Hershf.y.| 



Isthmus of Panama. 



263 



slight at the heads of deep bays. This seems to point to a slight 

 tilting of the interior toward the sea. Indeed, I am not certain that 

 the real nature of the movement was not a slight arching of the 

 Isthmus as in previous periods of disturbance. Away from the 

 coastal lands, especially in the high mountains, I found, instead of 

 traces of a recent subsidence, rather strong evidences of an uplift 

 which was probably contemporaneous with the submergence of the 

 coasts. 



The geologically very recent age of the depression is exem- 

 plified by the small amount of marine erosion which has been 

 effected on the precipitous headlands and mountainous islands. As 

 Mr. Hill has mentioned, the cliffs of marine erosion are only from 

 20 to 100 feet in height, and above them the land presents the type 

 of topography indicative of slow subaerial erosion. Since the coast 

 line reached its present position on the land slopes, the bench cut 

 by the waves is insignificant, comparatively speaking, and can not 

 have required a very long time. A movement and consequent 

 shifting of the shore-lines has certainly occurred recently. It was 

 not one of uplift of the land, for there are no raised shore lines. 

 Wave-cut benches above the present shore line (the single marine 

 shelf of the Middle Pleistocene base-level excepted) are totally 

 absent from the Isthmian country. As indicated by various phe- 

 nomena, the recent movement has been one of subsidence on the 

 coasts. 



The Parita Formation. — Around the head of shallow, sheltered 

 bays, there have been built up, since the subsidence, low coastal 

 plains, sometimes of considerable extent, even several miles in width. 

 The best developed is at the head of the Bay of Parita. It consists 

 of dark bluish gray silty muck and gray sand. This has been built 

 up to a level mainly a few inches above that of high tide, although 

 on the inland borders there are extensive tracts which are flooded 

 at spring tide, as at the Aguadulce Salt Works. The plain is 

 traversed by deep, narrow tidal channels. 



These flats must have required a considerable period for their 

 formation, and during it the coast-line has been remarkably perma- 

 nent. They are connected with low alluvial bottoms of small 

 extent along some of the rivers which enter the sea through the 



3 



