THE FORMATION OF THE MALDIVES AND LACCADIVES. 1795 
mound slightly domed in the centre and sloping outwards on all sides. The chiet currents 
from west and east striking approximately in the centre of the plateau, the banks would 
not naturally be expected to have their long axes lying about north and south but rather 
more in the direction of the currents. This would of course largely depend on the topography 
of the original mountains, but the currents, having to cross the bank somehow, would on 
either side of the valleys at any rate tend to spread out the detritus from the land or 
bank and to broaden the north and south ends of the shoals in an east and west direction, 
a condition of which there is no trace in their present contours save perhaps in Felidu 
alone. Yet at the same time it is quite reasonable to suppose that the banks commenced 
to grow up before the channels between them attained their present depths. There may 
even have been a slight additional lowering of the plateau as a whole in depth. The 
washing away would naturally be slower with the deepening of any bank by reducing the 
force of the currents, but the upward growths of reef at various points would tend by 
providing obstructions to accelerate them. The hard bottom, found in the channels, shows 
that they are still current-swept. The summit of a bank formed by the washing away of 
land or shoal would be almost flat, and there would not be in any case the much steeper 
slopes towards the sides, as found in these passages. There is no means of estimating this 
erosion since the original foundations for the banks commenced to be laid, but it may 
perhaps be safe to infer from the lesser soundings obtained that about 160 fathoms was 
the original depth of the plateau. 
My conclusion then is that an almost flat plateau at a depth of 160 fathoms was at 
one time formed, and that on this the banks severally arose. So far as sections have been 
run off atolls in the Indian and Pacific Oceans, there is in all a striking uniformity in 
slope, a gradual fall—that of the reef platform—to something under 50 fathoms, succeeded 
by a steeper drop to about 150 fathoms. The agreement in depth with our Maldive plateau 
suggests that this is the depth to which the oceanic currents have sufficient force to cut 
down the banks and prevent the fixation of the reef-building organisms, partially directly 
by their strength and partially indirectly by movement of mud and sand smothering them 
and preventing their growth’. Below this depth I consider that we have the volcanic basis 
of our banks, as Murray suggested; or, as I think, the remains of an ancient land. In 
commenting on the cutting down of land Sir A. Geikie postulated that the depth to which 
the currents would act “is probably nearly coincident with the lower limits of reef-builders.” 
Depths approaching 150 fathoms were not however imagined by anyone, but with increase 
of knowledge the depth at which it is known that the organisms may dwell has gradually 
been further and further increased. It has been pointed out by myself and others that 
reefs are largely formed by calcareous algae (Lithothamnion), and that the corals, which cover 
the reefs, feed mainly by their commensal algae. It follows then that the limit in depth, 
at which these may exist, is probably dependent on physical questions, chiefly on the power 
of sunlight to penetrate sea-water, the temperature being assumed to be favourable. There 
are no absolute experiments on this point, but the results of dredging point to a depth 
of 150 to 200 fathoms as the effective limit to which nullipores can live. 
1 This may appear at first sight to be inconsistent with posited on the reef-flat, but frequently off islands after a 
other views put forward in this paper. There is however a heavy gale or a change of wind large areas of corals on it are 
great difference between the deposition of sand and mud and __ found to have been killed by the mud and sand clinging to 
its smothering action on corals. No sand is normally de- the tissues and blocking up the mouth parts of the polyps. 
G. 23 
