HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 215 



eral beds of the Mamelon, the Vamos a Vamos, and the Mindi marls, 

 have no representation on the Pacific coast that I could find. 



The Panama (Barbacoas) formation is the only sedimentary rock 

 common to the Caribbean and Pacific sides of the present continental 

 drainage divide. The basic igneous rocks are common to the whole 

 area. 



The late Eocene and Oligocene (early Miocene) beds are undoubtedly 

 Caribbean sediments, and no structural evidence has been found that 

 they were deposited completely across the present area of Isthmian 

 land. Their composition, which includes volcanic debris and plant re- 

 mains, is such as to indicate that they represent the littoral debris of a 

 more extensive land which once existed, especially to the southward in 

 the direction of the Pacific. 



The Pleistocene or recent elevated coral reefs of the Caribbean side 

 as shown by Verrill are composed entirely of Atlantic species. 



Red Clays. — The classification of the surface red clay which conceals 

 all the formations of the Isthmus has caused the greatest perplexity. 

 The sub-aerial decay of the rocks on the Isthmus is simply enormous, 

 extending often to a depth of over 100 and seldom less than 50 feet. 

 The excessive rainfall and immense quantity of vegetal humus and 

 carbonic acid have no doubt expedited this sub-aerial decomposition. In 

 fact this decay is so great that the whole country is covered with a 

 mantle of red clay. It is largely the local residuum of the underlying 

 rocks, the basic igneous rocks, the Culebra clay, the greensand marls of 

 the Tertiary, the alluvial clays of the rivers, or whatever particular 

 formation they may locally overlie. In most regions which I have pre- 

 viously examined I have become able, by studying the character and 

 gradations of the residual material at the surface, to learn properly to 

 associate it with the underlying formation from which it had been 

 derived ; but upon the Isthmus I never became able to do so, and could 

 only distinguish their relations by the aid of artificial cuttings revealing 

 gradation into the underlying substructure. In extensive areas where 

 no exposures of the substructure are visible their geologic interpretation 

 is very difficult. Some of these red clays, particularly those derived from 

 the basic igneous rocks, are of homogeneous texture. Others show a 

 distinctly laminated character. This lamination was for a time thought 

 to be peculiar to the residuum of the substructure of the Culebra clays, 

 but I am unable to formulate a definite conclusion on this subject. The 

 residuals of the river deposits and of the Monkey Hill Tertiaries all 

 weather into similar looking red clays. Even on the Pacific slope, in 



