There are however in the north two islands the existence of which is most readily explained by volcanic action. I refer to the coral formations known as “The Brothers” and “Daedalus Shoal,” the former a pair of low islets, the latter a flat reef, rising out of the centre of the sea and surrounded by water hundreds of fathoms deep. They are extremely steep-sided cones, and what could form and support such structures far out from land is puzzling. A certain view of another section of the Rift Valley, that once seen can never be forgotten, seems to offer an explanation. After passing through the forests of the Kikuyu Plateau by the Uganda Railway one comes out into the open on the brink of the great escarpment of the Rift Valley and looks across a trough 3000 feet deep to the similar forest-clad heights of the Mau on the other side. The continuity of the valley is rudely broken by two volcanic cones rising abruptly in the middle of the flat bottom of the trough. On consideration the strangeness of their appearance in the middle of a valley passes away, one sees that the bottom of such a rift must be a zone of weakness of the earth’s crust where volcanoes might naturally be expected to arise. [Illustration: Diagram 8. Formation of atoll as a cap of coral growing on a mound of loose volcanic material. A original mound, B as cut down by the sea, C the atoll.] If the water were removed from the Red Sea Valley would not the appearance of The Brothers and Daedalus be very much like that of the two volcanoes of British East Africa, allowing for the steeper angle at which their materials would lie under water? Given such cones of loose volcanic ash, &c., wave action would quickly level down their summits until coral growth afforded protection and formed a cap of rock, part of which is now raised again above sea level as the islands on one of which the lighthouse is built. The ring-shaped reef of Sanganeb[63], opposite Port Sudan, which is outside the Barrier system and separated from it by water 400 fathoms deep, may be built on a similar foundation. Like the two coral reefs above it rises with extremely steep slopes from this deep water, and is the summit of a submarine pinnacle rather than hill.