HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 251 
encounters in going southward from the primitive rocks younger and 
younger eruptive strata. First, a broad streteh of old eruptive rocks, 
andesite, trachyte, basalt, etc., while the volcanoes, the youngest erup- 
tive formations of the land, rise for the greater part upon a still more 
southern parallel fissure. 
The presence of granitic débris in the rocks of Costa Rica, as shown by 
Professor Wolff's report upon my collections, clearly testifies that before 
Tertiary time there was a crystalline foundation against which its littoral 
sediments were 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 there existed a land nu- 
cleus of Pre-Tertiary formations. Тһе true granites brought down from 
the head waters of the Chagres may be a.further indication of an old Pre- 
Tertiary basement to the Isthmus, fcr no rocks identical] with these are 
known in the later intrusives, the so called “granites” of Gabb in Tala- 
manca and of the San Blas 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," 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. Тһе 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 
Y Op. ой. 
