HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 241 
upon them, occurred in Costa Riea. Furthermore, from the presence of 
feldspar in the Cretaceous near San José, we may go even further and 
presume the existence of a Pre-Cretaceous basement of igneous rocks in 
the region of Central Costa Rica. 
Proceeding eastward Dr. Evans has reported Archean “granites and 
syenites along the Chiriqui region.” * Dr, Gabb, in his manuscript de- 
scription of the geology of Talamanca, lying in the same general region 
but nearer the Panama boundary, also describes “ granitic” rocks, but 
shows that they are thrust up through the Tertiary strata of Miocene 
age. Granitic and syenitic rocks are reported from many places in 
that portion of the Isthmus east of Colon constituting the mountainous 
Cordillera of San Blas, lying adjacent to the east and west Caribbean 
coast. On a previous page it is shown, however, that many rocks men- 
tioned in this paragraph are not true granites. 
I was unable to visit the San Blas mountains, of alleged granitic 
origin, but from the study by Professor Wolff of large numbers of peb- 
bles brought down from the mountains by the waters of the Chagres, 
which drains them, it is clear that true granite undoubtedly enters 
largely into their composition. I 
The granitic ranges extending through seven degrees of longitude 
due east and west along the Venezuelan coast, from Puerto Cabello 
to the northeast end of the island of Trinidad, is a remarkable feature 
which is singularly harmonious with the east and west trend of the 
known older granitic axes of the Central American region. These 
granites are called Archean by Sievers and others, and are Pre-Tertiary. 
This chain of older granitic rocks lies almost due east, slightly north of 
the Cordillera de San Blas. 
These occurrences of supposedly older granites in the Central Ameri- 
can region, fragmentary as is our knowledge of them, at least indicate 
that, beneath the mighty heaps of volcanic débris constituting the mass 
of the region, there is an older basement of granitic rocks of earlier age 
than the oldest determinable sedimentary rocks of their respective local- 
ities, — probably Pre-Paleozoic in Guatemala, Pre-Cretaceous in Costa 
Rica and Venezuela, and possibly Pre-Tertiary in the San Blas region 
of the Isthmus. 
It is interesting to note that in each of the localities where the trends 
of these granites are known, —in Oaxaca, Guatemala, and Venezuela, — 
they occur as the massifs of east and west ranges. 
From the foregoing facts we may conclude that, possibly previous 
1 Op. cit. (work cited in footnote 1 on p. 297). 
