DECLIVITY AND TERRACES OF THE VALLEYS. veel 
dients of whose surfaces are very gentle. Thus, for a distance of 10 miles 
below Orizaba, the mean slope is 72 feet per mile, but this is made up of 
a number of steps, the surfaces of which are inclined very much more 
gently than the average slope given. For many miles above and below 
the section referred to in figure 1, the declivity averages 150 feet per 
mile. 
For the uppermost 4 miles the slope is 600 feet per mile, but of this 
mean there is a descent of 1,500 feet in the first mile. 
Of the greater steps, below the first of 1,500 feet, the most noticeable are 
those below Maltrata, Fortin, and Atoyac, where the great baselevels are 
separated by 500 feet or more. Their margins are everywhere in process 
of being incised by the streams which have not yet had time to cut the 
canyons farther back than a quarter or a half ofa mile. In these canyons 
are narrow cascades of a hundred or two hundred feet in height. Even 
the great canyon, leaving the edge of the tableland, near Esperanza, is 
Ficure 1.—Section along the Mexican Railway. 
In direct distance and showing some of the baselevels and inferior steps which characterize the 
slope of the valley. 
less than a mile long. Illustrations of the canyons, with their falls, may 
be seen in plate 4, figures 1 and 2, which show cataracts descending 
from the baselevels at Atoyac and near the city of Orizaba. The falls at 
Atoyae are nearly a hundred feet high, and descend through a short 
canyon from a terrace plain of the valley. 
The smaller steps which characterize the floor of the valley are often 
from only a few feet to 20, 40, or more feet apart. Their surfaces are fre- 
quently very extensive and appear to the eye nearly level. On account 
of the surface creep or washes, the terraces frequently merge into a slop- 
ing surface, but the terrace character is preserved in the more favored 
spots. Of the more prominent steps, 9 were measured between Cordova 
and Atoyac in a distance of 8 miles. These steps represent changes in 
the baselevel of erosion. As the streams descend from one platform to 
the next they are characterized by rapids. The surface of the terrace 
plains is composed of rounded gravel, or this with loam, which may 
