112 STEPHEN TABER 



In opposition to this view it may be urged that the east and 

 west ends of Porto Rico are rather irregular; that there is neither 

 topographic nor seismologic evidence of faulting; and that the 

 absence of Tertiary limestones from the ends of the island may be 

 due to submergence. The numerous reefs, projecting rocks, and 

 small islands extending eastward from Porto Rico as far as Anegada 

 are proof of subsidence at this end of the island; and at the other 

 end the water is relatively shallow between Porto Rico and Haiti, 

 averaging less than 450 m., while Mona and Desecheo islands rise 

 above the surface. Semmes attributes the present position of 

 the drainage divide to greater rainfall on the north side of the 

 island.^ 



ORIGIN OF THE ANTILLEAN TROUGHS 



The great troughs of the Antilles are bounded by fault zones, 

 and are characterized by great depth, by precipitous inclosing 

 scarps which show abrupt changes in slope at top and bottom, and 

 by relatively fiat floors that, instead of being graded like river 

 valleys, rise and fall throughout their length. Many of the deepest 

 places are close to the inclosing scarps instead of near the centers 

 of the troughs, and occasional horst-Hke elevations are also found 

 within the fault zones. The troughs are parallel rather than 

 convergent and in places are roughly arranged en echelon. 



All of the evidence is in accord with the conclusion that the 

 troughs have been formed through faulting. Moreover, they must 

 be attributed to subsidence of long narrow blocks rather than 

 elevation of the areas on each side; for the rock strata exposed 

 in the marginal scarps standing above sea-level were laid down in 

 relatively shallow water, and portions of the larger islands have 

 remained continuously above sea-level since pre-Tertiary time, while 

 the faulting is more recent. The subordinate depressions at the 

 base of the main scarps are also indicative of normal faulting, and 

 there is an absence of lateral pressure effects such as would be 

 expected to accompany thrust faulting. 



Vaughan is of the opinion that the faulting took place in late 

 Tertiary, probably Pliocene time, and he cites three kinds of evi- 



' D. R. Semmes, "The Geology of the San Juan District, Porto Rico," Scientific 

 Survey of Porto Rico and the Virgin Islands, N.Y. Acad. Sci., Vol. I, Part I (1919), p. 38. 



