348 INDEX TO THE STEATIGEAPHY OF NORTH AMERICA. 



With regard to Porto Rico Hill *^^ states : 



The mountains are composed largely of black or other dark-colored basic igneous vocks, 

 occurring as tuffs, conglomerates, and sUls 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 con- 

 figuration. Besides, much of this volcanic material 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 interbedded with the volcanic con- 

 glomerate. 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 limestone with Cretaceous fossils (Rudistes). * * * 



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 pUing 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 detected 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 iuto 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 movement was not completed until 

 after the close of the Miocene. In Pleistocene time the island suffered minor oscfilations of 

 elevation and subsidence, resulting in the present erosion and configuration of the coast-border 

 topography. 



F 17-18. CUBA AND ISLE OF PINES. 



The following notes have been prepared for this volume by T. W. Vaughan: 



Serpentines, granites, slates, and schists constitute the oldest rocks known on the island of 

 Cuba, except perhaps some limestones of doubtful Paleozoic age in the vicinity of Trinidad, and 

 form the basement upon which the subsequent geologic formations were deposited. Slates and 

 schists occur in small patches in the Province of Santa Clara, some 3 or 4 miles south of the 

 city of that name. They are much folded and metamorphosed, and apparently the serpentiae 

 has been intruded into them. Outcrops of serpentine occur in every province of the island. 

 The most westerly occurrence at present known is near Guanajay, in the Province of Pinar del 

 Rio. There are large areas in the vicinity of Havana and Guanabacoa, northwest of Matanzas 

 between Matanzas and Cardenas, and east of Cardenas in the vicinity of Hato Nuevo. Exten- 

 sive areas also occur in the central portion of the Province of Santa Clara and near the city of 

 Trinidad in its southern portion. Serpentine also constitutes the surface formation of large 

 areas in the northern and central portions of the Provinces of Camaguey and Oriente. 



Outcrops of granite were observed at the south coast of the Province of Oriente and south 

 of the city of Santa Clara. A granite porphyry of doubtful age occurs in Oriente Province. 



SemicrystaUine blue limestones of fine texture outcropping in the vicinity of Trinidad have 

 been considered as doubtfully of Paleozoic age. 



