26o 



University of California. 



[Vol. 2. 



below sea-level to about 50 feet above it. Some of it forms reefs 

 at the outer edge of the beach at low tide. 



The middle division is a false-bedded series of alternating layers 

 of fine gravel and coarse sand. It has a greater variety of pebbles 

 than that below and among the species represented are biotite 

 granite and separate foils of biotite, a mineral very rare in the 

 Isthmian country. Between the two gravel members, there is a 

 sharp but irregular line which affords some evidence of two 

 periods of deposition, separated by marine erosion. 



The upper third of the cliff consists of heavily-bedded, horizon- 

 tal, semi-lithified fine sand and silt of a light yellowish color with a 

 tendency to buff. In some layers there is a large constituent of 

 grains much finer than sand, so that these beds resemble loess. 



So far as I have been able to learn, the San Carlos plain of 

 aggradation is continuous, back of the immediate coastal lands, 

 with the Aguadulce-Santiago plain of denudation. While base- 

 leveling was in progress on the land along the coast, the eroded 

 materials were deposited, as the San Carlos formation, in the sea 

 just off-shore, and as fast as its surface rose to that of high tide 

 level, its area was added to that of the land as a continuation of or 

 appendage to the plain of denudation. 



The rather small size of the canon valleys eroded in the San 

 Carlos formation since its elevation indicate an age rather late in 

 the Pleistocene era. In quantitative terms, this erosion may be 

 directly compared with the post-Illinoin erosion in the Mississippi 

 basin and the erosion accomplished on the Red Bluff formation 

 of the Sacramento Valley. 



I wish to remark here that those who are not intimately 

 acquainted with climatic conditions on the Isthmus of Panama, may 

 be misled by statements of its supposed heavy annual precipitation, 

 which is said to vary for different parts of the country from 100 to 250 

 inches per year. It will be natural to infer that the streams must be 

 in a state of almost chronic flooding and erosion extremely active. 

 Now, on the southern side of the Cordillera de Veraguas, where are 

 situated the Pleistocene formations whose age I have endeavored 

 roughly to determine by erosion studies, there is a comparatively 

 dry belt, with six months in which scarcely a drop of rain falls, 



