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. — The igneous rocks in this section of the Isthmus 

 and their sequence may be grouped as follows : — 



1. The granites, constituting the massifs of the east and west Cor- 

 dilleras of San Bias 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 pimice 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 Barbacoas sec- 

 tion. The materials composing the formations are undoubtedly sedimen- 

 tary in arrangement, but primarily igneous in origin. All 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 debris, consisting of basic 

 eruptives, — including basalts (olivine, diabases, and dolerites), augites 

 (both andesitic and porphyritic), and trachyte tuffs of similar materials, 

 and 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 debris subse- 

 quently transported, distributed, and deposited over the latter. This 

 debris 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 a Vamos) 

 epoch of the Eocene Tertiary, the fossiliferous beds of which now lie 

 upon and contain the debris of this older period of igneous extrusion. 



Age of the Basic 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, 



