256 APPENDIX. 



joining Yucutan and Florida. The area of deposition seems 

 less intimately connected with the debouchement of the great 

 rivers, than with the course of the sea-currents; as is evident 

 from the vast extension of the banks from the promontories of 

 Yucutan and Mosquito. 



Besides the coast-banks, there are others of various dimen- 

 sions which stand isolated ; these closely resemble each other ; 

 they lie from 2 or 3 to 20 or 30 fathoms under water, and are 

 composed of sand, sometimes firmly agglutinated, with little or 

 no coral ; their surfaces are smooth and nearly level, shelving 

 very gradually to the amount of a few fathoms all round 

 towards their edges, where they plunge abruptly into the un- 

 fathomable sea. This steep inclination of their sides, which is 

 likewise characteristic of the coast-banks, is very remarkable : 

 I may give as an instance, the Misteriosa Bank, on the edges 

 of which the soundings change in 250 fathoms horizontal dis- 

 tance, from 11 to 210 fathoms; off the northern point of Old 

 Providence bank, in 200 fathoms horizontal distance, the 

 change is from 19 to 152 fathoms; off the Great Bahama 

 Bank, in 160 fathoms horizontal distance, the inclination is in 

 many places from 10 fathoms to no bottom with 190 fathoms. 

 In all parts of the world, where sediment is accumulating, 

 something of the same kind may be observed ; the banks 

 shelving very gently far out to sea, and then terminating 

 abruptly. The form and composition of the banks in the 

 middle parts of W. Indian sea, clearly show that their origin 

 must be chiefly attributed to the accumulation of sediment ; 

 and the only obvious explanation of their isolated position is 

 the presence of a nucleus, round which the currents have col- 

 lected fine drift matter. Any one who will compare the bank 

 surrounding the hilly island of Old Providence, with- the banks 

 in its neighbourhood which stand isolated, will scarcely doubt 

 that they surround submerged mountains. We are led to the 

 same conclusion by examining the bank called Thunder Knoll, 

 which is separated from the great Mosquito bank by a channel 



