C. CROSSLAND—RECENT HISTORY OF THE CORAL REEFs. 29 
This chain of hills, it is evident, were once islands and submarine banks 
lying parallel to the coast, their formation and position being a mechanical 
result of the opening of the rift valley. By marine erosion and the growth 
of Corals, these became levelled down and built up to a line of barrier reefs, 
part of which has now been elevated above the sea-level as the coast-ranges 
and as islands, other parts being elevated to such heights that rapid coral- 
growth has been possible on their summits. The deeper portions have probably 
been raised to within the limits of growth of reef corals by the accumulation 
upon them of the remains of marine organisms, which flourish especially on 
any elevation of the sea-bottom in the way described by J. Stanley Gardiner’. 
Thus, by degradation of the parts above sea-level and ageradation below, are 
formed the level lines of reefs of the barrier system of the present day. This 
mode of formation is probably the origin of the level band of coral-rock which 
lies between the alluvial gravel of the maritime plain and the deep sea, and 
to the presence of which this plain, consisting as it does of large gravel and 
sand to great depths, is due. (See p. 14, note at bottom of page.) 
Sept. 20, 1907. CyRIL CROSSLAND. 
EXPLANATION OF THE PLATES. 
Puate 1. 
Fig. 1. A pearling canoe and its occupants. 
2. Diver returning to the canoe with a captured pearl-oyster. 
8. Beach sandstone on the island of Sararat, in Khor Dongonab. 
(a). Sea horizon. 
(6). Hard coral-rock, eroded down to sea-level, which forms the foundation of the 
sand island and of the adjacent living reefs. 
(c) to (e). Band of beach sandstone, smooth and hard at (c), rough and moderately 
hard in the dark band, very soft at (e). 
(f) is the loose sand of which the island was originally composed entirely. 
4. Mouth of one of the shallow valleys which drain Rawaya and open into the North 
Bay. The opening lies between two low cliifs of coral-rock, one of which is shown 
in the background of the photograph. During the winter, fresh water percolates 
through the sand of the valley, which it consolidates into beach sandstone where it 
meets the sea-water. At A the sandstone has a smooth surface, at B broken and 
apparently undergoing reduction by the sea. This sandstone is found in all the 
valley mouths in this bay. 
Figs. 5 & 6. Part of a large heap of old pearl-shells which forms a considerable part of the 
island of Umm es Shekh, the Holy Island of Khor Dongonab. Presumably these 
are the remains of fisheries made in the days when this species was fished only for 
its pearls, not as now mainly for the mother-of-pearl shell. Neediess to say their 
value, which would be very great if the shells were fresh, is now nil. 
* “The Building of Atolls,” Proc. Camb. Phil. Soc. 1902. 
