HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 207 
with them. There are only two limestone horizons in this series. One 
of these, the coneretions of Vamos 4 Vamos, are of a secondary concre- 
tionary nature, largely mixed with volcanic débris, and the other — the 
Empire formation — according to Dr. Dall “has the appearance of the 
Everglades limestone of Florida, and has in part been deposited or 
recrystallized from a solution." With this exception masses of chalky 
or marine limestones of organic origin are conspicuously missing. 
In discussing. this Tertiary series as a whole we are confronted at the 
outset by uncertainty as to what constitutes its oldest beds. This con- 
fusion is ereated by the lack of final and conclusive evidence upon the 
stratigraphic position of the Culebra clays. It is my firm conviction, 
however, founded upon the general lithologie aspect of the Tertiary 
series of the Atlantic side and from their lay, inasmuch as we encounter 
newer and newer strata toward the Atlantic, that the Culebra clays 
represent the base of the Tertiary series of the Isthmus. Тһе Empire 
foraminifera are closely allied in age to those of Bujio, and hence it is 
but fair to conclude that the still lower lying Culebra clays belong to 
the earlier stage of this epoch of the Eocene. If my hypothesis be 
true that these clays are at the base of the Tertiary series, then it is 
probable that they belong to the earliest Eocene period. The Vamos á 
Vamos beds, according to Dr. Dall’s determination, are undoubtedly of 
Claibornian Eocene. Unfortunately the exact stratigraphic position of 
the foraminiferal beds of Bujio is concealed. From their position in the 
folded complex, I believe that they are below the Vamos beds. Dr. Dall, 
however, is of the opinion that the fossil foraminifera indicate that they 
belong above the latter, but from my observations upon the range of 
these forms in Jamaica, I believe my position the most tenable. 
The undonbtedly fossiliferous Tertiary beds lying above the Culebra 
clays occur in three or possibly four well defined subdivisions. The 
oldest or lowest of these are the Empire limestones, composed so largely, 
as we have shown, of foraminiferal remains. 
In the Vamos á Vamos beds we find the first display of a rich marine 
molluscan fauna, which gives a positive clue to geologic age. When I 
first saw the bluffs containing these fossils I thought they were igneous 
outcrops, so black was the color of the impure rocks, which, as shown 
by the petrographic investigations of Professors Wolff and Turner, is the 
result of the mixture of basic igneous material in them. Professor 
Wolff describes this rock as “a dark gray limestone containing shells in 
a calcareous cement containing fragments of hornblende, triclinic feld- 
spar, and augite from andesitic lava.” Evidently, from this descrip- 
