be over the whole outer slope largely discounted. All the corals and calcareous organisms, 

 obtained from the reef platform, markedly differ from those off surface reefs or from lagoons, 

 in that boring organisms are almost completely absent. Indeed all indications of change 

 point to the active gi'owth upwards of the whole bottom in this region of the seaward slope 

 rather than to the possibility of its being by any means washing away. 



Section II. The Formation of Lagoons. 



The matter dealt with in this Section has already been to some extent considered in 

 the preceding Chapters, but the subject is of wide interest in view of the various theories 

 that have been propounded to account for the formation of coral atolls. There are also a 

 few points to which attention should be more particularly drawn in the conditions at the 

 present time of some of the atoll lagoons in the Maldive group. 



Addu atoll''', firstly, is of interest in that its lagoon is filling up rather than increasing 



■ Jour. Linn. Soc. vol. xxviii. pp. I and 161. As it is, I am indebted to Mr Forster Cooper and Capt. Molony 



- The work in this atoll was carried on in the intervals of for all tlie soundings and most of the information about the 

 fever, and was not nearly as complete as I could have desired. lagoon. 



