Vol. 51.] AND PHYSICAL GEOLOGY OF THE WEST INDIES. 299 



sea deposits in the Mediterranean Miocenes, they may have been 

 laid down in isolated basins formed at different times by the sub- 

 sidence of various portions of the area to the required deptb. A 

 final decision upon this point cannot be arrived at until adequate 

 collections have been made from the beds above and below the 

 thalassic deposits, wherever these are found. So far as the evidence 

 goes at present, it is clear that their relations to the shallow-water 

 deposits are much the same wherever they occur, and that they 

 occupy, at least approximatively, a corresponding position in tbe 

 West Indian sequence. Thus, according to Prof. Harrison and 

 Mr. Franks, 1 the radiolarian marls in Trinidad unconformably over- 

 lie the San Fernando or Naparima (and possibly also the Nariva 

 Beds), just as the Oceanic Series in Barbados does the Scotland Beds. 

 Prof. Crosby's verbal description of the position of tbe Baracoa 

 marls in Cuba shows that they are there covered by the raised coral- 

 reefs just as are those of Barbados. In Trinidad they are either 

 Miocene or post-Miocene ; in Cuba they appear to be interstratified, 

 or closely associated, with Miocene deposits. These Cuban beds 

 are probably on the same horizon as the ' Pteropod Marl,' which 

 represents the deep-sea series in Jamaica. It is generally regarded 

 as Pliocene or even Pleistocene, 2 but this view does not rest on 

 precise or reliable observation. 



The Submergence of Panama. — There is another line of argument 

 which affords important help. Tbe Caribbean Sea is separated 

 from the Pacific by a ridge in places less than 50 miles broad, and 

 the summit of which is in one locality 154 and in another 287 feet 

 in height. A subsidence in this region of anything like the amount 

 that happened in Barbados would have submerged the whole 

 isthmus, and allowed the Atlantic and Pacific faunas to inter- 

 mingle. Now the evidence of the fossil mammalia of the Western 

 States of America conclusively demonstrates that in some part of the 

 Kainozoic period, North and South America were separated one from 

 the other in this way. As to the date of this separation opinions 

 differ greatly. One school of glacial geologists have seen in it a 

 chance of explaining the cause of the refrigeration of the climate of 

 the North Atlantic in the ' Ice Age ' ; for, through the channel 

 thus formed, the warm waters piled up in the Gulf of Mexico by 

 the main ' Equatorial Current ' could pass into the Pacific, instead 

 of flowing north as the Gulf Stream. For the diversion of the ocean- 

 currents of the Atlantic to be of any use for this purpose, it must 

 have happened in Pleistocene times. That date has therefore been 

 assigned to the submergence of the Isthmus of Panama. 



The evidence quoted in support of this view is twofold : — 1st, the 



1 Quart. Journ. Geol. Soc. vol. xlviii. (1892) p. 218. 



2 Jukes-Browne and Harrison, ibid. p. 221, discuss the question and conclude 

 that ' the final opinion of the Surveyors with respect to both these [the White 

 Limestone and the Pteropod Marls] seems to hare been that they are post- 

 Pliocene, or in other words Pleistocene.' 



