THE CRETACEOUS PERIOD. 143 



of bituminous clay, with a little limestone. Its fossils are chiefly 

 marine. The Austin formation, 600 feet or less thick, is limestone or 

 chalk, of marine origin. The Taylor formation, 600 feet or so thick, 

 consists of calcareous clay marls. The Navarro formation is similar 

 to the last in constitution, but contains some glauconite. Its thick- 

 ness is about 1000 feet. The Navarro formation is probably the 

 equivalent of much of the Upper Cretaceous farther east. The suc- 

 cession of beds is in reality much more complex than the preceding 

 statement would indicate, for some of the formations enumerated are 

 made up of many beds of different composition. The oil of the Cor- 

 sicana field of Texas is derived from the Montana series 1 (Webberville 

 formation). Locally, the system is much faulted as shown in Fig. 382. 

 The Cretaceous system of Texas is continued north into Arkansas 2 

 where each of the above series is present. Together they have an esti- 

 mated thickness of 1500 feet, though the original thickness was much 

 greater. The system also extends west into New Mexico, 3 where it 

 sometimes rests on the Red beds, and sometimes on Carboniferous 

 limestone. Locally, as in the Cerillos hills, the system contains coal. 

 The Cretaceous of the western Gulf region differs from the corre- 

 sponding system farther east, in its greater thickness, and in its greater 

 proportion of calcareous matter, chiefly in the condition of chalk. Of 

 limestone or chalk, the Cretaceous of the Atlantic coast contains little, 

 that of the eastern Gulf region (Alabama and Mississippi) more, and 

 that of Texas much. Nor is the chalk confined to Texas. The equiva- 

 lent of the Austin formation (the Niobrara chalk) extends far to the 

 north, and is the greatest chalk formation of the continent. Much 

 of the chalk resembles the gray chalk of Europe, and some of it the 

 white. Most of the American chalk, like the European, is made up 

 in considerable part of forammiferal shells. Fragments of coral 

 and of molluscan shells, the spicules of sponges, and coccoliths, also 

 abound. 



Unlike the Comanchean system, the Cretaceous has not its great- 

 est development in Mexico. While present in that country, it is less 

 widespread and less thick than the preceding system. 



1 Hill and Vaughan, Austin, Tex., folio, U. S. Geol. Surv., p. 7. 



2 Hill, Ark. Geol. Surv., Ann. Rept., 1888, Vol. II. 



3 Johnson, School of Mines Quarterly, Vol. XXIV, p. 332, and Keyes, Am. Jour. 

 Sci., Vol. XVIII, 4th series, p. 360. 



