THE GREAT FAULT TROUGHS OF THE ANTILLES 113 



dence, namely: (i) The faulting along the south coast of Cuba is 

 subsequent to old or middle Miocene, as the Miocene La Cruz 

 marl is abruptly cut off at the shore; (2) the sea has had time to 

 cut only narrow benches into the fault scarps that form shore hnes; 

 and (3) the biological evidence demands land connection in late 

 Tertiary time between Cuba, Santo Doniingo, Porto Rico, and 

 thence to South America.' 



The faulting may date from the Pliocene, as suggested by 

 Vaughan but the frequent occurrence of earthquakes along the 

 fault zones proves that the displacements are still continuing; and 

 the sea-waves, which have accompanied nearly all of the great 

 earthquakes with the exception of those originating in the Haitian 

 land valleys, indicate that these latest displacements have also 

 been vertical. Another evidence of recent displacements is 

 furnished by the character of the material on the sea-floor along 

 the fault zones. Off the northwest coast of Porto Rico broken 

 coral is found at a depth of 1,864 m. and the bottom is ''rocky" 

 at 3,221 m.^ 



The Antillean fault system, whether it continues into Central 

 America or not, forms one of the major structural features of the 

 earth; and the displacements now going on are similar to those 

 which have occurred in the past. The earthquakes are not limited 

 to the immediate vicinity of land areas and hence cannot be attrib- 

 uted to adjustments resulting from changes in surface-loading 

 due to erosion. That the faulting extends to profound depths is 

 shown by the persistence of the fault zones," by the great height of 

 the scarps, and by the extrusion of lava from some of the fault 

 fractures. 



The total volume of lava that has reached the surface along the 

 Antillean fault zones is insignificent as compared with the size of 

 the troughs; consequently there can be in this instance no causal 

 relation between volcanic activity and the foundering of the narrow 

 blocks, such as has been suggested for the Chris tiania Fiord in 

 southern Norway, the "rift valley" of eastern Africa and similar 

 troughs elsewhere. 



^T. W. Vaughan, "Geologic History of Central America and the West Indies 

 during Cenozoic Time," Bull. Geol. Soc. Amer., Vol. XXIX (1918), p. 626. 

 = United States Coast and Geodetic Survey Chart 901. 



