Hershey.] 



Isthmus of Panama. 



249 



the South American mainland, and at several places in the Antilles, 

 " which have been pushed up into the Tertiary strata, and now form 

 the core of great mountainous protuberances." The age of these 

 syenitic batholites is variously given as " mid-Tertiary " and "late- 

 Tertiary." It is probable that it was the uplift of the Isthmian 

 Territory due to the upthrust of the crystalline magmas of the 

 Cordillera de Veraguas and the Sierra San Bias which ended the 

 Tertiary sedimentation along the Panama Canal section, and hence 

 it may be provisionally considered middle Miocene in age. 



KESUMlS OF THE PKE-1'LIOCENE ISTHMIAN HISTORY. 



The "old land" or early representative of the Isthmus of 

 Panama lay mainly south of the present Isthmus. That it was a 

 land mass of considerable extent, is indicated by the heavy beds of 

 conglomerate formed from it. Its formations and history are 

 remarkably like those of the Coast Range Region of California, but 

 it will probably be more advisable to connect it with the Andean 

 Region on the south. The north-south ranges of mountains of 

 this old land may have been members of the system of Andean 

 ridges known to be of Cretaceous age and, to be made of just such 

 rocks as those of the Peninsula of Azuero. The eastern members 

 of these parallel north-south mountain ranges of Cretaceous age 

 persist in the northern portion of the present Andean region, but 

 the western members we may suppose have been destroyed by 

 erosion and subsidence, except this small remnant constituting the 

 Peninsula of Azuero. On the northern border of this old land were 

 laid down the formations which now make up the main body of the 

 Isthmus. 



During the Montijo epoch (probably early Cretaceous) the 

 coast was sinking to allow the conglomerate to overlap upon the 

 old land surface. An uplift added a belt of conglomerate to the 

 land, even throwing it up into mountains. Then ensued a long 

 period of sub-aerial erosion during which the mountainous land 

 was mainly reduced to a low undulating plain — presumably a late 

 Cretaceous peneplain. An extended submergence of the coastal 

 portion of this old land mass enabled the accumulation of many 

 hundreds of feet in thickness of Cretaceous(?) shales, fine sandstones, 



