COASTAL PLAIN TERRACES AND CHIVELA GEOLOGICAL CANAL. jl 
above sealevel, only steps of insignificant height occur. On the base- 
level plains of Chivela, in the mountain region, there are terraced plains 
at several altitudes between 700 and 850 feet, the last of which is 75 feet 
above the floor of the geological canal across the divide. 
Upon the Pacific coastal plain steps similar to those upon the Gulf side 
were observed. ‘These were most easily distinguished by their equiva- 
lent terraces within the river valleys. Along the San Geronimo river the 
terraces were seen at 250, 275, 800, and 325 feet above the sea; in the Rio 
Tehuantepec valley up to 400 feet, and in the Rio Verde up to 375 and 
400 feet. On the isolated ridge facing the city of Tehuantepec there are 
erosion shoulders or sea-cliffs and sea-caves at 400 feet. Sea-caves were 
‘also seen at other localities. 
GEOLOGICAL CANAL OF CHIVELA 
Exeayated out of the earthy sandstones which characterize the sum- 
mit of the divide, at a height of from 900 to 1,000 feet above the sea, 
there is a depression of less than a mile long between the two sides of 
the summit (see plate 3, figure 2, page 16). The floor of the narrow 
canal coalesces with the terrace plains of Chivela on the Gulf side. On 
the Pacific side there is a more abrupt descent across the lower remains 
of baselevels. The floor of the canal is 776 feet above the sea and is 
covered with from 4 to 8 feet of gravel, composed of quartz and soft 
sandstone, the latter being well water-worn, derived from the adjacent 
hills. These adjacent hills rise at an angle of 15 to 20 degrees, and the 
same surface gravel, thinly scattered over them, was seen to a height of 
150 feet at least above the floor of the canal; but the broad baselevel be- 
tween the higher mountain knobs and now denuded undulating hills, 
illustrated in plate 3, page 16, was evidently swept over by ocean currents 
passing through’the lately existing Tehuantepec straits. The elevation 
of this canal has been so recent that only short canyons have been 
cut into the baselevels and terrace plains, and the features of the canal 
itself have not been destroyed by the subsequent atmospheric denu- © 
dation. 
At the pass of Tarifa (Santa. Cruz), a dozen miles to the eastward of 
Chivela, there is a depression similar to that just described, through 
which the} formerly proposed ship railway was surveyed. On the Gulf 
side of the Tarifa pass there are also plains, at about the same altitude, 
to which those from Chivela extend. East of the Tarifa plains the 
mountain regions rise higher than in the vicinity of the canal. There 
are other current-swept depressions in this region, though probably at 
higher altitudes. Such a one is to be found some 60 miles or more to the 
