132REPORT OF BOARD OF. CONSULTING ENGINEERS, PANAMA CANAL.


inclusions, crystalline or vitreous refracting corpuscles. With this vitreous debris there are found, according to Mr. Cayeux, small bodies of organic origin, traces of Foraminifera, and spicuke of sponges. These spicuhe are partly amorphous (opal) and partly crystalized (chalcedony).
 This very fine dust (the largest particles of the d6bris are one-third mm. in diameter) is cemented together by a brown isotropic matter, considerably charged with oxide of iron, doubtless a vitrocrystal line cinder transformed into clay by metamorphosis. Numerous vacant spaces.
 19. Trachyte-The same rock as No. 1, but more altered, at least as regards the magnesian mineral. Crystals of sanidine of a considerable size interspersed with albite; a holocrystalline mass of anorthosis with disarranged streaks of a brown isotropic or almost isotropic substance, which is the result of the decomposition of a ferromlagnesian mineral (probably black mica); a little magnetite and oligist.
 The anorthose occurs frequently in flattened crystals, clearly formed, while in specimen 1 it generally occurs ini shapeless lamelke.
 The banks of the Rio Gigante are formed, according to the only specimen we have f roml them, of a basic cinerite, probably superposed on trachytic tuffs. We give here the diagnosis:
 Cinerite of basic rock. -Fine sand composed of crystals and zoned basic feldspar, of augite crystals, of small lapilli of a much altered, microlitic black rock, and of thinly scattered grains of quartz. The cement is brown and green clay, resulting from the decomposition of the lapilli.


 ADDITIONAL REPORT BY MESSRS. BERTRAND AND Zt#RCHER.
 [From 5S. Do. No. 188, 56th cong., st sess.]

 NOTE ON THE CULEBRA AND EMPERADOR CUTS.

 The works of the preparatory trench (cunette) and the wells accompanying them now enable us to understand with great certainty the composition and general nature of the soil to be removed. We can very happily say that the difficulties previously foreseen are greatly diminished by this more complete examination.
 In the first explorations the explorers were naturally very m-uch struck by the abundance of eruptive rocks, and it was supposed that the central frame of the Isthmus was formed of such rocks of great hardness, in which clay was found only in pockets more or less deep. The first works, on the contrary, by bringing to light a succession of clays of little solidity gave reason to fear the possibility of caving in and the necessity of very gentle slopes. Now, in reality, the axis of the Isthmus is formed of a series of sandstone, pudding stone, and marl of medium hardness, and the eruptive rocks spreading through these sheets or veins play only a secondary part. So far as hardness and solidity are concerned, it may therefore be said that the conditions will be more favorable than could at flst be hoped.
 The geological, formation of the Isthmus seems from the studies already made to display, as the most ancient formation, an eruptive breccia, improperly designated Gamboa sandstone.a7
 It will be met with only in the Emperador cut, where it will constitute less than one-seventh of the excavation. It would not reappear in the Culebra cut, even if the canalI were made on a level. It seems to support in discordance the alternation of pudding stone, sandstone, and mlarl, already mentioned. This system, which is probably of the Upper Oligocene age, ends at the top by a bed of lignites and a bank of lenticular limestone (kmi. 49), the fossils in which give reason to attribute it to the Mioccne. Subsequent eruptions, mostly of a basaltic nature, have given rise to veins, crossing the strata vertically, which, in places, have penetrated between the beds in long intrusive sheets, and which have cropped out in the hills (cerros). The importance of these eruptive rocks, so far as the excavations are concerned, is entirely secondary.
 a The Gamboa rock seems to be closely connected with the conglomerates of Bohiio, in which M. Douvill6 ascertained the presence of numinulites. It is likewise probable that the limestone containing numnulites ill the neighborhood of Emperador, described by Mr. Hill, ought to belong to the same lower series, the age of which is probably Lower Oligocene or Eocene. All these determinations of age, however, are given only provisionally, as the study of the materials collected is not yet finished.
 b The proportion of one-seventh has been calculated on the provisional cut estimated by the axis of the trench (cunette). As the eruptive rock is met with most in the deepest parts, where the width is less, it can be seen that this proportion ought to be considerably diminished, even allowing for the total excavation,


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