AGASSIZ: BERMUDAS. 253 



outside of the reef to a depth of twenty fathoms which are marked 

 rocky {>•). These spots are most probably the outcrops of £BoHan ledges 

 of the proto-Bermudiau hill lands projecting slightly above the sandy 

 bank bottom and forming a part of the broken ground. Beyond that 

 depth (twenty fathoms) the lead brings up what is called coral bottom, 

 made up in great part of a^olian sand and of fragments of Corallines, 

 Algjie, and the like. Tliere are also patches of this rocky bottom inside 

 of the reef ledges, as for instance close to the Western Blue Cut, where 

 all the hauls of the dredge only brought up small quantities of the bank 

 sand bottom. 



The bottom over the Bermuda Bank is quite uniform in character. 

 The greater part of it is covered with seolian sand of different degrees 

 of coarseness, and more or less mixed with fragments of coralline Algse 

 and of Millepores or Gorgonians. 



In other localities the surface of the old seolian rocky ledges is exposed, 

 and is comparatively bare of seolian sand, as in some of the sounds, and 

 the bottom may be called rocky. On this Oculinse grow in profusion in 

 the deeper waters of the sounds, or the more massive corals where the 

 sea has free access to the sounds. 



To the westward of Wreck Hill there is a small extent of bottom in 

 seven fathoms of water covered with very fine mud, much like the white 

 marl off Andros. A similar patch of marl occurs to the eastward of 

 Ireland Island. 



THE SERPULINE REEFS. 



Plates XXI. to XXVI. 



The serpuline reefs described by previous observers are perhaps the 

 most interesting structures of the Bermudas. Thev are most numerous 

 off tlie south shore, constituting miniature atolls and barrier and 

 fringing reefs apparently formed by the upward growth of SerpuL-p. 

 While Serpulse undoubtedly cover a great part of the surface of the 

 structures, yet Algse, Corallines, barnacles, mussels, and other inverte- 

 brates, are found to be fully as abundant as the Serpulee, which in many 

 cases play only a secondary part in the organic covering. In fact, it 

 would be as ci^rrect in some localities to call them Algse or Coralline 

 atolls. Neither the Serpuhe nor the A]ga\ nor any other organisms, 

 have to any considerable extent built up the vertical walls of the differ- 

 ent kinds of diminutive reefs so charactenstic of the south shore. The 



