National Geographic : 1899 Mar
PORTO RICO The playa plains are composed entirely of alluvium, derived mostly from the mountain formations, but also mixed with the debris of the adjacent white limestone hills, generally reddish in color, except that which is derived from the Pepino hills, which is a black calcareous soil. These extensive alluvial deposits are of a loamy nature, combining essentially the qualities of the residual soils, both of the clay mountains and the calcareous foothills, with the additional advantage of a more loamy phys ical structure adapted for better drainage and root penetration and general cultivation. The geologic history of the island may be briefly summarized as follows: The earliest positive chronology that can be fixed at present is Cretaceous time, when the island, in common with the other Great Antilles, was the site of active volcanism, which resulted in the piling up of vast heaps of igneous rocks now constituting its mass. At the close of Cretaceous time and during the beginning of the Tertiary this volcanic material was water-sorted and converted into marginal sea sediments, as represented in the stratified tuffs, conglomerates, and fossiliferous Cretaceous and Eocene rocks. The history of Porto Rico during Oligocene time is obscure, the vast thicknesses of white limestone of that age which occur in Cuba, Jamaica, and Santo Domingo not having as yet been de tected upon the island. It is supposed, however, that the island, together with the other Great Antilles, suffered great subsidence during this epoch. In late Tertiary time all the aforesaid rocks were uplifted and deformed into their present mountainous aspect, in common with the general Antillean uplift of that epoch. The exact period of this uplift in the later half of the Tertiary has not as yet been fixed, but it was largely accomplished before the close of the Miocene epoch. The tilted Pinones strata of Miocene age, at the northwest corner of Porto Rico, clearly show that the move ment was not completed until after the close of the Miocene. In Pleistocene time the island suffered minor oscillations of ele vation and subsidence, resulting in the present erosion and con figuration of the coast-border topography.* *Thle complicated geologic history of the Great Antilles is set forth in detail by the author in the Bulletin of the Museum of Comparative Zoology of Harvard College, which is now in type and will probably be published before this article appears.