40 THE ISTHMUS OF PANAMA, 



conclusion that at no remote period the two Ameri- 

 cas were completely separate, the ocean flowing 

 freely and uninterruptedly between them, and occu- 

 pying nearly all the space from Tehuantepec to Da- 

 rien, constituting, in fact, but one isthmus, although 

 known by different names. On the highest peaks, 

 which have been simply elevated by interior force, 

 marine shells of recent origin are frequently found. 

 The forces to which the formation is due, Colonel 

 Hughes supposes to have acted principally in two 

 different directions ; the one uplifting the main con- 

 necting link of the two continents, and the other 

 elevating the transverse ridges. From these axes 

 other diverging or radiating ridges have been thrown 

 out short distances in the direction of the lines of 

 least resistance. The valley of the Chagres can 

 scarcely be regarded strictly as one of denuda- 

 tion. It is probable that when this country was up- 

 heaved, there was no well defined outlet between 

 the summit and the Atlantic, but that the secondary 

 ranges were connected with low narrow ridges, in 

 consequence of which a series of lakes were formed 

 by the first rains, at different levels, falling succes- 

 sively towards the ocean. The accumulation of the 

 water at last broke through these slight barriers, and, 

 in the natural course of things, reduced the outlet to 

 a nearly uniform plane, the highest of these lakes, at 

 a distance of forty -four miles from the ocean, having 

 been but fifty feet above it. The valley of the Cha- 

 gres has evidently been formed, not by the deposi- 





