320 J. STANLEY GARDINEK. 



the most careful observations — many having been taken when dredging — 23 are left, all of 

 which show a decrease in depth of from 1 to 8 fathoms, the general reduction being 2 or 3 

 fathoms. 



It will be clear from the above remarks that Addu atoll differs from all others in our 

 group in the fact that it shows a decrease in depth in its lagoon. Minutely examining all 

 the factors as given above and comparing as far as possible with the chart, I estimate 

 that in the 65 years between Moresby's visit and my own there has been at the least 

 a decrease in depth of 2 fathoms over the whole area of the lagoon proper, i.e. about 

 15 square miles. The whole lagoon is about 22 square miles, and there must be taken 

 into account the inward extension of the lagoon reefs, a far more rapid increase. To suppose 

 therefore, for the 60 odd years since the survey, a deposit or filling in by coral growth of 

 2J inches a year over an area of 22 square miles does not seem to me to be excessive. 



To account for this decrease, it should in the first place be pointed out that Addu 

 atoll is markedly different in its topography to all the other banks of the group. From 

 my corrected chart I estimate that five-sevenths of its circumference are protected by land. 

 There are only fom- narrow passages — two extremely small — with an average dej)th over their 

 whole sections of probably less than 10 fathoms. There is thus not likely to be much 

 circulation of water in the deeper parts of the lagoon. The prevailing winds and cuiTents 

 are east and west ; from the latitude being almost that of the equator it is obvious that 

 they vary but little in different seasons of the year, inclining only somewhat to north-east 

 and south-west. Further the ordinary currents and winds never can have the same force 

 as in the more northern atolls, and hurricanes do not occur so far north or south. By 

 a glance at the chart, it will be at once apparent that in no case can the usual currents 

 or winds affect the lagoon, owing to the positions of the passages and the protection afforded 

 by the land. As important factors in Addu, which they certainly are in other atolls, they 

 may accordingly be neglected. The tidal rise and fall is stated to be 4 feet— on the lagoon 

 beaches certain measurements I made averaged 3 feet 8 inches, but it does not follow that 

 the rise within the lagoon would be as great as on the seaward sides of the islands — which 

 is certainly less than in any other part of the group. Lastly, the encircling reef of Addu 

 averages more in breadth than that of any other Maldivan atoll, and the tidal water in 

 crossing it would be more likely to be saturated with lime before ever it got to the lagoon, 

 so that there could be by its means little solution. 



The chief agent in this filling in of the lagoon of Addu atoll is coral growth. Its 

 luxuriance is quite without parallel in any of the described regions of the world, similar in 

 the size and closed character of the area. I have unfortunately no definite observations, 

 which serve to explain its cause. The water of the lagoon, owing to its protection from -svinds 

 and currents, would naturally be considerably clearer than in most basins. The periphery, 

 over which the surface fauna might be supposed to pass to the interior of the lagoon, is 

 only 8 miles, but I would emphasise the fact that all those kinds of corals that form the 

 reefs feed rather by their commensal algae. With regard to this it is probable that, from 

 the decaying matter of the lagoon flats and land, an especially abundant supply of carbonic 

 acid gas is provided. This would seem at first sight to be opposed to my argument that 

 the tidal water would be saturated with lime before ever it reached the lagoon. All the 

 carbonic dioxide would, I conceive, on account of the abundant growth of corals be utilised 

 by them, but the sea-water has a solvent action on calcium carbonate, independent of the 



