332 CORRELATION OF THE UPPER CRETACEOUS FORMATIONS 



two lower divisions of the Upper Cretaceous, the Earitan and Magothy, is 

 too insufficient to afford a basis of correlation with the Dakota and Benton, 

 but there seems to be a good reason, both in the general and in the detailed 

 aspect of the fauna, for correlating the Matawan with the Niobrara and 

 the Pierre, at least in part, and the Monmouth with the Fox Hills. 



The lowest Matawan contains the elements of one of the best character- 

 ized and widely distributed faunas of the entire Cretaceous. The diag- 

 nostic genus of this fauna is Mortoniceras, the analogue in the upper 

 Colorado of Prionotropis and Prionocyllus in the lower. Although iso- 

 lated species of Mortoniceras have been reported from strata as old as the 

 Albian and as young as the Campanian, the genus is abundantly present 

 only at the horizon which it characterizes. In the United States it is 

 entirely restricted to the basal Matawan of Maryland, the Merchantville 

 of New Jersey, the Tomhigbee sand member of the Eutaw, the Austin 

 chalk in the Gulf, and the Benton and Niobrara of the West. 



Conditions in the Western Interior during Upper Cretaceous time were 

 quite unlike those on the East Coast. In Maryland the ammonites are, for 

 the most part, restricted to the lower Matawan. The upper Matawan 

 faunas all present a littoral facies in which the ammonites are almost 

 or altogether absent. In the Western Interior, on the other hand, clear- 

 water conditions and the concomitant ammonite faunas persisted through 

 the Pierre. In fact, Mortoniceras and Baculites asper are the only 

 cephalopods common to the lower Matawan of the East Coast and the 

 Western Interior which have not been recognized in the Pierre as well as 

 in the Niobrara. The extinction of the sensitive group of Prionotropids 

 at the close of the Niobrara is especially significant. 



Although the restriction of Mortoniceras, together with Baculites asper, 

 to the Mortoniceras subzone in Maryland and to the Benton and Niobrara 

 in the Interior, is the only direct evidence for the contemporaneity of the 

 two horizons, the long-accepted correlation of the Niobrara with at least 

 the upper part of the Austin chalk confirms indirectly the supposed time- 

 equivalence of the horizons in question. Both the Niobrara and the Austin 

 chalk have clear-water faunas, and their correlation is based on the identity 

 of a large number of critical species, including Ostrea congesta-, Exogyra 



