626 



A. W. GRABAU TYPES OF SEDIMENTARY OVERLAP 



Inoceramus labiatus begins from 200 to 500 feet below the top of the 

 series, in an impure limestone averaging 50 feet in thickness. This lime- 

 stone (the Greenhorn) apparently represents the successful accomplish- 

 ment of the post-Dakota marine invasion, the underlying shales, except 

 in the southern area, showing little if any evidence of marine occupation. 

 In fact, it is not improbable that 800 to 900 feet of the Lower Graneros 

 shales and included sandstones of the northern region are chiefly of non- 

 marine origin, representing the continued deposition of fine material 

 during the period of continued southward retreat and early advance of 

 the sea. This would explain the increase in thickness northward of the 

 Graneros shales. The only fossils recorded from these shales in the 

 northern region are dicotyledonous plants and fish scales. 



The Carlisle group overlies the Greenhorn (Inoceramus labiatus) 

 limestones and constitutes the upper member of the Benton division. 

 This group consists mostly of clays, with some limestones and sandstones. 

 It is as a whole not very fossiliferous, but certain beds are characterized 

 by Prionocyclus wyomingensis and Prionotropis woolgari. 



The Eagle Ford beds are succeeded by the Austin chalk with a thick- 

 ness of 600 feet in central Texas, but 1,500 feet on the Eio Grande. In 

 Colorado this series is represented by the Niobrara formation, with 

 Inocoramus deformis and Ostrea congesta. This is 700 feet thick in 

 southern Colorado, where it rests on about 200 feet of Benton, North- 

 ward, in the Black hills, where the Carlisle has a thickness of from 500 

 to 700 feet, the Niobrara is only about 200 feet thick, thinning to 100 

 feet toward the northwest. 



Above the Niobrara comes a great thickness of clay shales, the Pierre. 

 These have a recorded thickness of 4,000 feet in southern Colorado, in- 

 creasing to over 7,000 feet in the Denver region, but decreasing to 2,700 

 feet in the Bighorn mountains and to 1,200 feet in the Black hills. Beds 

 of sandstone become intercalated in the thicker sections, as at Denver, 

 where a bed of sandstone from 100 to 350 feet thick occurs near the 

 middle. The succeeding Fox Hill beds, which are mainly sandstones, 

 have an average thickness of 300 feet or less, though increasing to 1,000 

 feet in the Denver region. Marine fossils occur, together with plant 

 remains, the whole series grading up into the great non-marine Laramie 

 formation. 



In the northern region the Austin chalk grades up into the Taylor 

 marls and Eagle Pass or Navarro formation. These are the Exogyra 

 ponderosa beds. The Taylor marls are about 700 feet thick in southern 

 Texas, and 600 feet in central Texas. The overlying Navarro has a 

 thickness of 4,300 feet on the Eio Grande and consists mainly of sand- 



