THE BAHAMA ISLANDS 17 



covered by sand dunes. (Plate VIII.) It is the only instance of the kind 

 which was discovered in the Bahamas, but it is evident that when this reef was 

 formed it occupied a lower level than at present, and that it has been raised 

 to its present position since its formation. 



Structure and Age. 



Xowhere were these sedimentary beds observed to lie otherwise than in a 

 horizontal position. Cross-bedding due to wave and atmospheric action was, 

 of course, prevalent, but this did not interfere with the general horizontal 

 attitude of the deposits. Although the marine beds have been elevated since 

 their deposition, they now occupy a position a few feet above and parallel to the 

 one in which they were laid down. From the fossil contents of both the 

 ffiolian and aqueous beds, Dr. Dall has determined that the deposits are Eecent. 



Erosion. 



Signs of erosion are visible on every side throughout the Bahama Islands. 

 In fact, it is evident that processes of destruction are much more important 

 here than those of construction. The upper surface of the limestone rock out 

 of which the Bahamas are built shows everywhere signs of solution. The 

 solving agents are both carbon dioxide brought down by the rain from the 

 atmosphere, or humic acids contained in the soil. Wherever these reagents 

 attack the rock a differential erosion tak«s place, the softer parts are dissolved 

 and carried away, leaving irregular cavities which in time fill with soil and 

 form the pot-holes mentioned above. In other places where the solution has 

 not advanced so far, a rubbly or rough surface is the result. These results, 

 however, are insignificant when compared to the more important work of 

 underground waters which during some period in the past, when the region 

 stood higher than to-day, dissolved the subterranean rocks of the Islands 

 and left them in a cavernous and honey-combed condition. The ocean-holes, 

 banana-holes, and many of the brackish pools mentioned above are due to this 

 subterranean solution. 



As there are no true rivers in the Bahama Islands the mechanical erosion 

 is practically confined to the work of waves. The Islands situated as they are, 

 well out in the Atlantic are subjected to a perpetual attack from the sea. A 

 glance at the map (Plate X) will suffice to convince one that the whole 

 archipelago has suffered severely from the effects of wave erosion. The Islands 

 are being slowly worn away and broken up into keys and rocks and these in 

 2 



