108 PORTO RICO 



It is not my intention to burden this paper with geological 

 detail, but inasmuch as all the cultural and natural aspects are 

 intimately associated with the geologic structure, a few words 

 upon this subject are absolutely essential — in fact, I have seen 

 no region where these phenomena were so intimately related. 



The chemical and physical composition of the soils are two 

 of the chief factors producing vegetal differences in the southern 

 United States and tropical America, altitude being next, and 

 rainfall, owing to its general abundance, the least appreciable one. 

 Inasmuch as the soils of Porto Rico, with the exception of that of 

 the playa plains, are all residual — the surface decay of the under- 

 lying rock — it is impossible to make a clear presentation of the 

 forestry conditions without presenting a few elementary geo- 

 logical descriptions. The chief and most radical differences in 

 flora (excepting altitude, which- is relatively a less important 

 factor in Porto Rico) are those occurring between clay and cal- 

 careous soils, especially in the tropics, where the latter is of an 

 open-textured white limestone which abounds from Florida 

 southward, but is not common in the United States. 



The mountains are composed largely of black or other dark- 

 colored basic igneous rocks, occurring as tuffs, conglomerates and 

 sills of hornblende-andesite, cut by dikes of diorite. While these 

 rocks are of volcanic origin, there are nowhere any signs of recent 

 or late geologic volcanism, such as craters, unburied lava flows, 

 cinder cones, etc., all original volcanic forms of topography 

 having been destroyed by erosion, to which are due the present 

 features of configuration. Besides, much of this volcanic ma- 

 terial has been worked over into sediments in prehistoric ages 

 and now occurs in well defined strata. 



Included in this mass of volcanic rocks are two limestone 

 formations, interbedded with them and relatively inconspicuous 

 in area. One of these, found on the crest of the island near 

 Cayey and Aibonito, is a black bituminous shaly limestone in- 

 terbedded with the volcanic conglomerate. This calcareous 

 horizon is fully 1,000 feet thick, apparently upholds the crest 

 of the Sierra, and weathers into soils noted as the best tobacco 

 lands on the island. The other is a light gray crystalline lime- 

 stone with Cretaceous fossils (Rudistes). It is seen outcropping 

 on an east and west line from near Cabo Rojo to fifteen kilo- 

 meters north of Ponce on the Adjuntas road, and has no special 

 agricultural value, but the natural vegetation is always notice- 

 ably different where these rocks occur. 



