FORESTS OF PORTO RICO. 5 



elevation of 1,341 meters (4,398 feet), while ''Mt. Giiilarte," com- 

 monly considered second to El Yunque, is 1,204 meters (3,950 feet). 



The many lateral ridges which diverge from the central mountains, 

 mostly from the north side, are commonly very steep-sided and nar- 

 row-crested, and the valleys are deep, V-shaped, and almost devoid 

 of level bottom land. Rock outcrop is generally infrequent, except 

 toward the outer portion, where the ridges are often capped with hard 

 limestone. 



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

 colored igneous rocks, which occur in the form of tuffs, conglomerates, 

 silts, and an occasional dike of diorite. Their volcanic forms have 

 been destroyed by erosion. The material thus worked over into sedi- 

 ment in prehistoric ages now occurs in well-defined strata. Two rela- 

 tively inconspicuous limestone formations also occur, one black, bi- 

 tuminous, and shaly, and the other light gray and crystalline. 



As a result of the almost uninterrupted action of an abundant pre- 

 cipitation, a high relative humidity, and a warm temperature, rock 

 weathering at the higher elevations is more rapid than erosion, as 

 shown by a soil mantle of unusual depth and almost no bare indurated 

 rock here. The characteristic soils are deep, reddish clay loams and 

 tenacious red clays. So cohesive, unctuous, and compact are these 

 soils that they are able to maintain themselves in an almost vertical 

 position. Cultivation, in consequence, is in many places carried on 

 to the very tops of the ridges and on the steepest slopes, yet evidence 

 of excessive erosion and landslides is surprisingly inconspicuous. 

 At the lower elevations the sandy character of the soil and the more 

 common occurrence of outcrop show that the rate of rock erosion has 

 exceeded that of weathering. 



THE CORAL LIMESTONE BELT. 



The belt of coral limestone is several miles wide in places and on its 

 interior border overlaps the igneous rocks. This area is of sedimen- 

 tary origin. Where rock solution has been the most active agent of 

 decay, it retains the general form of a table-land. Where erosion 

 has been the most active only isolated conical hills remain. In 

 certain parts of the island the limestone extends directly to the 

 water's edge, where it terminates in steep scarps, often 100 feet or 

 more in height, notably on the south coast west of Ponce and on the 

 north coast west of Quebradillas. Elsewhere on the island the rem- 

 nants of this formation stand as steep, sloping, solitary mounds or 

 domes, which rise singly or in chains above the coastal plain. 



Along the junction of the central mountains and the limestone belt 

 is a distinct line of weakness marking the former shore line. Strong 

 valley lines are developed there, separating the two physiographic 

 regions. These ' ' parting valleys " are especially well developed on the 



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