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 aeolian ledges 

 of the proto-Bermudian 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 aeolian sand and of fragments of Corallines, 

 Algae, and the like. There 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 aeolian sand of different degrees 

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

 and of Millepores or Gorgonians. 



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

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

 the bottom may be called rocky. On this Oculinae 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. They are most numerous 

 off the south shore, constituting miniature atolls and barrier and 

 fringing reefs apparently formed by the upward growth of Serpulae. 

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

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

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

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

 would be as correct in some localities to call them Algae or Coralline 

 atolls. Neither the Serpulae nor the Algae, nor any other organisms, 

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

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



