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 deseriptions of Gabb 
that the same phenomena are extensively developed throughout that 
region. A study of the hydrographic chart of the Chirigui 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 deseribed 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 Atlantie, 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. І 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 сайопв 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 mear sea level 
to its present height of from 700 to 900 feet, but the fact that it is 
composed of rolled volcanic débris somewhat modifies this hypothesis, 
for it would be just as reasonable to imply that this plain had been built 
up by voleanie ejecta, and hence I shall not present a positive opinion 
that any continental elevation may be interpreted from its occurrence. 
There is little reason to doubt that the entire Isthmian region after 
