HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 207 



with them. There are only two limestone horizons in this series. One 

 of these, the concretions of Vamos a Vamos, are of a secondary concre- 

 tionary nature, largely mixed with volcanic debris, 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 created by the lack of final and conclusive evidence upon the 

 stratigraphic position of the Culebra clays. It is my firm conviction, 

 however, fouuded upon the general lithologic 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. The 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 a 

 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 undoubtedly 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 a 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- 



