288 BULLETIN 103, UNITED STATES NATIONAL MUSEUM. 



The great extent and relatively uniform height of a coral-reef 

 terrace between 30 and 40 feet above sea level favors the interpreta- 

 tion that the geologically Recent shift in position of strand line has 

 been without pronounced crustal deformation. 



The relations of the off-shore reefs to the platforms on which they 

 grow will now be briefly considered. A detailed description of the 

 reefs is unnecessary here, as it would be only a repetition of that 

 already given by A. Agassiz ^ and the accounts contained in the 

 West Indies Pilot.^ It need only be stated that the best developed 

 off-shore reefs on the north coast are the Colorados Reefs, between 

 Bahia Honda and Cape San Antonio ; and that off the south coast 

 the best are those between Trinidad and Cape Cruz and those east 

 and west of the Isle of Pines. Mr, John B. Henderson has devoted 

 attention to the Colorados Reefs in his ''Cruise of the Tomas 

 Barrera." Have the reefs ofi the south coast grown up on the 

 surface of preexistent platforms or are the platforms due to infilling 

 behind a reef during subsidence ? 



The area between Trinidad and Cape Cruz will be considered 

 first. The fact that the reefs form disconnected hillocks or mounds, 

 sometimes of mushroom shape, above a plain surface, which in places 

 is 50 miles wide along a line perpendicular to the shore, while on 

 the seaward side of the reefs there are large areas of shallow platforms, 

 without any margining reefs, seems conclusive evidence against the 

 platform having been caused by infilling behind reefs. 



The following, in my opinion, is the correct explanation: The lit- 

 toral geologic formations from Cape Cruz to Trinidad are mostly 

 upper Oligocene or Miocene marls and limestones which dip under 

 the sea at relatively low angles. They dip into the Cauto Valley, 

 which is a gently pitching syncline, and into its seaward continua- 

 tion, the Gulf of Guacanayabo. The embayment northeast of Boca 

 Grande passage is probably also synclinal in structure. The abrupt 

 undersea termination of the platform is most reasonably explained 

 by a submarine fault which runs from Punta Sabanilla, at the mouth 

 of Cienfuegos Harbor, to Cape Cruz. The coral reefs have grown 

 up on the surface of a plain underlain by geologic formations that 

 were gently tilted seaward and faulted along the line indicated. 



That the Isle of Pines was joined to Cuba during Pleistocene time 

 has, I believe, been shown in a convincing manner. As the Mocene 

 and upper Oligocene formations from Batabano to Pinar del Rio dip 

 • under the sea at low angles they must underlie the flat bottom of 

 the Gulf of Batabano. That the submarine slope from East Guano 

 Key to off Cape San Antonio is determined either by a fault or by a 

 very steep flexure is clearly indicated, as off the south shore of the 



- 1 Bull. Mus. Comp. Zool., vol. 26, pp. 133-136, 1S94. 



2 West Indies Pilot, vol. 1, pp. 199-332, 1913 (U. S. Hydrographic Office). 



