BIGKLOW. — SUOAL-WATER DEPOSITS OF THE BERMUDA BANKS. 567 



coral sand, though fragments of millepores play a considerable role as 

 beach builders. 



The detached ledges which run parallel to the south shore enclose a 

 more or less clearly marked lagoon, with a depth of about six fathoms, 

 where all conditions are as favorable for marine organisms as possible. 

 Here mollusks, echinoderms, tube-buikling worms, gorgonians, corals, 

 millepores, corallines, and nullipores flourish, and it is from this area 

 tliat the beaches of the south shore receive most of tlieir material. 

 The amount of detritus added by the destruction of the cliffs varies in 

 different places, but the total addition must be enormous, because the surf 

 is often very violent. Most of the beaches, however, are swept clean of 

 any of this fiue calcareous silt, most of which is probably carried out and 

 de[)0sited in deeper water. Such parts of it, however, as are added to 

 tlie beaches, as at Tucker's Town, are often blown far inland, to form 

 large dunes. The gorgonians, which are luxuriant just off the shore, can 

 of course add nothing but their spicules to the deposits, and the corals, 

 being mostly porite-?, meandrinas, and other massive forms, are not easily 

 ground up. The offshore reefs probably furnish most of the tubes of the 

 red Serpulae and other worms and a few mollusk shells ; but they are 

 so well protected by incrusting Serpulae and algae that the surf cannot 

 exercise any very vigorous destructive action on them. 



Besides the beaches of the south shore, there are others on the north shore 

 — as at Siielly Bay — and in Castle Harbor, — as at Tucker's Town and 

 at Cooper's Island, — all of which consist of shell sand of the type already 

 described, with the addition of a large proportion, probably 25 per cent, 

 of very fine calcareous dust derived from the destruction of the neighbor- 

 ing cliffs and ledges. Here there is little or no surf action, and the slope 

 of the beaches is not nearly so steep, being only about five degrees. 



In the second class of localities fall also the landlocked lagoons or 

 sounds. These are all more or less nearly enclosed bodies of water, of 

 considerable size and moderate depth, standing in communication with 

 the sea through several passages. The most important are Castle Har- 

 bor, St. George's Harbor, Harrington Sound, Great Sound, and Little 

 Sound. It is generally agreed that they originated as depression.s, or 

 sinks, whose walls became broken .through by denudation and the me- 

 chanical action of the sea. Several such sinks, now in process of for- 

 mation, are occupied by marshes or marshy ponds, an example being 

 I.'evonshire Marsh. Some, perhaps many, of the channels which connect 

 the sounds with the open sea originated as partially subterranean channels, 

 such as to-day connect Harrington Sound with the surrounding waters. 



