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ME. JUKES-BROWNE AND PROF. HARRISON 



deeply submerged the bulk of tbe Gulf Stream water would still be 

 deflected instead of passing through the opening. As the Gulf Stream 

 is a surface-current, and its depth where crossed by the 'Challenger' 

 in May 1873 was ascertained to be 100 fathoms, it seems safe to 

 conclude that a fairly wide gap 600 feet deep in the centre would 

 be sufficient to give it passage. But we think it highly probable 

 that the depth of water over parts of Panama, Costa Kica, and 

 Nicaragua was much more than 100 fathoms. Some of the 

 Pliocene limestones of Jamaica may well have been formed in 400 

 to 500 fathoms, and as they have been raised to levels of between 

 2000 and 3000 feet above the sea they may be taken to indicate 

 an upheaval of from 4800 to 6000 feet (800 to 1000 fathoms). If 

 anything like this change occurred also in Central America very 

 little of it could have been above the sea-level in Pliocene time, for 

 very little of it is now 1000 feet above that level. 



Our belief that there was free communication between the 

 Atlantic and Pacific at a late period in Tertiary time is strengthened 

 by the very nature and contents of the raised Oceanic Deposits. 

 Radiolarian ooze is at the present time specially characteristic of 

 the tropical parts of the Pacific Ocean. It occurs chiefly between 

 the depths of 2300 and 2900 fathoms, but as both red clay and 

 Globigerina-ooze occur at the same depths its formation is clearly 

 dependent on other conditions besides those of depth and surface - 

 temperature. The fact that these three different kinds of oceanic 

 ooze all occur in the Pacific and replace one another at these depths 

 suggests that the Oceanic Deposits of Barbados which include the 

 same three varieties were formed in water of about the same depth 

 (2000 to 3000 fathoms), and strongly supports the view that the 

 Caribbean Sea was then as much a part of the Pacific as of the 

 Atlantic Ocean. 



It may fairly be asked, however, how this could be if at that 

 time the Atlantic Equatorial current traversed the Caribbean Sea 

 and set into the Pacific. Would not the organic deposits of this 

 sea be still likely to have an Ailantic aspect ? To this it may 

 be replied that the Equatorial current would doubtless pass 

 over the southern part of Central America, south of the high plateau 

 of Guatemala, and that there would probably be an incurrent stream 

 from the Pacific over the Isthmus of Tehuantepec bringing Pacific 

 water into the Gulf of Mexico and the Antillean area. This would 

 introduce the radiolaria which now swarm in the Pacific, and they 

 would doubtless flourish in the warm waters of the intra-American 

 Seas. 



At the present time, though the greater part of the Caribbean 

 Sea is over 2000 fathoms deep, no radiolarian ooze is found upon its 

 floor. The deposit now accumulating in this sea is a Glohiyerina- 

 ooze of a reddish-brown tint, while in the shallower waters around 

 the Caribbean islands a white pteropod ooze is the usual deposit. 

 Both often contain radiolaria, but never in any considerable 

 quantity. 



These facts show how great a change has taken place in the 



