EARLIER TERTIARY (EOCENE AND OLIGOCENE). 713 



Directly above a conglomerate that occurs near the railroad station at Gatun is a brownish 

 impure calcareous clay or argillaceous sandstone, apparently containing much debris of volcanic 

 rocks and numerous fragments of shells. Many perfect fossils are to be found, but the extremely 

 crumbling nature of the rock makes it almost impossible to obtain good specimens. Such 

 as were collected have been determined by Dr. Dall as of probable Oligocene age and newer 

 than the foraminiferal beds of Pena Blanca, and the rocks are to be regarded as near the 

 base of the succeeding Oligocene formations that occur at numerous places between Gatun 

 and the bay, notably at the Mindi Hills and at the Monkey Hills. 



The conglomerate under these sandstones contains many fossils that are the same as those 

 of the Bohio and Vamos Vamos beds, but it is beheved that the conglomerate represents the 

 base of the Gatun formation and that the Eocene fossils present were derived from the older 

 rocks and so do not indicate the true age of the conglomerate. 



Soft sandstones of a dirty greenish-gray color, derived from igneous rocks with a calcareous 

 and clayey cement, are the most abundant rocks of the Gatun formation. Associated with 

 them are shales, impure marls, greensands, and, at certain horizons, fine white tuffs composed 

 of pumice, probably of the composition of a rhyolite. * * * The rocks are all well con- 

 solidated, though in a few rare cases sandy layers are found which crumble on exposure to the 

 air. These are the beds that have been referred to frequently as "indurated clays." The 

 term is a misleading one, since true clays make up but a small part of the formation. Indura- 

 tion is, a term applied to the process by which sandstones or argillaceous rocks are converted 

 into quartzites or slates by heat or minerahzed solutions accompanying the intrusion of igneous 

 rocks. None of these conditions existed in the vicinity of Gatun. The rocks are of sedimentary 

 origin and were deposited on the sea bottom at some distance from the shore in the form of 

 sands and clays. Their subsequent hardening into rock is the result of simple cementation by 

 calcareous solutions contained in the sea water and through pressure. Certain beds are harder 

 than others, since the nature of their constituents favored more complete consohdation. The 

 beds, however, are not to be regarded as unconsolidated. They are all "rofck," though in some 

 instances soft enough to be loosened with a pick. 



These beds extend northward with httle change in hthologic character to the Monkey 

 Hills in the vicinity of Colon. Fossils collected in the more northerly locahties indicate that 

 the beds in this region are still younger than those at Gatun. No sharp breaks have been 

 observed, however, and it is believed that practically continuous sedimentation took place 

 from the time the Gatun beds were deposited until the close of later sedimentation. 



The rocks belonging to the Culebra formation have been exposed by excavation from 

 Las Cascadas to Pedro Miguel along the line of the canal, the best exposures being in the vicinity 

 of Culebra. 



The formation consists of a series of soft shales, with sandy, conglomeratic, and calcareous 

 layers abundant. Limestones, as at Empire, Las Cascadas, and in the railroad cut between 

 these two places, are also indicated at several points by borings. Of these the occurrences at 

 Empire (Camacho) are undoubtedly the most massive, the others being present apparently as 

 lenses in the shales and sandstones. Although some thick beds of very homogeneous pure 

 clay shales occur, most of the formation is richly carbonaceous, and at a number of places in 

 the cut lenslike seams of lignite have been found and remains of trees and plants are abundant. 



The age of the Culebra formation can not be fixed with absolute certainty. Fossils col- 

 lected at a number of points between Las Cascadas and Pedro Miguel were not sufficiently 

 characteristic to determine the age of the beds, but at the lock site at Pedro Miguel collections 

 were made at two points less than 100 yards apart that supply more definite information. The 

 fossils from the more northerly point, 30 feet vertically above the other horizon, are regarded 

 by Dall as representing the Oligocene, probably a reef deposit. The material from the lower 

 and more southerly locality is pronounced Eocene by Dall. From the prevailing southerly 

 dip of the beds in the region the fossils from the two localities were beheved in the field to belong 

 to the same horizon. The lack of continuous outcrops, however, makes it impossible to assert 

 positively that this is so. The only fossils collected by Hill from the Culebra formation came 



