HILL : GEOLOGY OF THE ISTHMUS OF PANAMA. 177 



We reach the banks of the Chagres at Gatun, 6 T G °o miles from 

 Colon. This river, flowing through banks of unconsolidated terraues, 

 reminds one of the Little Missouri and Ouachita of the wooded region 

 of Arkansas, but it carries a strong volume. Its banks, sometimes 40 

 feet in height, are of red clay and it has no wide flood plains or con- 

 spicuous second bottoms, such as mark the rivers of the Atlantic slope 

 of the United States. In fact, one will find little comfort in studying 

 its valleys in search of benches and terraces such as mark our own 

 coastal sti'eams. Sometimes I thought I could detect to the west of 

 Gatun at Barbacoas and Pena Negra one bench about 40 feet above the 

 bed, but even this is indistinct. The great playa or swamp (Miller 

 and Young's swamp) between San Pablo and Bujio also may represent 

 older levels of the Chagres, corresponding to estuaries at the time the 

 coast swamps were shallow littoral sea bottoms. From one mile below 

 Bujio, to near its mouth, the stream flows through the Monkey Hill Ter- 

 tiaries. Above that point its tortuous course is mostly through volcanic 

 rock. From Gatun to Bujio the road continues in swamps occasionally 

 running close to the foot of the Quebrancho Hills, and then breaks 

 away across the swamp again. Lion Hill and Tiger Hill, which are parts 

 of this group, are not over 300 feet in height and are all deeply serrated 

 and cut by erosion. 



At Bujio Station (Colon miles) the first igneous rocks are encoun- 

 tered in crossing the Isthmus from the Caribbean side, here consisting 

 of volcanic tuffs (Bujio formation) and the marine sedimentaries of the 

 Caribbean side of the Isthmus, containing fossil invertebrates apparently 

 interbedded with them. There is still another sedimentary formation 

 outcropping southward, which may also belong with the Atlantic sedi- 

 mentaries, which will be discussed later under the head of the Culebra 

 Clays. 



Before entering upon a description of the igneous rocks of the central 

 portion of the Isthmus, let us examine a little more thoroughly the 

 sedimentary rocks of the Caribbean side between Colon and Bujio, as 

 seen in the exposures of the cuttings of the Panama Canal from Bujio to 

 Colon. 



The Foraminiferal Marls. — About one tenth of a mile northwest of 

 Bujio there is a small mound-shaped hill, spoken of by the French en- 

 gineers of the canal as a " mamelon." Upon this hill is built the house 

 of the Chef de Section of the canal. Like all other exposures the surface 

 of this hill is residual red clay, but excavations at its base reveal a 

 peculiar calcareo-arenaceous marl abounding in numerous foraminiferal 



