FORMATIONS ON GULF SIDE TEHUANTEPEC ISTHMUS. 29 
lumbia red loams are from 5 to 10 feet thick in this section, with the un- 
derlying fine quartz gravels (mostly subangular) ofabout equal thickness. 
It rests unconformably on the Lafayette red loams, in which there is a 
streak of clayey matter one to two feet thick. ‘The hills immediately 
on the coast are covered with loose 
sand in the form of dunes. Be- 
tween the coast and the mountain 
zone this Columbian formation 
occupies all the lower depressions 
of the undulating plains, while 
occasional ridges of the Lafayette 
red loams and gravels rise through 
them. At Jaltipan, as noted be- 
fore, the hills are composed of 
arkose, which may be a local 
representation of the Lafayette. 
While the Columbia and upper 
loams prevail eastward of the 
Jaltepec, beyond that river, in the higher hills, the Lafayette is frequently 
observed with its surface eroded and succeeded by the Columbia gravels 
and loams. These accumulations are best exposed between the 146 and 
152-kilometer posts. The Lafayette is typically a red sand or sandy 
loam, more or less distinctly laminated, although frequently no bedding 
is seen. It sometimes includes layers of sandy clay, which may be 4 
feet thick. In many places the base of the bed is represented by 
eravel, which, however, may appear in other portions of it. When no 
other structure is seen, a vertical cleavage is apt to prevail, which, in 
this as in other Lafayette and Columbia loams, is a physical distinction 
from the underlying Coatzacoalcos formation. These red loams are 
also Hable to be case-hardened. The stratification is nearly horizontal, 
although varying two or three degrees. Near the 136-kilometer post 
the Lafayette formation is notably faulted with a local dip of 10 degrees, 
and near by there is a dip of 6 degrees. On many portions of the 
coastal plain the Lafayette is wanting, and the Columbia rests directly 
upon the Coatzacoalcos formation, while on the inner portion of the belt 
no unconformity between the Lafayette and exposed underlying Coatza- 
coalcos formation was recognizable; but this feature may be due to the 
failure of observations. 
After passing up the canyon of the Malatengo the baselevel step, above 
700 feet, is characterized by a country abounding in hills 50 feet in height 
and separated by broad, shallow depressions. These higher hills are 
Figure 6.—Junction of the Columbia (C) and Lafayette 
Formations (L). 
As shown at the 17-kilometer post of the Tehuan- 
tepec railway. 
V—Butu. Gron. Soc. Am., Vou. 9, 1897 
