14 PHYSIOGRAPHY AND GEOLOGY 



flows with the tide. Others have do such connection and remain dry except 

 when filled with rain-water. These holes range downward in size until they 

 merge with the general rough surface of the country. The smaller ones 

 having a depth of 5 or 10 feet usually contain rich soil and are used to grow 

 bananas ; hence the name. 



Throughout the Bahamas the surface rock is extremely rough and is 

 carved into innumerable cavities and holes. When these openings are about 

 the size of a flower pot they are known as " pot-holes " ; although they are 

 due to solution and not to mechanical erosion. These pot-holes are filled 

 with rich soil and in them the pineapple plant is cultivated. 



GEOLOGICAL FEATURES. 



The Bahama Islands are built out "of sediments derived from the destruc- 

 tion of coral and other calcareous organisms. Nothing is known regarding 

 the geology of the platform on which these Islands rest, but, if it is similar 

 to other ridges of the West Indies, its geology is complex and the veneer of white 

 calcareous sand simply covers it like a shroud. These sedimentary rocks, out 

 of which the Islands are built, have been deposited by three agencies, air, water, 

 and. organisms. Each one of these types will now be considered. 



^OLiAx Deposits. 



These deposits include the dunes and rock-ridges described in a previous 

 section. These dunes are composed of half solidified calcareous sand blown 

 lip from the sea-shore, while the ridges of hard rock are ancient dunes cemented 

 to solid limestone. They both contain fossil land-shells and have a cross- 

 bedded structure. This cross-bedding may be seen wherever the rock has 

 been excavated. The fresh surfaces thus exposed weather quickly, but as the 

 various layers do not decay at the same rate, the cross-bedded structure is 

 brought out in a striking manner. Such exposures may be seen in the various 

 quarries of Nassau and in the approach to the bridge leading to Government 

 House (Plate IV). This cross-bedding can also be seen from the sides of the 

 road where the highway leading to the south passes over the summit of Blue 

 Eidge (Plate. 11, Fig. 3). 



Fossil land-shells are frequently discovered in these seolian deposits. A 

 representative collection was secured and submitted to Dr. Dall who has dis- 

 cussed it in the succeeding chapter. The manner in which these shells 

 are entombed is most interesting. The surface of the dunes supporfe a scant 



