UPPER CRETACEOUS. 651 



the marl rests directly on the massive upper strata of the chalk, the transition being very 

 abrupt; but south of Waxahachie there are several feet of calcareous arenaceous marl inter- 

 vening between the two divisions which are included with the chalk. 



The thickness of this formation is difficult to determine with accuracy. Southeast of 

 McKinney it is 625 feet; at Waxahachie, 550 feet; and southeast of Sherman, Dallas, and 

 Waco it is estimated to be very nearly 600 feet thick. At Austin, according to the log of the 

 Manor well, it is about 410 feet thick, but the exact thickness has not been determined. 



The Cretaceous section above the Austin chalk in the Black Prairie region of Texas is 

 composed almost entirely of unindurated layers, firm rock strata being very few and exceptional. 

 The beds are principally calcareous clays which weather so rapidly into a mantle of thick black 

 soil that continuous sections of them can not be seen or measured with accuracy, and hence 

 their thickness, sequence, and general character must be judged largely from well holes. These 

 unindurated uppermost Cretaceous strata in places, as at Corsicana, are fully 2,000 feet thick. 

 They can be differentiated, after a manner, into two general formations, the lower of which 

 may be termed the Taylor and the upper the Navarro. In some places, notably in the north- 

 eastern corner of the State, the upper formation, which will be described later, is separable into 

 several members. 



The Taylor formation consists of calcareous clay marls which are locally known as "joint 

 clays." These are blue-black in the substructure but weather into a deep regolith consisting 

 of a whitish-yellow subsoil and a dense black soil which characterizes the main "black waxy" 

 belt of the Black Prairie. The regolith is jointed, laminated, and friable throughout, having a 

 crackled appearance when dry. Because of their rapid surface disintegration the character of 

 the unaltered beds is seldom seen except when their material is brought up by the well digger 

 or is exposed in freshly cut ravines or creeks. To the eye this clay marl is generally lacking in 

 siliceous material, although a portion may be detected by chemical analysis. The bituminous 

 matter which characterizes the Eagle Ford formation and the grains of glauconite which are 

 found in the rocks of the overlying Navarro beds are also less notable. Their accessory 

 constituent is lime, in a chalky condition. The marls are of fine consistency, compact, and 

 apparently massive until their laminated character is developed by exposure. 



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Owing to lack of outcropping sections for measurement it is difRcult to ascertain the 



exact thickness of this formation or to separate it from the overlying Navarro formation. In 



the latitude of Austin, as determined from the well at Manor, the beds are about 650 feet thick. 



In north Texas we have no reliable records by which to estimate their thickness. At Corsicana 



there are at least 1,500 feet of clays above the Austin chalk, a portion of which belong to the 



Taylor marls. Taff gives the Taylor marls nearly 1,000 feet in Williamson County, but this is 



probably overestimated. 



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The Navarro formation includes the highest beds of the Cretaceous which outcrop along the 



eastern portion of the Black Prairie region and of the northern margin of the Rio Grande Plain. 



This formation is the upward continuation of the Taylor marls, the one passing into the other 



by gradual transition. The chief lithologic differences are that the clays, chalks, and sands 



composing the Navarro formation contain more or less sand and glauconite, while the Taylor 



marls are apparently free from those substances. Besides these lithologic differences, there are 



conspicuous changes in the fossils. The latter become more plentiful in the Navarro formation 



and have the same character as those of the Upper Cretaceous of the New Jersey and Alabama 



regions, of which they are the equivalent and continuous beds, being especially marked by 



Gryphsea vesicularis, Exogyra costata, 'Ostrea larva, Sphenodiscus lenticularis, and numerous 



other forms. 



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In the southwest corner of Arkansas and in the Red River counties the uppermost Creta- 

 ceous beds show considerable stratigraphic differentiation, consisting of beds of glauconitic 



