Central American Geology. 421 



its starting-point the study of the fossils brought by M, Zurcher. 

 The authors also refer to the works of Messrs. Hill, Dall, Wagner, 

 and Boutan in this connection. 



The Panama memoir is illustrated by two coloured sections — one 

 a general section of the whole 74 kilometres of the canal ; the 

 other, on a larger scale, of the great cutting (Emperador and 

 Culebra) between the 48th and 56th kilometres. The range in 

 time covered by these sections extends from the Tongrian with 

 Nummulites to the Quaternary, the Aquitanian and Miocene being 

 represented, but the Pliocene being absent. 



General Structure of the Isthmus : Bock of Gamhoa. — The oldest 

 formations occur in the centre of the Isthmus, wliere their outcrop 

 forms a large undulating plateau ; these pass under more recent 

 beds on both sides, the general arrangement being a very flattened 

 anticline. The central part is formed by a volcanic rock — the rock 

 of Gamboa, which is described as consisting of breccias and lava- 

 fJows, with which at the northern end is associated a bank of 

 Nummulites. Whether we regard the rock of Gamboa as of 

 'Tongrian' or 'Aquitanian' age (i.e. Oligocene), it represents the 

 most ancient formation of the Isthmus. Wagner has spoken of 

 Permian grits on the Pacific coast, but these are merely trachytic tutfs 

 and breccias reddened by decomposition. Mr. Hill has endeavoured 

 to assign to these same tuffs a Cretaceous age ; we shall see, however, 

 that they are clearly superposed on the rock of Gamboa, and even on 

 Miocene fossiliferous limestones. 



The Atlantic Slope.— Taking the breccia and the associated beds 

 with Nummulites as a datum-line, we find in the direction of Colon 

 a series of marine beds more and more recent as we approach the 

 coast. This series, which is perfectly continuous, contains the debris 

 of older eruptive rocks, but without any lava-flows, and extends in 

 time from the ' Aquitanian ' to the Lower Miocene. Mr. Dall had 

 previously determined the fossils of some of these beds as Eocene 

 (Claiborne), but their superposition to beds with Orbitoides is certain. 

 The cnlcnire a. Orbitoides itself, associated with the sandstones and 

 clays of Vamos, are regarded as Aqnitanien marin. The highest beds 

 on this side, excluding the Quaternary, are the argiles de Jlindi of 

 Lower Miocene age. The fauna, which is very rich and well 

 preserved, represents that of the Miocene of the Antilles. 



South-West Slope :. Cutting of Culebra. — Passing to the other side 

 of the central anticline, we find a series of beds almost symmetrical 

 with the preceding, but which differ in containing brackish inter- 

 calations with vegetable matter and abundance of eruptive rocks. 

 In the coloured section these are generalized under the heading 

 " Tufs et gres de la Culebra (Aquitanien schisteux et lignitifere)." 



I'he whole of this ' Aquitanian ' series is pierced by the ' andesites 

 de la Culebra' of Upper Miocene age, and the peaks of the Cerro 

 Culebra are composed of these same andesites. Above this very 

 extensive and varied series of beds of ' Aquitanian ' (i.e. Upper 

 Oligocene) age, there come the equivalents of the Lower Miocene 

 of the Atlantic side, reinforced by beds of trachytic tuffs. 



