E. Hoice — Geology of the Isthmus of Panama. 215 



Age of the Obispo breccias. — The only definite statement 

 that canbe made in regard to the age of the Obispo breccias 

 is that they are older than the oldest sedimentary rocks of the 

 region, which, as will be shown presently, carry a fauna consid- 

 ered by Prof. William H. Dall to correspond to the Claiborne 

 Eocene. Although it is not improbable that the eruption of 

 these andesitic breccias marked the beginning of Tertiary his- 

 tory in the isthmian region, there is no direct evidence that the 

 rocks are not still older. 



MM. Bertrand and Znrcher* included with the breccias that 

 have been described as the Obispo, certain other fragmental 

 igneous rocks exposed near Bohio, calling the whole " Roche 

 de Gamboa.-' From fossil evidence found at Bohio they corre- 

 lated the Gamboa rock with the Oligocene of southern Europe. 

 Reasons are given in a later paragraph for believing that the 

 breccias at Bohio are intimately associated with the oldest sedi- 

 mentary rocks and separated from the Obispo breccias by an 

 unconformity. 



Bohio Formation. 



The name Bohio is proposed for the oldest sedimentary for- 

 mation recognized on the isthmus and includes certain beds 

 occurring at Bohio, Tamos Tamos, and in the vicinity of Gatun. 

 This name is suggested in order to avoid confusion in referring 

 to the same rocks described by Hill and Bertrand, but not rec- 

 ognized by them as being parts of one formation. 



What are believed to be the lowest beds of this formation 

 are conglomerates exposed in the lock site partly excavated by 

 the French near Bohio, and breccias in the quarries at the 

 Bohio railway station. A mile and a half to the west, at a 

 locality known as Tamos Tamos, rocks of the same age are 

 exposed on the south bank of the French canal, and similar 

 rocks, although no fossils have been found among them, occur 

 along the Panama Railroad near Tiger Hill. On the west 

 bank of the canal nearly opposite the mouth of the Gatuncillo 

 the formation is again exposed and the beds carry abundant 

 fossils. At intermediate points borings have shown the pres- 

 ence of rocks lithologically the same as those at Tamos Tamos 

 or near Gatun, but their few fossils have been indeterminable. 

 To the southeast of Bohio borings in the vicinity of Bnena 

 Tista and San Pablo have indicated the presence of beds like 

 those at Bohio, while a few isolated patches of conglomerate 

 that rest on the Obispo breccias in the neighborhood of Mamei, 

 Gorgona, and Matachin, are in all probability a part of the 

 Bohio formation. 



* Etude Geologique sur L'Isthme de Panama, p. 5. 



