3i6 The America?i Geologist. May, mon 
limestones were then in part removed by erosion; the rigidity and 
thickness of the rocks having a great influence on the structure it 
should be supposed that the parts of the Maltrata limestone that are 
intensely folded are in regions where erosion took place before the 
principal uplift. 
"The change in the strike of the strata should be referred to the 
circumstance that the erogenic force acted not only at right angles to 
the longitudinal axis of the mountains, but also at right angles to their 
breadth. 
"The central plateau of Mexico is a phenomenon wholly secondary 
and should not be attributed to great lateral fractures, but was formed 
by the filling up of the higher valleys of the ancient mountains by 
masses of eruptive rocks, volcanic sands and modern alluvium. Where 
the valleys are not filled up, that is to say where eruptions were lack- 
ing, the slope appears steeper because the sediments descend by 
faults in steps toward the east and the west." 
There is added some brief notes by Ordofiez on the eruptive 
rocks from the lava flows and dikes of the region. 
The author's discussion of the age and correlation of the sedimen- 
tary rocks is of special interest, on account of its bearing on the 
geology of the Texan region, although it has no great importance in 
his principal thesis on the structure and origin of the central plateau. 
His opinions concerning the age of the Texan Comanche series will, 
doubtless, be given considerable weight in Europe, where the in- 
fluence of Roemer's stratigraphic error of fifty years ago is still felt. 
These opinions ought not, therefore, to be allowed to pass unchal- 
lenged, especially when it can be shown that the data brought forward 
in their support are largely erroneous. 
The Necoxtla slates at the base of the Orizaba section consist most- 
ly of yellowish, gray and red argillaceous slates, becoming more cal- 
careous above. The thickness has not been determined, and as they 
are entirely unfossiliferous, their age is uncertain, though they are 
correlated on lithologic and stratigraphic grounds with the slates and 
sandstones of Zapotitlan, whose fauna has been partly described by 
Nyst and Galeotti and by Felix and Lenk. According to Aguilera's 
preliminary studies their fauna indicates the Upper Neocomian and 
Apt-Urgonian. 
The overlying Maltrata limestones, with an estimated thickness of 
600 meters, are gray or black in color and contain much flint in the 
form of layers and nodules. Fossils are very rare, including some 
bivalves seen only as sections of the shell, an undescribed Nerinea and 
an ammonite, which the author thinks is probably Acanthoceras? 
justinae Hill, but as he also compares it with Scaphites of the group 
of Scaphites 7'entricosus Meek and Hayden, an Upper Cretaceous 
species that has nothing whatever to do with Hill's Texan ammonite 
from the Trinity division of the Comanche series, it is evident that 
this identification has no value. 
The highest member of the section, the Escamela limestone, also 
