bo 
8 J. W. SPENCER—GREAT CHANGES OF LEVEL IN MEXICO. 
blocks thrown out of the voleano of Orizaba. Many of them have been 
rounded by water action, while a few, from 3 to 5 feet in length and 
near the mountains sometimes 10 feet in leneth, are angular. ‘hese 
boulders were lodged in the sea while the tuffaceous deposits were being 
assorted by the waters. Their appearance upon the surface is now due 
to the atmospheric denudation removing the fine materials and leaving 
gravel-covered surfaces or pavements of boulders. Below Solidad the 
eravel is mixed with red or chocolate-colored loams. These red loams 
and underlying gravels cover the undulating plains for nearly 25 miles 
to the sand dunes which separate the plains from the coastline. The 
relationship of all these superficial beds is shown in section in figure 5. 
From this preliminary survey it may be observed that the lower 
gravels at the edge of the mountains appear to have been a delta de- 
posit accumulated on the eroded surfaces of the older Tertiary limestone 
when the region became depressed to sealevel. The angular deposits 
represent great volcanic eruptions burying the gravels, and (with the 
materials more or less weathered ) they resemble the till of glacial regions, 
These voleanic eruptions, of which the lava is seen in other localities, 
are provisionally regarded by Mexican geologists * as belonging to epochs 
from the Pliocene period to modern days. 
Resting on the volcanic rocks there are extensive deposits of tuffaceous 
loams, which correspond in position to the Lafayette of the north, that 
rests on old Miocene surfaces. These tuffaceous accumulations are super- 
ficially eroded, as is the typical Lafayette, and they are succeeded by 
gravels and red or chocolate-colored loams of the surface, which have 
not been subjected to a great amount of erosion, as is the case with the 
Columbia formation. Thissuccession makes it not unreasonable to cor- 
relate the deposits described with those of the Lafayette and Columbia 
of northern Mexicoand Texas. Their accumulations appear to rise to a 
similar height of nearly 1,600 feet, and it is possible that some of the 
gravels and red loam of the mountain valleys of higher altitudes may 
belong to the Columbia period. 
ON THE GULF SIDE OF THE TEHUANTEPEC ISTHMUS 
On the coastal plain of the Tehuantepec isthmus the Lafayette and 
Columbia formations are extensively developed. At the 17-kilometer 
post of the Tehuantepec railway, two miles in a direct course from the 
Gulf, in some cuts through ridges from 25 to 30 feet high, the juxtaposi- 
tion of the two formations is well illustrated, as in figure 6. The Co- 
* JoséG. Aguilera by Ezequiel Ordofez: Datos Para laGeologiade Mexico. Also orally mentioned 
to the writer by Sefor Aguilera. 
