EXTENT AND TOPOGRAPHIC EXPRESSION OP THE TERRANES. 311 



rocks change in various places from Carboniferous to Silurian or to Red 

 beds, in accordance with the diverse areal geology of the land during the 

 time of their deposition; but these rocks were entirely buried in central 

 Texas beneath the Cretaceous, whose western shoreline extended to the 

 nucleal Rocky mountain region. 



To the southward, in the vicinity of the Brazos, the Trinity limestone 

 terrane (the Glen Rose beds) begins to outcrop (see vertical sections on 

 plate 13), sej^arating the sands into an overlying and underlying forma- 

 tion. These limestones increase in thickness southward until they are 

 nearly 500 feet at the Colorado river. The reappearance of this intercala- 

 tion of the limestone members of the Glen Rose beds in a small area in 

 Arkansas near Murfreesboro is peculiar. 



* The Goodland limestone everywhere west of Arkansas forms an escarp- 

 ment overlooking interiorward the lower Trinity division. South of the 

 Brazos it thickens into the tripartite formations of the Fredericksburg 

 division, but its scarps of indurated limestone continue to be the forma- 

 tive factor in determining the topography of the Texas region. As it is 

 the stratum of greatest resistance, it expresses itself in escarpments of 

 dip-plains, mesas and buttes. 



Tlie Kiamitia cla3'S, which are a type of the marly clay members of 

 the Comanche series, are expressed in the topography as blackland 

 prairies, limited coastward by the scarp lines of the harder and more 

 resisting Duck Creek and Fort Worth beds. 



The Dakota or Lower Cross Timber saixds participates in this east-and- 

 west strike throughout Indian Territory and northern Texas and the 

 southern deflection through the latter state. Its narrow outcrop sur- 

 mounts the prairies as low, rounded, timbered iron ore hills. Its strike is 

 split longitudinall}^ into two belts by the faulting described. The half 

 north of Red river thus split off caps the hills south of fort Washita and ex- 

 tends eastward to the Missouri, Kansas and Texas railroad. The southern 

 half of the ribl)on of outcrop constitutes the hills south of Denison. 

 Their line of eastward strike is cut into by Red river near the eastern 

 border of Grayson county, and that stream virtually flows in these beds 

 as far east as Pine blufl" at the northeast corner of Lamar county, where 

 the river first deflects northward upon the Washita division and then 

 southward into later deposits. Near Pine blufl" the Dakota, with its east- 

 and-west strike, crosses into Indian Territory and finally disappears under 

 the bed of Little river at Morris ferry, which is about 10 miles east of the 

 Choctaw line. This formation was undoubtedly laid down at marine level, 

 and is not of lacustral origin, as argued in the correlation papers of the 

 United States Geological Survey,'^ for I found at Pine bluff Ammonitid?e 



*Bull. U. S. Geol. Siirvej^ no. 82, pp. 122 et .seq. 



