FORMATIONS OF PACIFIC COASTAL PLAINS OF TEHUANTEPEC. 23 
feet of water-worn pebbles below, which are overlaid by 3 feet of the red 
loam. These accumulations rest upon decayed gneiss. Near by, the 
river exposes the gravels and loams filling an old valley, where the thick- 
ness is 50 feet without reaching the underlying surface. The thickness 
of the deposits usually varies from 10 to 20 feet of loam above and from 
2 to 8 feet of gravel below. On the higher undulations of the plain these 
accumulations are sometimes wanting and low bosses of the underlying 
rocks come to the surface. The stratification, while often apparent, is in 
many places obscure, especially where the loams become case-hardened 
when they stand in vertical walls. In the vicinity of the rivers and of 
Ficure 3.—Section along Rio San Geronimo. 
B, Eroded surface with valleys excavated out of decayed basaltic rocks ; L, Marl with water-worn 
pebbles; C, Grayels and loams resting unconformably upon the marls. 
the mountains, the greatest amount of the gravel was seen. This forma- 
tion of gravels and loams has been a great leveller of the plains by burying 
the old valleys and hollows; accordingly, the old courses of the streams 
have often been changed. Thus the Tehuantepec river leaves the broad, 
open plain, where its former course has been buried, and flows through 
a narrow channel between two high isolated hills rising out of the plains 
near the sea. 
The formation thus described as covering the Pacific coastal plain is 
identical with that forming the upper surface of the Gulf plains (the Co- 
lumbia), while the underlying marly lmestones are regarded as the 
equivalent of the lower mechanical deposits occurring on the Gulf side 
of the isthmus (the Lafayette). 
Later ForMATIONS oF THE ATLANTIC CoasTaAL PLAIN oF TEHUANTERPEGC 
COATZACOALCOS FORMATION 
Character, extent, and relations.—The reason for a nomenclature so diffi- 
cult arises from the isthmus not possessing important towns which would 
appear upon an ordinary map, while the Coatzacoalcos river is the great 
feature of the coastal plain, which is extensively underlaid by the de- 
posits here named the Coatzacoalcos formation. It is fine grained, lam- 
