206 J.S.Gardiner — Origin of Coral Reef s. 



fathoms. The study of the charts and of the survey work of 

 the last thirty years gives much additional evidence, and the 

 Maldives themselves give clear indications. Lastly, a depth of 

 about 150 fathoms is that at which the steep slope of most coral 

 reefs passes into a more gradual slope, and would seem to be 

 the critical depth to w^iich the main oceanic and tidal currents 

 act, this depth decreasing off the smaller reefs and increasing 

 off the larger. At the same time it is scarcely necessary to 

 suppose that the whole area was cut down to 200 fathoms 

 before reefs commenced to build up, though such a depth would 

 seem to be critical for a bank of about this size, so situated in 

 a fully exposed area. Seeing the absolute impossibility of sub- 

 sidence affording any explanation, examining all considerations 

 and taking the indications that the area itself gives, I conclude 

 that an almost flat plateau at a depth of 140 to 170 fathoms 

 w^as at one time formed by the erosion and denudation of an 

 original land mass or more probably series of masses. 



We have now to examine the means by which the existing 

 reefs have been built up on this plateau to the surface. Here 

 I am on firmer ground. Of course, as already mentioned, cer- 

 tain parts may from the first have been higher and have served 

 as foundations for lime-secreting organisms, but such elevations 

 are not necessary for the foundations of our reefs. In the first 

 place the observations of my expedition from upwards of 300 

 dredgings, largely made to settle this point, prove that the regu- 

 lar reef-building corals do not live below 30 fathoms in any 

 such luxuriance as would be requisite if a reef were to be built 

 up. Reef corals feed mainly or almost entirely by their com- 

 mensal algae. Hence their limit in depth depends on the 

 penetrability of sunlight through sea water, and this, judging 

 from marine algae, must be about 150 fathoms in the tropics. 

 In all probability, however, the light is not of sufficient intensity 

 for any of these organisms to actually live below 75 fathoms, 

 to upwards of which depth flourishing banks of Lithothamnion 

 have been found in the East Indies. 



The question in reality is not one presenting any real diffi- 

 culty. Given an upstanding bank, on which the movement of 

 mucl and sand be not too great t:o choke the animal larvae, we 

 know from the examination of many such that it would soon 

 be covered by animal life. Every part even of our immense 

 bank at 150 fathoms w^ould be thus overgrown, since a rela- 

 tively strong current would be passing over it and thoroughly 

 churning up and mixing the pelagic life on which the sedentary 

 organisms so largely feed. This state could not, however, con- 

 tinue as the bank grew up towards the surface. The current 

 over it would increase in proportion to its upw^ard growth. 

 Less and less food would tend to reach the organisms on its 



