96 



PORTO RICO 



STBEAM AND VOM'ANIC ROCKS, MAYAGUEZ 



ruptly toward the south. While the general axis of uplift ex- 

 tends east and west, the mountains do not now present a well- 

 defined and continuous summit crest, although various terms are 

 popularly used indicating a feature such as the "central back- 

 bone range," etc. The chief approach to such an axial feature 

 is an irregular line of summits which can be drawn about two- 

 thirds the distance across the island from east to west between 

 the headwaters of the streams flowing to the north and south 

 coasts. This feature, however, which follows more nearly the 

 southern than the northern coast, is a line of separated peaks and 

 passes. High peaks exceeding in altitude this so-called divide 

 project at various places from the lateral ridges which extend 

 between the parallel streams flowing from this drainage divide. 

 The main crest line extends from Mayaguez on the west through 

 Aibonito and Adjuntas to Humacoa on the east. This is called 

 the central Cordillera west of Aibonito and the Sierra de Cayey 

 to the east of that town. Another crest line bifurcates from this 

 main ridge near the center of the island like a letter K, so that 

 there are practically two crest lines in the eastern half of the 

 island. This northern branch is the Sierra Luquillo, which 

 practically extends from westward of the San Juan-Ponce mili- 

 tary road to the northeast cape. This range, which 'decreases 

 in altitude to the westward, contains the highest island summit, 

 El Yunque, and is nearly separated from that of the Sierra Cayey 

 by the valleys of the Rio Guarabo, which flows west into the 



