SHOALING OF THE LATER CO^[ANCHE SKA. 517 



ison section, two miles north of Main street, where they form a slight escarp- 

 ment with overlying dip plane. They are best shown, however, at Fort Worth, 

 where the characteristic structure of their alternating dimension layers and 

 marly clays is shown in the bluff north of the public square, at the quarries 

 near the Union depot, and in the Texas and Pacific railway cuts, as well as in 

 the Union Pacific cut at Hodge station, three miles north of the city. They 

 are also displayed in Indian territory and in Cooke, Tarrant, Denton, McLen- 

 nan, Bell, Williamson and Travis counties. Two hundred miles southwest- 

 ward, at Del Rio, near the mouth of the Pecos, they are very pure chalks. 

 The railroad cut in West Austin is another typical locality. Four miles 

 west of El Paso, at the corners of Texas and New Mexico on the Mexico 

 line, the Washita limestone is seen, greatly broken and disturbed. The 

 formation is distinguished by the occurrence of many unique and character- 

 istic species, like the large Macraster elegans, Roemer; Ammonites leonensis, 

 Conrad; Gryphcea washita ; G. sinuata ; Ostnea carinata ; and other species 

 mentioned in my check-list. 



This terrane,- together with the Duck Creek limestone and clays, constitute 

 the typical Neocomian of Marcou as described at Fort Washita, a fact of 

 importance, inasmuch as it is near the top of the Comanche series and far 

 above the Comanche Peak and lower divisions, which must be older. 



The lithologic and stratigraphic features of these beds show shallower 

 sedimentation than the underlying Duck Creek chalks and deeper deposition 

 than the overlying Denison beds ; they are sublittoral in characteristic 

 features, indicative of shallowing which continues into the next terrane. > 

 These beds are also an important economic landmark, for they occupy a 

 hypsometric position in which artesian wells can always be obtained. 



The Denison Beds. — The Fort Worth semi-chalky beds are overlain in 

 the Red river district by a series of shallower deposits of laminated arena- 

 ceous clays (the Arietina clays), at the base grading upward into sandy clays 

 and occasional limestones, the chalky element of all the underlying Comanche 

 series having finally disappeared. The detail of these beds, as seen with 

 slight variation in Grayson, Cooke, and Denton counties and in Indian 

 territory, presents a threefold division. At the base they are composed of 

 a blue marly clay weathering brown, with occasional layers of immense, 

 rounded fissile indurations, generally brown in color. Above these the beds 

 are more sandy and ferruginous, oxidizing into ironstone and almost indis- 

 tinguishable from adjacent Dakota sandstones, but separated from them by 

 the uppermost bed of impure yellow limestone, \vhich underlies Main street 

 in Denison. 



At Austin the sediments, almost pure clays and limestones, are void of 

 silica and most of the littoral fossils, and from thence to the Rio Grande at 

 Del Rio are represented by marly clays (tlie Exogyra arietina clays of my 



