660 INDEX TO THE STEATIGRAPHY OF NORTH AMERICA. 



Exogyra costata Say. The Black Creek would appear, therefore, to represent all the Upper 

 Cretaceous deposits of the Chattahoochee region subjacent to the beds carrying the above- 

 named species. This would mean the correspondence of the Black Creek to about the basal 

 one-half of the Selma chalk, where that formation is most fully developed in western Alabama 

 and east-central Mississippi, and to all of the Eutaw formation of the same region. According 

 to Berry, on the evidence of the plants, the Black Creek includes also representatives of at least 

 a part of the Tuscaloosa formation of Alabama, which forms the basal division of the Cretaceous 

 in central and western Alabama and northeastern Mississippi. 



The Peedee sand is correlated, on the evidence furnished by invertebrates, with the Mon- 

 mouth of the Maryland-New Jersey Cretaceous, with the beds of the eastern Gulf region car- 

 rying Exogyra costata Say (which would include the upper one-half or two-thirds of the Ripley 

 formation of the Chattahoochee region, the upper half of the Selma chalk, and the Ripley 

 formation of northern Mississippi), and with the Navarro formation of Texas. 



I-J 13. COLORADO AND NEW MEXICO. 



Concerning the Cretaceous of southwestern Colorado, Cross ^^' states : 



The lowest Cretaceous formation recognized is the Dakota, which alone, of all the divisions, 

 has the lithologic character common in other parts of the Rocky Mountain region. 



Succeeding the Dakota comes a very homogeneous clay shale formation more than 1,000 

 feet thick, which from the invertebrate fossils found at several horizons must be supposed to 

 represent the Benton, Niobrara, and a part of the Pierre formations as distinguished at the 

 eastern base of the Front Range in Colorado. But the fossil-bearing strata are neither suffi- 

 ciently numerous nor weU enough developed to serve as horizons for a satisfactory subdivision 

 of this shale formation, which was named the Mancos shale in the TeUuride folio. 



The next higher distinguishable series in the Cretaceous section is the variable complex 

 whose more massive sandstones cause the principal scarps of the Mesa Verde. Holmes named 

 this series after the Mesa Verde in the Hayden reports and his designation is here accepted. 

 It was not used, however, on the Hayden map. The Mesaverde formation consists of alter- 

 nating sandstones and shales with several seams of excellent coal. The invertebrate fossils, 

 which are not uncommon at several horizons in the shales and sandstones, range throughout 

 the Montana group, and hence give no ground for a reference of the Mesaverde strata to the 

 Fox Hills, as was done on the Hayden map. 



Above the Mesaverde formation occurs another formation of clay shale, reaching an 

 observed thickness of nearly 2,000 feet, which is very much like the Mancos shaie but con- 

 tains fewer fossils. The only identifiable form thus far found in this shale occurs also in the 

 Mancos shale, so that this division is still apparently below the true Fox HiUs. This formation 

 is called the Lewis shale. Holmes designated it the "Sand Shale group." 



Stni above the Lewis shale is a second series of sandstones, shales, and coals, bearing some 

 resemblance to the Mesaverde formation but differing in detail. The lowest member of this 

 complex is the " Pictured Cliff sandstone" of Holmes's San Juan section, which he placed in the 

 Fox HiUs upon the evidence of invertebrate remains. The remainder was referred by Holmes 

 to the Laramie, but without fossil evidence. The present survey has also failed to bring to 

 hght valid ground for assigning any of the beds in question to the Laramie, while there is some 

 reason to believe that more than the lower sandstone belongs to the Montana group. 



Between the uppermost quartzose sandstones of the Cretaceous and the Puerco marls 

 (Eocene), which are well developed below Durango on the Animas, there occurs a series of 

 beds not recognized by Holmes. These strata are composed mainly of andesitic debris, tuffs, 

 or conglomerates, and it has been proposed by the writer to call them the Animas beds or 

 formation. The fossil plants obtained from the tuff layers clearly indicated that the beds may 

 be correlated with the Denver, Middle Park, Livingston, and other post-Laramie formations, 

 which paleontologists refer to the Mesozoic, although they are stratigraphically shown to be 

 later than the great revolution which terminated the conformable succession of Cretaceous 

 sediments. 



