18-4 



BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



Station is reached, where the stream of the same name has cut down 

 through the clays to a black rock similar to that of Bujio. 



At Tavernilla Station (Colon 21.5 miles) the Canal Company has cut 

 into a small hill apparently composed of red clay. At its base is the 

 first undoubted exposure of a peculiar rock which may be called tem- 

 porarily the Barbacoas formation. 



At Barbacoas (Colon 23 miles) the railway crosses the Chagres upon 

 the bridge resting on a vertical bluff of some 30 feet in height. Rising 

 above the track is a hill some 40 feet or more, making in all an expos- 

 ure of some 70 feet of the rocks which dip slightly northward towards 

 the Caribbean coast. 



Figure 6. Section across Chagres River at Barbacoas. 



The first sight of this bluff created the impression of a stratified chalky 

 limestone, but closer examination showed it to consist of a loosely 

 cemented, white earthy rock, composed of firm fine particles, apparently 

 siliceous, but water sorted, and showing distinct lines of lamination, in 

 alternating degrees of fineness and coarseness. Throughout the mass 

 were numerous white specks of a softer material, which seemed to be the 

 small rounded decomposing pebbles of rhyolite. Macroscopically this 

 earth strongly resembled the Radiolarian beds of Cuba, and the volcanic 

 glass deposits of the Great Plains region of the United States. 



I was also fortunate in finding the basement relations of the Barba- 

 coas beds at the village of San Pablo, half a mile to the west of Bar- 

 bacoas. About 100 yards west of the station of San Pablo the white 

 Barbacoas formation grades down into a mass of brownish rock, studded 

 with fragments of decomposed light blue eruptive pieces embedded in it. 

 It has an earthy texture and is easily cut with a hammer. It is of light 

 specific gravity, and apparently a long decomposed conglomerate of vol- 

 canic material. This we may call the San Pablo phase of the Barba- 



