26 J. W. SPENCER 
GREAT CHANGES OF LEVEL IN MEXICO. 
These formations cross Texas and extend over the Mexican plains. The 
Lafayette reaches an altitude of 1,700 feet at Monterey. How much of 
the mechanical deposits in the higher valleys belong to the Lafayette 
series is not known. The two formations have similar characters, being 
composed of red or chocolate colored loams, with bedding often indis- 
tinct, except where traversed by lines of pebbles. The surface is com- 
monly case-hardened, so that the materials stand in natural walls, subject 
to more or less vertical cleavage. The underlying portion of both series 
frequently consists of water-worn gravel, especially in the vicinity of 
great valleys. The accumulations appear to have originated by the 
rapid deposition of the residuum left from the decay of rocks of various 
ages, from those of the Archean crystallines to the Tertiary limestones. 
As the earthy products of rock decay generally resemble each other, so 
the Lafayette and Columbia deposits have remarkably uniform charac- — 
teristics. Neither formation is usually more than 20 feet thick, except 
where it fills buried valleys. The Columbia material is derived from that 
of the Lafayette, the surface of which was enormously denuded before 
the latter period. The newer formation differs from the older principally 
by the finer grained deposits or by the somewhat more perfect separa- 
tion of the sandy particles from the unassorted mass. 
IN CENTRAL-EASTERN MEXICO 
Landward of Vera Cruz the Geological Survey of Mexico has mapped 
a belt of Tertiary rocks. underlying the coastal plain. Sections of these 
may be seen along some of the streams, especially those crossed by the 
Interoceanic railway between Vera Cruz and Jalapa, where the white 
marls and limestones frequently come to the surface. The limestones 
appear at various points as far as H] Palmar, about 15 miles in a direct 
distance from the sea, where the altitude is about 2,550 feet. The strata 
of the white limestones are more or less upturned, and they are succeeded 
by thin horizontal layers of white marl, which are frequently noticed 
beneath the red loam near the coast. Higher up the escarpment, at about 
2,800 feet, the marl, having a thickness here of from 2 to 6 feet, is hori- 
zontal, and rests upon the eroded surface of basaltic rocks, as shown in 
section in figure 4. Owing to the similarity of material, this upper marl 
is not always easily identified from the lower Tertiary beds. The basaltic 
rocks along this escarpment are in close proximity to several great vol- 
canoes, which have been in activity since the Pliocene period. At the 
low altitudes, and at many places higher up, where the marl does not 
form the surface, the country is covered with red loams, or these under- 
laid by gravels. While the line of the Interoceanic railway follows a 
promontory of the Mexican escarpment, the Mexican railway passes up 
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