248 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



disconnected from it but comparatively recently, gives us the key to 

 the formation of coral heads and of the ledge flats. Many of these 

 patches are not as yet covered with coral growths of any kind, and their 

 origin can still be plainly traced. From these we pass to more distant 

 patches, in somewhat deeper water, on which Millepores, Corgonians, 

 Algae, and corallines have begun to obtain a foothold, but of which the 

 ledge structure is still apparent. Some of the ledges and patches on the 

 north side of Spanish Point show admirably the passage from the jeolian 

 rock cliffs, which have fallen into the sea covered only with a thin coat- 

 ing of Alga?, to ledges with a more abundant growth of Corallines and 

 Algae, and finally to patches with corals and Gorgonians at a greater 

 distance from the shore. 



The mouth of Wreck Bay is protected by a number of islands running 

 across the opening of the harbor, the remnants of the land which once 

 connected Somerset Island with the main island. The rocks and islets 

 to the westward of Mangrove Bay are the continuation to the south of a 

 series of ledges which connected it once with Ireland Island. Traces of 

 the proto-Bernuulian land are found in the many patches of ledges to 

 the westward of Ireland Island which extend towards Green Flat and 

 thence to the west of Mangrove Bay, reaching out to Cow Ground Flat. 

 The patches between Wreck Bay and Daniel Island reach out to Elies 

 Flat and connect with the Chub Cut Flat. They are the northern 

 boundary of an extensive sound bounded on the south and west by the 

 flats reaching to the westward of Hogfish Cut, and sweeping northerly 

 east of the Chub Heads to join the Western Ledge Flats. Similarly, 

 south of Chub Cut, Elies Flat is the sunken boundary of a smaller sound 

 bounded on the north by the western extension of the Cow Ground Flat. 



On the western ledges we find large patches of sand intervening be- 

 tween the ledges on which corals grow ; sometimes these sand patches 

 form long sand bars with coral-bearing ledges only on the windward 

 edges, the lower ledges having been triturated into sand which is more 

 or less shifting according to the direction of the wind. 



The formation of the sand flats from the disintegration of the a?olian 

 rock ledges shows how little material the corals have supplied to form 

 the flats ; they often come up close to low-water mark, and yet no coral 

 sand islets have been formed anywhere on the ledge flats, either near the 

 outer reefs or on the interior flats. These sand patches gradually pass 

 into the ledges forming the outer flats, where the coral growth is most 

 abundant and gradually diminishes on the sloping ledges of the sea face 

 to a depth probably of twelve fathoms. The finest corals and Gorgonians 



