1905.] AMPHIBIANS AND REPTILES. 237 



America, viz. Colombia and Yenezviela) I use the name of Great 

 Antillia, the term Antillia having ah^eady been used by others. 

 The present Gulf of Mexico remained below the sea, and was 

 larger than it is now, covering the Atlantic Tierra Caliente of 

 Mexico, Yucatan, and, according to Hill, the main part of Florida. 

 If correct, the latter point is important. 



It seems also probable that the Mexican- Central American land, 

 during the Miocene epoch, extended considerably farther west- 

 wards than the present Pacific coast, taking in with almost 

 certainty the Revilla Gigedo Islands. 



Late Miocene, or early Pliocene, comprise a time of subsidence, 

 resulting in the present features. Severance of the Antilles into 

 the present islands, which since have undergone comparatively 

 unimportant changes of shape and extent ; separation of Florida. 

 Lower California became a peninsula, owing to the formation 

 of the Gulf of California. The Revilla Gigedo Islands, still later 

 the Tres Marias, are remnants of the subsiding land. Yucatan 

 appeal's at the beginning of the Pliocene epoch*. The Isthmus 

 of Panama is limited to its present narrow dimensions. 



A few words remain to be said about the volcanic activity and 

 other changes affecting the configiiration of the Mexican Plateau. 

 A tremendous dislocation, at the latest in Eocene times, produced 

 the Eastern Sierra Madre, composed entirely of Cretaceous lime- 

 stones, raised up high, forming the elevated eastern rim of the 

 plateau, and falling off abruptly towards the Atlantic lowlands. 



In the Eocene epoch began also the enormous outburst of 

 volcanism, raising the Western Sierra Madre, piling up gigantic 

 masses of igneous rocks, mostly andesite, and lavas, which con- 

 tinued to spread over a vast part of the country during most of 

 the Miocene epoch, and, more locally, even in historic times. 

 Most of the plateau is now covered with the Quaternary debris, 

 sand, &c., which overlie the eruptive masses and the older 

 calcareous or limestone formations. These accumulations of more 

 or less sandy soil form plains, mostly treeless. They are of great 

 extent, in the northern half, from Texas to Zacatecas. In the 

 middle, say from Guadalajai-a to Puebla, exist a great number 

 of smaller plains or " valles," that is to say fertile plains, 

 intei'rupted or partly surrounded by the outcropping hills of 

 volcanic formation, and they contain a fair number of lakes. In 

 the south of Mexico, in the States of Oaxaca and Guerrero, 

 such plains are rare or absent. Trees are scarce or absent on the 

 plateau ; it is an idle fable that it was well-wooded in historic 

 times. The bordering high Sierras and their slopes are well- 

 wooded, densest on the moist, Atlantic side. The eastern, 

 southern, and western Tierra Caliente is covered with luxurious 

 growth, either forming continuous forests or showing the features 

 of savannahs. 



The plateau is dry, verging towards prolonged droughts, 

 interrupted by few, occasionally torrential, rains. The Atlantic 

 * See footnote to p. 242. 



