24 POPULAR SCIENCE MONTHLY. 



doors of the Gulf of Mexico and the Caribbean Sea, to which dejDth 

 the drowned valleys can be traced, were continental plains, like 

 those of the Mississippi or Amazon of the present day, with perhaps 

 some shallow lakes or small seas. The backbone of this West Indian 

 bridge was near the Atlantic side, and remains of it are seen in the 

 ridge dissected during the epoch of high elevation by the rapidly 

 descending streams, which now constitute the chain of the Wind- 

 ward Islands. 



The plains now forming the floors of the Gulf of Mexico and the 

 Honduras and Caribbean Seas were apparently drained into the 

 Pacific Ocean across the Tehuantepec Isthmus, Honduras, ISTica- 

 ragua, Panama (see map, page 000), and other low depressions farther 

 south. The writer's recent explorations in the Tehuantepec Isthmus 

 confirm this hypothesis. Plateaus of Mexico and Central America, 

 rising to from six thousand to ten thousand feet, are there reduced 

 in height for a distance of more than sixty miles, so that we find half 

 a dozen passes as low as eight hundred feet above the sea. During 

 the earlier period of the elevation of the Antillean region the Tehuan- 

 tepec Isthmus was a strait in which a deep-water fauna was living. 

 Later the strait was transformed into land troughs, fragments of the 

 old base-level floor of which are still extant, and through them nar- 

 row geological canals were formed when the low Mexican plains had 

 again sunk beneath the gulf waters. This question of the elevation 

 of the Mexican and Central American barriers would carry us be- 

 lyond the limits of the present paper, so that all that can now be said 

 of them is that they were elevated at a very recent date, correspond- 

 ing to the subsidence of the West Indian region, to heights reaching 

 from six thousand to even more than ten thousand feet. This eleva- 

 tion was a sort of compensation in the terrestrial balance. 



The Date of the Continental Bridge. — Over the West In- 

 dian region there are many widely distributed geological formations 

 which were acumulated in early Miocene times. During the follow- 

 ing late Miocene and Pliocene periods these formations were lifted 

 above the sea and became lands of great extent. This elevated con- 

 dition continued so long that the country became enormously de- 

 nuded, and was reduced to valleys and low base-level plains, some of 

 which now appear to constitute the gradation plateaus beneath the 

 sea. On the sinking of the land, after the Mio-Pliocene period of 

 elevation, new deposits (the Lafayette) were accumulated in the 

 valleys, the age of which is provisionally placed at the end of the 

 Pliocene, although some might regard it . as early Pleistocene, for 

 there is no sharp line of dernarcation characterizing the limits of 

 these formations. But these deposits immediately underlie those of 

 the Glacial period of the north. Subsequently the land of the West 



