272 BULLETIN: MUSEUM OF COMPAEATIVE ZOOLOGY. 



division walls of the irregular pot-holes of the ledge, and in places they 

 overhang to a very considerable extent. The coating of Algge and of 

 Serpulae on some of the walls of the interior pools is quite thin, as upon 

 breaking off the edges we came upon the ringing aeolian rock. On the 

 northeast edge of the ledge a deeper pot-hole has formed a regular bar- 

 rier reef, the edges of which are covered by Serpulae, Algae, and Corallines. 

 On the interior of the ledge their growth is less vigorous. Outside of 

 North Rock there are a few ledges, both to the southwest and to the 

 northeast. Those towards the inner side of the reef are the outer reef 

 patches, on which grow Millepores, Gorgouians, and the usual growth of 

 corals, Algae, and Corallines, which have completely hidden the nature of 

 the ledges. The ledges which form the continuation of North Rock to 

 the east and west, on the contrary, still plainly show that their structure 

 in no way differs from that of the North Rock Ledge. Outside of North 

 Rock " broken ground " extends to ten or twelve fathoms, but no patches 

 of corals could be seen bevond five or six fathoms. We mitjht call the 

 outer ledges of the reef near North Rock a flat of coral heads and of 

 Gorgonians. On the North Rock Ledge fiat, Zoanthus, Millepores, Algae, 

 and Corallines are most flourishing. 



Winding our way towards St. George Island from the North Rock, 

 we picked our passage between the many mushroom-shaped ledges. 

 Many of them came to the surface or nearly so, and on two of the ledge 

 patches rather nearer the inner line of the ledge flats than to the North 

 Rock I broke off a piece of hard ringing aeolian rock in every respect 

 similar to that of the North Rock Ledge flats. 



We could not have a better example of the true nature of the reef 

 ledge flats and patches than is exhibited by the North Rock Ledge and the 

 adjoining patches. Those of the ledges nearest to the North Rock show 

 the hard ringing aeolian rock which marks the North Rock Ledge ; some 

 of the patches aire separated from it by water five to ten fathoms deep. 



We can easily imagine the whole of the ledge flats of the vicinity 

 to have been made up of ax)lian rock ledges and pinnacles, very much 

 like those off the north shore, on which corals, Gorgonians, and the like 

 have little by little become attached, and have finally grown over not 

 only the sides, but the upper surface after it became eaten away well 

 below low-water mark. 



There may have been on the northern reef flat ledges lower lines of 

 hills than those now existing on the Bermudas, or low hills like those 

 separating St. George and Castle Harbor, or along the west end of St. 

 George Island and on the north side of Harrington Sound. 



