C. CROSSLAND 
RECENT HISTORY OF THE CORAL REEFS. LG 
level and remain down for one or more days. Further, the mean level is 
about three feet lower in summer than in winter. 
Tt is found that there is a true tide of a few inches, but that this is 
swamped by the greater changes of level induced by changes of barometric 
pressure. The “tides” are therefore quite irregular and cannot be predicted. 
My own observations have been made daily for about six months at 
Dongola, where, owing to its position near the head of a very long bay, the 
effect of the strength and direction of the winds is very marked, but even 
there the wind is not the only factor in producing changes of level. 
The difference in the level of the whole of the Red Sea in the winter and 
summer is a result of the monsoons in the Indian Ocean. The total 
difference between highest water in winter and lowest in summer is about 
six feet. 
JAUSES OF THE FORMS oF THE PRESENT REEF SYSTEM. 
The theory advanced is that—(i.) As the region is one where considerable 
upward movement (at least 500 feet) has occurred since modern reefs began 
in the Sea, and where elevation is still in progress, Darwin’s subsidence 
theory cannot be called into account for the existence of the barrier reefs 
at a distance from land and separated from it by deep water, or of the atoll 
forms of certain reefs. (ii.) The relations between elevated and still living 
reefs show that the foundations of both are ranges of sandstone hills partly 
below sea-level. (iii.) The forms of the reefs are due to the balance of 
agerading over degrading agents, the former being the growth of coral, 
nullipores, &e., and the latter the corrosive action of the sea and the rotting 
caused by boring organisms, ce. 
A note on the probable cause of the changes effected after the upheaval of 
a coral-reef, whereby it takes on the hardness, homogeneity, and crystalline 
structure characteristic of “ Coral-Rag,” is appended. 
HVIDENCES OF ELEVATION. 
The existence of the coast-plain and its breadth, which on the Sudan 
coast averages five miles, are against any considerable movement of depression 
during its formation. The raised reefs indicate upward movements of 
1500 feet or more, and more recent changes are recorded as raised beaches 
and old erosion-lines or cliffs, of which examples are found throughout the 
length of the Sudan coast. 
At Agig, a mile inland, but only a few feet above the sea, is a raised beach 
in the form of a long ridge of rolled pebbles of crystalline rock corresponding 
to that of coral-fragments which borders the present shore. Both are 
surmounted by a line of graves, the seashore having been, apparently, a 
favourite place of sepulture through all ages. Even the design of the graves 
LINN. JOURN.—ZOOLOGY VOL. XXXI. 2 
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