BIGELOW. — SHOAL- WATER DEPOSITS OF THE BERMUDA BANKS. 583 



mixture of marl, and of various sorts of silt, calcareous and siliceous 

 spicules, and the like. The organisms chiefly active in the formation of 

 the shell sands are corallines, mollusks, tube-building worms, millepores, 

 and Foraniinirera, varying in their relative importance in different 

 localities. Algae probably form the greatest mass of the sand, the most 

 important genera being those described from the Castle Harbor sands. 

 Large sand-dwelling Foraminifera form nearly half of the entire bottom 

 deposits in certain restricted localities, the most abundant forms being 

 Orbitolites duplex, Orbiculina adunca, Bulimina, Cornuspira foliacea, 

 Pulvinnlina menardii, Textularia concava, T. luculeuta, Ammodiscus 

 tenuis, and a species of Trochammina. 



The chief deposits of shell sand occur in rather shoal water, or in 

 the channels or other places where there are tidal currents, or at least 

 strong currents of water. They may mark areas either of the growth 

 of organisms or of deposition ; the former being localities, such as parts 

 of Castle Harbor and Castle Roads, where sedentary organisms, chiefly 

 corallines, Foraminifera, and mollusks, flourish in the sand. It is in 

 such places that the larger Foraminifera reach their greatest abun- 

 dance. But these localities of growth are comparatively rare and re- 

 stricted, by far the greater part of the shell sands marking areas of 

 deposition of the coarse detritus from the reefs and ledges and from 

 the littoral zone. Such sands are coarsest along shore, for example 

 off St. Catherine's Point, and on the shoalest bars, as in Great Sound, 

 where the action of the surf and of currents reaches its maximum ; they 

 decrease steadily and regularly in coarseness as the depth increases, until, 

 as in Hogfish Cut, they may be very fine indeed, though still washed 

 clean of fine ooze by the tides. In these sands, built of the organic 

 fragments from the neighboring ledges, corallines and millepores are of 

 the greatest importance, while mollusks and Foraminifera constitute but 

 a small portion of the deposits. The reefs, as in true coral islands, con- 

 tribute far more to the sands of the las;oon than do the sand-dwellinor 

 organisms, despite the local abundance of the latter. Heilprin ('89), it is 

 true, thought that " all the sand being formed at present is derived from 

 the destruction of existing land masses," that is, either marl or coarser 

 limestone detritus ; but I believe that the organic mains, which are con- 

 stantly settling down from the reefs to the bottom, are in much greater 

 bulk. It is, however, exceedingly difficult to estimate the relative im- 

 portance of the marl and the shell-sand deposits over the plateau, since 

 they always occur together, though in varying proportions. No doubt 

 the formation of marl over many parts of the plateau is rapid ; so rapid 



