20 J. W. SPENCER—GREAT CHANGES OF LEVEL IN MEXICO. ° 
terraces or greater baselevel (of erosion) plains, the hypothesis of the 
recent elevation of Mexico seems to have become an established theory. 
GEOLOGICAL BASEMENT oF Mwxican TopoGRAPHY 
The Mexican plateau is very extensively underlaid by rocks of the 
Cretaceous formations, and consequently it appears that the region was 
generally reduced to sealevel about the close of that period, so that the 
older features scarcely affect the topography due to the late changes of 
level of land and sea. To the late Cretaceous or early post-Cretaceous 
denudation is due the reduction of the older highlands to base-plains, 
often only a few miles wide and separated by ridges, which are frequently 
themselves the remains of higher baselevels of erosion. The plateau 
valleys are often hundreds of miles long, and are frequently connected’ 
with each other. ‘These post-Cretaceous baselevels, however modified by 
more recent accumulations, often of lacustrine origin, now constitute the 
plateaus of at least the eastern-central portion of Mexico. The formation 
of this topography, being in part modified by lacustrine deposits, must 
have lasted for a long time, perhaps throughout a considerable portion 
of the Tertiary period. Valleys such as those below Hsperanza and 
Catorce, from 40 to 70 miles long and excavated out of the margins of 
the great plateau, represent the molding of physical features during 
greatly changed baselevels of much more recent date and of compara- 
tively short duration. Afterward the excavating forces became largely 
restricted in their actions, and the valleys were partly occupied by de- 
posits accumulated either in embayments or over flood-plains of the 
valleys, reduced to or beneath baselevel of erosion. The subsequent con- 
ditions have favored the production of vast numbers of terrace steps, or 
a succession of inferior baselevels, often molded out of loose materials, 
and now haying short canyons retreating into them. 
The basement of the coastal plain is largely composed of light colored 
or variegated limestones and marls which are more or less fossiliferous. 
Some of these beds are referable to the Miocene period, but others are 
said to contain Miocene forms more or less commingled with Pliocene 
and recent species,* but the different horizons haye not been separated 
by the Mexican geologists. That the older Miocene strata may be ex- 
tensively succeeded by newer Miocene or Pliocene beds does not seem 
improbable, for deposits of the later period were found by the writer in 
the Tehuantepec isthmus. Whether or not true Pliocene beds commonly 
succeed the older Miocene formations, the Tertiary light colored lme- 
* José G. Aguileria by Ezequiel Ordoiiez: ‘Datos para la Geologia de Mexico.” Mexico, 1893, 
p. 40. 
