186 bulletin: museum of comparative zoology. 



Little Bahama Bank are two depressions, one to the north of Little 

 Abaco, the other to the south, with a maximum depth of three and 

 three quarters fathoms. These sinks are depressions iu the general 

 topography of the banks, and have no definite relation to the islands 

 and to their edging reefs. The slopes to the west of Cat Island and 

 of the open banks, such as the Crooked Island, the Caicos, and Turk's 

 Island Banks, are very gradual until they reach the edging coral reef in 

 from four to six fathoms, whence the pitch is more rapid and quite sudden 

 from the 20 fathom line to the 100 fathom line. There is nothing on 

 the Bahamas which exactly corresponds to the sounds of the Bermudas, 

 such as Harrington Sound, Castle Harbor, and the Great Sound, the land 

 which once may have fringed the greater sinks on the Bahamas having 

 disappeared by erosion, only flats of a more moderate depth indicating its 

 former existence. 



As is well known, a depth of fifty to sixty fathoms has been assigned to 

 some of the lagoons in the Maldives, of twenty-five fathoms at Keeling 

 Island by Darwin, and of twenty fathoms at Matilda atoll. In the 

 composite atolls, like those of the Mahlos Mahdoo atoll, and other of 

 the Maldives, the depth in the smaller atollons is stated by Darwin to 

 be not more than five to seven fathoms, while the depth of the greater 

 lasoon — of the bank itself — is sometimes as much as two hundred and 

 fifty to three hundred feet. May this not be due to causes similar to 

 those which in the Bermudas have formed the sounds, attaining a depth 

 of nearly sixteen fathoms, and in the Bahamas have formed deep chan- 

 nels between adjacent but more distant banks and islands'? Are the 

 atollons perhaps, only on a larger scale, such formations as the Serpulse 

 reefs occurring off the south shore of the Bermudas,^ which are purely 

 mechanical structures, and in the formation of which the Serpulae or 

 corals in the case of the Maldives and Mahlos Mahdoo atoll have had 

 but a limited share? 



The islands and islets of the two archipelagos named may be only the 

 summits of a bank of very irregular outline, upon which corals have 

 established themselves. Subsidence has taken place there to a very 

 considerable extent, as stated by Darwin, but the position of the corals 

 and the shape and distribution of the reefs have only been affected in a 

 limited manner by it and are not due to it any more than in such locali- 

 ties as the Bermudas and the Bahamas, The depth between Horsburgh 

 atoll and the southern end of the Mahlos Mahdoo atoll (over two hundred 

 fathoms) is no more surprising than the great depth of the channel be- 



1 A. Agassiz, Am. Journ. of Science, June, 1894. 



