226 K Howe — Geology of the Isthmus of Panama. 



Caribbean, calcareous rocks similar to those near Las Cascadas 

 and Empire are exposed at many places between Cruces and 

 Dos Bocas, a distance in a straight line of about twelve miles. 

 Between Cruces and Palo Grande compact limestone similar to 

 that at Empire rests on carbonaceous shales like many in the 

 Culebra beds. The rock is partly crystalline and contains in 

 places fragments of shells. About one mile above the mouth 

 of the Chilibre is the lower end of a winding gorge through 

 which the Chagres flows for nearly twelve miles, the entrance 

 to the gorge being a mile below Dos Bocas. The river, 

 entrenching itself in an old valley, encountered between the 

 points mentioned limestones and calcareous sandstones which 

 offered greater resistance to erosion than the rocks to the north- 

 east and southwest, so that its former meandering course has 

 been preserved very perfectly, and at the outside curves of the 

 meanders are nearly vertical cliff exposures of massive cal- 

 careous sandstone and limestone, in places more than one hun- 

 dred and fifty feet high. Although the length of the gorge 

 following the river is approximately twelve miles, the belt of 

 hard rocks traversed is only about four miles wide. 



The rocks are all of a light cream or buff color and range 

 from partly crystalline limestones to coarsely granular rocks 

 composed of broken shells, sands, specks of magnetite and 

 occasional pebbles of igneous rocks held in a calcareous cement. 

 Certain layers are well bedded, others massive, while cross- 

 bedding is not uncommon. Fossils, Ostrea and Pecten, are 

 abundant, but I was unable to find any determinable species ; 

 at a locality about a mile below Alhajuela a bed composed of 

 broken corals and fragments of molluscan shells was found. 



The similarity of some of these rocks and their fossils with 

 those observed near Las Cascadas is striking, and Bertrand and 

 Zurcher regarded the two occurrences as of the same age.* I 

 agree with this opinion in so far as the Chagres and Las Cas- 

 cadas beds are correlated with the fossiliferous rocks exposed 

 by the canal at Kilometer 10 near Gatun, whose fauna, as has 

 been said, Dall considers Claiborne Eocene ; the French geolo- 

 gists have compared the Chagres rocks with beds near Mar- 

 seilles that grade from Upper Oligocene to Miocene. Bertrand 

 states, on the authority of M. Boutan,f that the limestone 

 near Dos Bocas resembles in part certain marls occurring at 

 Pena Blanca, and that it contains foraminifera. I am not pre- 

 pared to say that this is not the case, but the structure of these 

 beds as noted both by Bertrand and myself would carry them 

 beneath the limestones of the gorge, and they should corre- 

 spond closely in position to the limestones observed near 

 Cruces and therefore be comparable to the Empire limestone. 

 *Op. cit, p. 10. fOp. cit., p. 10. 



