32° ‘J. W. SPENCER 
GREAT CHANGES OF LEVEL IN MEXICO. 
eastward,at San Fernando pass, which is at the summit of the proposed 
Tonala railway. 
BroLtocicaL Eviprncre or INrEROCEANIC CONNECTION 
There is a striking resemblance between the littoral fishes, mollusks, 
and echinoderms of the West Indian waters and those of the Pacific, ac- 
cording to the studies of the late Dr G. Brown Goode, although there is 
absolutely no resemblance between the deep-water fishes on the two sides 
of Central America. The intermingling of the molluscan fauna was long 
ago pointed out by Dr W. B. Carpenter, who identified 35 species out of 
1,400 Pacific forms as occurring on the Atlantic side of this region. Sub- 
sequent discoveries have increased the number to about a hundred species, 
according to Mr Charles T. Simpson. This distribution of modern fauna - 
_ was only to be expected, after studying the physical and geological struct- 
ure of the region, or vice versa; but it is satisfactory that both lines of 
evidence have been obtained. 
SUMMARY AND CONCLUSIONS 
The coastal plains of Mexico are continuous with those of the United 
States, and rise to a height of 1,600 or 1,700 feet before they abut against 
the foot of the high plateau; but on the inner margins these plains are 
only about 325 feet above the sea on the Tehuantepec isthmus. 
The high plateaus of 6,000 or 8,000 feet or more were formerly broad 
baselevels, surmounted by cerros, ridges, and voleanic cones. On the 
Tehuantepec isthmus the mountain zone is reduced to a breadth of 25 
miles and a height of 1,000 feet, and it is crossed by short channels at 
lower altitudes. The margins of the higher plateaus are penetrated by 
deep valleys for a distance of 40 or 60 miles or more, showing features 
of denudation very different from the broad baselevels of the plateaus, 
and also marking a later epoch of a very much shorter duration and 
more recent than some of the Pliocene volcanic eruptions. 
The gradients of the valleys may average from 75 to 150 feet per mile, 
and in the uppermost few miles of their amphitheaters as much as 600 
feet per mile; yet they are not simple slopes, but are characterized by 
several greater baselevels of erosion and a yast number of terrace steps 
of inferior height which are covered with gravels and loams that refill in 
part the older valleys. The edges of these baselevels and terraces are 
dissected by deep, narrow canyons of too recent origin to have penetrated 
any considerable distance. 
These forms impress three great features of erosion upon the modern 
