14 The American Geologist. Jan. i89o 
of the granitic and older limestones (pala)ozoic) of central 
Texas. Southwest of the Colorado it is made up of flint, the 
quantity of which is so great that it exceeds the power of one's 
imagination, to consider the enormous amounts of chalk 
deposits of the Lower Cretaceous which must have been 
destroyed during its deposition. Another problem awaiting 
future solution concerning the Lignitic area is whether the 
ancient base level which the gravel represents did not also 
include the Black Prairie region next to be described. There 
is also some evidence that while the eastern border of it was 
thus included, the main portion of the black waxy area was a 
nearly allied, but slightly earlier epoch. The chief and pre- 
valent structure of the Lignitic area, however, is that of alter- 
nations of unconsolidated sands and clay of a thousand or 
more feet of thickness of the extensive formation known as 
the Eo-Lignitic, or basal Tertiary. These sands contain 
minute black specks of glauconite or limonite, which, owing 
to the porosity of the formation, quickly undergo hydrous 
oxidation, lixiviation and segregation, giving the countr}' its 
red color and causing the stratified beds of workable iron 
ores. Topographically and historically then it may be con- 
sidered the remnant of an ancient plain which has been much 
degraded by atmospheric agencies and alternations of post- 
Tertiary subsidences and elevations. 
The Black Prairie Region. 
Immediately interior of the Sandy Lignitic area, radically 
different, lies the Black Prairie, the richest and largest con- 
tinuous body of agricultural land in Texas, and hence the 
most important from a cultural as well as scientific aspect. 
It extends in an unbroken body across the state from Indian 
territory to Mexico. The narrowest portion of its area is about 
twenty miles in width west of Austin where the Colorado 
transects it. From that point, however, it widens in both 
directions until its broadest margins — over one hundred miles 
in width — rest upon the Red and Rio Grande respectively. 
The topography of the area was well defined by Dr. Ferdi- 
nand Roemer, some forty years ago, as the "Sanftwellige 
hiigle land," or gently undulating region. When viewed from 
a distance it is apparently level, but upon closer inspection it 
is found to consist of many gentle undulations, which seem to 
differentiate it throughout its extent from the topography of 
