18 J. W. SPENCER—GREAT CHANGES OF LEVEL IN MEXICO. 
haye a depth of more than 30 feet. KEyen this loose material has not 
been removed from the floor of the valley, but only from the newly 
formed valleys or canyons near the margins of the succeeding platforms, 
thus showing the elevation of the recent baselevel of erosion ; as the de- 
posits, which were not formed at sealevel, could only have been accumu- 
lated by the streams depositing their loads in sluggish waters or by their 
meandering over the floor of the plains when these were reduced to the 
baselevels of erosion. The gravel-covered floors are seen up to an eleva- 
tion of 6,500 feet above Maltrata, and after passing through the canyon 
to ~°> Ngperanza, water-worn gravels occur on the edge of the plateau 
at uvut 8,000 feet above the sea. 
In proceeding from Vera Cruz by way of Jalapa to the summit of the 
Mexican plateau (about 8,200 feet), the road, after passing the coastal 
plain, ascends over various baselevels of erosion (which are noticeable 
upon the face of the escarpment at altitudes of about 2,550, 3,650, and 
ys Miles 
Ficure.2.—Section between Saltillo and Monterey. 
Showing steps in the valley. 
4.300 feet), above which it passes up heavy grades along the face of the 
plateau, and near the summit it enters a canyon, which is excavated out 
of basaltic lavas about half a mile in length. 
Beyond the abrupt margin of the tableland the country, as far as Puebla, 
is similar to that beyond Esperanza—that is, so level that it appears flat 
to the eye, but crossed by ridges already referred to. The surface is more 
or less covered with layers of wind-blown volcanic ashes. Beneath it 
there is commonly a loam with layers of water-worn pebbles or again 
with fragments of volcanic rocks. Very wide areas are distinguished by 
marls of lacustrine origin. 
The northeastern portions of the plateaus of Mexico have a character 
similar to the region westward of Vera Cruz. The valley descending from 
Catorce (6,000 feet) to Saltillo and Monterey illustrates a succession of 
baselevels with short canyons cutting backward into the margins of each 
of these steps, In a general way, these are illustrated in figure 2. The 
