296 Geologic Story of the Colorado River .— Hill. 
gion with wide valleys, comparable to the chalk downs of 
England. 
Here the parting between the upper and the lower Creta¬ 
ceous formations is clearly shown and may be traced north or 
south across the state. Although the Dakota or lower Cross 
Timber sands are not represented here the succeeding finely 
laminated basal shales of the Benton or Niobrara epoch are 
found resting unconformablv directly upon the Vola limestone 
above described. The center of the city is built upon the Nio¬ 
brara chalk (Austin limestone of Shumard). Succeeding the lat¬ 
ter and forming the eastern half of the county is 1,000 feet of 
Ponderosa marls or laminate calcareous clays from which the 
great body of residual soils known as the black lands are de¬ 
rived. These phases, basal clays, chalk and marls are widely 
distributed and uniform, and if we add to them the Hilgard 
section above the basal Tombigbee sands, they will represent 
what has until my previous announcement 1 been considered 
all of the American Cretaceous or the Meek and Hayden 
section. 
The uppermost or arenaceous beds as represented in 
New Jersey, Alabama and Arkansas were mostly eroded dur¬ 
ing the post-Cretaceous land epoch and the early Tertiary sub¬ 
sidence. The. total thickness of the upper Cretaceous forma¬ 
tion in the Arkansas Texas region—sands, chalk marls, chalk 
clays and basal sand—I estimate to be from 1,500 to 2,000 
feet. 
Like the lower Cretaceous epoch the upper was closed by 
an upward movement and a succeeding continental epoch but 
of slighter elevation and shorter duration than the mid-Creta¬ 
ceous land. This is attested by the unconformity between 
Cretaceous and Tertiary, as my studies in Arkansas and Texas 
during the past year showed. The movement preceding this 
uplift was slow and long continued as proved b}^ the great 
thickness of more shallow marls and sands succeeding the 
chalk. 
THE TERTIARY. 
Near the eastern edge of Travis county the Cretaceous is 
succeeded by the basal Tertiary formation or Lignitic beds of 
Hilgard. These sediments are composed of debris of the 
1 Am. Journal Science, April, 1887. Am. Naturalist, Feb., 1887. 
