THE COAST-BORDER REGION. 11 



very short distance from their coastal borders, notably the Portugues, 

 near Ponce on the south, and the Ahasco on the west. 



THE COAST-BORDER REGION. 



The demarcation between the rugose-angular topography, the soils, 

 and the geologic features of the central mountain regions and the 

 coastal belt is well defined, aud the most unobservant traveler remarks 

 the radical natural differences which take place upon passing from the 

 mountains into the lower lying coastal plains and foothills, especially 

 upon the south side. 



The coast-border topography comprises a narrow belt of low hills and 

 plains encircliug the main or mountainous mass of the island, and 

 broken in continuity upon the northeast, southeast, and west by spurs 

 of the central mountains which run across it into the sea. This border 

 region of itself is an exceedingly diversified area, presenting two 

 conspicuous major types of relief, coast hills, and playa plains, and 

 generally a third type, which may be called parting valleys. (See 

 PL II, fig. 1.) 



THE HILLS. 



Seen from the sea, the coast -border topography at the southwest end 

 presents the aspect of a low tilted dissected bench or plain, bluffing 

 rather abruptly at the water's edge, with its summit gently rising 

 toward the foot of the ribbed and corrugated front of the mountains. 

 Here and there a stream from the mountains cuts across this coast 

 bench and severs it into blocks of hills. The wide alluvial plains of 

 these stream valleys are frequently of a much greater area than the 

 hills. The hills resulting from the dissection of this bench, of what- 

 ever shape, either round or oblong, have regular slopes void of the 

 strong vertical corrugations and knife-edged salients which character- 

 ize the mountains. These hills are also distinguished from the interior 

 mountains by their entirely different geologic composition, and on the 

 south coast by their vegetation. 



On the north the coast hills stand as steeply sloping solitary mounds 

 or domes, rising singly or in chains above wider extents of plain lying 

 between them and the mountain front. The citadels of San Juan are 

 built upon a hill of this character; others rise to the east and west 

 of the city as far as Eio Grande and toward Arecibo. They probably 

 do not exceed 500 feet in height at their interior side toward the 

 mountains, but exact measurements were not made. 



Along the shore from the southwest cape of Porto Rico to within 3 

 or 4 miles of Ponce, except where occasionally broken by playas, coast 

 hills are finely developed. These hills, like those of the north coast, 

 are the remnants of what was once a steeply slanting bench plain. 

 The slant is from the central mountains toward the sea, where the hills 

 are in some places terminated by a steep scarp or sea bluff 100 feet in 

 height. The interior side scarp of these hills is bordered by a valley 



