I04 STEPHEN TABER 



mately concentric extends nearly three times as far. The trough 

 is deepest near its western end. 



On the south the trough is bounded by a great scarp that rises 

 steeply to the plateau-like ridge on which Porto Rico and the 

 Virgin Islands stand, the average slope being about one in thirteen. 

 The rectilinear north coast of Porto Rico and the steep submarine 

 scarp descending from it are indicative of faulting. This coast is 

 bordered, near the west end of the island, by a long line of high 

 cliffs which evidently represent a fault scarp, for the youthful 

 topography of the plateau back of the cUffs and the gorge-like 

 valley of Guajataca River near its mouth testify to a recent eleva- 

 tion of the land, whereas a long period of time would be required 

 for the sea to cut such cliffs in gently dipping rock strata. A 

 wave-cut bench at the foot of the cliffs marks the latest uplift of 

 the land, amounting to several meters; and a similar sea- terrace 

 bordering Desecheo Island proves that this recent elevation 

 extended at least that far westward.' The abrupt change in the 

 slope near the loo-fathom contour about 8 km. north of the sea- 

 chffs and the occurrence of earthquakes at points lower down along 

 the slope indicate the presence of a zone of faulting, and suggest that 

 the descent into the trough is not accomplished by a single fault 

 scarp. 



The fault zone along the south side of the Brownson Trough is 

 possibly the eastward extension of the Cayman Islands-Sierra 

 Maestra-North Haiti fault zone, but if so, there is a sharp flexure 

 in the trend of the zone immediately north of Mona Passage, and 

 the downthrow changes to the opposite side. The topography 

 of this critical region, in so far as it has been revealed by soundings, 

 suggests that the ends of the two zones overlap and intersect; and 

 this may explain the origin of the pecuhar submarine valley, which, 

 heading in Aguadilla Bay, extends northwestward into the Brownson 

 Trough. This valley was described and figured in a previous 

 paper.^ At Aguadilla the coast has been slowly sinking during the 

 last half-century, while at Mayagiiez, 23 km. farther south as well 



'Harry Fielding Reid and Stephen Taber, "The Porto Rico Earthquakes of 

 October-November, 1918," Btill. Seis. Soc. Amer., Vol. IX (1919), p. 120. 

 ^ Ibid., pp. 118-21 and Plates 13 and 14. 



