HILL : GEOLOGY OF THE ISTHMUS OF PANAMA. 255 



phenomena along the intervening coast line between Colon and Port 

 Limon, there can be but little doubt from the descriptions of Gabb 

 that the same phenomena are extensively developed throughout that 

 region. A study of the hydrographic chart of the Chiriqui lagoon 

 shows that the vast number of islands in that region, as well as the 

 adjacent mainland, are marked by numerous low escarpments and flat- 

 topped terraces corresponding to those of the Monkey Hill level. Maack 

 has also described certain low hills near the mouth of the Atrato which 

 are strongly suggestive of the continuation of the Monkey Hill bench 

 in that direction. 



It is difficult to correlate the phenomena of the Pacific side with 

 those of the Atlantic, owing to the great difference in the lithologic 

 composition of the respective coasts, resulting in the variation of the 

 topographic expression, as well as the fact that the entirely different 

 tidal waves have produced on the opposite sides at synchronous epochs 

 quite different details of topography. I have called attention, how- 

 ever, at Panama and the islands of Panama Bay along the entire coast 

 to Punta Arenas, as well as on the Coiba and Jicaron Islands, to the 

 persistent occurrence of a low bench about 100 feet above the tide. 

 These phenomena everywhere present the appearance of an ancient 

 base levelled plain, corresponding in altitude to the Monkey Hill level of 

 the Caribbean coast. It is also interesting to note that, extending far up 

 the Reventazon River of Costa Rica, there are extensive playas or flat 

 valleys in the canons which apparently represented the old base level 

 of that stream at the time the Monkey Hill bench was down to the 

 sea. The occurrence of these levels on both sides of the continent is 

 very suggestive of the fact that a general uniform regional elevation, 

 whereby these areas, which were near sea level at the time that the 

 swamp levels were beneath its surface, have been brought up to their 

 present height. 



A still older and higher base level may be possibly represented in 

 the San Mateo Plain, lying between Punta Arenas and the Aguacate 

 range of mountains of Costa Rica. This certainly has the aspect of 

 an old base levelled plain which has been elevated from near sea level 

 to its present height of from 700 to 900 feet, but the fact that it is 

 composed of rolled volcanic debris somewhat modifies this hypothesis, 

 for it would be just as reasonable to imply that this plain had been built 

 up by volcanic ejecta, and hence I shall not present a positive opinion 

 that any continental elevation may be interpreted from its occurrence. 



There is little I'eason to doubt that the entire Isthmian region after 



