GEOLOGY AND PALEONTOLOGY OP THE CANAL ZONE. 609 



genera became extinct in the Atlantic during upper Miocene time, 

 according to the present information, but, they persist in the Indo- 

 Pacific region. It, therefore, seems that the fauna of Carrizo Creek 

 migrated to the head of the Gulf of California after the extinction 

 of these forms. 



PLIOCENE AND LATER. 



Subsequent to the Miocene there have been many oscillations 

 of the West Indian area, and during perhaps Pliocene time there 

 was profound deformation. Zeogeographic data in the opinion of 

 several investigators seem to demand former connection, prob- 

 ably during late Miocene or Pliocene time from Yucatan across 

 Cuba to Haiti, Porto Rico, and the Virgin Islands; from Honduras 

 to Jamaica; and from Anguilla to South America. It also 

 appears that St. Croix was once joined to Anguilla and to the 

 eastern end of the Virgin Islands. There are certain geologically 

 late fault-lines which perhaps date from this time and the 

 severance of the old ridges into the islands we now know may be 

 largely due to movement along them. One of these fault lines forms 

 the northern boundary of the Bartlett Deep, and passes between 

 the east end of Cuba and the west end of Haiti. Another tectonic 

 line which forms the south side of the Bartlett Deep seems to con- 

 verge toward the former in the Windward Passage. A down-thrown 

 block between these lines has separated Cuba and Haiti and pro- 

 duced the Bartlett Deep. Probably there was also faulting or 

 flexing between Cayman Ridge and the southern shore of Cuba, west 

 of Manzanillo Bay, while either faulting of flexing may have sep- 

 arated Cuba and Yucatan. There is evidence of a downthrown 

 fault block between St. Thomas and St. Croix, the two sides converg- 

 ing toward Anegada Passage. This will account for the deep of over 

 2,400 fathoms north of St. Croix, and the severance of St. Croix 

 and the St. Martin Plateau group of islands from the Virgin group. 



There are three kinds of evidence that bears on the age of this 

 faulting, namely: (1) In eastern Cuba, as the Miocene La Cruz 

 marl is abruptly cut off at the shore line in the vicinity of the Morro 

 at the mouth of Santiago Harbor, the faulting must be subsequent 

 to old or middle Miocene; (2) as the sea along the fault shores has 

 been able since the faulting to cut only narrow benches into the fault 

 planes on the up- thrown side, the faultplanes are physiographically 

 young; (3) the biologic evidence, in the opinion of most of those 

 who have recently considered it, demands land connection in late 

 Tertiary time between Cuba, Santo Domingo, Porto Rico, and thence 



