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AMERICAN FORESTRY ASSOCIATION. 



209 



The name "parting valley " the writer 

 has given to certain long and narrow 

 valleys which sometimes occur between 

 the foothills and the front central mount- 

 ains. Some of the streams, as they 

 emerge from the mountains and cross the 

 lower country, tend either to bend along 

 the mountain front as they pass from it 

 or to send out laterals parallel to the 

 same. The erosion attendant upon such 

 phenomena produces long parallel val- 

 leys at the junction of the mountains 

 and foothills. Parting valleys of this 

 character are especially well developed 

 on the south side of Porto Rico, such as 

 the plain of Saba Grande and the de- 

 pression of Guanica lagoon. Other 

 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. 



Several features which are more de- 

 veloped upon the other Great Antilles 

 are exceptional or lacking in the config- 

 uration of Porto Rico notably, interior 

 mountain valleys, bordering benches of 

 elevated coral reef, the coast lagoons or 

 lakes, and the mangrove swamps. The 

 interior mountain valleys of Porto Rico 

 are not conspicuous features, nor are 

 they completely closed (without drainage 

 outlets), like those of Jamaica. 



Elevated reef benches or seborucco, 

 which in Cuba form the narrow coast rim 

 of hard rock and protect a softer interior, 

 thereby producing the excellent pouch- 

 shaped harbors, are but faintly developed 

 in Porto Rico. This material was seen 

 only at the entrance of San Juan Harbor. 

 The coast lagoons or lakes are collec- 

 tions of water in swales of the coastal 

 plain on the north and in parting valleys 

 of the type of Guanica, previously de- 

 scribed. Mangrove swamps are ex- 

 tensively developed around the interior 

 margin of San Juan Harbor. 



In the Southern United States and the 

 Antilles, where altitude is not a control- 

 ling factor, the chemical and physical 

 composition of the soils are two of the 

 chief factors producing vegetal differ- 

 ences. Inasmuch as the soils of Porto 

 Rico, with the exception of that of the 



playa plains, are all residual (the surface 

 decay of the underlying rock), it is 

 impossible to make a clear presentation 

 of the forest conditions without a few 

 remarks upon the nature of the rocks. 

 Inasmuch as all cultural and natural 

 aspects are intimately associated with 

 geologic structure, a few words upon 

 this subject are absolutely essential to a 

 complete understanding of the subject. 

 But in a brief review, such as this, having 

 called attention to the omission, we may 

 speak briefly, not of the history, but only 

 of the present condition of the soils. 



The chief and radical differences of 

 flora in Porto Rico occur between the red 

 clay mountain soils and the calcareous 

 foothill soils, the latter being of the 

 open-textured white limestone type 

 which abounds from Florida southward, 

 but is not common in the United States. 



The mountain areas present but little 

 if any barren indurated rock surface, 

 but are covered with a deep red soil, to 

 which vegetation clings tenaciously. 

 This mountain soil is one of the most 

 marked features of the island, and to it 

 are largely due many of its agricultural 

 and forest conditions. Were it less te- 

 nacious and sticky than it is (and lan- 

 guage can hardly convey an idea of the 

 unctuousness of this stickiness, which is 

 especially disagreeable in a road mate- 

 rial) the mountain slopes of Porto Rico 

 would now be washed and dreary wastes 

 of barren rock. This mountain soil is 

 mostly red ferruginous clay, accom- 

 panied by much pebble and other rock 

 debris. It is naturally ameliorated by 

 the vast amount of humus derived from 

 the native vegetation. Decay is so rapid 

 under perpetual warmth and moisture 

 that the volcanic rocks quickly rot and 

 weather into soils of this character. 

 The regolith or decayed superfice of the 

 rocks is unusually deep on these moun- 

 tains, extending down 50 or 100 feet, 

 correspondingly affording a splendid 

 medium for root hold and penetration. 



Owing to this soil the mountains were 

 originally wooded and are now cultivated 

 to their very summits, verticality of slope 

 presenting no obstacle to cultivation in 



