232 E. Hoioe — Geology of the Isthmus of Panama. 



The relation of the tuffs and fine breccias in the vicinity of 

 Panama to Ancon Hill suggests that the pyroclastics were 

 derived from a volcanic center now marked by the massive 

 porphyry of Ancon. The tuffs, as has been said, are well 

 stratified and surround Ancon Hill except in the vicinity of 

 Sosa; they dip away from Ancon Hill at all points and in a 

 few instances have been found to be cut by dikes that appear 

 to radiate from the hill as a center. No local source or center 

 of eruption for the tuffs on the Atlantic slope has been found, 

 and it is possible that they may have been derived from the 

 Ancon eruptions, the distance between the points being only 

 twenty miles. The extremely light pumiceous character of 

 the San Pablo deposits would favor this view. 



The age of the acid eruptives is fairly well shown at several 

 places. Near Panama evidence from borings quoted by Ber- 

 trand* indicates that the acid tuffs are younger than the 

 Culebra beds and rest upon them. I was unable to find bor- 

 ing records at the localities mentioned by Bertrand, but the 

 field relations fully justify Bertrand's view. The Culebra 

 beds disappear beneath the surface of the lower Bio Grande 

 Valley near Pedro Miguel, and at Miraflores the acid tuffs are 

 well exposed at a number of points. Borings made in explor- 

 ing for dam sites between Sosa Hill and Corazal and also 

 across the mouth of the Pio Grande indicate the presence of 

 sediments similar to those of the Culebra beds at approxi- 

 mately seventy or eighty feet below sea level, and one boring 

 near La Boca showed a dike of rock similar to that of Ancon 

 Hill cutting these sediments. On the Atlantic side between 

 San Pablo and Tabernilla there is no surface evidence bearing 

 on the age of the acid tuffs. Borings for a dam site near San 

 Pablo indicate rocks similar to those. of the Bohio breccias, 

 beneath what I believe to be tuffs of the acid series, but the 

 advanced decomposition of the rocks makes it difficult to dis- 

 tinguish one from another, and the supposed tuffs may be 

 sandstones of the Bohio formation. It was mentioned, in 

 describing the Gatun occurrence of the Monkey Hill beds, 

 that beneath the uppermost conglomerate borings revealed the 

 presence of one or more beds of extremely fine white pumi- 

 ceous tuff, and recent excavation for a road from the old Gatun 

 railway station to the encampment at the top of the hill has 

 exposed the tuffs. They are clearly interstratified with the 

 sediments, but as previously stated, it is difficult to determine 

 the base of the Monkey Hill beds, so that the tuffs may be 

 regarded as lying either at the top of the Bohio or at the base 

 of the Monkey Hill formation. In either case there can be 

 no doubt that they are younger than the Eocene sediments and 

 not Cretaceous, as supposed by Hill. 



*Op. cit., 8-9. 



