April 16, 1920] 



SCIENCE 



395 



SPECIAL ARTICLES 



THE TERTIARY FORMATIONS OF PORTO RICOl 



In 1914, the ISTew York Academy of Sci- 

 ences commenced a scientific survey of Porto 

 Rico and the Virgin Islands. The outcome 

 of this work has been a series of reports, 

 covering geology and other branches of in- 

 vestigation. The imxwrtant geological con- 

 tributions which have been published are: 



1. " A Geological Eeconnoissance of Porto 

 Eico," by C. P. Berkey, Ann. N. Y. Acad. 

 8ci., Vol. XXVI., pp. 1-70, 1915. 



2. " Geology of the San Juan District," by 

 D. E. Semmes, IST. Y. Acad. Sci., Sci. Surv. 

 of P. E. and the Virgin Islands, Vol. I., pt. 1, 

 pp. 83-110, 1919. 



In the summer of 1916, the writer, working 

 imder the auspices of the New York Academy 

 of Sciences, made a detailed study of the 

 northwestern portion of the island (Lares 

 District). The results of that survey, to- 

 gether with the conclusions of Berkey, 

 Semmes, and other geologists who have 

 worked in Porto Eico, are outlined in the 

 present paper. 



General Outline. — E. T. HilP showed that 

 the central core of Porto Eico is made up of 

 a volcanic complex, with sediments of Cre- 

 taceous age, and with coastal belts of a white 

 limestone (Pepino Formation) of Tertiary 

 age. In 1915, Berkey^ showed that the cen- 

 tral mountainous complex (Cretaceous) is 

 overlain unconformably by the Tertiary lime- 

 stones of the north an south coasts (Arecibo 

 Formation). The Tertiary in timi is over- 

 lain disconformably by a limited coastal belt 

 of solidified dune sands and beach deiwsits 

 (San Juan Formation) of Pleistocene to 

 Eecent age. He called the Cretaceous com- 

 plex the " Older Series " ; the Tertiary and 

 Pleistocene formations the " Younger Series," 

 and pointed out that the unconformity sep- 

 arating these two series is a profound one, 



1 Presented before the Geological Society of 

 America, Boston meeting, December 29-31, 1919. 



2 Porto Rico, Nat. Geog. Mag., Vol. X., pp. 93- 

 112, 1889. 



the chief break in the geologic succession of 

 the island. The work of Berkey, Semmes, 

 and others has added much to our knowledge 

 ,of the geologic structure of the island, espe- 

 cially of the Older Series rocks. However, the 

 Yoimger Series is best developed in the north- 

 west comer of the island, and it was not until 

 work here had been completed that a detailed 

 statement of the Tertiary formations could 

 be made. 



The Tertiary Formations. — The Tertiary 

 formations are essentially a series of white 

 limestones, part massive or reef -like, part well 

 stratified. The beds are for the most part 

 undisturbed, and dip gently seaward at angles 

 of 4° to 6° on the north coast, and 10° or 

 more on the south coast. Except locally, 

 where slumping or slight warping has oc- 

 curred, or faulting (on the south coast) these 

 dips represent the initial angles at which the 

 beds were deiKJsited. 



The Tertiary formations were laid down 

 upon a slowly subsiding old land surface of 

 considerable relief. The valleys of this old 

 land surface were invaded by the sea during 

 the initial submergence, and in them were 

 deposited gravel, sand, mud, lignitic clay, and 

 marl. Such deposits, with their alternation 

 of fresh water, brackish water, and marine 

 fossil faunas, now form the basal shale mem- 

 ber of the Tertiary groups of the north and 

 south coasts. Compared with the overlying 

 limestones, this basal shale is local in distri- 

 bution, and very variable in thickness. 



The maximum thickness of the Tertiary 

 group in the northwest part of the island 

 (Lares District) is nearly 4,000 feet. On the 

 south coast, Berkey^ estimates the thickness 

 at 3,000 to 4,000 feet. Evidence obtained in 

 the Lares District seems to show that these 

 beds were never deposited vertically to any 

 such thickness, but are somewhat analogous 

 to the fore-set beds of a delta. The lime- 

 stones represent a series of fringing reefs 

 whose maximum growth . was outward rather 

 than upward. It is believed that at the 

 period of maximum submergence in Tertiary 

 time, the central mountain chain of the 

 island was not submerged. During sub- 



