34 Bulletin 39 206 



ceue, had far reaching consequences and to them we must ascribe 

 even the sudden and marked change between the fauna of the 

 Alum Bhiff and Chesapeake series of the eastern United States. 

 It probably resulted in a jjartial closing of the Atlantic-Pacific 

 straits and a disarrangement of the direction of oceanic currents. 

 The strongly clastic character of the Gatun formation as compar- 

 ed with the more shaly Uscari, is also a result of the elevation of 

 the land. 



The Pliocene w^as a period of high elevation and, in Costa 

 Rica, of intense volcanic activity. The coarse conglomerates, 

 or the so-called boulder clays, which have perplexed most geo- 

 logists v\^ho have visited Costa Rica, were formed from the 

 erosion of these Pliocene lava flows and deposited as flood-plain 

 material. In a few localities where these conglomerates reached 

 the sea, they contain intercalated shale beds carrying marine 

 fossils of late Pliocene age. 



