ot J. W. SPENCER—GREAT CHANGES OF LEVEL IN MEXICO. 
ing from 776 to 820 feet, by short geological canals, the floors of which 
are covered by gravel which forms continuous layers with the materials 
of the terrace plains on the Gulf side of the ridge. This oceanic con- 
nection, or rather the somewhat earlier and broader strait, was as old 
as the Columbia (mid-Pleistocene epoch) and admitted the Pacific litto- 
ral mollusks and other fauna to the Mexican gulf. It also continued 
nearly to the modern epoch, as indicated by the shortness of the modern 
canyons. 
Finally, as shown by the excavations of the short canyons at all alti- 
tudes in Mexico, to even more than 10,000 feet, the almost modern ele- 
vation of the region should be strongly emphasized. The molding of 
the low baselevel or current-swept floor of the Tehuantepec divide ante- 
dates the Columbia formation or the mid-Pleistocene depression of the 
locality. This subsidence was followed by a gentle elevation and exca-- 
vation of the narrow short valleys, which later became geological canals. 
Thus the region was depressed during at least a part of the early Pleis- 
tocene epoch, when great valleys were being formed in the Antillean 
region and elsewhere to the east—a repetition of Mio-Pliocene conditions. 
In the undulating continental movements extensive tilting of the 
mountain region is characteristic. How much of the recent extreme rise 
is due to gentle undulations from lower baselevels and how much from 
below sealevel has not been determined, as the features are due to both 
conditions. While the superficial formations of Mexico appear continu- 
ous with those of the United States, they are not relatively as much de- 
nuded by recent atmospheric actions as their stratigraphic equivalents 
farther north, even though of greater altitude, thus suggesting a later 
continuation of the upheaval in Mexico than on the coastal region of the 
United States. 
