244 APPENDIX. 



from some plans and accounts to be an atoll, but from others 

 this does not appear to be the case ; not coloured. Secondly, 

 Cosmoledo : ' this group consists of a ring of coral, ten leagues 

 in circumference, and a quarter of a mile broad in some places, 

 inclosing a magnificent lagoon, into which there did not appear 

 a single opening' (Horsburgh, vol. i. p. 151) ; coloured blue. 

 Thirdly, Aldabra : consists of three islets, about 25 feet in 

 height, with red cliffs (Horsburgh, vol. i. p. 176), surround- 

 ing 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 descrip- 

 tion 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. 



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, published 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 distance of two miles from the 

 shore ; in some parts, however, the reef must be attached, 

 since Lieut. Boteler (Narr. vol. i. p. 161) describes a passage 

 through it, within which there is room only for a few boats. 

 Its height, as I am informed by Dr. Allan, is about 3,500 

 feet ; it is very precipitous, and is composed of granite, 

 greenstone, and quartz; coloured blue. — Mohilla : on the 

 S. side of this island there is anchorage between a reef and 

 the shore in from 30 to 45 fathoms (Horsburgh, vol. i. 



