National Geographic : 1899 Mar
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.