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Show 1 9 0 5 .] AMPHIBIANS AND REPTILES. 2 3 7 America-, viz. Colombia and Venezuela) I use the name of Great Antiilia, the term Antiilia 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 westwards 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 Eocene times, produced the Eastern Sierra Madre, composed entirely of Cretaceous limestones, 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 continued 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 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 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. |
