[Footnote 54: More correctly a _compound_, not mixture of carbonates, is formed, viz. dolomite, CaMg(CO₃)₂.] [Footnote 55: Examples of all three are well shewn on the map opposite page 136.] [Footnote 56: I.e. in the great oceans. As shewn later, volcanic mounds and islands are found in the Red Sea, and may have formed the foundations of certain of its reefs.] [Footnote 57: Gardiner, J. S., “The building of Atolls,” _Proc. Camb. Phil. Soc._ or The Challenger Society’s _Science of the Sea_, John Murray.] [Footnote 58: In some lagoons quantities of sand are formed of great numbers of the minute shells of lowly organisms, the foraminifera. This is, of course, a very different thing to that resulting from breaking up of coral and shells.] [Footnote 59: 1912 was a rich year! 40 millimetres fell on Oct. 22nd and 20 millimetres in December, about 2½ inches altogether.] [Footnote 60: J. W. Gregory, _The Great Rift Valley_, John Murray, 1896.] [Footnote 61: The ranges bordering the sea are from 4000 feet to 8000 feet high, but of course were much higher in the days when the Rift opened.] [Footnote 62: Recently, that is, geologically speaking.] [Footnote 63: Such ring-shaped reefs, rising out of deep water, are termed Atolls. Sanganeb is a small example compared to those of the Indian and Pacific Oceans.] [Footnote 64: I have marked other harbours as they are interesting geologically, but there are only three miserable villages on this whole coast in addition to the two towns. There is no village at Trinkitat, but the cotton from Tokar is loaded on to _sambûks_ here.] [Footnote 65: I.e. hills of Archean, ancient rock, as opposed to the newer sandstones, gravels and limestones.] [Footnote 66: The Eastern Desert of Egypt, southern section, describes a part of the Red Sea coast range north of the Sudan boundary, but essentially similar to the rest.] [Footnote 67: Two borings, made in the hope of obtaining water two miles inland, near Port Sudan, were carried down through 300 metres, nearly 1000 feet, of this kind of material.] [Footnote 68: “Mersa” in Arabic = anchorage.] [Footnote 69: Arabic Hamâma = pigeon. The shape of this hill is a cone with a cubical block on its apex, hence the appropriate name Pigeon Hill. “Khor” is used for an inlet of the sea, among other meanings.] [Footnote 70: Actually many of these shells retain some of their colour! e.g. the red of the common _Spondylus_, the bands of _Strombus fasciatus_ and the mottlings of _Cyprina tigrina_ are quite distinct on some specimens from 100 feet above, and on others dug out 15 feet below sea level.] [Footnote 71: _All_ the corals are of existing species, but the identification is less easy than in the case of the shells.]