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 
been 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. Тһе principal discoloration is a light greenish 
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 tho 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 débris 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 Rica, 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, Vamos á 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, Curaqoa, 
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 glauconitio and 
lignitie material derived from the plants deposited contemporaneously 
