238 APPENDIX. 



38 fathoms is laid down within the reef. In the same 

 chart, the space of open water within the reef in some 

 parts is even more than three miles wide : the land is bold 

 and peaked; this island, therefore, is encircled by a well- 

 characterised 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 

 (Nar., 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, in from 30 to 45 fathoms, between a reef 

 and the shore (Horsburgh, voL i. p. 214); in Capt. Owen's 

 chart of Madagascar, this island is represented as encircled ; 

 coloured blue. — Great Comoro Isld. is, as I am informed 

 by Dr. Allan, about 8,000 feet high, and apparently 

 volcanic ; it is not regularly encircled ; but reefs of various 

 shapes and dimensions jut out from every headland on the 

 W., S., and S.E. coasts, inside of which reefs there are 

 channels, often parallel with the shore, with deep water. 

 On the N. -western coasts the reefs appear attached to the 

 shores. The land near the coast is in some places bold, 

 but generally speaking it is flat ; Horsburgh says (vol. i. p. 

 214) the water is profoundly deep close to the shore, from 

 which expression I presume some parts are without reefs. 

 From this description I apprehend the reef belongs to the 

 barrier class ; but I have not coloured it, as most of the 

 charts which I have seen, represent the reefs round it as 

 very much less extensive than round the other islands in 

 the group. 



