FORESTS OF PORTO RICO. 7 



the mangrove is only slightly developed, but there are in places 

 extensive saline plains too low and wet for cultivation, where rank 

 grasses, a few scattered acacias, or low, succulent, salty herbs con- 

 stitute the only vegetation. 



The coastal plain proper is elevated but a few feet above the sea, 

 and has but a slight gradient toward the mountains. It terminates 

 rather abruptly at the foothills, except in the valleys of the larger 

 rivers. These plains are entirely sedimentary, having been laid 

 down when the island stood at a somewhat lower level than now. 



The coast-plain hills are isolated, low, and dome-shaped. Some 

 have been nearly buried by the alluvial deposits of the rivers ; others 

 rise 100 feet or more above the level of the plain. 



The soil, except on the hills, is largely a fine, rich alluvium, sandy 

 in places, and is almost entirely under cultivation or in pasture. 



DRAINAGE. 



It would be difficult to find another country of its size so well 

 watered as Porto Rico. Within the mountainous area are many 

 swift-flowing streams which have cut for themselves deep, steep- 

 sided valleys. In their upper courses they traverse steep, angular 

 gorges, where numerous cascades and cataracts are to be found, par- 

 ticularly in the Sierra de Luquillo. The peculiarity of the drainage 

 system where it passes from the central mountain into the limestone 

 region has already been described. Within the coastal plain the 

 valleys are broad, with considerable areas of bottom land through 

 whcih the rivers pursue a meandering course. The streams flowing 

 north from the main divide are much more numerous and longer than 

 those from the south side, and they likewise carry a much greater 

 and more constant volume of water. The island is reported to have 

 upward of 1,300 named streams, of which the Rio de la Plata is con- 

 sidered the longest, about 45 miles. None of the rivers is naviga- 

 ble, except for small boats, and then chiefly in their tidal reaches. 

 They, nevertheless, are of „ tremendous importance as a source of 

 domestic water supply, and their power possibilities are also very 

 considerable. 



CLIMATE. 



Though Porto Rico is well within the Tropics, it has an equable 

 and comfortable climate, for the modifying influences of the ocean 

 are accentuated by its position in the direct path of the North 

 Atlantic trade winds. These counteract the enervating effect of the 

 high temperature and humidity, the occasional periods of sultry and 

 oppressive weather invariably occurring when they fail. They vary 

 in direction from northeast to southeast, usually coming from east or 

 east-southeast. Their average velocity from month to month is 



