1905. | AMPHIBIANS AND REPTILES, 237 
America, viz. Colombia and Venezuela) I use the name of Great 
Antillia, the term Antillia having already 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 further 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 
appears 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 configuration of the Mexican Plateau. 
A tremendous dislocation, at the latest in Kocene 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, d&e., 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 Guadalajara to Puebla, exist a great number 
of smaller plains or “valles,” that is to say fertile plains, 
interrupted or partly surrounded by the outcropping hills of 
voleanic 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 isan 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, 
