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. 



TJie 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 greeusand beds of the Caribbean sedimenta- 

 ries as at Monkey Hill, in that they consist of alternations of distinct 

 laminae 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. 



TJie 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 



Figure 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 



