6 AXXALS NEW YORK ACADEMY OF SCIENCES 



sees slopes of this kind with angles of thirty to forty degrees from the 

 horizontal under cultivation. 



The islands of Culebra and Vieques lie to the east of Porto Rico and 

 are said to be similar in structure and makeup, but neither of them was 

 visited on this expedition. 



The streams of the island are numerous and surprisingly large for the 

 size of the areas drained. According to Mr. Wilson's description^ of the 

 water supply there are twelve large streams flowing north, four flowing 

 west, seventeen flowing south and six flowing east. Besides these there are 

 said to be 1,300 small streams, and on account of the heavy rainfall on 

 portions of the island many of them are of larger size than such an area 

 would usually afford. The main divide runs near the southerly margin 

 of the island, so that about one-third of the drainage is tributary to the 

 Caribbean Sea on the south, and about two-thirds to the Atlantic Ocean 

 on the north. This unsymmetrical position of the dividing range is an 

 abnormal feature, the cause of which is the subject of discussion in an- 

 other portion of the report. Because of the prevailing trade winds, the 

 rainfall is very unevenly distributed. The east end and the north side 

 are comparatively humid and plentifully watered; in contrast, the west 

 end and especially the south side are comparatively arid. In order to 

 overcome partly the shortage of water, a large system of irrigation is now 

 being developed on the south side of the island. At the east end, north- 

 east of El Yunque, there is an annual rainfall of 123 inches. On the 

 other hand, at Cabo Rojo, at the other extreme on the southwest side of 

 the island, it is exceedingly dry. and in occasional years there is said to 

 be not a single drop of rainfall. Other parts of the island vary between 

 these extremes. The wettest months are September and November. 



In most districts, the underlying rock is compact enough to discourage 

 much deep water circulation and the stream run-off is correspondingly 

 responsive to the rainfall. In the northwest corner of the island, on the 

 broad limestone belt extending from Aguadilla to Camuy, there is a pre- 

 vailing tendency for the surface waters to sink into underground chan- 

 nels, leaving the surface very much more poorly watered than even the 

 somewhat scanty rainfall would lead one to expect. In some cases, streams 

 developed on the more compact rocks of the interior districts completely 

 lose themselves in underground channels upon entering the limestone 

 belt, and in some cases do not again come to the surface for several miles. 

 Elaborate caves and channel-like caverns are common and, in the north- 

 ern belt of limestone countr}-, there are thousands of such occurrences 



5 H. M. Wilson : "Porto Rico ; its Topography and Aspects." Jour. Am. Geog. Soc, 

 Vol. 32, p. 220. IflOO. 



