Hershhs .] 



Isthmus of Panama. 



and during the rainy season, the streams are not more flooded than 

 those of the Sacramento Valley in the winter season. It is un- 

 warranted to assume that erosion has been very much more active 

 here than in the Mississippi Basin or the Sacramento Valley. This 

 applies only to the comparatively dry belt above mentioned. 



On the whole, I think we are justified in provisionally placing 

 the San Carlos formation in the Middle Pleistocene. 



The Mariato Formation. — On the eastern side of the Gulf of 

 Montijo, about seven miles north of the mouth of the Torio River, 

 there is a grassy and level but dissected old coastal plain, occu- 

 pied by the hacienda of Mariato. It is elevated 20 to 40 feet 

 above sea-level. On the seaward margin it has been much 

 eroded, and at "the port" it is seen to be composed of hori- 

 zontally stratified, reddish colored clay, which is very sandy above 

 and gravelly below. The pebbles are well rounded, but mostly of 

 soft rock which can be broken with the point of a knife. This 

 marine deposit rests on an ancient submarine shelf carved from 

 the Santiago formation, upon whose nearly flat surface it thins out 

 in passing inland. 



This Mariato coastal plain has the same relation to the neigh- 

 boring mountains, to a low coastal plain of later age, and to the 

 present rivers and bays, as the San Carlos Plain. L ithologically, 

 the Mariato formation is much like the Red Bluff in California 

 (which means little), and it has been eroded to about the same 

 extent as the latter under approximately similar conditions, which 

 is significant. 



THE MIDDLE PLEISTOCENE UPLIFT. 



If all the fragmentary coastal plains, the Pleistocene peneplain 

 on the Pacific side of the Isthmus, and the base-level on the Carib- 

 bean side at the foot of the Cordillera cle Veraguas, were uplifted at 

 the same time, as the evidence indicates, we have a definite and per- 

 haps important orographic disturbance in the middle part of the 

 Pleistocene era. This was not a mere epeirogenic uplift, without 

 deformation, of the entire Isthmus, but a broad arching of the 

 country in a system outlined in previous disturbances. The actual 

 amount of uplift along the line of the present shores was quite 



