HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. ДОГ 
We reach the banks of the Chagres at Gatun, 61605 miles from 
Colon. This river, flowing through banks of unconsolidated terranes, 
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 Atlantie 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 streams. 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 Culebr: 
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 
