30 J. W. SPENCER—GREAT CHANGES OF LEVEL IN MEXICO. 
composed of red loams and gravels which are referred to the Lafayette. 
The pebbles consist of quartz and sandstone, as before described. The 
hollows are occupied by the unconformably overlying materials referred 
to the Columbia. In some localities, as in the vicinity of the 2012 to the 
2082-kilometer posts, there are sections exposed to a depth of 30 feet, 
showing nearly this thickness of perfectly rounded boulders or more 
properly large gravel—in size often two feet in diameter—which is re- 
markable, as the locality is too far from the higher lands to suggest their 
transportation by ordinary floods, but they lie in what was the pathway 
of the ocean currents passing through the former straits of Tehuantepec. 
These gravels rest on the sandstones referred to before as of probably 
early Tertiary age. 
The character and the unconformable succession of the deposits, wher- 
ever found resting on the eroded Tertiary formations, and the similarity - 
_of the remarkably uniform surface features, point to the distribution of 
the American Lafayette and Columbia formations throughout Mexico 
to the Tehuantepec isthmus, although the commencement and ending of 
their epochs may not have been perfectly synchronous over the extreme 
limits of distribution. 
The extensive erosion of the surface of the Lafayette is an important 
feature, and its assigned equivalent on the Pacific side—the pebble-bear- 
ing marls at the base of the mountain—has been mostly removed, as 
well as all of the Tertiary stratified deposits, in this case due to the 
denudation of the two great periods of elevation. 
TERRACES ON THE CoAsTAL PLAINS 
Terraces or terrace plains and baselevels of the higher valleys have 
already been described. ‘The inclined surface of the coastal plains from 
the sea to the mountains is not distinguished by a uniform rise, but is 
made up of a number of steps. The margins of the terrace platforms 
are characterized by sweeping hills and knolls, often from 50 to 100 feet 
high. When these are ascended the country is again seen to be a rising 
plain, with the inferior terraces often obscured, but these may be pre- 
served in the shallow valleys. Among the more conspicuous terrace 
steps and plains there is one of about 300 feet above the sea, just east of 
Solidad, and the front margins of some of the more noticeable higher 
terrace plains were found at heights of 450, 625, 800, 1.120, 1,400, 
and 1,560 feet above sealevel, the last named being the summit of the 
plain. 
The coastal plain of the Tehuantepec isthmus is characterized by sim- 
ilar steps, but the margin of the plain being reached at about 3825 feet 
2 
