WEST INDIES. 257 



only seven miles wide, and 145 fathoms deep. There cannot 

 be any doubt that the Mosquito bank has been formed by the 

 accumulation of sediment round the promontory of the same 

 name ; and Thunder Knoll resembles the Mosquito bank, in 

 the state of its surface submerged 20 fathoms, in the inclina- 

 tion of its sides, in composition, and in every other respect. 

 I may observe, although the remark is here irrelevant, that 

 geologists should be cautious in concluding that all the out- 

 Iyer s of any formation have once been connected together, for 

 we here see that deposits, doubtless of exactly the same nature, 

 may be deposited with large valley-like spaces between them. 

 Linear coral-reefs and small knolls project from many of 

 the isolated, as well as from the coast banks ; sometimes they 

 are irregularly placed, as on the Mosquito bank, but more 

 generally they form crescents on the windward side, situated 

 some little distance within the outer edge : — thus on the Ser- 

 ranilla bank they form an interrupted chain which ranges 

 between two and three miles within the windward margin : 

 generally they occur, as on Roncador, Courtown and Anegada 

 banks, nearer the line of deep water. Their occurrence on 

 the windward side is conformable to the general rule, of the 

 efficient kinds of corals flourishing best where most exposed ; 

 but I cannot explain their position some way within the line 

 of deep water unless it be that a depth somewhat less than 

 that close to the outer margin is most favourable to their 

 growth. Where the corals have formed a nearly continuous 

 rim, close to the windward edge of a bank some fathoms sub- 

 merged, the reef closely resembles an atoll ; and if the bank 

 surrounds an island (as in the case of Old Providence), the 

 reef resembles an encircling barrier-reef. I should undoubtedly 

 have classed some of these fringed banks as imperfect atolls, 

 or barrier-reefs, if the sedimentary nature of their foundations 

 had not been evident from the presence of other neighbouring 

 banks, of similar forms and of similar composition, but with- 

 out the crescent-like marginal reef. In the third chapter, I 



S 



