THE GREAT FAULT TROUGHS OF THE ANTILLES 103 



island and probably originated on the south flank of the Grande 

 Hilera or in the central valley of Haiti. This valley, according to 

 Jones, was formed by folding and erosion rather than by faulting.^ 

 The earthquake of April 23, 1916, which damaged buildings at 

 Boya, Guerra, and Bayaguana, seems to have originated near 

 latitude 18° 45' N., longitude 69° 45' W. 



The separation of Cuba and Haiti has been attributed by 

 Vaughan to the downthrow of a block between two faults; one 

 forming the north side of the Bartlett Deep, the other forming the 

 south side and converging toward the former in the Windward 

 Passage.^ The evidence summarized in the present paper indicates 

 that the separation of the two islands is to be correlated with the 

 formation of the series of depressions, including the Vega Real, 

 which mark the Cayman Islands-Sierra Maestra-North Haiti 

 fault zone, and which have probably originated through differential 

 displacement of relatively narrow blocks or wedges within that zone. 



A narrow trough, 3,058 to 4,353 m. in depth, separates the 

 Island of Haiti from the Bahama Banks. It extends eastward, 

 in a great arc convex toward the north, from a point north of the 

 Windward Passage and enters the Brownson Trough northeast of 

 the Samana Peninsula. The topographic characteristics of this 

 trough and its parallelism to the two Bartlett Trough fault zones 

 suggest that it has originated through faulting; but no geologic 

 evidence of faulting is known and Httle seismologic evidence is 

 available except for the extreme eastern end where it joins the 

 Brownson Deep (see page 106). 



THE BROWNSON TROUGH 



The Brownson Trough, containing the deepest sounding made 

 in the Atlantic Ocean, parallels the Porto Rico- Virgin Islands ridge 

 on the north. Its shape is not known in detail for as yet few 

 soundings have been made in these waters. The 4,000-fathom 

 contour surrounds a narrow area, about 320 km. long in an east- 

 west direction; and the 3,000-fathom contour which is approxi- 



' W. F. Jones, op. cit., Jour. Geol., Vol. XXVI (1918), pp. 735-36 and Plate V. 

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

 During Cenozoic Time," Bull. Geol. Soc. Amer., Vol. XXIX (1918), pp. 625-26. 



