16 PHYSIOGRAPHY AND GEOLOGY 



seolian and marine rocks is shown more distinctly than in any of the other 



localities visited hy the Expedition. Along a canal which has been cut to 



admit water in a salt pan, some excellent sections are shown. The lower 



part of the section contains great quantities of marine fossils, while above, 



land forms are equally abundant. These two deposits are not separated by 



any pronounced break, but if the section could be traced for any distance it 



is probable that an unconformity would appear. 



The surface of the banks is everywhere covered with a snow-white mantle 



of calcareous sand and mud derived chiefly from the erosion of coral reefs. 



In the region south of Tongue-of-the-Ocean these sediments are heaped into 



sand bores by the currents. In the bights and along the A\'estern shore of 



Andros, as well as along the western shore of Abaco, on the l)ottom of Wide 



Opening and other places, there is a peculiar deposit of finely divided calcareous 



mud known as '*' white marl." This has the consistency of chalk and may, in 



fact, be considered to be a modern chalk deposit. The following is an analysis 



of this white marl from Middle Bight, Andros, which the Bureau of Soils, U. S. 



Department of Agriculture, has been kind enough to furnish : 



Potash (K.O) 0.306 



Soda (Na,0) 2.12 



Lime (CaO) 47.50 



Magnesia (MgO) 2.85 



Iron and Alumnia (Fe & Al) trace 



Nitrogen (N) 0.054 



Phosphorus pentoxide (PoOJ 0.123 



Sulphur trioxide ( SO3) 0.37 



Chlorine (CI) 2.97 



Silica (SiO,) 3.22 



Carbon dioxide (CO,) 40.48 



99.993 



Organic Deposits. 



Throughout the Bahama Islands coral polyps are actively engaged in 

 building up fringing reefs against the shore, while coral heads in both isolated 

 and small clusters are scattered promiscuously over the more sheltered l)anks. 

 As these were discussed in a previous section, they will not be taken up further 

 in this place. There is, however, on the little island of Green Cay, situated on 

 the southeast margin of Tongue-of-the-Ocean, a dead reef which lies per- 

 manently above the level of low tide. This reef, which extends along the shore 

 for about a quarter of a mile, is composed of a number of different species 

 of corals and extends in toward the center of the Island, where it becomes 



