Geographic Character: Southern Division. 115 
(628 km); the Grijalva, 350 miles; the Fuerte, 340 miles (547 km); the Usu- 
macinta 330 miles (531 km) and several others. The topographic conditions 
of the country are such as to cause these streams, in their progress toward 
the sea, to be continually precipitated in the form of cascades'). 
The isthmus of Tehuantepec is narrow and low averaging little over 
400 feet (128 m) above sea level, and rising to scarcely 3000 feet (914 m) 
even in the ridge which, on the Pacific side, forms a sort of connecting link 
between North and Central America. From this ridge the country clopes 
northward to the Gulf of Mexico through a series of terraced cretaceous 
formations, which were deposited when a sound connected the Atlantic and 
Pacific Oceans in Mesozoic time. Later when the marine deposits had been 
upraised the chalk clifis became overlaid in several places by Tertiary material. 
East Mexico which comprises the four states of Chiapas, Tabasco, Campeche 
and Yucatan consists of two distinct physiographic division, viz., the Central 
American orographic system (Chiapas, Tabasco) and the marine limestone 
division of Campeche and Yucatan. Chiapas has its Sierra Madre on the Pacific 
coast, which forms a northern continuation of the Guatemalan system. With 
an average altitude of 5000 feet (1524 m), two volcanic peaks Tacana and 
Xoconochco rise respectively 11,500 (13,300) feet (3505 m) and 7900 feet 
(2408 m). East of the Sierra Madre is an undulating plain, well watered and 
wooded. — Yucatan possesses no mountains except a ridge 400 to 500 feet 
(120—150 m) high. The country is a tableland of blunt peninsular shape 
consisting of limestone rocks, which abound in caverns of which Loltun per- 
haps is the most famous. The Great Bank of Yucatan is a submarine ex- 
tension of the peninsula and is being constantly added to by the activity 
of coral. 
In the Peninsula of California, the central Sierra de la Giganta forms 
a series culmininating in a peak about 8000 feet high, consisting of a granite 
mass which shows signs of glacial action in the form of moraines, especially 
on the side facing the Gulf. Here the ranges are separated by the San Jose 
del Cabo valley from a non-fossiliferous limestone ridge, beyond which volcanic 
stratified red rocks prevail on the shores of the Gulf of California. 
“The Geology of Mexico has been but imperfectly studied. In the higher 
ranges the prevailing formations are granite, which seem also to form the 
foundations of the plateaus, above which rise the traps, basalts, mineral-bearing 
porphyries, and more recent lavas. Hence LYELL’s theory that Mexico consisted 
originally of granite ranges with intervening valleys subsequently filled up to 
the level of the plateaus by subterranean eruptions. Igneous rocks of every 
geologic epoch certainly form to alarge extent the superstructure of the cen- 
tral plateau. But the Mexican table-land seems to consist mainly of meta- 
morphic formations which have been partly upheaved, partly interpenetrated 
and overlaid by igneous masses of all epochs and which are chiefly represented 
1) Bureau of American Republics: Mexico, a geographical Sketch. Washington 1900, p.8. 
g*+ 
