INDIAN OCEAN. 
251 
some places, inclosing a magnificent lagoon, into which 
there did not appear a single opening’ (Horsburgh, vol. i. 
p. 151); coloured blue. 1 Thirdly, Aldabra : consists of 
three islets, about 25 feet in height, with red cliffs (Hors- 
burgh, vol. i. p. 176), surrounding a very shallow basin or 
lagoon. The sea is profoundly deep close to the shore. 
Viewing this island in a chart, it would be thought to be 
an atoll ; but the foregoing description shows that there 
is something different in its nature ; Dr. Allan also states 
that it is cavernous, and that the coral-rock has a vitrified 
appearance. Is it an upheaved atoll, or the crater of a 
volcano ? — uncoloured. 2 
Comoro Group. — Mayotta, according to Horsburgh 
(vol. i. p. 216, 4th edit.), is completely surrounded by a 
reef, which runs at the distance of three, four, and in some 
places even five miles from the land ; in an old chart, pub- 
lished by Dalrymple, a depth in many places of 36 and 38 
fathoms is laid down within the reef. In the same chart, 
the space of open water within the reef is in some parts 
even more than three miles wide : the land is bold and 
peaked ; this island, therefore, is encircled by a well- 
characterized barrier-reef, and is coloured pale blue. — 
Johanna: Horsburgh says (vol. i. p. 217), this island from 
the N.W. to the S.W. point, is bounded by a reef, at the 
1 [The islands on the ring have been upraised about 10 feet. — 
Capt. Wharton.] 
2 [ Aldabra is an upraised atoll 22 miles long ; the lagoon is nearly 
dry at low water. The height of the rock on the encircling islands 
is 20 feet, and it descends on both sides to the water for that distance 
in a cliff, though on the lagoon side the coral is much disintegrated by 
the mangroves. This is the only island in the Indian Ocean where 
the gigantic tortoise, of a distinct species, exists. — Capt. Wharton. 
Horsburgh’s account is misleading, as neither the red cliffs nor high 
forests were to be found. It is entirely composed of coral-rock with 
a fine growth of mangroves, inclosing an extensive but shallow lagoon. 
There is a narrow riband of 5 fathoms water running 3 miles into the 
vagoon from the N.W. corner.— Lieut. Chas. Smith.] 
