Hkrshky. I 



Isthmus of Panama. 



-59 



In this formation near Aguadulce are many small, smooth, sub- 

 angular pieces of limonitic chert and hardened clay which have a 

 peculiar semi-glazed surface and a uniform light brown color. A 

 great number of these same water-worn brown pebbles are scattered 

 over the plain far from the sea, particularly in the vicinity of the 

 more important stream courses. They appear to be the river 

 deposits of a certain age, as none were formed before or since. It 

 is a singular fact that very similar pebbles of semi-glazed brown 

 river gravel are widely distributed in the drift of northwestern 

 Illinois, where I have described them under the name of "Freeport 

 gravel," and they occur in southern Missouri and Arkansas, where 

 I have traced them into connection with the Lafayette deposits. 

 No suggestions of correlation are intended by this comparison. 



The Aguadulce formation represents a slight local depression 

 immediately preceding the uplift of the Pleistocene peneplain and 

 may be classed as Middle Pleistocene. 



The San Carlos Formation. — At many places on the Pacific 

 side of the Isthmus are remnants of a coastal plain of aggradation 

 which has been uplifted 20 to 100 feet above high tide level and 

 partly destroyed by marine action. A beautiful example is the 

 San Carlos plain, lying between 30 and 50 miles west-southwest of 

 Panama, and extending from a 100-foot white sea-cliff inland prob- 

 ably five miles to the base of a mountain mass. Since its uplift, it 

 has been dissected by narrow canon valleys, but the inter-stream 

 portions in general remain intact so that from a distance it looks 

 like a remarkably level plain. The divides are flat instead of 

 slightly arched as in the case of the Pleistocene peneplain. As 

 might be expected, the sea-cliff shows that the plain is composed 

 of a marine deposit of mainly horizontally stratified fine sand and 

 beach gravel. 



The sea-cliff about one mile east of San Carlos exposes the for- 

 mation to perfection. In the maximum thickness of 100 feet there 

 is included three divisions. At the base is a bed of very coarse 

 gravel and boulders up to several tons in weight, which is well lith- 

 ified and composed largely of the porphyritic and crystalline vol- 

 canic rocks of the Isthmus, in particular a red porphyry. It is of a 

 dull brownish color. The surface is very irregular, rising from 



