LAGOON FORMATION, 319 



atoll lagoon the slope of this same reef is nearly perpendicular, the depth immediately attained 

 being generally greater than 8 fathoms, whence the bottom slopes to over 20 fathoms in 

 about 150 yards. 



The lagoon flat against the land in Addu atoll presents elsewhere more or less the 

 same characters, but the deep inner channel is not found. Its edge against the lagoon is 

 well-defined, precipitous, covered with coral and at about the low tide level. Within it there 

 may be a slight hollowing out for 2 or 3 feet, but this shallow basin partakes rather of 

 the nature of a boat channel and is in no way comparable to a velu. An analogy to the 

 north-eastern velu is only found in the north-west horn, where a portion of the bay has 

 almost been cut off from the atoll lagoon by the growth of coral knolls. The upper part 

 near its head is a fairly clear basin, covered on the bottom with muddy sand. Towards 

 its mouth, where it opens into the lagoon, these coral heads stud the whole surface, arising 

 per]5endicularly — perhaps overhanging at the top — from 10 fathoms or even deeper. Many 

 of the knolls have grown together, so that a passage to the head of the bay is at the 

 present time difficult for a vessel of any size, if not impossible. A basin or false velu is 

 thus being formed by the upgrowth of a reef within the lagoon. Without having seen the 

 growth of shoals in this horn it would have been difficult or impossible to diagnose the 

 formation of the outer lagoon reef off the other (north-east) horn of the atoll. The analogy 

 is, however, so close that there can be no reasonable doubt, but that it arose in the same way. 



The coral growth, taken as a whole, in Addu lagoon is most extraordinary, and in vigour 

 quite surpasses anything in my experience elsewhere either in the Maldives or in the Pacific 

 Ocean. On all sides of the lagoon the reefs appear to have grown out into its basin, and 

 fresh patches of coral are everywhere off them being built up towards the surface. The 

 reef patch, charted by Moresby in 1836 to the north of the south-east passage, has become 

 joined to the reef, and the horns on both sides of this same channel have grown further 

 into the lagoon. Mr Forster Cooper in addition found a shoal patch with 6 fathoms im- 

 mediately where the same passage embouches into the lagoon. Otherwise in the southern 

 channels I could see no change either in depth or breadth, but the natives report that the 

 two northern ones are now impassable for their vessels, while they seem to have been 

 regularly used when Moresby surveyed the atoll. 



The bottom of the lagoon was found by my companion, Mr Forster Cooper, to be covered 

 with rubble and sand, one small patch of mud only existing at 27 fathoms to the head 

 of the north-east horn. The rubble consisted of coral masses and shells on which were 

 growing large trees of Dendropli i/lli.a, colonies of Madrepora and Pocillofora, Polytrema, 

 Pohjzoa, great masses of Alcyonacea, Halimeda and sponges. The sand consisted merely of 

 finer ftugments of coral and other organisms, Halimeda remains and a few fi:-ee bottom-living 

 Foraminifera. Sedentary and indeed all other organisms were on this class of bottom singularly 

 scarce. 



Mr Forster Cooper investigated with soundings all parts of the lagoon, paying more 

 particular attention to those, which were charted as having more than 30 fathoms of water. 

 Making every allowance for the possibility that the soundings may have been too low — of 

 which, nevertheless, there was no indication — the greatest depth obtained by my companion 

 was 32 fathoms, where 36 fathoms ought to have been found. Where 39 fathoms are charted, 

 only 31 fathoms were obtained. Isolated soundings may mean nothing on account of difficulty 

 of fixation — in this atoll intensified by considerable changes in the land. Eliminating all but 



G. 41 



