Geologic Story of the Colorado River—Hill. 295 
all Texas including the post paleozoic axis and the Jurassic 
and Triassic basins of the west. It was one of the grandest but 
heretofore least appreciated events of American geologic 
history. 
The close of this epoch was marked b}^ the elevation of the 
great monoclinal plain extending in an extensive level plain 
over the central portion of the state. At its eastern edge, how¬ 
ever, its eastwarclly dipping strata suddenly bend beneath the 
upper Cretaceous. This plain is transected by the rivers but 
the divides are always mesas, and the isolated hills always of 
the. butte type of structure. Where the Colorado flows parallel 
with the strike the western scarp of the formation consists of 
benches resembling the Quaternary lake terraces of Utah, but 
really the result of unequal resistance of the layers. Where 
the river transects the formation deep canons and mountains 
mark its course. These mountains, as beautifully seen to the 
west from Austin, are the result of faulting and erosion, and 
often assume large proportions. Thej^ form the eastern border 
of the monocline and extend south-westward to the Rio 
Grande. Some of the canons are in fault lines. Their walls 
are usually of a rich cream-colored chalky limestone resemb¬ 
ling the Caen stone of France. Just west of the city of Austin 
they are of metamorphosed chalk, in which can be seen at in¬ 
tervals bands of flint nodules. It is from this interesting ho¬ 
rizon that the beautiful calcite fossils recently described b}^ Dr. 
Roemer, were found, and not in the Austin chalk, as he in his 
far-away home supposed them to be. 1 The soil and cultural 
aspects of the region are entirely different from those of the 
upper Cretaceous, agriculture being possible only in alluvial 
valleys, and uplands poorly adapted for drouth. 
THE UPPER CRETACEOUS FORMATION. 
At the city of Austin the Colorado emerges from the rugged 
canons and mountains and enters a beautiful undulating re- 
1 See Paleontologische Abhandlungen, Herausgegeben von W. Dames 
und E. Ivayser. Vierter Band. Heft 4. Berlin 1888. pp. 281-296. Dr. 
Ferd. Roemer describes and figures in his usual excellent manner an 
interesting fauna of beautiful calcite fossils from this lower 
Cretaceous formation but includes with it several forms from entirely 
different horizons, and assigns the whole to the Austin chalk, which 
is 500 feet above it and separated by a half dozen distinct horizons. 
‘Such has been the nature of most paleontologic deductions by non¬ 
residents when based upon collections of others. 
