2.54 



University of California. 



[Vol. 2. 



That this plain does not represent the original constructional 

 surface is demonstrated partly by the total disappearance of the 

 cones and craters, but better yet by the fact that at many places 

 among these montanas, the structural features are beautifully dis- 

 played, especially through the presence of thick, originally nearly 

 horizontal lava sheets, now dipping steeply in the opposite direction 

 from the slope of the supposed ancient plain which bevels their 

 upturned edges. Indeed, the volcanic strata of the foot-hills north 

 and west of Santiago have a prevailing northerly dip or toward the 

 cordillera, only a part of which dip I think was original. 



It has been shown above that there exists over the entire width 

 of the Isthmus west of the head of the Bay of Parita (the Penin- 

 sula of Azuero possibly in part excepted), an uplifted, deformed 

 and nearly destroyed ancient plain of erosion. In the compara- 

 tively short time in which, it is known, this plain was formed, it is 

 impossible that it can have been the work of the sea. Marine 

 erosion proceeds rapidly relatively to the land surface exposed 

 to its action, but very little of it is acted on by the sea at one 

 time. The construction of broad submarine shelves requires an 

 extremely long time and a sinking coast with the accumulation off 

 shore of thick sediments. In the light of recently acquired knowl- 

 edge on the subject, I do not think it is necessary to indulge in 

 any elaborate discussion to demonstrate that the leveling of the 

 Isthmus was done by the ordinary agencies of sub-aerial erosion — 

 that the now dissected plain was one of land surface denudation, 

 or, in other words, a peneplain. 



The age of the supposed upper peneplain may be roughly fixed 

 by a study of the newest formation involved in the denudation and 

 by a comparison of the erosion accomplished since the uplift of the 

 peneplain with that of certain portions of the United States where 

 physiographic studies are well advanced. 



The newest formation under the plain seems to have been the 

 alkaline granite and syenite batholites of the cordilleran region, of 

 an age certainly newer than the Eocene and probably about middle 

 Miocene. Some time was required for the removal of a consider- 

 able thickness of strata from over the crystallines, and this may 

 throw the time of uplift of the peneplain forward to at least the 

 late Miocene. 



