~ 9A J. W. SPENCER 
GREAT CHANGES OF LEVEL IN MEXICO. 
inated, calcareous clay of brownish color weathering to a bluish hue. 
Its characteristics are uniform. ‘The stratification is horizontal, with 
some slight undulations. In some sections a few miles south of the Rio 
Jaltepec, these clays rest upon decomposed gneisses, but the underlying 
beds to the north are rarely exposed. These horizontal beds, near the 
80-kilometer post, cover the floor of the old baselevel out of which rise 
the upturned beds of white limestone (noted before as probably early 
Tertiary). The clays were seen as far as the 35-kilometer post a point 
only 7 miles in direct course from the Gulf. This formation underlies 
most of the coastal plain for a breadth of 65 miles or more. In the rail- 
way cuts only the lower portion of the sections are composed of these 
clays, so that seldom more than 10 or 15 feet of their thicknesses is shown 
at any locality. The surface is overlaid by a mantle of Lafayette or of 
Columbia deposits. : 
Fauna and age—A considerable number of fossils were obtained close 
by the 124-kilometer post, near San Lucretia (at the crossing of the Rio 
Jaltepec), where the altitude is 125 feet; near the 104-kilometer post, at 
180 feet above the sea; at the 70-kilometer post and 35-kilometer post, 
at 30 or 40 feet above the sea. The fossils collected were kindly deter- 
mined for the writer by Dr W. H. Dall, who gives the list as follows: 
Sabella (tube) (70).* Fusus sp. (fragment) (70). 
Dentaliwm sp. (like megathyris, Dall) (70, Murex sp. (fragment) (70). 
124). Trophon sp. (like triangulatus, Cpr.) (70). 
Conus (young, like leoninus) (70). Trophon sp. (124). 
Pleurotoma albida, Perry (70, 35). Phos sp. (70, 35). 
Pleurotoma (like ostrearum) (70, 124). Mitra (like fulgurita, Rye.) (70). 
Plewrotoma sp. (70, 124). Mitra striolata, Lam. (124). 
Pleurotoma (like cedonulli, Rve.) (124). Cancellaria (like modesta) (70). 
Pleurotoma (like henikeri, Say) (124). Cancellaria centrota, Dall (70). 
Drillia sp. (70, 124). Lunatia sp. (124) 
Cancelluria sp. (124). Natica (like canrena) (70). 
Scaphella dubia, Brod. (70, 124). Solarium sp. (70). 
Marginella (like cineracea, Dall) (70). Xenophora caribea, Petit (70). 
Marginella (like suecinea, Conr.) (70). Pecten (like glyptus, Verr.) (70). 
Olivella mutica, Say (70). Amusium lyoni, Gabb (70). 
Niso interrupta, Say (70). Pinna sp. (fragment) (70, 124). 
Scala retifera, Dail (70). Ostrea sp. (fragment) (124). 
Phalium globosum, Dall (70). Loripes sp. (young) (70). 
Dalium (like solidwm, Dall) (124). Aslarte smithii, Dall (70, 124). 
Daphnella sp. (fragment) (70). Arca spencert, Dall (70). 
Glyphostoma gabbi, Dall (70). Leda acuta, Cony. (70). 
Mutulella fusiformis, Gabb (70). 
* The figures refer to the nearest kilometer post along the line of the Tehuantepec railway. 
