220 E. Howe — Geology of the Isthmus of Panama. 



Lupia perovata Conr. Cytherea 



Glyptostyla panamensis Dall. Mactra 



Turritella gatunensis Conr. Corbula 



Marginella sp. Tellina 



Natica ( cf. eminula Conr. ) Leda sp. 



Pleurostoma sp. Pyramidella sp. 

 Dentalium sp. 



In addition to these there are fragments of Ostrea sp., Liocar- 

 diii?ft, and Pecten, and a small ribbed Cardium was noted. 



From the locality on the left bank of the canal near Gatun 

 practically the same fossils were collected as were found at 

 Tamos Tamos with the addition of a Cadidus. 



There can be no doubt that the rocks at Tamos Tamos, near 

 Gatun, and at intermediate points as shown by borings, belong 

 to the same formation, and they have been so regarded by 

 Hill and Bertrand. The relation of these rocks, however, to 

 the peculiar breccias and conglomerates at Bohio is not so 

 clearly shown. Bertrand considers that the breccias at Bohio 

 belong to the same series as those occurring in the central part 

 of the isthmus which I have described as the Obispo formation, 

 and he correlates them with the European Tongrian, that is, 

 the base of the Oligocene in the Paris Basin. section. This 

 determination is based on fossils obtained from a boring at 

 Kilometer 24*36. Here, according to Bertrand,* small num- 

 mulites of the Oligocene type are associated with Orbitoides 

 that appear to be the same as those of the Pena Blanca marls 

 ( to be described later ). The breccia, a part of the " G-amboa 

 rock, " is regarded, therefore, as certainly Oligocene and prob- 

 ably Aquitanian. In the geological profile accompanying 

 Bertrand's report a special tint is given to the breccias, in order 

 to accentuate the difference between these beds and the higher 

 series, and they are referred tentatively to the Tongrian. 



Hill appears to regard the breccia and perhaps the conglom- 

 erates at the French lock site as parts of an early igneous forma- 

 tion older thati the Tamos Tamos beds. My own observations 

 make me believe that the conglomerates at the lock site repre- 

 sent the lowest portion of tlie formation of which the Tamos 

 Tamos beds and those in the vicinity of Gatun are the upper 

 parts. This opinion is in accord with the local structure, the 

 prevailing dip of the conglomerates, about 15 degrees to the 

 northwest, being sufficient to carry them below the beds at 

 Tamos Tamos. From a boring made by the Canal Commission 

 at the Bohio lock site (Hole 24-b, K 24), fossils were obtained 

 at elevations of from 20 to 40 feet below sea level that are 

 believed by Doctor Dall to represent an Eocene horizon. 

 They consist of fragments of Zuci?ia, Lima, Pecten, Cardium, 



* Op. cit., p. 5. 



