EAST COAST OF AFRICA. 
257 
main land, a little S. of Zanzibar, there are some banks 
parallel to the coast, which I should have thought had been 
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 and roughened, un- 
doubtedly by water. These are generally about 50 feet above the 
sea. Several isolated hills of coral stand on these plains, their bases 
being undermined and worn precisely as the present cliffs, and their 
flat summits present the same appearance. 
‘ The whole thickness of the coral of Zanzibar must be very great. 
‘ The coast of the mainland about Zanzibar is similar to the island, 
and, as far as I know them, Pemba, Monfia, and the coast far north 
and south are the same. 
‘ 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 and 
rocks on them, remnants of a former upheaval ; others afford a found- 
ation to coral sand-banks that are dry high at low water, and others 
are perfectly smooth and covered at high water, being just awash at 
low tide. Of the second of these, are the reefs referred to by you at 
page 258 as described by Lieut. Boteler as sand-banks. That descrip- 
tion is erroneous. 
‘ One island, mentioned in the beginning of the century, had by 
Capt. Owen’s time (1825) been reduced to a sand-head always visible. 
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. 
‘There is, indeed, one instance that, if not isolated, might have been 
of use. He describes a particular shoal as being a 1 knoll with deep 
water all round,’ and in his chart, 7 fathoms is marked on it and 25 
fathoms around. That patch has now only 1J fathoms on it and 20 
fathoms round. 
‘ This, altogether, looks like upheaval of the whole bottom ; but as 
in most instances our soundings agree remarkably well, I cannot think 
