HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 209 
cene or recent age; as also are the elevated coral reefs of Colon, com- 
posed of common Caribbean species, 
The Igneous Rocks, — Тһе igneous rocks in this section of the Isthmus 
and their sequence may be grouped as follows : — 
l. The granites, constituting the massifs of the east and west Cor- 
dilleras of San Blas or Choco, which borders the coast from Puerto 
Bello eastward, the rocks of which have been mentioned by many 
writers, and the gravels which are brought down so abundantly by the 
Chagres from its headwaters. 
2. The rhyolitic tuffs and pumice occurring on both sides of the Isthmus, 
as the Panama and Miraflores formations of the Pacific coast and islands, 
and as the Barbacoas and San Pablo formations of the Darbacoas sec- 
tion. Тһе materials composing the formations are undoubtedly sedimen- 
tary in arrangement, but primarily igneous in origin. АП known data 
indicate that they were produced during a period of acidic eruption 
previous to the extrusion of the basic igneous rocks, and before the 
Eocene period. 
3. The third igneous group comprises the basic igneous rocks, — occur- 
ring as intrusives, eruptives, tuffs, and rolled débris, consisting of basic 
eruptives, — including basalts (olivine, diabases, and dolerites), augites 
(both andesitic and porphyritic), and trachyte tuffs of similar materials, 
&nd one boulder bluff of hornblende augite, andesite, or porphyrite. 
These rocks occur in situ between Mamei and the extreme south coast 
of Panama parallel to the outer margin of the Gulf. While they un- 
doubtedly represent different eruptive events, they practically belong to 
one generic epoch, which might be termed the period of the basic erup- 
tives. "The more massive basic intrusives were first pushed through the 
older Panama (Barbacoas) formation, and their rolled débris subse- 
quently transported, distributed, and deposited over the latter. This 
débris makes its earlier positive occurrence as the Bujio conglomerate, 
the age of which can definitely be placed during that portion of the early 
Tertiary as immediately preceding the Claiborne (Vamos 4 Vamos) 
epoch of the Eocene Tertiary, the fossiliferous beds of which now lie 
upon and contain the débris of this older period of igneous extrusion. 
Age of the Basie Igneous Rocks. — Structurally the section indicates 
that there was a great area of basic igneous eruption, the cross section 
of which reached from one hundred miles south of Panama, in line with 
the points of Panama Bay, to the Mamei, near the geographic centre of 
the Isthmus. The former elevations which these igneous intrusions 
attained, through the vicissitudes natural to its greater area, have, 
