ABSTRACTS OF PAPERS 139 



ernment of the Island, a study of tbe geology of the region was undertaken 

 as one phase of the investigations. The first geological party to enter the 

 field consisted of Doctors Charles P. Berkey and Clarence N. Fenner, who, 

 during the summer of 1914, completed a reconnaissance of the island. Since 

 that time an average of two parties a year have been sent into the field. 



In his report^ Doctor Berkey noted the occurrences of terraces 100 to 200 

 feet above the present sea level, particularly on the south coast near Guayama, 

 and attributed their origin to wave action. Subsequent investigators have 

 substantiated this conclusion. Mr. A. K. Lobeck, however, after a study of the 

 physiography of the island, concluded that there has been only a slight differ- 

 ential uplift of the western end of Porto Rico in very recent time, the maxi- 

 mum change being at Rincon, on the west coast, where an elevation of 40 feet 

 occurs. 



During the past summer a survey of the southwestern quarter of the island 

 was made by the writer, that section being one which appeared favorable to 

 the solution of the question of recent changes of level in Porto Rico. The 

 evidence gathered in this study is summed up as follows : 



1. One-half mile southwest of Juana Diaz, on the north bank of the Jaca- 

 guas River, the folder Tertiary beds are beveled and a deposit of silt, sand, 

 and gravel 2 to 12 feet thick covers the surface. In this surface covering, 

 at an elevation of 130 feet, are found numerous Stromhus pugilis. 



2. At kilometer 72.5 on the Ponce-Penuelas road recent marine fossils are 

 found in finely stratified material of estuarine character. In this deposit a 

 layer of black mud averaging one foot in thickness occurs at a depth of from 

 2 to 5 feet below the surface. In this black mud are found Stromhus pugilis, 

 Liicina jamaiccnsis, Lucina tigrina, Area tuberculosa, and Byssoarca ziehra. 

 These fossils are also found in other parts of this deposit, the elevation of 

 which is 180 feet. 



3. Across the west branch of the Canas River, just east of the above locality, 

 the same species of fossils which occur at locality "2" are found in the strati- 

 fied sands and gravels at a depth of 3i/^ feet below the surface and an eleva- 

 tion of 160 feet. 



4. Southeast of Yauco, 1^^ miles, in the Rio Yauco ^\alley, abundant fossils 

 are found in the surface covering of the river valley at an elevation of 150 

 feet. The fossils include Murex elongatus. Area rhombea, Lucina tigrina. Area 

 tuberculosa, Turritella imhricata, Pecten nucleus, Venus caneellata, Osirea 

 virginica, and Perna sp. 



5. East of Yauco, one-eighth of a mile, the pre-Tertiary rocks are truncated, 

 and in the gravel and sand which mantels the beveled strata are found Area 

 tuberculosa and Lucina tigrina, occurring at depths of 1 to 2 feet below the 

 surface. The elevation at this point is 200 feet. 



6. On the coast southeast of Yauco a terraced surface bevels the Tertiary 

 limestone at an elevation of 60 to 160 feet, the inner margin being marked in 

 places by clifling. The following fossils are found on this surface: Stromh'iis 

 aceittrinna, Fissurella nodora, Area rhombea, and Turbo pica. 



7. Just north of the lighthouse at Guanica the Tertiary limestone is beveled 

 by terraces at levels of 10, .50, and 150 feet, and in the surface soil on the two 



3 Geological reconnaissance of Porto Hico. Annals N. Y. Aoadomv of Sciences, vol. 2(5. 

 1010. 



