BIGELOW. — SHOAL-WATER DEPOSITS OF THE BERMUDA BANKS, 573 



numbers. These living organisms, particularly mollusks, millepores, 

 and Foraminifera, are adding constantly to the bottom deposits, except 

 in some parts of the basin, where silt from the limestone ledges is 

 deposited in such large quantities as to practically choke them. In the 

 deep channels on the south, such as Castle Roads, the tidal currents, 

 setting now in one direction and now in the other, make conditions for 

 sedentary animal life still more favorable, by the great access of fresh 

 sea water and the continual scouring wliich they efifect. In St. George's 

 Harbor the conditions are somewhat different. This is much more 

 nearly enclosed, and the channels which connect it with Castle Harbor 

 and with the outer lagoon are very much shoaler than its main basin, 

 the result being that the circulation of water is not rapid, and a con- 

 siderable quantity of terrigenous detritus settles to the bottom, giving 

 the sand its characteristic gray color. This detritus, added to the 

 calcareous silt from the outer lagoons, is in sufficient quantity to prevent 

 any very active development of animal life on the bottom. In Dolly's 

 Bay and Mullet Bay the same process has gone so much further that the 

 whole bottom is covered to a considerable depth with the blue mud, which 

 is chiefly of terrestrial origin. In St. George's Channel the tidal current 

 is strong enough to scour the bottom of most of this fine material, leav- 

 ing little but the large fragments. 



The much larger basin at the western end of the islands, known as 

 Great Sound, agrees, in many of its topographic features, with Castle 

 Harbor. Like the latter, it probably originated as an enormous sink, 

 which, through the combined af^tion of subsidence and of the denudation 

 of its surroundmg hills, has become open to the sea. In its present form 

 it is open on the north by a passage about a mile and a half broad, and 

 on the west by the narrow cuts which separate Ireland Island, Somerset 

 Island, and the main island. The southern part of this large basin is 

 practically cut off by several small islands, forming the smaller basm 

 known as Little Sound, while Hamilton Harbor forms a narrow arm on 

 the east. The greatest depth of Great Sound, about eleven fathoms, is 

 near its southern border; passing thence northward, the water shoals 

 steadily until at the narrowest point, between Ireland Island and Spanish 

 Point, many of the ledges are nearly awash at low tide, thus forming a 

 natural bar. Through this a narrow ship channel has been blasted. 



The character of the bottom in the different regions of Great Sound 

 varies considerably. Near the entrance (Station 459) there are numerous 

 luxuriant patches of gorgonians, sponges, millepores, and corals, which 

 alternate with spaces of white shell sand, composed chiefly of calcareous 



