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edges, rising above tbe water and some of them inhabited.’ 
He indicates further difficulties in applying the theory 
of subsidence to the Cliagos Bank, especially pointing out 
that the Six Islands atoll, within a few miles distance, has 
not been affected ; still he admits that the ‘ Saya de Mallia 
Bank appears to have the characters of a submerged atoll, 
having a central depression of 65 fathoms surrounded by a 
rim which has only 8 to 16 fathoms on its eastern side, 
but 22 fathoms on the western.’ On the whole, however, 
he considers that * most of the coral formations of the 
Indian Ocean mark areas of elevation rather than of rest ; 
certainly they are not evidence of subsidence.’ 
In regard to the explanation of the formation of 
lagoons by solution of the interior parts of the reef, and 
by the more rapid growth of the corals on its periphery, as 
being more directly in the track of food-bearing currents, 
Mr. Bourne observes : — ‘ Neither of these explanations has 
completely satisfied me. That sea-water exercises a solvent 
action upon carbonate of lime does not admit of doubt, and 
that the scour of tides, combined with this solvent action 
of the water, does affect the extent and depth of a lagoon is 
obvious. But I challenge the statement that the destructive 
agencies within an atoll or a submerged bank are in excess 
of the constructive. It would be nearer the mark to say 
that they nearly balance one another. In the first place the 
carbonate of lime held in solution by sea-water is deposited 
as crystalline limestone in the interstices of dead corals or 
coral debris. Anyone who is acquainted with the struc- 
ture of coralline rock knows how such a porous mass as a 
mceandrina head becomes perfectly solid by the deposition 
of lime within its mass. This deposition can only be 
effected by the infiltration of sea-water. In reckoning the 
solvent action of sea- water, therefore*, account must be 
taken of the fact that a not inconsiderable proportion of 
the carbonate of lime held in solution is re-deposited in the 
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