512 R. T. HILL — THE COMANCHE SERIES. 



The Grypluea Rock and Walnut Cktys.—The Paluxy sands are everywhere 

 succeeded throughout their extent by a stratum of grypheate oysters, occur- 

 ring sometimes in solid masses from ten to fifty feet thick, in some places 

 imbedded in a calcareous matrix. This terrane is sometimes underlain and 

 overlain by yellow laminated clay marls containing Exogyra texana, Roemer. 

 Hence the Gryphcea rock and Exogyra clays must be discussed as one ter- 

 rane. The yellow clays also contain occasional flags of hard, crystalline 

 limestone, composed largely of shells of Exogyra texana. For these the name 

 of Walnut clays is proposed, after their characteristic occurrence at Walnut, 

 Bosque county. 



At Comanche peak the beds encircle the base of the butte, forming a well- 

 marked bench around the mountain. Below them are the timbered Paluxy 

 sands. The stratum is here fifty feet thick, and composed entirely of the 

 shells of a small GnjjjJicea resembling G. incurva of Europe, but as yet not 

 differentiated from the various species called G. pitcheri in our nomenclature. 

 The shells are more or less loosely cemented, and form one of the most unique 

 rock-sheets in the region. This stratum extends from the Trinity to the 

 Lampasas, and is l)eautifully exposed in the counties of Parker, Wise, Hood, 

 Erath, Comanche, Hamilton, Coryell, Bell and Lampasas, forming a founda- 

 tion for the Walnut clays, whose exposure is coincident with it. 



The Walnut clays, or Exogyra texana beds, overlying and underlying the 

 Gryphcea beds, are alternating strata of thin limestone flags and yellow clay 

 marls, accompanied by inconceivable numbers of Exogyra texana, Roemer 

 {= Ostroea virgula, Goldfuss, and Exogyra inatheroniana, D'Orb.), the lowest 

 and first unmistakably Cretaceous form in the Comanche series. These clays 

 weather into an exceedingly fertile, chocolate-colored soil, forming the chief 

 agricultural lands of the Fredericksburg division. In extent these beds coin- 

 cide with the Gryphcea breccia. North of the Lampasas they are separated 

 from the Glen Rose beds by the Paluxy sands. South of that river they 

 rest directly upon these sands, and constitute a prominent topographic bench 

 or plain near the summit of the buttes, as seen west of Austin, in Travis 

 county. 



Tfie Comanche Peak Chalk— OYer]y'mg the Walnut clays and succeeding 

 them rather abruptly there is a more chalky terrane, for which Dr. Shumard 

 proposed the name of the Comanche Peak beds. This chalk is hard, but 

 readily disintegrates, and usually occurs as the slope or escarpment of the 

 buttes and mesas. It is exceedingly fossil iferous, and its numerous and 

 characteristic species are given in my check-list. The thickness of this bed 

 averages about 100 feet in central Texas, but it thins rapidly to the north- 

 ward and thickens to the southward. The beds grade upward into the 

 Caprina limestone, from which it is differentiated, however, by displaying 

 more regular and frequent lines of stratification, by crumbling nature, and 



