DECLIVITY AND TERRACES OF THE VALLEYS. 19 
mountain ridges bounding the plateau valley, before leaving the table- 
land, are from 2 to 4 miles apart, and are composed of Cretaceous lime- 
stones bent and folded in such perfect flexures as are rarely seen else- 
where. The descent from 6,000 to 5,400 feet is over broad terraces and 
steps, where the valley falls more abruptly from step to step, but each 
of these terrace steps widens out again on approaching their frontal 
margins. Terraces or deltas were frequently seen, as shown in plate 4, 
figure 3, between the elevations of 4,000 and 3,750 feet, near which the 
streams descend through a succession of canyons for 12 miles along the 
windings; but in the valley there are many terraces; 7 were me ~ned, 
the margins of which are dissected by the wilder forms of the caixyons. 
This section reaches down to about 3,000 feet above the sea. Baselevel 
plains were noted as having considerable developments at 2,600, 2,500, 
and 2,300 feet, and others were also observed at lower levels, until the 
extension of the coastal plains of Monterey was reached at 1,700 feet 
above tide. 
The same kind of baselevel plains occurs at even higher altitudes than 
that of the general Mexican plateau. ‘Thus at Salazar, 25 miles west of 
the City of Mexico, a baselevel of 10,635 feet is reached. This valley- 
plain is a mile wide, bounded by spruce-covered mountain ridges. The 
descent from it, on both sides, is abrupt, and the margins are incised 
with short canyons. In proceeding northward, the first descending step 
is 300 feet to a broad baselevel below. Following the railway from the 
Salazar summit down to Maravatio (6,700 feet), 114 miles distant, twelve 
distinct baselevels of the greater types alone were noted. ‘The descent 
from the margin of one platform to the next varied from 200 or 300 feet 
to 100 feet or less, and generally the descent was through a barranca or 
canyon (seldom or never a mile long) to small valleys rapidly broaden- 
ing out into the lower plains from 2 to 4 miles wide. ‘The floors of these 
plains commonly consisted of layers of loam, very often underlaid by 
gravels, which also frequently appear at the surface. Some of the loams 
consist of volcanic ashes. The loams are often replaced by lacustrine 
marls. North of Puebla (9,000 feet) similar short, youthful canyons are 
being excavated across the higher ridges. 
On the Tehuantepec isthmus there are fine illustrations of new canyons 
being formed at low elevations, for from the marginal plains of the geo- 
logical canal across the divide at 778 feet above the sea there are short 
canyons descending 350 or 400 feet. 
From all that has been seen of the elevated baselevels of erosion, even 
up to nearly 11,000 feet, and of the various steps in the valleys being 
covered with gravels and loose materials, and from the general feature of 
short canyons retreating into the edges of the various steps, whether of 
