SUMMARY1There have been three successive lines of barrier reef along the Red Sea Coast, which, by continual uplift, have become:1A range of sandstone hills rising from the alluvial maritime plain;2A fringe of limestone along the present coast‐line;3The present barrier system.2These three ridges were formed by folding and faulting of sedimentary‐rocks which overlay the bases of the Archean hills at the time of the great movement which opened the Red Sea section of the Great Rift Valley.3The northern ends of several sections of the present barrier reefs are elevated above sea‐level, and examination of these, especially the one forming Rawaya Peninsula, and of the hills forming the maritime plain, enables us to reach the above conclusions.At the same time Rawaya gives evidence of a seaward movement as well as uplift, Khor Dongonab and some at least of the channels within the Barrier Reefs being recent fault‐depressions, not merely the original anticlinal fold formed by the opening of the Rift Valley.The harbours and other fissures in the coast are due to the same secondary faulting.4The maritime plain has had two maxima of seaward extension, the first being before, the second after the growth of coral on the second and third barriers. Owing to elevation nothing has been added to its seaward slopes since the formation of the features of the coast by secondary faulting.5The filling in of valleys and the completion of the connection of the second barrier with the maritime plain has been largely due to blown sand. The process is continuing, an extensive plain near Dongonab showing perfect uniformity in its formation. The effect of climate and vegetation on the formation of the plain is described (Plate 34).6Of the possible mode of formation of the widely distributed gypsum deposits the theory favoured is by the evaporation of a shallow sea which is presumed to have existed before the deep Rift Valley was made. The recent and still continuing deposition of gypsum beneath the Rawaya salt field is an interesting actual example of this method.7The restriction of coral growth within Khor Dongonab, a very exceptional it' not unique state of things in the Red Sea, is shown to be the result of abrasion by silt‐carrying currents. Considerable effects follow from comparatively feeble currents which curry very little sand, but that this is sufficient to prevent the fixation of coral larvae is borne out by the comparison of newly fixed spat of those mollusca which do flourish on these current‐swept banks with those of the Aviculidæ,e.g., which flourish on the coral areas.8That on its west shore Khor Dongonab is extending itself at the expense of the land in spite of the presence, in some abundance, of coral andLithothamnia.The presence of these must not be assumed to imply the addition of material to a reef unless their growth can more than counterbalance the forces of