216 E. Iloive — Geology of the Isthmus of Panama. 



Lithologic character of the Bohio rocks. — Lack of contin- 

 uous outcrops and sudden changes in the character of the beds 

 within short distances have prevented an altogether satisfactory 

 study of the Bohio formation. Boring records have assisted 

 at a number of places, but even with their help the relations 

 at Bohio are not easy to make out. 



Directly south of the village of Bohio on the opposite side of 

 the river is a hill about seventy-five feet in elevation, through 

 the middle of which a cut was made by the French for the 

 construction of a lock. The sides of the cut are now more or 

 less covered by vegetation but still present excellent exposures 

 of the rocks composing the hill. The section shown is about 

 fifty feet in thickness and consists of beds of coarse conglom- 

 erates, gravels, and sands that strike 1ST. 25° E. and dip about 

 14° to the northwest. Crossbedding is common and the beds 

 of finer textured rocks are frequently lenticular, The con- 

 glomerates contain many bowlders a foot or more in diameter 

 associated with coarse gravel or cobbles and held in a gritty 

 matrix that is of the same character as the beds of finer sand- 

 stone. The bowlders are of a number of kinds of eruptive 

 rock, the commonest being hornblende-andesites with less 

 abundant hornblende augite-andesite, augite-andesite and latite 

 porphyry; nearly all are coarse-grained and strongly porphy- 

 ritic. It is noteworthy that of the bowlders examined few 

 resembled rocks observed in other parts of the isthmus. The 

 matrix in which these bowlders lie and the associated sandstones 

 are composed of finer debris of the same rocks ; quartz is so 

 rarely present as to be negligible. It is worth noting in this 

 connection that the bowlders of the conglomerate are of com- 

 paratively fresh rock while the matrix and intercalated sand- 

 stones are invariably decomposed, the alteration being to a 

 complex aggregation of epidote, serpentine, kaolinite and seri- 

 cite that causes the rocks to have a distinctly soapy feel. This 

 is a feature common to most of the fragmental rocks not of 

 direct volcanic origin, wherever alteration has not progressed 

 so far as to result in the formation of the surface red clays. 



Not more than a quarter of a mile north of the conglom- 

 erates exposed at the Bohio lock site, outcrops of a peculiar 

 greenish brown rock are to be seen on the right bank of the 

 Chagres beneath the village of Bohio. The hills to the north 

 and east are composed of the same rock, and just back of the 

 village, quarries are located from which building stone has 

 been obtained for the Panama Kailroad bridge piers and other 

 purposes. The quarrying operations have exposed a vertical 

 section of the rock about fifty feet thick. The rock is com- 

 posed of fragmental materials of volcanic origin ; the imbedded 

 fragments are largely of pyroxene-andesite of a number of dif- 



