BIGELOW. SHOAL-WATER DEPOSITS OF THE BERMUDA BANKS, 577 



and other organisms are growing, and they furnisli a great deal of ma- 

 terial to the bottom sands. The depths of the lagoons are fairly uniform 

 and moderate, being nowhere over twelve fathoms. The more important 

 lagoons from which I have examined samples are Mangrove Bay, the 

 Cow Ground Flats, Green Bay, Murray Anchorage, the Ship Channel, 

 and Brackish Pond Flats. 



Mangrove Bay, though open to the sea, is nearly surrounded by 

 Somerset Island and, on the north, by a shoal flat, making it another 

 enclosed catcli basin, with a greatest depth of six fathoms. The bottom 

 at Station 319, in five fathoms, is fine gray sand, containing many large 

 fragments of millepore and oculina. evidently washed from the neighbor- 

 ing flat. Tlie bulk of the sand consists of broken shells, sea-urchin plates 

 and spines, and fragments of worm tubes, with about 20 per cent of very 

 fine calcareous silt. The presence of great numbers of the calcareous 

 spicules of sponges and gorgonians, and of the tests of small Foramini- 

 fera, show that it is of reef origin ; a small proportion of it, however, is 

 apparently terrigenous silt, or " blue mud," and there are few living 

 organisms. Most of this bottom is evidently derived from the surround- 

 ing shoals, being composed of various animal remains, together with a 

 large amount of the fine silt resulting from the erosion of the limestone 

 ledges. The rest of the material is of terrigenous origin, and resident 

 organisms have had little or nothing to do in the formation of the 

 deposit. 



Murray Anchorage is the largest of the submerged lagoons, covering 

 an area of about ten square miles. It extends from the shores of St. 

 George's Island on the southeast to the Three Hill Shoals on the north, 

 a distance of three or four miles. On the southwest it is bounded by 

 Bailey's Bay Flats, and on the northeast by the boundary reefs to the 

 north of St. George's Island. The bottom is level, with very few shoals 

 or ledges, and the greatest depth, in the Ship Channel on the southern 

 edge of the lagoon, is ten fathoms. The deposits over the bottom of 

 the lagoon vary considerably in character in different localities, although 

 they are everywhere white sand. North of Murray Anchorage, between 

 Three Hill Shoals and the boundary reef at Station 324, in ten fathoms, 

 this white sand consists of exceedingly minute particles, and has the ap- 

 pearance of a white, chalky ooze. Fragments of algae, millepores, and 

 gasteropod and lamellibranch shells, with a few echinoid fragments and 

 pieces of worm tubes, are distinguishable ; also a few Foraminifera. 

 Over 70 per cent of the mass, however, consists of minute calcareous 

 particles, forming a true marl. There are very few living organisms of 



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