582 POPULAR SCIENCE MONTHLY. 



miles into Honduras and a hundred miles into ISTicaragua; but at 

 Panama they are reduced to a width of five or eight miles. The 

 Pacific coastal plains of Panama are wider than those on the Carib- 

 bean side ; bu.t, generally speaking, the plains upon the western side 

 of the table-lands are much narrower than those upon the Atlantic 

 side. In ISTicaragua they are reduced to eighteen miles in width, 

 and even at Tehuantepec, where they set in from the Pacific Ocean 

 and indent the plateau region, their breadth is only twenty-five 

 miles (see map. Pig. 7, page 588). The coastal plains represent the 

 lands which have been most recently elevated above the sea. AVhen 

 they were covered by the oceanic waters the Isthmus of Tehuantepec 

 was only twenty-five miles across, and still less where penetrated by 

 dissecting arms of the sea. The Panama Isthmus was reduced to 

 five miles across. 



Chakacteristics and Teeraces of the Valleys descending 

 FROM THE High Plateaus. — The plains, having no barriers to ob- 

 struct their drainage, and terminating at the margins of the high 

 plateaus, indicate former base levels of erosion, and their great 

 altitude shows the extent to which they have been elevated above 



* ^.•.^,*-v' 



flG. ;j.— TiiL, Uas-i. Li.vi.]. ..i; Ei.KVATLii Fl'm.i: (.1- Tin; Sai.azaj: Simmii -i ini. Interna- 



TIUNAL KaILWAV. 



the sea. Their final surfaces may have been due in part to shallow 

 accumulations of lake deposits leveling up the slight undulations 

 of the old base-level surfaces, or these irregularities have at times 

 been smoothed over by wind deposits. These superficial accumula- 

 tions do not modify the general inference that the floors of the 

 great table-lands were the eflects of atmospheric agents, acting 



