4 AXNALS XEW YORE ACADEMY OF SCIENCES 



son Deep cii the other is approximateh' twenty-eight thousand feet. Re- 

 garded in this way. the Island of Porto Rico belongs to one of the higher 

 relief features of the earth. 



It is in reality a Ijadly eroded summit of a great mountain belonging 

 to an east-west chain or range including the Greater Antilles. The gen- 

 eral structural features of the islands are consistent with this east-west 

 axial trend which is expressed in the topography of the central Cordillera, 

 extending from the west end near Rincon, where it starts abruptly from 

 the water's edge^ to Fajardo^ Avhere it terminates in El Yunque, the high- 

 est point on the island. The mormtain range, however, is not so simple a& 

 this statement would lead one to believe, for there are in reality two 

 ranges or branches toward the east, one of which is known as the Sierra 

 de Luquillo, culminating in El Yunque, and the other, wdiich is best de- 

 veloped in the divide between Cayey and Guayama, is called the Sierra de 

 Cayey. The military road crosses this latter branch near Aibonito over a 

 pass that reaches above 2,000 feet. To the westward, the double character 

 of the mountain ranges is not so pronounced, but there is a semblance of 

 it in the spurs that reach the sea rather abruj)th^, one near Rincon and 

 the other near ^Mayaguez. The exact elevations of the higher mountains 

 have not been accurately determined, the values given on the older maps 

 being evidently too great. The revised approximate elevations are: for 

 El Yunque, at the eastern end of the island, 3,750 feet; for El Guilarte, 

 wdiich stands to the west of the Arecibo road, 3,610 feet. The highest 

 point in the Sierra de Cayey is about 3,000 feet. Many points are nearly 

 as high as those given, and all of the roads that cross the island reach 

 elevations on the divide that are in excess of 2,000 feet. Many of tlie 

 roadways in the interior districts reach elevations over 2,225 feet. 



Although the island as a whole has a mountainous aspect, and although 

 much of the interior is very rugged and picturesque, there is usually a 

 comparatively gentle or smooth topograph}^ along the coast, and some of 

 the marginal areas are almost perfectly flat. These are uniformly at the 

 mouths of the larger rivers and represent river alluvium or delta-like 

 deposits, — they are known in the island as playas. 



The aspect of the island as a whole is moderateh' rugged. Although 

 the major portion of the rock inakeu]^ is igneous, and although there is 

 considerable complexity of structure represented in all parts of the range, 

 all of the surface forms are of erosional origin. The relief is that of 

 early maturity in the interior and perhaps late maturity in portions of 

 the coastal districts. Exceedingly steep slopes are the rule in all parts 

 of the island where tliere is any consiflcrable relief, and one of the most 

 surprising things is tlie way tlie soil clings to these slopes. One often 



