DENISON BEDS AND THEIR FAUNAS. 



331 



limestone contains a characteristic fauna which has important bearing 

 upon our subsequent discussion. This fauna is as follows : 



Fauna of tlie Main Street Limestone. 



Klngena ivacoensis, Roenier. 

 Orthopsis, sp. no v. 

 Holaster sp. 



Holectijpus planatus, Roemer. 

 Holaster simplex, Shumard. 

 Ostrea quadriplicata, Shumard. 

 Gryphsea mucronata, Gabb. 

 Exogyra arietina, Roemer. 



Exogyra sinuata, Marcou. 



*' drakei, Cragin. 

 Janira texanus, Roem.er. 

 Protocardla texanum, Conrad. 

 Hoplites texanus, Cragin. 

 Turrillites hrazoensis, Roemer. 

 Pachyma austinensis, Shumard. 

 Protocardla texanum, Conrad. 



It will be noted that this fauna is different from the species or associa- 

 tions of the underlying Denison beds. It is that of the Exogyra arietina 

 clays and Kingena limestone of our Austin section. The whole thick- 

 ness of the underlying Denison beds is an intercalation barely repre- 

 sented in the Colorado section. It is also noticeable that it presents a 

 similar association (notspecies) of Echinoids and Grj^pheate and Exogy- 

 rate ostreid?e which characterize tlie beds of the Washita division below 

 the Marietta bed. In other words, it marks a return of clearer water 

 environment and with a time-modified fauna of the lower Washita which 

 disappeared from this shore upon the slight elevation and shallower 

 impure sedimentation of the Denison beds. 



(j-eneral Renuirks on the Sections. — The Denison beds below the Main 

 street limestone with their littoral faunas thin out with great rapidity 

 southward or off-shore from Red river, the 328 feet at Denison only being 

 represented, if at all, by the ten feet of marly calcareous clays below the 

 Kingena zone at Austin, and even in this the littoral species have mostly 

 disappeared. These facts afford us a standard of appreciation of the 

 conditions under which the marly calcareous clays which are so common 

 in the Comanche series were deposited. They represent the off-shore 

 (archibenthal) or finest physical and chemical sediments and are not 

 littoitil. The}" should be represented near the Cordilleran front, as at 

 the Ouachita shore, by thicker arenaceous, littoral, plant-bearing terranes. 



It will also ])e seen that the nearer-shore Denison section is composed 

 of a great many more lithologic members than the Austin, being seven 

 in number instead of three at the latter place. These seven beds at 

 Denison are really represented by only tAVO members at Austin, the 

 Exogyra arietina clays and the Fort Worth limestone, while the upper- 

 most member of the Austin section, the Shoal creek limestone, has no 

 representative whatever at Denison. It may have been once deposited 

 there (under what conditions cannot be said) ; but, if so, it was com- 



