ABSTRACTS OF PAPERS 47 



unconformably overlying all the older formations. The south half of the dome 

 is composed of volcanic breccia cut by many dikes. Blocks of the former 

 Cretaceous rock have sunk into the breccia. 



The origin of the Cretaceous dome, one of the most perfect in the world, is 

 believed to be a laccolithic intrusion into the basal Paleozoic rocks. The 

 breccia is believed to iill a pipe through which the laccolith broke and reached 

 the surface at about the close of the Upper Cretaceous period. A sill in the 

 basal Cretaceous can be traced completely around the dome. 



Eead from manuscript. 



GREAT FAULT TROUGHS OF THE ANTILLES 

 BY STEPHEN TABER 



(Abstract) 



A series of fault zones extending along arcs that run approximately east and 

 west have determined the major relief features in the region of the Greater 

 Antilles. Displacements along these fault zones have resulted in the forma- 

 tion of great trough-like valleys. The fault troughs are for the most part 

 submerged beneath the Atlantic Ocean and Caribbean Sea and are, therefore, 

 protected from erosion. They are characterized by great depth, precipitous 

 inclosing scarps, abrupt changes in slope at the top and bottom, and relatively 

 flat floors that, instead of being graded like river valleys, rise and fall through- 

 out their length. The deepest places are close to the foot of the inclosing 

 scarps rather than near the center of the trench, while horsts are also present 

 along the fault zones. 



Practically all of the severe earthquakes which have occurred in this region 

 during the last four centuries have originated along these fault zones, and they 

 have been caused by vertical displacements, as is shown by the sea waves 

 which have accompanied them. 



Presented without notes. 



The paper was discussed by Dr. T. Wayland Vaughan, Dr. I. C. White, 

 and Prof. W. H. Hobbs, with reply by the author. 



Discussion 



Prof. W. H. HoBBS : I have taken great satisfaction in listening to this ex- 

 cellent paper of Professor Taber, for the reason that in common with that of 

 Pi'ofessor Louderback, read at the morning session, it indicates entire corre- 

 spondence with the views which I have long held concerning the nature of 

 block faultings, namely, that adjustments within any region are comparable 

 to those which occur when floating blocks are disturbed by water waves. 

 Troughs due to faulting do not begin and end gradually, as long supposed, but 

 are interrupted suddenly. The interruption of the Bartlett Deep by the island 

 of Haiti and the continuation of a depression to the northward of that island ; 

 the cross-trough of the Anegada Passage, all require this interpretation and 

 this only. The smaller adjustment within the larger earth blocks. have also 

 been well established by Doctor Taber. 



