180 PROFESSOR J. W. JCJDD. 



Over the greater part of the lagoon floor, however, there would appear to he a turf- 

 like growth of the green calcareous alga Halimeda. In the shallower portions of the 

 lagoon this Halimeda growth appears to be most vigorous and constitutes a green 

 carpet on the lagoon floor ; l)ut in deeper portions of the lagoon, the Halimeda 

 appears to be dead and decaying. Towards the edges of the lagoon and around the 

 various shoals in it, the mass of living and dead Halimeda fronds gives place to the 

 usual shore-sands largely composed of foraminifera. 



We have fortunately the means supplied to us by the ol)servations of Mr. Fjnckh 

 (see page 14G) of estimating the rate of growth of Halimeda, and of comparing it with 

 that of various genera of corals. A tuft of Halimeda forming a compact boss 2^ inches 

 (55 millims.) high, and 3^ inches (80 millims.) in diameter, grew up in six weeks ! 

 No coral of which the rate of growth has been studied has anything like this power 

 of rapid development. Lithothamnioii too, seems, in spite of its ubiquitous character, 

 to be a much more slowly growing organism than Halimeda. It would appear that 

 while in some parts the growth of corals was choked by the covei'ing oi' Halimeda, in 

 other places the corals conquered in the struggle for life, and continued to grow 

 upwards. A great part of the lagoon of Funafuti is tlius proved to be covered by a 

 mass of calcareous algre, and this mass of vegetable matting has a remarkable 

 resemblance to a peat-ljog. The upper surface is green and living, but, below, the 

 mass is dead and decaying. Tlie deptli of this mass of living and dead vegetable 

 matter appears to be between GO and 70 feet, and it lies between and around upgrow- 

 ing bosses of coral rock that form shoals. 



The rate of growth of the tuft of Ha.limeda studied by Mr. Finckh may not 



improbably have been exceptionally rapid, for the conditions were perhaps unusually 



favourable in the shallow waters where the experiment took place. But it is possibly 



not an excessive estimate to take 1 inch per annum as the rate of accumulation of the 



broken fronds of Halimeda on the lagoon floor. At this rate of accumulation the 



mass of vegetable matter on tlie floor would represent a period of about 800 years. 



If we take 15 tathoms as the depth at which the vigorous growth of reef- forming 



corals goes on (and this is in conformity with the results obtained by Professor 



A. Agassiz and Mr. Stanley Gardiner), a subsidence of 25 fathoms (150 feet) must 



have taken place in this 800 years, being at the rate of less than 20 feet per century. 



We have compared this mass of vegetable matter, alive on its surface and dead 



below, to a peat-bog, and there is another respect in which the analogy holds good. 



In a peat-bog the lower portions are undergoing slow chemical changes, various gases 



being given off, the proportion of carbon to the other constituent elements showing a 



progressive and constant increase as we go downwards. Dr. Hinde has pointed out 



that in going downwards through the mass of Halimeda fronds on the lagoon floor, 



we find first the organic matter disappearing, and the fronds becoming white and 



friable, and afterwards these fronds becoming more solid and dense by a secondary 



deposit of calcium carbonate. The exact nature of the changes taking place 



