4 PHYSIOGRAPHY AND GEOLOGY 



from corals and other calcareous organisms and rest on a shallow, submerged, 

 platform, which is separated by deep submarine troughs from the neighboring 

 land-masses of North America and the West Indies. 



PREVIOUS INVESTIGATION. 



Although there has been considerable written about the Bahamas in books 

 of travel and in popular magazines, this group has received less careful geologic- 

 al stud_y than almost any other portion of the West Indies. 



For our knowledge regarding the form of the submarine bottom on which 

 the Bahamas rest, and its relation to North America and the West Indian 

 regions, we are chiefly indebted to the excellent charts published by tlie British 

 Admiralty, the U. S. Coast and Geodetic Survey, and the IT. S. Hydrographic 

 Office. These charts, by indicating a large number of soundings, bring out 

 very clearly the figure and character of the platform from which the Bahama 

 Islands rise. 



Capt. R. J. Nelson, E. E., was the first to adequately describe these Islands 

 and to bring their true nature to the attention of geologists.' He regarded 

 them as composed of calcareous sand which had been thrown up by the waves 

 to form beaches, and later picked up by the winds and piled into dunes. He 

 saw no evidence of either uplift or subsidence during recent time, and concluded 

 that the Islands had remained stationary in their position during the present 

 epoch. In this connection he says : " Whatever may be the real foundation of 

 the Bahamas, whether, like the West Indian Islands generally, they are indebted 

 to igneous agency for their existence as elevated masses, or otherwise, there is 

 no evidence of such elevation having taken place either in the Bahamas or 

 Bermuda. On the contrary, the total absence of coral-reefs in mass, or even 

 of detached coral blocks, above the tide-line leads us to the supposition that no 

 upheaval has taken place during the present epoch. . . . The fact of detached 

 blocks of coral being found in the rock at considerable distance from the sea- 

 coast at the tide-level, proves that no subsidence has taken place during the 

 present epoch. Conch-shells also, either dispersed or in beds, are found by the 

 well-diggers in the solid rock at about the sea-level, and thus bear evidence 

 to the same fact." ° It was from this paper of Capt. Nelson that Darwin and 

 Dana drew their facts when later they described the Bahamas in their discus- 

 sions on the origin of coral islands. 



' On the Geology of the Bahamas. Quar. Jour. Geol. Soc. London, 1853, vol. ix, 

 pp. 200-214. 



■hoc. cit., pp. 212-213. 



