150 REPORT OF BOARD OF CONSULTING ENGINEERS, PANAMA CANAL.


 General trflctre-Thie Gawtboa rock.-The oldest formations were found in the central part of the Isthmus, where their horizontal position forms a large undulating plateau, slightly inclined in its whole extent toward the Atlantic and dipping quite rapidly at its two extremities under the more recent sedimentary strata. The general aspect is therefore that of a wide, low, anticlinal arch.
 The central part is formed by the volcanic rock of Gamboa, which presents itself almost everywhere as breccia, the cement of which is clearly eruptive, and often of an entirely analogous nature to that of the embedded fragments. True overflows of eruptive rock alternate with the breccias. According to Mr. Hill there are two distinct formations, and at Matachin the breccias can be seen resting uncomformably on the massive rock. Such unconformities are, at the same time, very difficult to ascertain and of little significance. However, everything seemed to us to show an intimate connection between the overflows and breccias.
 The probable age of these rocks can be determined from the following considerations: Onl one hand, 6n the southwest slope, toward Obispo, we are unable to perceive any clear demarcation between the upper breccias, mixed with the volcanic rock, and those which form a part of the Aquitanian series of the Culebra; onl the other hand, andesitic flows, intercalated in this Aquitanian series, are identical with certain elements of the Gamboa rock; in ally, a more precise fact is furnished by the nummulites and the orbitoides which Mr. Donvilhe recognized near Bohio.
 The outcrops of Bohio are not, it is true, in continuity with the principal outcrop of the Gamboa rock, which along the plain of Tavernilla is overlain with trachytic tuf. They have,moreover, partly the appearance of conglomerates altered by water; thus, in the rocks of the banks of the Chagres River a part of the embedded fragments, rounded or angular, seem to be rocks of a peculiar aspect and of a strange origin. It should be noticed, however, that a microscopic examination reduces the differencee and seems to show that they are rocks of the same composition, but consolidated-at a greater depth and torn off from a neighboring crater. At least the breccias of the quarry of Bohio, formerly worked for the railroad construction, have a less decomposed cement, which per~its their clearly eruptive nature to be established. It is, as Mr. Boutan says, a true solidified volcanic mud.
 The association of the breccias of Bohio with the Gamboa formation seems to us unquestionable; and, on the other hand, it is certain that under these breccias the boring at kilometer 24.36 discovered strata containing nummulites. These are very small, of the Oligocene type, and they are associated with orbitoides, which seem to be the same as those of the Aquitanian limestone formation of Peiia Blanca. There is, therefore, a confirmation of the gradual change indicated on the other slope of the anticlinal. arch. The Gamboa rock is certainly Oligocene and probably Aquitanian.
 It is important to clearly specify that, Tongrian or Aquitanian, the Gamboa rock represents the oldest formation of the Isthmus. Wagner has spoken of Permian sandstone on the Pacific coast, but this is only tuff and trachytic breccias reddened by decomposition. Mr. Hill is tempted to assign this tuff to the Cretaceous age. We shall see that they are clearly superposed onl the Gamboa rock and even on the fossiliferous Miocene limestone.
 Atlantic slope.-If, taking the breccia and the associated nummulitic strata as a starting point, we pass toward Colon on the Atlantic we meet with a series of marine strata of an age more and more recent as we approach the coast.
 These are first glauconitic clays, rich in Foraminifera and full of debris of the neighboring volcanic rock. Mr. Hill says that near Pefia Negra these clays lie in discordance against the breccias.
 It seems probable, from what precedes, that this nonconformity is local and due to a change of the course 'of the waters rather than to the movements of the strata. Mr. Dali determines the fossils of these clays to be Oligocene.
 These strata, from which we get no fossils, have, moreover, an imperfect development. They, are almost immediately overlaid by the orbitofdes limestone of the Peia Blanca. "These orbitoYdes, saysM.Do el6 "belong to the genus 'Lepidocyclina,' Gflmbel; furthermore, all


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