The rule established by Dampier and quoted by Humboldt directs us to expect great depth near a coast formed by high perpendicular mountains. Here, as in the rest of the Zanzibarian archipelago, the maritime line, unlike the west Atlantic islands Tenerife and Madeira, is composed of gently rolling hills. Yet seven fathoms are often found within a stone- throw of the land, whilst the encircling ledges are steep-to, marked in the charts 1/100 and 1/140. Evidently, then, the corallines are perched upon the summits of a submarine range which rises sharp and abrupt from abysmal hollows and depressions. As usual too in such formations, the leeward shore line of the island, where occur the lagoon entrances, is more varied and accidented than the eastern. At Pemba this feature will be even more remarkable. The windward coast, in common with many parts of the continental seaboard, suffers especially from June to August from the Ras de Marée (Manuel de la Navigation et la Côte occidentale de l’Afrique), a tide race, supposed to result from the meeting of currents. It is a line of rollers neither far from nor very near the shore. The hurling and sagging surf is described to resemble the surge of a submarine earthquake; and the strongest craft, once entangled in the send, cannot escape. It would be useful to note, as at West African Lagos, the greater or less atmospheric pressure accompanying the phenomenon, and to seek a connection between it and the paroxysms of the neighbouring cyclone region. At all times sailors remark the ‘shortness’ of the waves and the scanty intervals between their succession. This peculiarity cannot be explained in the usual way by shoals and shallow water causing a ground-swell.