330 R. T. HILL — GEOLOGY OF RED RIVER. 



ically. They represent a zone of excessive ferrugination in the section. 

 At Denison* they are from 100 to 130 feet thick. Their summit, for con- 

 venience, may be placed at a zone locally known as the " quarry lime- 

 stone " band, where the unique Ostrea qiiadrupUcata has its greatest devel- 

 opment. Its fauna passes into that of the overlying Paw Paw clays. 

 The sands rapidly thin out to the southward, not occurring in the Trinity 

 section. 



The North Denison sands are inseparable from the overlying Paw Paw 

 clays. These clays are of a light drab color, very thinly laminated and 

 contain many beautiful fossils, a species of Turritella predominating, 

 the nacreous substance of which is preserved unaltered, like some of the 

 fossils of the Claiborne Tertiary, and which fall to pieces upon exposure 

 to the air. The clays are exposed in the banks of Paw Paw creek, in the 

 southern and eastern portion of the city of Denison ; at the mineral 

 spring on the head of Duck creek, in northwest Denison, and elsewhere. 



The species of the North Denison sands and Paw Paw clays have not 

 been completely described, but the following are recognizable as consti- 

 tuting their fauna : 



Fauna of the Paw Paw Clays and North Denison Sands {Denison Beds in part). 



Ostrea quadriplicata, Shumard. Protocardia texana, Conrad. 



Corbula sp. Pholadomya (?) postextenta, Cragin. 



Axinsea sp. Cyprimera sp. 



Volsella sp. Anchura mudgeana, White. 



Tapes sp. Turritella sp. 



Cytherea sp. Ammonites emarginatus, Cragin. 



Tellina sp. 



The Denison beds terminate in the beds of the Main street limestone, 

 not over thirty feet in thickness, which underlies the main street of the 

 city; strikes in an east-and-west direction, and also outcrops on the 

 bluffs of Paw Paw creek, south and east of the city. This limestone is 

 coarse grained, of irregular hardness and composed of minute shell frag- 

 ments ; is of a dull yellow color upon weathering, but is white upon 

 fracture, and burns into a very impure lime with hydraulic properties. 

 It is separated by thin beds of clay. 



The Main street limestone is everywhere surmounted unconformably 

 by the coarse ferruginous sands and silicious iron ores of the Dakota. 

 This is the only positively known break in the sedimentation from the 

 beginning of the lowest (Trinity) Cretaceous beds. The Main street 



*The "Denison marl" of Taff: Fourth Ann. Rept. Geol. Survey of Texas, p. 273. 



