abstracts and discussions op papers bs 



Session of Thursday, December 30 



The Society convened at 9.15 o'clock a. m., with President Coleman in 

 the chair. 



The Society proceeded immediately to the consideration of scientific 

 papers. 



titles and abstracts op papers presented before tpie morning 

 session and discussions thereon 



stages in the geologic history of porto rico 



BY CHESTER A. KEEDS 



(Abstract) 



In March, 1915, Prof. C. P. Berkey in his report, "Geological Reconnoissance 

 of Porto Rico," Annals New York Academy of Sciences, stated that an early 

 Tertiary peneplain separated an "older" from a "younger" series of formations 

 in Porto Rico. Observations made in the field during the following June and 

 July suggest that more recent erosion cycles and periods of uplift and subsi- 

 dence are also in evidence, and that they are responsible, for the most part, 

 for the present configuration of the island. The positive and negative oscilla- 

 tions of the strand-line were induced in all probability by diastrophic move- 

 ments within the island mass and adjacent sea-floor. 



Previous to the early Tertiary peneplanation beds of shale, limestone, con- 

 glomerate, and volcanic rocks were intruded by igneous masses, producing a 

 basal complex of folded and faulted strata and igneous intrusions. Following 

 this peneplanation no igneous intrusion or ejectments have occurred, for none 

 were observed in the basal lignitic shales and overlying chalky and cross- 

 bedded limestones of the younger series. Folding and faulting are, however, 

 in evidence in certain places in the beds of the "younger" series. 



Evidences of marine transgression are to be seen in the marginal coastal 

 plains and the wave-cut escarpments. Suggestions of an earlier transgression 

 are to be seen in the even-crested cliffs and headlands on certain coasts and 

 the isolated hills, ridges, and plateaus of near-by tracts. This older plain is 

 typically developed in the district to the south and west of Tauco. 



While successive transgressions and recessions of the sea were affecting the 

 coastal and marginal tracts, subaerial denudation was busily sculpturing the 

 present mature topographic features of the interior out of the exposed portion 

 of the older warped peneplain and accompanying monadnocks. Features which 

 indicate that stream erosion has been varied are to be found in the abandoned 

 erosion levels in valley walls and at water gaps on rivers like the Descalabrado 

 and the Jacaguas. At one stage the lower courses of these rivers were filled 

 with sand and gravel. At the present time these sediments are being eroded 

 and at some points the stream bed was observed to be 50 to 60 feet below the 

 base of the gravel deposits. 



The stages in the geologic history of Porto Rico were varied in length and 

 kind of work performed. 



