282 MARNE BIOLOGY OF THE SUDANESE RED SEA. 
slope suddenly changes, soundings passing from five to nine or ten fathoms in 
a few yards. This suggests that we have come upon the lower portion of 
the usual reef precipice, and this is very nearly the case. 
The diagram illustrates roughly the results of marine denudation upon a 
mass of limestone or sandstone in the presence or absence of vigorous coral 
growth, the line marked 1 being the outline of the mass as it was originally 
elevated ; 3, after wave-erosion under the protection of growing coral, 
resulting in the formation of a reef flat, slightly hollowed out near shore and 
typical precipitous edge of growing coral ; 2, the effect of wave and current 
erosion when protection by living organisms is only partial. 
The former case is that universal in the Red Sea, and found in the North 
Basin of Khor Dongonab, the latter the peculiarity of the Main Basin. 
Currents—A glance at the chart of the Khor shows how its peculiar 
contour must result in the formation of especially strong currents whenever 
there is any small change of sea-level outside. The Red Sea is not nearly 
of so constant a level at this latitude as at Port Sudan or Suakin. Records 
have been kept at Dongonab for several years, and show that the difference 
between maximum and minimum levels amounts to 80 cms., but that the 
alteration of level rarely or never exceeds 50 ems. in the 24 hours. Con- 
sequently there are no violent tide-races formed as there would be with a 
tide of say 1 or 2 metres only. The strongest local current I have actually 
observed ran at perhaps 3 knots, but this was rather exceptional. We have 
indeed a good example here of the considerable effects produced by currents 
of quite moderate strength and carrying very little silt. That these banks 
have been, and are being, carved out by currents is obvious on inspection, 
their areas of smooth rock, bare or covered with a very little sand or here 
and there by nodules of Lithothamnia, could be produced in no other way. 
I have had occasion to look for soft patches for anchoring on these banks, 
and on seeing a bottom of sand have sent down a diver with a crowbar to 
test its depth. It has almost always been found only an inch or two deep, 
and too coarse and clean to be brought up by a dredge-net that would retain 
quantities of ordinary sea-sand or mud. The water is generally clear, pearl 
shells, half hidden among weed or sponge, being picked out by the natives, 
using a “ water telescope,” at depths of from three to ten fathoms. The 
former is the average depth at which shells can be seen all the year round, 
the latter rather exceptional, but both depths are of course a long way short 
of that at which 3 white plate would be visible. The dirtiest water is due 
to wealth of micro-plankton rather than to mud. The absence of coral can 
only be attributed to the destruction of its newly-settled larvee by the 
rasping of current-swept silt, not to the current alone; and the distribution 
of other organisms as well as of corals agrees with this theory. Coral reefs 
are found where the currents are least strong, and this in spite of the fact 
