SUMMARY AND CONCLUSIONS. 33 
topography, namely: (1) The old baseleveling of the outlines of the 
plateaus, though in part molded before it was completed in post-Creta- 
ceous times; (2) the excavation of large valleys, which, however, did not 
dissect portions of the coastal plain observed, thus indicating an inferior 
elevation at that time, with various baselevels showing pauses in changes 
of altitude ; and (3) the subsequent submergence of the Columbia and ter- 
race episodes, followed by the late elevations and development of modern 
canyons. 
The second class of erosion was modified in the Tehuantepec isthmus 
where the low dividing ridge was a broad baselevel channel or current- 
swept strait, indicating that the lower altitudes were depressed to near 
or below sealevel during part of the period, while afterwards it was dis- 
sected by the short channels forming the canals of the Pleistocene epoch. 
The older Tertiary limestones of the coastal plain were greatly de- 
nuded before the deposition of the succeeding marls, gravels, and loams, 
with volcanic debris, in the Lafayette or provisionally late Pliocene 
epoch. On the Tehuantepec isthmus, resting upon the upturned and 
eroded surfaces of older strata, the Coatzacoalcos formation occurs in 
horizontal beds. This contains an off-shore or deep-water fauna be- 
longing to the late Mio-Pliocene period. Thus the isthmus is shown to 
have been a strait during a period when the West Indian and adjacent 
portions of the continent were elevated: and subjected to a long-contin- 
ued denudation. The Lafayette formation seems to have succeeded the 
Coatzacoalcos in the Tehuantepec region, without any considerable phys- 
ical disturbance intervening, but on the Pacific side the mechanical ma- 
terials were replaced by a white soft limestone, with water-worn pebbles. 
‘The denudation following the Lafayette epoch was here at baselevel and 
has not only widely removed this formation, but even affected the un- 
derlying formations. The Lafayette formation extends over the coastal 
plain of central-eastern Mexico to the foot of the tableland. 
The Columbia formation, of similar materials, and including volcanic 
blocks, rises to an altitude as great as that of the Lafayette. Some of 
the accumulations in the higher valleys, whether estuarine or fluviatile 
(deposited at low baselevels) occur up to heights of nearly 8,000 feet, 
and may belong to this mid-Pleistocene epoch, as we do not yet fully 
understand the limits of the continental and mountain movements 
which have given rise to the recent differential elevation of the plateaus 
and the higher parts of the coastal plains. 
The coastal plains are characterized by terrace-steps which have their 
margins sculptured into hills, and the streams descending from one 
platform to another form small canyon-lke valleys. 
The baselevel of the Tehuantepec divide is dissected, at heights rang- 
