BERKEY, GEOLOGICAL RECOXXOISSAXCE OF PORTO RICO 17 



that tlie numerous different levels represented by many of the small 

 plains referred to above were necessarily connected with any of these 

 subsidence changes. Additional comment on the hillock topography, so 

 strongly developed in some parts of the area, is made in connection with 

 discussion of drainage in another part of this paper. 



The Arecibo formation is of Tertiary age. So far as identifications of 

 the fossils have gone, they appear to confirm the opinion that the larger 

 part of the formation belongs to the Oligocene epoch. These determina- 

 tions were based largely on collections made in the heavy limestone beds 

 and reefs in the vicinity of the Quelbraclillas Eiver. The shale beds 

 lying at the base of the series, and exposed farther to the south in the 

 vicinity of Lares, are certainly somewhat older and probably belong to 

 the Eocene epoch. There are higher beds developed rather irregularly 

 that doubtless represent still later time, referred by Hill to the Miocene 

 epoch, but these determinations must be left to future detailed study of 

 the formation as a whole. 



It is considered eminently fitting to refer to some of the chief varia- 

 tions which have especially strong development in certain localities, by 

 special locality designations, such as, — San Sebastian shales. Ponce chalky 

 limestones and marls, Juana Diaz shales and marls, Guanica coral reefs, 

 Quebradillas reef limestones, etc. 



The correlating of all these and other local representatives of the Are- 

 cibo formation is a work that can be done only by extensive and detailed 

 stratigraphic study and paleontologic comparison. This is one of the 

 larger pure-science problems awaiting future investigation. 



Older Series 



Below the Arecibo formation and forming the surface in the interior, 

 beyond the Arecibo margin, the island is made up of an exceedingly com- 

 plex series of many different kinds of rocks. They include chiefly varie- 

 ties of igneous rocks, both extrusive and intrusive, both fragmental and 

 massive, ranging from small stringers or dikes or flows to large boss-like 

 masses that cover many square miles in area. In addition to the igneous 

 rocks of these types, there are numerous shale beds and conglomerates of 

 rather massive habit aggregating a very great thickness, and with them 

 are associated limestones and foraminiferal beds of considerable variety. 

 A study of the rocks of this series for the purpose of determining their 

 character and origin indicates that practically everything in the older 

 series except the limy portions of the shales or the limestones proper are 

 more or less directly of igneous origin. The coarser materials and those 

 least affected by anv secondary processes are the tuffs which are of direct 



