194 Profeasor E. Hull — Changes of Level in Mexico. 



somewhat in height in the Mexican plains and the isthmus, sloping 

 downwards from the continental tracts towards the Tehnantepec 

 Isthmus. Thus the coastal plains of Mexico (which are continuous 

 with those of the United States) descend from a level of 1,600 to 

 1,700 feet to 325 feet in the isthmus, where they abut against the 

 base of the high plateaux. It is inferred that these plains represent 

 pauses in the emergence or subsequent submergence of the region, and 

 represent on the land the features represented by the " drowned 

 plains and river-valleys " of the Atlantic coast, the West Indian 

 Islands, and the Mexican Gulf. They are generallj^ covered by loam, 

 gravel of rounded pebbles, or calcareous marl with shells ; some- 

 times of lacustrine, sometimes marine, origin. Most important 

 amongst the Tertiary deposits is the " Coatzacoalcos formation " 

 of the Tehuantepec Isthmus on the northern side, and so called from 

 the river of that name which traverses the formation. It consists 

 of calcareous clay, of uniform texture and horizontal stratification, 

 containing a large number of marine forms collected by the author 

 and determined by Dr. W. H. Ball. Of the fossils found 34 per cent, 

 are not known to be living forms, so that they may be regarded as 

 of late Miocene or early Pliocene age ; but as the characters of the 

 living fauna of the deeper waters of the Gulf of Mexico are only 

 partly known. Dr. Dall suggests that the fossils collected represent 

 a much larger percentage of living forms than those named in the 

 list given in the memoir of the author. It does not appear to what 

 altitude this formation extends, but it has suggested to Professor 

 Spencer that there may have been many Pliocene connections 

 between the Gulf and Pacific Ocean throughout a distance of 200 

 miles during the greatest submergence. Inland from Vera Cruz 

 a belt of Tertiary limestone and marl has been mapped by the 

 Geological Survey of Mexico, reaching levels of 2,350 to 2,800 feet 

 above the sea, which may not improbably be representative of the 

 Coatzacoalcos formation. 



The "Lafayette" and "Columbian" formations, eiich uncon- 

 formable to the other, are found occupying eroded basins in these 

 middle Tertiary strata, rising to high altitudes, and indicating 

 deep Mid -Pleistocene depression, on the supposition that they 

 represent deposits formed along the sea-margins of the period. 

 In any case, however, the inference of a deep submergence during 

 the Pliocene stage seems inevitable. According to the author, the 

 Lafayette formation seems to have succeeded the Coatzacoalcos in the 

 Tehuantepec region without any considerable physical disturbance 

 intervening ; but on the Pacific side the mechanical materials were 

 replaced by a white soft limestone with water-worn pebbles. It would 

 be of interest to know whether this limestone contained mai'ine forms, 

 and if so, whether of existing species. We must now refer to the 

 position and nature of the divide (or neck of land) which at a late 

 Pleistocene epoch formed the floor of the connection between the 

 Gulf of Mexico and the Pacific. The details given by the author 

 are illustrated by photographic views. The Tehuantepec highland is 

 at this spot reduced to the narrow width of only 25 miles, bounded 



