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



The Fort Worth limestones, characterized by whitish and very sH^^litly 

 yellow tints, begin to show at their top strong ferruginous colors. This 

 ferrugination is quite sudden, and the limestones become less frequent 

 and alternating, and clays and sands begin to predominate. This ap- 

 parently trivial feature is really an important one, for it indicates change 

 in the condition of sedimentation, and, as I shall show, it is accompanied 

 by certain faunal changes, throwing light on the Austin section. This 

 change is perceptible in the beds as far southward as fort Worth. 



The Denison beds at Denison consist of the following members : * 

 Main street limestone ; Paw Paw shales ; North Denison sands ; and 

 Marietta marls. 



The Marietta beds, the lowest member of the Denison beds, consist oi 

 friable, brown laminated clays. Throughout their extent in the Red river 

 region they are marked, a few feet above their base, by peculiar segre- 

 gations, pseudo-concretions, in the shape of large, thin lenses of indu- 

 rated silicious limestone, which are often four or five feet in diameter 

 and from one to two feet in thickness, and which split into fissile laminae 

 upon weathering. These beds are marked by the association of several 

 interesting species, the most characteristic of which is that of Alectryonla 

 carinata, associated with Gryphsea pltcheri, Morton, and the beautiful 

 echinoid Leiocldaris hemigranosus of Shumard, which occurs near the 

 base. This is the horizon which, in my first section of the Texas Creta- 

 ceous, I described as that of Gryphsea pltcherl, associated with Ostrea 

 carmata.'f These fossils occur in thin brecciate slabs, good specimens of 

 which are in the cj^llections made by me for the United States Geological 

 Survey and the Texas State Survey. It was also from these beds on 

 Little Fossil creek, six miles north of fort Worth, Texas, that I collected 

 the types of the rare species Stearnsla robblnsii, White, Dalliaconcha in- 

 vaginata, White, and Ophioglyphia texana, Clark,J the horizon and geologic 

 position of which has not hitherto been published, I believe. 



The ferruginous character of these beds extend southward as far as 



*MrJ. A. Taff misinterpreted my definition of the Denison beds in the phiral, and inferred that 

 I meant by them only the basement bed of the group of beds. He uses the term in the singular 

 for its lowest member. Proceeding upon this erroneous impression, probably justified by my im- 

 perfect definitions, he presented an erroneous criticism of my work in the American Geologist of 

 December, 1892. Since that writing he has visited and published on the Denison section, and no 

 better proof of the correctness of my conclusions can be given than an examination of the details 

 of his sections in the Fourth Annual Report of tJie Texas Geological Survey. In this report he 

 speaks of the lower (Marietta) bed without definition as the "Denison beds," the "Denison marl," 

 the " Denison bed," etcetera. The term •' Denison beds," as used for the group, has precedence and 

 should not be confused with Mr Taff' s application of the term. (See Third and Fourth Ann. Repts. 

 Texas Geol. Survey, and Am. Geologist for November and December, 1892.) 



t Am. Jour. Sci., April, 1887. 



JProc. Phila. Acad. Sci. 



