HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 211 



facts strengthen our inference that the most marked volcanic episode of 

 the Isthmian region took place during the late Eocene epoch. 



The excessive occurrence of this igneous material in the sedimentary 

 Eocene formations of the foraminiferal marls and Vamos a Vamos forma, 

 tion below Bujio, may be accounted for by one of two hypotheses. It 

 was either "wash "from pre-existing igneous ' masses, or contemporane- 

 ously ejected from active vents. The suddenness with which the abun- 

 dance of the material ceases, as shown by its absence in the Mindi beds, 

 gives preference to the latter theory. 



Late Tertiary " Syenitic " Intrusions. — There is every reason to believe 

 that, in addition to the older granites and basic extrusions we have de- 

 scribed, there is a line of pseudo granitic or syenitic rocks exposed in 

 the mountains of eastern Costa Rica, the San Bias, and around Sierra 

 del Marta, and the Antilles, which have been pushed up into the Tertiary 

 strata, and now form the core of great mountainous protuberances. The 

 evidence concerning the existence of these rocks is as follows. 



The axis of the chain of high peaks of eastern Costa Rica rising to 

 11,000 feet, according to Gabb, 1 is composed of a granitoid mass which 

 has been pushed up through the Tertiary strata. 



Unfortunately no specimens are available by which the exact mineral- 

 ogic nature of these rocks can be verified. Mr. Gabb says : 2 — 



" This rock is decidedly syenitic in structure, containing almost no mica but 

 an abundance of hornblende, resembling closely a similar rock which I found 

 abundantly and under like circumstances in the island of Santo Domingo. It 

 is usually, however, of finer grain than the West Indian rock, and in the man- 

 ner of its relation to the overlying rocks, it differs in rarely or never sending 

 out dykes through them. . . . 



" The granite mass runs to the east in the form of a long narrow tongue 

 leaving the high ridge of the Cordillera, which trends to the southeast, and 

 finally disappears under the (Tertiary) slates, just before reaching the Tilori 

 River. To the northwest it follows the general direction of the mountain 

 summits in a valley of from three to five miles wide on the Talamanca side." 



A similar occurrence of apparently the same kind of rock occurs near 

 the coast of San Bias, as can be ascertained from the detailed itineraries 

 and surveys of Messrs. Carson and Bowditch, who accompanied the 

 Selfridge explorations. 8 



Cross sections, via the drainage ways and dividing cols, across these 



1 Unpublished MSS. in library of U. S. Geological Survey. 



2 Ibid. 



8 See Reports of Carson and Bowditch, p. 118. Report of Explorations and 

 Surveys for a Ship Canal, Isthmus of Darien, by T. 0. Selfridge, U. S. N., 1870. 



