Hershev.] 



Isthmus of Panama. 



267 



South of Acajutla, the low coastal plain is represented, but at 

 and near La Libertad, in San Salvador, the land next the coast is an 

 elevated plain, much dissected by canon valleys and remarkably 

 like an uplifted and dissected Pleistocene peneplain. It rises rapidly 

 to the foot of the mountains several miles distant inland. South of 

 here the immediate coastal lands consist chiefly of the low plain, 

 especially along the coast of Nicaragua. At Corinto, a low, narrow 

 ridge of reddish colored stratified rock like sandstone forms a rim 

 outside of the low, coastal plain, which latter is miles in width and 

 traversed by broad crooked estuaries or salt-water channels so 

 common in the low coastal plain region. The sandstone ridge at 

 Corinto seems to be a remnant of another coastal plain now largely 

 submerged and the present low coastal plain built on it. 



It is evident to me that from the Isthmus of Tehuantepec to the 

 mainland of South America, the Pacific coast has had the same 

 history. During the Pleistocene era there have been two coastal 

 plains formed. The first or Middle Pleistocene plain was mainly 

 one of erosion inland and aggradation on the seaward border, and 

 was more extensive than the present coastal plain. Then there 

 was a tilting of this plain toward the sea, with local variations of 

 the disturbance. As a general thing the plain was elevated, and 

 dissected by stream erosion inland and largely destroyed by marine 

 erosion on the seaward margin. About at the close of the Pleisto- 

 cene period, a movement of depression submerged the seaward 

 portion, forming islands from some of the isolated low mountain 

 masses which formerly stood as monadnocks on the plain. Subse- 

 quently the sea built another coastal plain over the portions of the 

 old plain which were but slightly submerged. 



The subsidence of the Pacific coast of Central America and the 

 Isthmus of Panama was very slight, probably nowhere amounting 

 to as much as several hundred feet. On the Mexican coast and 

 farther north it seems to have been greater in general but more 

 local in character. The occurrence of such a slight subsidence in 

 the Recent period along over 3,000 miles of coast is remarkable, 

 and indicates that the disturbance was one of a continental nature. 



Berkeley, California, 



February p, igoi. 



