hill: geology of THE ISTHMUS OF PANAMA. 193 



Cerro Cordo, Cerro Pruja, Cerro Mitra, and Cerro Keyo, which surround 

 the basin. These clays likewise filled the so called Culebra Pass and 

 extend into it, but not far beyond the col, towards the Pacific side. It 

 is their presence which has caused the engineering difficulties accom- 

 panying the cutting at this place. 



The combined section exposed in the pass between Cerros Lirio and 

 Culebra, and on the slopes of Culebra hill above the lowest cutting in 

 the canal (246 feet) to the summit (623 feet), and brought up by the 

 drillings of the Canal Company from 28 feet below sea level, reveals 

 nearly 500 feet of this clay. The lithologic character can best be seen 

 in the fresh exposures of the latest of the excavations of the Canal 

 Company, in progress while I was upon the ground. They consist of a 

 mixture or rather alternations of laminae of dark drab clay and sand, 

 and resemble in general aspect the clays of the lower Eocene which have 

 such extensive development through Alabama, Mississippi, Arkansas, 

 and as far south as Tampico in Mexico. They have such an ancient look 

 that I should hesitate to consider them of recent origin. 



The following section was made from the lowest diggings of the canal 

 in the Culebra Pass to the summit of Culebra Mountain on the east 

 side of the pass, as shown in the illustration. (See Plate XII.) 



Section of the Culebra Pass, beginning at the line of the lowest Canal diggings, January 

 19, 1895, altitude 246 feet, to summit of Culebra, altitude 623 feet. 



Cut 1. a. Dark brownish black, laminated, unctuous clay shales with alterna- 

 tions of sandy layers, much finely disseminated lignitic matter, and containing 

 imprints of dicotyledonous leaves. These beds are greatly distorted, and have a 

 prevalent dip to north. Canal bottom in these clays. Thickness, 5 feet. 



b. Oxidized bed of similar material to a, soft earth of mealy consistency, resem- 

 bling decomposed tuff, with a layer of very small pebbles at base, and an indurated 

 clay layer at top, containing boulders of apparently igneous rock. According to 

 Prof. Wolff this material is a gray tuff, with fragments of feldspar and basic ande- 

 sitic lava, consisting of a gray agglomerate of small fragments closely pressed to- 

 gether. The cement is a fine chert material with solid grains of black ; the feld- 

 spar crystals are quite perfect, but the rocks appear to be sedimentary. The small 

 black pebbles fonnd in the clay are, according to Wolff, chert, cherty tuff (with 

 feldspar fragments), silicified tuff with large feldspar fragments, and finegrained 

 black siliceous shale with feldspar fragments. Thickness, 5 feet. 



Cut 2. c. Similar beds of clay to 1 and 2. 20 feet. 



A few hundred feet east of the line of the above section, in the lowest cutting 

 of the canal (Cuts 1 and 2), two more interbedded sheets of the tufaceous material 

 were observed, both with a strong dip towards the Pacific. These were only a 

 few feet in thickness and one of them was strongly faulted down to the southward. 

 The clays clearly lie against it, and between it and the Cerro Lirio, to the north, 

 which is of a similar nature. 



