OLDER GEOLOGY OF THE TEHUANTEPEC ISTHMUS. 21 
stones form a physical unit beneath the coastal plains of the Gulf of 
Mexico, and are unconformably succeeded by the mechanical deposits, 
which are of much importance and which will be considered later. 
OLDER GroLoGy oF THE TEHUANTEPEC IsTHMUS 
The backbone of the Tehuantepec isthmus consists of a dark blue semi- 
crystalline limestone abounding in white quartz veins. The rocks are 
very much dislocated and crushed. ‘They are similar to the limestones 
in adjacent parts of Mexico, which the Mexican geologists assign to the 
Cretaceous period.* ‘This formation gives rise to the bold interrupted 
ridges of the divide rising above the baselevels from 600 to 1,000 feet 
higher. These limestones form two or three ridges and several outliers. 
Along the Tehuantepec river and other localities gneisses appear be- 
neath the limestones. The topographic forms of these and other occa- 
sional older rocks are involved in the sculpturing which affected the 
limestones, and accordingly their features have not a distinct impor- 
tance. The Pacific slope of the limestone ridges is principally composed 
of dislocated and metamorphosed shales. The shales between the bold 
limestone hills become like hydromica schists. Apparently occupying 
a basin in these shales there is a soft shaly laminated sandstone slightly 
erystalline with quartz veins. ‘These deposits form the surface of the 
divide, beyond which, upon the Gulf side, the sandstone and shales are 
less metamorphosed. The margin of this zone, which corresponds to a 
baselevel, isincised with canyons. The valley along the railway, drained 
by the Malatengo river, is about 4 miles long in a direct course and de- 
scends about 400 feet to the coastal plain, which extends up the valley 
of the river. The bolder features occur where the canyon cuts through 
thick-bedded quartzite-looking rock which rises out of the shale forma- 
tion of the district. Between this point and the Jaltepec river there are 
some other outliers of decayed gneiss and of limestone, rising through 
the sandy rocks of the inner portion of the coastal plain, which is cov- 
ered by superficial deposits. 
Beyond the Rio Jaltepec there are occasional hills of whitish compact 
limestones, rising to a height of perhaps a hundred feet, which are the 
remains of an extensively denuded surface. These beds were tilted to 
considerable angles before the general denudation of the region. In ap- 
pearance and physical position these hills of limestone so closely resemble 
the white limestones of Tertiary age west of Vera Cruz and of the West 
* Over a large portion of Mexico surveyed there is a remarkable absence of sedimentary and eal- 
careous rocks older than the Cretaceous period, although limited exposures of Carboniferous for- 
mations have been discovered in Chiapas. 
TV—Butn. Gron. Soc. Am., Vor. 9, 1897 
