16 J. W. SPENCER 
GREAT CHANGES OF LEVEL IN MEXICO. 
between the higher portals, one of which is shown in plate 38, figure 2, 
and this is further reduced by channels at 776 and 820 feet above the sea 
(see plate 3, figure 2). There are the remains of a lower baselevel at 600 
feet on the Pacific side and extensive baselevel plains from 800 to less 
than 700 feet on the Gulf side. The summit of the divide is an old 
baselevel of earthy sandstones, molded by the rains into a series of hum- 
mocks, as shown in plate 3, first illustrated by Mr J. J. Mitchell at a 
similar pass a dozen miles away.* Interrupted ridges of limestone rise 
several hundred feet higher on the dividing ridge. 
Some 50 miles to the eastward, within the limit of the Tehuantepec 
depression, across the great Mexican and Central American plateaus, 
there is the pass of San Fernando, on the proposed line of the Tonala 
and San Juan Bautista railway, which has a height of only 2,681 feet, . 
with a broad baselevel on the Gulf side at from 2,350 to 2,250 feet ~ 
above the sea. The pass from the gulf of Honduras to the Pacific ocean 
is also reduced to about 2,700 feet. 
DeEcLivity AND TERRACES OF THE VALLEYS DESCENDING FROM THE HIGH 
PLATEAUS 
The valleys descending from the high tablelands of Mexico afford an 
opportunity for studying geomorphy such as is not seen in eastern Amer- 
ica, on account of the greater degradation of the inferior plateaus of the 
east, which have been so modified that the important valleys no longer 
present immature or youthful forms. 
The Mexican railway between Vera Cruz and the City of Mexico passes 
through one of the grandest stretches of scenery in the world, and the 
beauty is due to the magnitude and youthfulness of the geological forms. 
The valley leaves the highlands near Atoyac at an elevation of 1,512 feet 
above the sea and extends for a distance of 40 miles in a direct line, 
where it ends in an amphitheater, near the head of which is Esperanza, 
on the margin of the plateau at an altitude of 8,042 feet. This valley 
of the Rio Atoyac (a tributary of the Rio Blanco) and the Rio Blanco 
may be widened out to a breadth of two miles or more where tributaries 
join it, so that the large cities of Cordova and Orizaba are built on the 
floor of the valley. The valley is bounded by steeply rising mountain 
slopes which are so modified as to indicate that their age is far beyond 
the canyon stage. The floor of the valley shows at least four great 
pauses, each of sufficient length to allow of the production of extensive 
baselevels of erosion and a vast number of similar small steps, the gra- 
* J, G, Barnard and J, J, Williams: Report on Isthmus of Tehuantepec, Appleton’s, 1852. 
