REPORT OF' BOARD OF CONSULTING ENGINEERS, PANAMA CANAL.15


62 and 63, then it disappears to give place to gray trachytic tufts, which increase more and more as they approach the coast. The second point is the incontestable superposition of the calcareous tufts, discovered in the borings of kilometers 62 and 64.10. The trachytic tufts are therefore more recent than at least one part of the limestone containingPctnsheroce, i. e., than the base of the lower Miocene. They are associated with trachytic rocks (Cerro Ancon, near Panamna), and are certainly of marine origin; for, in the identical rock of Barbacoas, Mr. Caycux found a foraminiferous specimen and Mr. Boutan brought from the Panama coast a small gastropod (cerite?) in a specimen full of Foraminifera and associated with 'trachytic tufts.
 We have said that Mr. Hill considered the trachytic tufts as very ancient and presumably Cretaceous. The other observers (Wagner and Boutan) had only indicated that these tufts must be previous to basic eruptions. The first of these suppositions can not be sustained any longer. As to the second opinion, it is only exact if we distinguish, as we shall, two series of basic eruptions, the tufts being, from what precedes, certainly of a later age than the Gamboa rock.
 The stratigraphic position of the trachytic tufts and their marine origin leads us to see in them the representative of the argillaceous series of Monkey Hill, where strata of tuftaceous appearance are intercalated, rich in crystal, and containing especially orthoclase. This result is important, because it shows us the Isthmus as formed by a symmetrical bulging, whose summit does not coincide with the line of the ridge, but is situated about in the middle of the isthmnus. The center of the bulging is formed by the volcanic breccia (ancient eruptive series), and on both sides strata of the same age sink symmetrically toward the two oceans. Only, on the Atlantic side, the series is certainly marine and without volcanic intercalations; on the Pacific side it is partly marine and rich in eruptive intercalations.
 According to the data collected by Mr. Wagner it seems probable that the breccia of Gainboa reappears in the islands of the bay of Panama.- That would lead us to suppose that the central bulging of the Isthmus has not had, since the Oligocene epoch, the importance which the difference of composition of the two slopes would at first tend to give it, and that the true cause of the difference is the existence of a coast line in the southeast. There is, perhaps, here a very interesting consideration reserved for the history of the formations of the Isthmus.
 Uentrcd part-Plateau of Alkafela.-It remains now to consider the central part. The canal cut shows only a mighty development of the breccia of Gamboa, With a fragment of lignitic schists still preserved at its surface, near Matachin. But in the railroad cut we find trachytic tufts superposed on the breccia, and these tufts- seem to be inseparable from those which are shown near the bridge of Barbacoas and are identical with those of Panama. The central part presents here the Gamnboa breccia, directly covered by trachytic tufts.
 If we proceed toward the east, following the valley of the Chagres, we find at first, in the river bed near Las Cruces, an outcroppng of compact limestone with polypiers, which represents perhaps the same Aquitanian level as at Emperador. But farther up the river the bed of the Chagres and the entire plateau as far as Alhajuela are occupied by compact and well-str atified calcareous sandstone, where we again find the fauna of kilometer 49. There is, first, the Pecten subpleurowectes, then the /urritella tornata; there is also a large Clypeaster, of a type which is different f rom the Aquitanian type of Schio; there are also the large Bivalves, among them the Mfftilus mickelini. The rock is quite similar to that of our u ropean mollasse (Fr.), and the character of the fauna is very similar to that of the strata of Carry, near Marseille (Upper Oligocene changing into the Miocene). The Clypeaster and the Turritella connect rather the whole with the Miocene, which is, however, only an insignificant difference and a question without importance considering the continuity of the deposits. Trhe microscopic analysis shows in much of the volcanic d bris (without recognizable orthoclase) bryozoaires and numerous Foraminifera, among them a globigerine.
 In ascending beyond Alhajuela we find, according to Mr. Canelle, white limestone, determined by Mr. Boutan as partly identical with the rock of Pefia Blanca and containing here and there large orbitoides, This limestone sinks gradually and very regularly under the preceding series.


153