40 



A-Y-VJ.Z.? XEW YORK ACADEMY OF SCIEXCES 



island from Juana Diaz past Ponce at a short distance to the north, cross- 

 ing the Ponce- Arecibo road at K-4.S. and thence westward, crossing the 

 Ponce-Penuelas road at K-10. This is the only large fault actiiallT ob- 

 serred that is necessarily of recent age. although a few others are inferred. 

 It must be of very late Tertiary age. because the chalk}' Ponce beds are 

 abruptly cut off by it. The older rocks of the pre-Tertiary are lifted with 

 respect to the younger series forming the present coastal margin wherever 

 tiiis fault has been seen. It has been traced by us from Juana Diaz to 

 the vicinity of Penelas. a distance of about 12 miles. What becomes of 



Fig. 13. — Crumple*! shales as seen along the layufia road r.ecr rJ.e summit of the range 



it at either end is not yet determined, but it is believed to extend much 

 farther in both directions. 



The physiographic habit of the island as a whole tends to support the 

 view that the fundamental structural form is that of a large fault block, 

 with the principal fault displacement and uplift along or near the south- 

 erly margin, tilting the whole mass gently northward. If this disturb- 

 ance took place, as seems to be indicated by the fault described, in very 

 late Tertiary time, accompanying the emergence from the sea, it would 

 account for the abnormalities of Tertiary rock distribution as well as the 

 unsymmetrical position of the main drainage divide. In any case, how- 

 ever, the fault block structure is a very late development and is superim- 

 posed on the other more complex and older structures of the mass. 



