110 PORTO RICO 



The playa plains are composed entirely of alluvium, derived 

 mostly from the mountain formations, hut 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 oeologic 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.* 



*The complicated geologic history of the Great Antilles is sot forth in detail by the 

 author in the Bulletin of the Museum of Comparative Zoology of Harvard College, 

 which is now in type anil will probably be published before this article appears. 



