206 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



Caribbean and Pacific sides of the Isthmus. (2) The fossiliferous Ter- 

 tiary beds of the Caribbean side. (3) The Pleistocene beds deposited 

 synchronously on both sides. 



Of the first class the only rocks found were those of the so-called 

 Panama formation, which name will now be used to include the analo- 

 gous deposits of Barbacoas, San Pablo, and Miraflores. These, as has 

 beeu stated, are so distorted and concealed by later igneous protrusions 

 and deposits, that almost nothing can be ascertained of their antecedent 

 relations. Lithologically they certainly disagree in every generic char- 

 acter from the Tertiary and Pleistocene sediments. They are composed 

 almost entirely of rhyolitic and andesitic volcanic material, of whitish 

 colors singularly free from the darker basic igneous minerals and the 

 ferruginous, bituminous, and glauconitic colors everywhere so abundant 

 in the later formations. The principal discoloration is a light gi-eenish 

 tint, occurring sparsely, as if the chloritic element was but faintly devel- 

 oped in the rocks. 



There are other facts which lead me to believe that these rocks 

 belong to a much older series than those of the fossiliferous Tertiary 

 sediments, and that they are a part of an extensive Pre-Tertiary forma- 

 tion, which has been largely concealed by the later events, and which 

 occurs widely beneath the volcanic debris of Panama and Costa Rica. 

 In the peninsula of Nicoya, and where erosion has cut down to the very 

 base of the plateau of Costa Puca, there is a great series of rocks which 

 have such petrographic resemblance to this Panama formation that I 

 am prone to believe they are of the same epoch. 



The fossiliferous Tertiary sediments, including the Culebra, Empire, 

 Gatun, Yamos & Vamos, Mindi, and Monkey Hill formations, I con- 

 sider to have been deposited along a littoral margin of the Caribbean 

 Sea. These sedimentaries are of the Eocene and Oligocene epochs of 

 the Tertiary. The beds of this series containing marine fossils, accord- 

 ing to Dr. Dall's determinations, like those of the Claiborne-Tejon 

 epochs of the Eocene, the Vicksburg Oligocene of the United States, 

 and the Caribbean Oligocene ; the last corresponds paleontologically 

 with a group of sediments now found around the peripheral lands of the 

 Caribbean in the Bowden beds of the Great Antilles, Trinidad, Curacoa, 

 and in the Chipola beds of West Florida. 



As in the United States, the Tertiary rocks of the Caribbean side 

 seem to represent an aggregation of muddy sediments composed of 

 impure mixtures of sand and clay, accompanied by glauconitic and 

 lignitic material derived from the plants deposited contemporaneously 





