578 POPULAR SCIENCE MONTHLY. 



ent epochs since the early Tertiary period, and the Last was so recent 

 as to j)lace it ahnost in modern times. 



In searching for the geological water ways, the phenomena of the 

 recent elevation of Mexico and Central America became so apparent 

 as to establish the theory that the uplifting of the American barrier 

 to its great altitude occurred principally since the period when the 

 "West Indian islands formed a high plateau bridge between the 

 eastern portions of the two American continents (as described by the 

 writer in The West Indian Bridge between JSTorth and South Amer- 

 ica).* Indeed, the discovery of the recent elevation of Central Amer- 

 ica and Mexico forms a most important sequel to the story of the now 

 submerged West Indian lands, for it shows that Central America 

 was low at the time when the Antilles stood at a great height — 

 though it has subsequently been elevated, while the eastern region 

 has been largely submerged. The discoveries of these great changes 

 of level in recent periods are the necessary complementary phe- 

 nomena of those found in the Antillean region. 



Physical Features of Mexico and Central America. — The 

 Tahle-Lands. — Much the larger area of Mexico and Central America 

 is occupied by plateaus from six to eight thousand feet above the 

 sea. Indeed, some of them have an altitude of over ten thousand 

 feet. These elevated table-lands rise abruptly for thousands of feet 

 above the inner margins of the coastal plains, which gradually slope 

 upward from the seashores. The elevated plains, traversed by many 

 mountain ridges, are, to the eye, apparently level. Out of their 

 floors there also rise numerous island-like hills called cerros, as well 

 as great volcanic cones. Thus the volcanoes of Popocatepetl and 

 Iztaccihuatl, capped with perpetual snow, tower to a height of eight 

 thousand feet or more above the Mexican plateau, as shown in Fig. 1. 



The floors of the plateaus are substantially base levels of ero- 

 sion — that is to say, the more ancient land surfaces were in olden 

 days ground down by the action of the rains and rills to such low 

 elevations above the sea that the streams could not further deepen 

 their channels, whereupon the mountains were worn down into roll- 

 ing plains. The hills and the mountains traversing the table-lands 

 are often only the remnants of higher plains which have, so far, 

 escaped destruction. Thus these elevated plateaus themselves demon- 

 strate their subsequent great elevation above the sea. 



In proceeding from Mexico into Central America, the high 

 plateaus are not only necessarily of smaller extent, but they are 

 more broken, and on the Isthmus of Panama they are almost replaced 

 by mountain ridges rising from one to three thousand feet, except 

 along a few lower passes. 



* Appletons' Popular Science Monthly, 1898, vol. liii, pp. 10-30. 



