REPORT OF BOARD OF CONSULTING ENGINEERS, PANAMA CANAL.


break them up. They separate like marl or granulate like sand, and from the practical point of view it might appear that this phenomenon must cause unfavorable conditions in the cuts. But we may reply that experience has demonstrated that deep excavations, left exposed for more than two years to weather, have stood perfectly without revetment. It would seem that the unstable cohesion mentioned above is more stable in mass than in small fragments, and that the sides of a cut do not act like an isolated fragment. Indeed, this is a question which we should not neglect, especially with regard to the line of the canal, and which may tend to increase the anticipated importance of substantial revetments. Perhaps we may be able to establish a connection between this condition and the rather curious fact that terraces of the great cut seem, during the later work, to stand better the steeper the slopes are. They stand better, at least momentarily, with a steeper slope because they are more massive and better sustained; and to this we must evidently add that the surface exposed to the action of the water andconsequently the influence of this action are so much less as the slope is steeper.
 Alluvial soils and Quaternary phenomena.-In conclusion we must add a word on the Quaternary formations which occur in the great alluvial plains where the valley of the Chagres broadens. They have, unfortunately, a thickness sufficiently great in the region of the projected dams to prevent us from reaching the subjacent rock for the purpose of getting foundations. We know very little about this subject. However, Mr. Zfircher was able to ascertain the existence of an old terrasse" near Bohio, composed of gray, compact, and sandy clay, below which the borings have struck a thick bed of gravel mixed with argillaceous sand. The continuity and the homogeneousness of this clay bed have been established for a considerable area. This layer secures, therefore, a stable base for a great earthen dam. But it is difficult to draw, solely from the existence of this terrasse," any precise conclusion concerning the history of the formation of the valley.
 From the view point of recent earth movements Mr. Hill observed that near the mouth of the Chagres a part of the clays of the plain are of marine formation. But we can not draw any conclusion from this, since we are here almost at the actual level of the sea. Mr. Hill observes, besides, the traces of an ancient plain of marine denudation (ancient base level) on the top of Monkey Hill and the Mindi Hills. This indicates a rise, relatively recent, but of an undetermined age. The absence of the Pliocene along the Atlantic coast, except at one point on the-coast of Costa Rica,'shows, on the other hand, that the sea at this epoch reached less far than at present. The indications are, therefore, opposed to this theory, and only denote an alternation of rising and sinking movements without permitting us to draw any precise conclusion. On the Pacific slope the bay of Panama, with its islands standing isolated, seems to show a more marked tendency to submersion, and the impression is confirmed by the examination. of the contours, which again seems to suggest an ancient submerged valley.
 Re'snre'-Cross section.-The central bulging is formed by eruptive Oligocene rocks. In a kind of depression of the bulging ridge are lodged the calcareous sandstones of Alhajuela and the trachytic tuffs. On the two slopes the eruptive breccias pass into the conglomerates, where strata containing nummulites are intercalated; it is the Tongrian or the lower Aquitanian. Then, on the Atlantic side follow, first with "a rather steep slope, afterwards in undulations, and finally horizontally, the successive strata of a continuous sedimentary series-calcareous rocks with cebitoides, the glauconeous layers of Vamos, the sandstones of Gatun, which are equivalent to the mollasse" (Fr.) of Alhajuela, and the clays of Monkey Hill. On the Pacific side is reproduced almost symmetrically the same characteristics of a rather abrupt slope, followed by more or less marked undulations; but the marine series is replaced by a partly marine series with lignites, where the tuffs and the eruptive flows play an important part. The most recent basic rocks, which are presumably Miocene, fill a series of vertical chimneys between Pedro Miguel and the Culebra, and show their preserved expanse only on the Pacific edge of the bulging, along the more abrupt slope of the formation.


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