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 REPORT OF BOARD OF CONSULTING ENGINEERS, PANAMA CANAL. 155

which raised the strata of the Isthmus. Furthermore, we find, according to Mr. Hill, on the Atlantic coast of Costa Rica, proof that these movements are anterior to the Pliocene formation. We have here, therefore, a confirmation of the fact that these rocks belong to the Miocene period.
 Superficial red clay.-We have now to mention a formation which has no special age, but which is, however, the best known and best shoxfn on the Isthmus, and one which at once impresses itself on our mind when we speak of the great cut; we mean the red clays. These clays, peculiar to the tropical climates, are the result of a disintegration of the strata by atmospheric influence. When this disintegration is complete it leaves a red, ferruginous, unstratified mud; but it contains all the intermediate .degrees, and the incipient decomposition may have left certain characteristics of the strata, and particularly the division into well-defined banks. That is to say, we are in the presence of an apparently sedimentary system with lithologic characteristics and seemingly corresponding to a special formation. This notably occurs in the upper part of the great cut, where the red clays are still frequently well stratified. They resemble very much a more special sedimentary system which places all the others in discomformnity, as we distinguish there more layers with intercalated breccias. They represent, nevertheless, a species of more or less advanced alteration.
 When there exists in the series an impermeable clay bed it is natural that the alteration stops at this layer. The series of red clays often rests on a clay bank which would, especially in the case of abundant infiltration, favor sliding. The danger is especially great when the slope of the layer lies toward the cut. In that case a feeble shock, or even the weight of the material alone, may be sufficient to make these clays slide on their inclined and lubricated bed; and if their mass is considerable it is believed that, when once set in motion, nothing could stop it. We have lately seen, though in other regions but under similar conditions, an impressive instance of these phenomena in the sliding of. the Gouffre, on the banks of the Gardon River, in the Alais basin. We have seen, even in Panama, an equally famous case, although on a much more reduced scale, in the landslide of Cucaracha. The precautions to be taken in order to prevent similar accidents are well known and have already been applied with success.
 Besides we may add that the arrest of decomposition at an impermeable layer which serves as a base for the red clays is more theoretical than real. Indeed, there is rarely a distinct line of demarcation; more often the clays carry with them their substratum on all intermediary passages. In that case a movement of the whole is not to be feared, and the danger Would be confined to some local landslides. At Cucaracha the level of the clays was reached long ago, and the great width given to the old excavation is already a guaranty. It is, however, advisable, because of the slope mentioned and the thickness of the clays, to reenforce and rather to exaggerate, on this point, the measures to prevent sliding.
 Alterations of the strata in depth.-Here is the place to speak of another mode of alteration, less apparent and probably also due to special conditions of the climate. The clayey bed which stops the deep changes of the terrain does not entirely prevent the circulation of the waters in the deep strata, and from this circulation a very curious and special phenomenon results, which has been made evident by cutting thin slices for microscopic examination. Rocks apparently compact and solid disintegrate rapidly when wet. After in immersion of a few minutes a slight pressure, even of the finger, crumbles them, and this has even occurred in the case of an eruptive rock. To be able to make the thin slices it was necessary to immerse them in oil and not in water. This fact is explained, to a certain point, when we examine these cuts under the microscope. All of these rocks are traversed by argillaceous filaments which feebly depolarize light and which form with crystalline residues the body of the rock. This clay, according to Mr. Cayeux, differs considerably in appearance from that which he had occasion to examine in the rocks of our country, and appears frequently to be the product of decomposition, although very difficult to distinguish from true argillaceous deposits shown in other samples. In the eruptive rocks it is the vitreous particles that seem to have undergone this decomposition.
 Whatever may be the chemical cause, the fact is certain. Rocks apparently solid and freshly taken from deep shafts possess only an unstable cohesion, and it requires only a slight effort to


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