14 J. W. SPENCER—GREAT CHANGES OF LEVEL IN MEXICO. 
struction of the Antillean Continent,” * the suggestion that the drainage 
of the basin of what is now the Gulf of Mexico crossed the Tehuantepec 
isthmus into the Pacific ocean seemed so probable that the writer visited 
the region early in 1895, in order to ascertain if the hypothesis was sus- 
tained by the physical and geological features of Mexico. The results 
were so confirmatory that the phenomena will be briefly described here. 
7 
PuysicaL Frarures oF MExtco 
COASTAL PLAIN OF EASTERN MEXICO 
A coastal plain similar to that of the Atlantic states forms a zone in 
front of the Mexican plateau. Itis simply a continuation of that of Texas. 
On leaving the coast, the inclination of the plain is often uniform and so 
gradual that the rise is scarcely noticeable. In other localities there is 
-a succession of low steps or surface-washed terraces ; or, again, the country 
may be gently undulating. The landward border of the coastal plain is 
sharply marked by the escarpments of the plateaus which rise thousands 
of feet above it. From the Rio Grande the coastal plain extends for 350 
miles to above Monterey, where the elevation of 1,700 feet is reached. 
West of Tampico the plain reaches for a distance of 125 miles from the 
Gulf, and forms an embayment in the plateau region which rises from 
4,000 to 6,000 feet above tide. Along the Interoceanic railway the inner 
edge of the coastal plain at Colorado station has an elevation of 1,625 
feet above the sea. Along the Mexican railway, back of Vera Cruz, its 
breadth is reduced to 50 miles, with an elevation of 1,560 feet at Paso 
del Macho. 
ATLANTIC COASTAL PLAIN OF THE TEHUANTEPEC ISTHMUS 
The isthmus separating the Atlantic and the Pacific waters has a breadth 
of 140 miles, of which the Atlantic side of 90 miles is characterized at 
first by low flats with lagoons, and then by rolling plains which grad- 
ually rise to an elevation of 325 feet on entering the mountain zone. It 
is crossed by a large winding river called the Coatzacoalcos, probably 200 
miles long, or more than double its direct course. At 60 miles (in direct 
course, at San Lucretia, at the mouth of the Rio Jaltepec) the surface of 
the river is about 55 feet above the sea. From 20 to 24 miles (direct 
course) farther up, the river leaves its canyon section at about 325 feet 
above the sea. The valley of the Malatenga tributary is less than 4 
miles long (direct course), with the canyon section a mile in length, and 
*J. W. Spencer: Reconstruction of the Antillean Continent, Bull. Geol. Soe. Am. vol. 6, 1894, 
pp. 103-140. 
