HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 251 



encounters in going southward from the primitive rocks younger and 

 younger eruptive strata. First, a broad stretch of old eruptive rocks, 

 andesite, trachyte, basalt, etc., while the volcanoes, the youngest erup- 

 tive formations of the laud, rise for the greater part upon a still more 

 southern parallel fissure. 



The presence of granitic debris in the rocks of Costa Eica, as shown by 

 Professor Wolffs report upon my collections, clearly testifies that before 

 Tertiary time there was a crystalline foundation against which its littoral 

 sediments wei'e deposited. If we are correct in our assumption that the 

 Panama formation of Rhyolitic tuffs are of Pre-Tertiary age, (and in this 

 we are borne out by the independent observations of Maack and Garella,) 

 we have the proofs that in the Isthmus itself thei'e existed a laud nu- 

 cleus of Pre-Tertiary formations. The true granites brought down from 

 the headwaters of the Chagres may be a further indication of an old Pre- 

 Tertiary basement to the Isthmus, for no rocks identical with these are 

 known in the later intrusives, the so called " granites " of Gabb in Tala- 

 manca and of the San Bias range recorded by the Selfridge Expedition 

 as " syenites " being a different class of rocks, which have been intruded 

 through the Tertiary, as will presently be demonstrated. 



Karsten, Sievers, and other students of the Venezuelan coast, also 

 assign to a Pre-Tertiary origin the east and west granitic coast of Vene- 

 zuela. These in their east and west strike are singularly in harmony 

 with the prevalent trends of the Isthmian region. 



These fragmentary data are sufficient to indicate that there may have 

 been in Pre-Tertiary time a basement barrier of granitic rocks in an east 

 and west arrangement which outlined the Central American and Isth- 

 mian regions, and constituted an ancient buttress against or upon which 

 the later mountain folding has originated. 



Epochs of Igneous Activity. — There is every evidence that vulcanism 

 was active in the Central American and Isthmian region in late Creta- 

 ceous, or possibly earlier in the vast interval of Pre-Tertiary time, for 

 which we have no paleontologic data to fix a chronology. According to 

 Felix and Lenk, 1 it was at the close of the Cretaceous, near the begin- 

 ning of the Tertiary, that the gigantic line of Mexican volcanoes had 

 their conception, and the Central American chain of volcanoes was like- 

 wise piling up its vast heaps of material during this epoch, while the 

 Andean extrusions may have been in full blast. The rhyolitic tufas of 

 the Panama formation are almost beyond all reasonable doubt of Pre- 

 Tertiary age and show the existence of extensive vulcanism. The pebble 



i Op. cit. 



