HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 191 
which have been partially destroyed by erosion and covered by later 
deposits in the whole section. At no point could I find any trace of 
modern lava flows or erupted material In all cases they appear to be 
the intrusive necks of igneous rocks or buried flows. 
These rocks constitute the lowest strata exposed in the Mata Chin 
section. 
The Red Clays. — Before leaving this section mention must be made 
of certain red clays seen along the railway between Juan Grande and 
Gorgona. These differ from the homogeneous residual clays of the 
igneous rocks, and the greensand beds of the Caribbean sedimenta- 
ries as at Monkey Hill, in that they consist of alternations of distinct 
laminz of white and red material. They do not present the sedimental 
irregularities of river alluvium, and strongly resemble certain phe- 
nomena to be further described in the discussion of the Culebra 
section. 
Just south of Mata Chin the line of our section leaves the Chagres 
River, and takes up the course of the Obispo. The main Chagres makes 
a great bend at this point, almost a right angle, its headwaters deflect- 
ing eastward into the unexplored interior region, while in that portion 
of its course between Mamei and the mouth of the Obispo it follows 
the gorges we have described. 
The Bench at Mata Chin. — There is some slight evidence of an old 
planation surface representing an ancient level seen in a high bench sur- 
rounding a pointed mountain to the west of Mata Chin. This bench as 
shown in the accompanying illustration (Fig. 9) is fully 150 feet above 
Ficurn 9. Topogaphy in Vicinity of Mata Chin, showing supposed Ancient 
Bench of Chagres. 
the present river, and may be, geomorphically speaking, a continuation 
and correlative expression of the drainage curve during the period of 
the Monkey Hill base level. How far up the Chagres similar phe- 
nomena may extend cannot be said, but this is the last of the pos- 
sible planation surfaces indicative of old river history to be seen in 
crossing the Isthmus. Thirty-three miles from Colon the gradient of 
