H KRSHKY. ] 



Isthmus of Panama. 



257 



ridges thus formed have an even and gentle slope toward the 

 ocean. That they represent a plain of sub-aerial erosion is evident 

 from the presence on the summits of many of them of gold-bearing 

 river gravels, the remnants of old delta-like alluvial fans formed at 

 the mouths of the mountain valleys. 



The Pleistocene peneplain rises very gradually and evenly from 

 about 25 feet above high tide level at Aguadulce to 370 feet at 

 Santiago, a distance of 40 miles. From Santiago it has a more 

 rapid slope to the head of the Gulf of Montijo, and in consequence 

 it is deeply dissected and to a great extent destroyed in this region. 

 Here is an excellent area for studying the question of its age, as it 

 shows the maximum erosion. Canon valleys from 100 to 200 

 feet deep and 100 feet to half a mile wide have been excavated by 

 insignificant streamlets. Over areas as large as a township in the 

 United States, the divides have been so generally reduced that it is 

 difficult to detect any trace of the plain. Yet the altitude at which 

 it is due is known from neighboring areas. 



It is true that the slope, the softness of the formations, the 

 absence of heavy vegetation, and the division of the year into one 

 extremely dry and one extremely wet season, favor rapid erosion, 

 but time is required to make a valley even on a precipitous moun- 

 tain-side. Undoubtedly such a late age as the opening of the 

 Recent period, the Wisconsin epoch or the Iowan epoch of United 

 States geology, can not be entertained. It is doubtful even if all the 

 erosion could have been accomplished since the opening of the 

 Illinoian epoch. The Kansan epoch appears to have been too 

 remote, and as for the Lafayette epoch, which closed the Pliocene 

 history in the Eastern United States, it is out of the question. 

 Considering the favoring conditions of erosion, I am averse to 

 placing the uplift of the peneplain so far back as the early Pleisto- 

 cene. So we will provisionally identify it as Middle Pleistocene 

 in age. 



PLEISTOCENE FORMATIONS. 



There is in various parts of the Aguadulce-Santiago plain a bed 

 of exceedingly finely divided clay which dries into coarse grains 

 by the development of fine cracks. It is of a very light gray or 



