HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. all 
facts strengthen our inference that the most marked volcanic episode of 
the Isthmian region took place during the late Eocene epoch. 
Тһе excessive occurrence of this igneous material in the sedimentary 
Eocene formations of the foraminiferal marls and Vamos 4 Vamos forma. 
tion below Bujio, may be accounted for by one of two hypotheses. 16 
was either * wash" from pre-existing igneous masses, or contemporane- 
ously ejected from active vents. Тһе suddenness with which the abun- 
dance of the material coases, 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 Blas, 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,' 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: س2‎ 
“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, ëmer 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 Blas, as can be ascertained from the detailed itineraries 
and survoys of Messrs, Carson and Bowditeh, who accompanied the 
S fri 1 9 1 rati 8 
Selfridge explorations. 
Cross sections, via the drainage ways and dividing cols, across these 
1 Unpublished MSS. in library of U. S. Geological Survey. 
2 Ibid. 
3 See Reports of Carson and Bowditch, p. 118. Report of Explorations and 
Surveys for a Ship Canal, Isthmus of Darien, by Т. O. Selfridge, U. S. N., 1870. 
