Zanzibar seems to me to have undergone several motions of subsidence & upheaval, the latter being the latest; it appears now to have been for many years nearly stationary.²

The island at present is surrounded with a nearly perfectly flat, dead, altered coral ledge, more or less dry at low water, without doubt the result of long action of the sea on the upheaved ancient and compressed coral of which the island is principally formed. This action has worn away the sea face of the land to the level of low water for a distance, in some instances, of 1$\frac{1}{2}$ mile inside the original high water line, which now remains as a steep rim, dropping to 10 & 20 fms almost immediately, with, (on the outside of the island) 100 fms. within $\frac{1}{4}$ of a mile. I could see no sign of this ledge extending seaward, though there is living coral on its steep face visible a few feet below at low water, but this is not abundant, as it is on some of the detached reefs off the island.

The present high water line of the island at the back of this flat area is, for the major part of its perimeter, a cliff of the same old coral from 10 to 20 ft in height, undermined by the waves, and overhanging, in some places, to a marvellous extent, showing the hardness and cohesion of the material, and giving a notion of the long period of time necessary to wear it away. As a further proof of this is the fact of very few lately detached pieces being seen at the foot of the cliffs, though the blocks, when they do fall, must be large, and not easily moved by the sea.

In most parts of the island, the tops of these low cliffs run back from the sea, nearly level, for a greater or less distance, showing water worn coral whereever the surface rock is exposed, and indicating another stationary period or one of very slow upheaval. Out of this level the higher lands of the island rise.

Zanzibar is intersected by what may be regarded as 3 lines of hills running north & south, the highest of them being 450 ft above the sea.

I regret to say that I cannot say of what formation these hills may be; I cannot call to mind any rock beyond the coral, which crops out at considerable heights, (in one instance 250 ft) but there is a good deal of hardened clay or mudstone which generally appears in the ravines &c and on the bare sides of the hills, but there may be other rock lying under this.

The valleys, or rather flat plains, between the ranges of hills, are mostly (particularly to the south.) coral, worn & roughened undoubtedly by water. These are generally about 50 ft above the sea. Several isolated hills of coral stand on these plains, their bases being undermined and worn precisely as the present cliffs, & their flat summits present the same appearance.

The whole thickness of the coral of Zanzibar must be very great.³

The coast of the mainland about Zanzibar is similar to the island, and, as far as I know them, Pemba, Monfia & the coast far North & South are the same⁴

The outlying and detached reefs are of two kinds, those growing up with living coral; and those of dead coral, like the island washing gradually away. Of these latter many still have level islets & rocks on them, remnants of a former upheaval; others afford a foundation to coral sand banks that dry high at low water; & others are perfectly smooth & covered at high water, being just awash at low tide. Of the second of these, are the reefs referred to by you at Page (?) as described by Lieut Boteler as sandbanks; That description is erroneous.⁵

One island, mentioned in the beginning of the century, had, by Capt Owen’s time, (1825) been reduced to a sandhead always visible.⁶ Now (1874) even this has entirely disappeared, and the reef on which it stood is flat and bare.

This is the only instance in which I have been able to make any reliable comparison between Capt. Owen’s chart and mine, as to reduction of reefs.

As to the perhaps still more interesting question of growing coral, I have been unable to make any such, as Owen’s work was so cursory and hurried that it is impossible to know whether he struck the shoalest part of a reef.⁷ There is indeed one instance that, if not isolated, might have been of use. He describes a particular shoal as being a “knoll with deep water all round”, & in his chart, 7 fms is marked on it & 25 fathoms around.⁸ That patch has now only 1$\frac{1}{2}$ fms on it and 20 fms round.

This, altogether, looks like upheaval of the whole bottom, but as in most instances our soundings agree remarkably well, I cannot think that that can be so. On the other hand the reef is so small and the bottom so clear, that is is difficult to understand how they could have missed the shoaler water if it existed then, as it is very plain to see.

Other reefs with from 7 to 10 fms on them, seem not to have altered. Rodriguez has a true fringing reef of great extent, round the volcanic mass of the island.⁹ At one end upheaval has been at work, as water worn coral crops up in the flat plains between the basaltic hills.

W J L Wharton | Commander R.N.