STRUCTURE OF THE TURKS ISLANDS 269 



condition of the island when large lagoons occupied its interior. 

 Whilst a region of sand-dunes lies behind the beach at the north end 

 of the island, illustrating the constructive action of the waves and 

 of the winds, extensive bare rocky surfaces in the interior represent 

 the work of the coral reef. The same features are presented by 

 Cotton Cay. Here again islets of seolian sandstone, now existing as 

 elevations, thirty to forty feet high, at its two extremities, have 

 been joined by the reclaiming agencies of the waves and of coral- 

 reef growth ; but the lagoons have long since been cut off from the 

 sea, and are now represented by shallow ponds in the centre of the 

 island. 



No Recent Change of Level in the Turks Islands. — Of 

 changes in level, whether of elevation or of subsidence, I found no 

 evidence in this region. No old erosion lines came under my notice, 

 except in one or two places where it was apparent that they had 

 been merely cut off from the sea by the intervention of a beach or of 

 a reef-flat. That the level of the sea has remained much the same 

 for a long period is indicated by the circumstance that the platform 

 of reef-rock and ordinary beach-sandstone, on which the seolian 

 sandstone reposes, has been formed with the sea at its existing 

 level. 



Probable Destruction in Great Part of the Original 

 Islands of the Turks Group. — There must have been great 

 destruction of the original islands of seolian sandstone before the 

 modern process of reclamation was made possible by the shoaling 

 of the great bank from which the present islands rise. The low hills 

 and ridges of the larger islands and the small rocky cays are but the 

 scanty remains of what may have been two or three large islands of 

 seolian rock that occupied almost all the bank. At a uniform depth 

 of nine or ten fathoms the bank extends seven or eight miles eastward 

 of Eastern and Pear Cays, and then plunges down into the depths. 

 All this submerged area was once covered by land of seolian forma- 

 tion. The same may be said of the two long tongues that extend 

 north and south from the opposite extremities of the group at a 

 depth of about ten fathoms for about seven miles in each case. The 

 same is also true of the inter-island tracts of shallower water. All 

 this land has disappeared. 



The same has been inferred by A. Agassiz for the whole 

 Bahamian Region. — All the questions raised by the consideration 

 of the Turks Islands are issues raised in connection with the seolian 

 rocks of the Bahamas as a whole. The original much greater extent 

 of the land-surface of seolian rocks, which has been postulated by 

 me in the case of the Turks Islands, was assumed twenty years ago 

 by A. Agassiz for the whole of the Bahamian archipelago. Before 

 the last subsidence of 300 feet, which, as he infers, affected the whole 

 region (a point on which the present writer, as already shown, is not 

 in agreement with him), the present banks were occupied, as he 

 holds, by " one huge irregularly shaped mass of low land." How- 

 ever, as I have pointed out, several of the eastern banks are now 

 separated by passages 1000 to 2000 fathoms deep. We cannot, 

 therefore, postulate for that period a continuous land- surface over 



