

14 FOREST CONDITIONS OF PORTO RICO. 



cut through the limestone hills. Parting valleys of a similar character 

 are developed in many places around the rest of the island, although 

 perhaps not quite so extensive in area. 



MINOR AND EXCEPTIONAL FEATURES OF CONFIGURATION. 



Several features which are more developed upon the other Great 

 Antilles are exceptional or lacking in the configuration of Porto Eico — 

 notably, interior mountain valleys, bordering benches of elevated coral 

 reef, the coast lagoons or lakes, and the mangrove swamps. The inte- 

 rior mountain valleys of Porto Eico are not conspicuous features, nor 

 are they completely closed (without drainage outlets), like those of 

 Jamaica, but are local widenings of the stream valleys which formerly 

 reached Flack water a considerable distance within the marginal area 

 of the mountain mass, when the present coast bench was submerged 

 beneath the sea. The valley of Caguas is the most conspicuous exam- 

 ple of this type. This is a wide amphitheater a considerable distance 

 within the mountain area, and its bottom is filled with old alluvium. 

 It stands at present about 250 feet above the sea. 



Elevated reef benches or seborucco, which in Oub.a form the narrow 

 coast rim of hard rock and protect a softer interior, thereby producing 

 the excellent pouch-shaped harbors, are bat faintly developed in Porto 

 Eico. This material was seen only at the entrance of San Juan Har- 

 bor, but studies of the littoral were not extensive. San Juan, Jobos, 

 and Guanica, however, are the only pouch- shaped harbors of Porto 

 Eico, and probably their general absence is largely due to the lack of 

 the elevated reef formation. The coast lagoons or lakes are collections 

 of water in swales of the coastal plain on the north and in parting val- 

 leys of the type of Guanica, previously described. Mangrove swamps 

 are extensively developed around the interior margin of San Juan 

 Harbor. 



GEOLOGIC FORMATION AND SOILS. 



In the Southern United States and the Antilles, where altitude is 

 not a controlling factor, the chemical and physical composition of the 

 soils are two of the chief factors producing vegetal differences. Inas- 

 much as the soils of Porto Eico, with the exception of that of the 

 play a plains, are all residual (the surface decay of the underlying 

 rock), it is impossible to make a clear presentation of the forest condi- 

 tions without a few remarks upon the nature of the rocks. It is not 

 the intention to go into geologic detail, but inasmuch as all cultural 

 and natural aspects are intimately associated with geologic structure, 

 a few words upon this subject are absolutely essential. PI. Ill shows 

 the undergrowth and substructure on the river Brujo. 



GKEOLOGY. 



The mountains are composed largely of black or other dark-colored 

 basic igneous rocks, occurring as tuft's, conglomerates, and silts of 



