624 A. W. GRABAU TYPES OF SEDIMENTARY OVERLAP 



age which have overlapped the Trinity. Here, as in the case of the 

 Cheyenne sandstone, the simple application of the principle of progres- 

 sive overlap will give the right solution of the problem. 



The Kiamitia clay is equivalent to the Kiowa shales of Kansas, both 

 being at the Fredericksburg- Washita boundary. The Caddo, Bokchito, 

 and Bennington formations, however, are later Washita beds. Here, 

 then, we have clear evidence that the Dakota retreat, beginning in central 

 Colorado and Kansas at the commencement of Washita time,, reached 

 the Arbuckle Mountains region only toward the middle of that period, 

 after nearly 700 feet of additional strata had been deposited in this more 

 southern region. 



It is not at all improbable that while the Kiowa (Kiamitia) clays were 

 forming the upper Edwards limestone of southern Texas and Mexico was 

 still being deposited. 



In northern Texas the Washita division is about 500 feet thick and 

 consists of clays, marls, and some limestone beds, the whole resting con- 

 formably on the Goodland limestone. At the base lie the Kiamitia clays 

 with Gryplisea corrugata, while the top is formed by the Grayson marls, 

 which are apparently conformably succeeded by the Dexter sands of the 

 Woodbine (Dakota). In the Austin region the corresponding deposits 

 (Georgetown, Del Kio, and Buda) are chiefly limestones, some of them 

 even chalk of foraminiferal origin; hence it is not surprising to find 

 the series much thinner in that section. The top of the section, more- 

 over, is here marked by an erosion interval, and hence the whole of the 

 Buda (elsewhere 100 feet thick) is not shown. This erosion interval is 

 important as indicating the extent to which the Dakota retreat took 

 place, the Austin region being lifted into dry land. 



The Dakota of Texas is known as the Woodbine. In the northern 

 section it is at least 600 feet thick, at Denison about 500 feet, and at Fort 

 Worth about 300 feet. Near Waco it has thinned to 45 feet, and on the 

 Brazos it has disappeared altogether. Hill states that it apparently rests 

 unconformably on the Grayson marls and Main Street limestone of the 

 Denison beds of the Washita division. "The upper beds pass by in- 

 separable transition from sands into sandy clays and finally into the 

 bituminous clays of the Eagle Ford formation. This transition is so 

 gradual that no exact line of separation can be drawn between the Wood- 

 bine and Eagle Ford formations. The parting is arbitrarily established 

 at the zone of Exogyra columbella, which is considered as the top of the 

 Woodbine formation."* The disconformity at the base of the section 

 can not be great, if it exists at all. Of course, the emergence at the 



* R. T. Hill : Twenty-first Ann. Rept. U. S. Geological Survey, pt. vii, p. 296. 



