22 J. W. SPENCER—GREAT CHANGES OF LEVEL IN MEXICO. 
Indies (Lower Miocene or Oligocene) as to suggest that they had their 
origin at a period not distantly separated from that of the Antillean 
limestones. Casts of Corbis and other shells were obtained, but. the 
Species were not determinable. However, the age of the rocks is newer 
than the Cretaceous limestones and the shales of the Tehuantepec isth- 
mus, some of which, at least, are probably Eocene. 
In the study of the geomorphy of the Tehuantepec isthmus we have 
thus seen that the great general period of the degradation of the country 
to the baselevel of erosion affected the Cretaceous limestones (and the 
few remnants of older rocks) and the subsequent formations of shales, 
sandstones, and compact white limestones. Thus the modern history of 
the physical changes does not date back further than the extensive low 
continental conditions of the early or mid-Tertiary period, although 
more recent faultings and thrusts may have brought the mountain zone — 
-into greater prominence. 
The volcanic features of Sierra Misappe and the cone of Tuxtla, on the 
southern coast of the gulf of Mexico, adjacent to the isthmus, are in part 
modern, although in part they may date back to the Pliocene period, 
but they do not form directly an important link in the general study of 
the changes of level of land and sea. 
Later FORMATIONS OF THE PactFic CoAasTAL PLAIN oF ‘TEHUANTEPEC 
Subsequent to the mid-Tertiary baseleveling, which constituted the 
foundation of the plains of the isthmus, there has been the deposition 
of two other formations. ‘The older of these is a belt of white or varie- 
gated marly limestone, containing within it water-worn pebbles of lime- 
stone, marble, gneiss, etcetera. This zone skirts the foot of the mountain 
region, and is itself somuch denuded as to be preserved only in the more 
ancient hollows, which it fills, or in other protected places. These accu- 
mulations were observed to an altitude of 320 feet, and possibly the lime- 
stone in the valley of the Rio Verde, near the hot spring, at an altitude 
of 400 feet, may belong to the same formation. 
This more or less mechanical limestone bears strong evidence of a second 
baseleveling of the region, which upon the more open plains is not sep- 
arable from the older denudation. The altitude during the epoch of de- 
nudation was low. 
The relationship of this limestone to both the underlying floor of the 
coastal plain and the succeeding mantle is shown in section in figure 3. 
The surface formation of the plains is a reddish loam derived from the 
weathered rocks of the region. Along the rivers many sections are well 
exposed. Thus near San Geronimo station the formation consists of 2 
~ 
