The formation of the island is madrepore, resting upon a core or base of stratified sand-stone grit, disposed in beds varying from 1·5 to 3 feet thick. The surface gently inclines towards the sea, and the lines of fracture run parallel with the shores. Three distinct formations occur to one crossing the breadth.[35] The first is a band of grit-based coralline, which runs meridionally, and is most remarkable on the eastern side. This portion, featureless and thinly inhabited, is protected from the dangerous swell and the fury of the Indian Ocean by a broad reef and scattered rocks of polypidoms. The band thins out to the north and south: in the centre, where it is widest, the breadth may be three to four miles, and the greatest height 400 feet. The coral-rag is mostly white and of many shapes, like fans, plants, and trees: the most usual form is the mushroom, with a broad domed head rising from a narrow stem. The texture is exceedingly reticulated and elastic; solid masses, however, occur where neighbouring rocks meet and bind—hence the labyrinth of caverns, raised by secular upgrowth and preserving the original formation. The ground echoes, as in volcanic countries, hollow and vault-like to the tread; the tunnels are frequently without issue for drainage, and when the rain drips in, the usual calcareous phenomena, stalactites and stalagmites, appear. Many of these caves are found on the coast as well as on the island. The carbonate of lime is very pure, and contains brown or yellow-white crystals. A stony valley, sunk below the level of both flanks, is said to bisect the island from north to south. Into this basin fall sundry small streams, the Mohayra and others, which are lost through the crevices and caverns, and in the cracks and fissures of the grit. There are other drains, forming, after heavy downfalls, swamps and marshes, whence partly the great insalubrity of the interior. The western part of Zanzibar, with its wealth of evergreen vegetation, appears by far the most fertile. It is a meridional band of red clay and sandy hills, running parallel with the corallines of the eastern coast. Here are the most elevated grounds. I found the royal plantation Sebbé or Izimbane, 400 feet (B.P.) above sea-level, or a little higher than the Bermudas. The least productive parts are those covered with dark clay. Heavy rains deposit arenaceous matter upon the surface, and the black humus disappears. On this side of the island also many streamlets discharge into the sea, bearing at their mouths mangrove beds, whose miasmas cause agues, dysenteries, diarrhœas, and deadly fevers.