HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 241 



upon them, occurred in Costa Rica. Furthermore, from the presence of 

 feldspar in the Cretaceous near San Jose, 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 Archgean " 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 Bias, 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 Bias 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. 



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 Archaean 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 Bias. 



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 debris 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 Bias 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. 237). 



