BIGELOW. — SHOAL-WATER DEPOSITS OP THE BERMUDA BANKS. 563 



the sea, and he has shown beyond question that the reefs and ledges are 

 composed of aeolian limestones, being the remnants of bluffs or cliffs, 

 uiidercut and demolished by the sea, a process now taking place actively 

 on tlie south shore. 



The most important point to be borne in mind in any study of the 

 physiography of the Bermudas is that the ring-like outline of the reefs 

 bears no relation whatever to an atoll ; and that Bermuda is not in any 

 ' true sense a coral island, although coral grows there in some profusion. 

 There is no coral reef on the plateau ; and we have no reason to sup- 

 pose that Bermuda ever showed any more of the characters of an atoll 

 tlian it does to-day. Coral fragments are rather rare in the aeolian rock, 

 though other organic remains are recognizable in abundance. Mr. Agassiz 

 says ('95, p. 228) tliat he never saw a piece of coral in Bermuda above 

 high-water mark where its presence could not be explained by storm 

 winds or waves. The living corals have added very little to the vertical 

 height of the reefs, forming merely a thin covering, which, with the 

 encrusting masses of serpula tubes, protects the limestone of the ledges 

 against the attacks of the surf. 



Although the general features of the Bermudas, both geographic and 



geologic, are so well known, but little attention has been paid to the 



bottom deposits. Almost the whole information which we possess on 



this subject is contained in the brief accounts of the Challenger expe- 



vdition and of Mr. Agassiz's explorations. 



Murray and Reynard ('91, p. 48) analyze four samples of coral sand 

 and mud from as many localities inside the reef, and find that calcic car- 

 bonate composes the following proportion of the deposits. No. 1, from 

 the northern half of Murray Anchorage, in nine and a half fathoms, 

 95.43 per cent ; No 2, near the Ship Channel, in five fathoms, 91.09 per 

 cent; No. 3, on the Cow Ground Flat, 90.18 per cent; and No. 4, in 

 Murray Anchorage, in six fathoms, 86.77 per cent. In three of the 

 samples calcareous algae and their broken-down parts composed over 

 50 per cent of the mass, and Foraminifera, 10 to 30 per cent, the bal- 

 ance consisting of serpuline, gasteropod, and lamellibranch fragments, 

 corals and millepores, ostracods, and plates and spines of echinoderms ; 

 1 to 2 per cent was of mineral origin, fine particles of quartz and mica; 

 and 2 to 10 per cent was fine flocculent amorphous dust. Siliceous 

 remains are in small amount. These authors especially emphasize the 

 importance of algae as sand builders. Wyville Thomson ('77) describes 

 the bottoms in from four to ten fathoms as coral muds and sands, com- 

 posed of the triturated fragments of algae, corals, and polyzoans, together 



