606 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



Colorado, and it is probable that the type of I. flaccidus came from about the 

 same horizon. I. umbonatus and I. exogyroides are reported from the Austin 

 limestone of Texas, and Baculites asper (?) occurs in the same formation. 

 The European /. involutes, which is very closely related to, if not identi- 

 cal with, I. umbonatus, is also confined to the Emscher Mergel, according to 

 Schliiter, which appears to be the homotaxial equivalent of the Niobrara. 



The fact that within the Colorado formation experience has shown the 

 various species of Inoceramus to be good guide fossils for the different 

 zones gives this evidence of a few species greater weight than it would 

 have otherwise. 



The name Colorado formation has come into general use for the 

 combined equivalents of the Fort Benton and Niobrara, and it is a very 

 convenient term, especially in the regions where the lithological differences 

 are not clearly marked. 



A fragment referable to Scaphites ventricosus, obtained on the southeast 

 spur of Electric Peak, makes it probable that the shales there also belong 

 to the upper part of the Colorado formation. The same horizon is repre- 

 sented at Cinnabar Mountain, just north of the Park, though no Cretaceous 

 fossils from that place are included in these collections. Professor Meek 

 examined fossils obtained there in 1872 and listed 1 Scaphites ventricosus, 

 Baculites asper (?), and undetermined species of Thracia, Trigonia, 

 Inoceramus, and Ostrea. 



There are two other localities in the northern part of the Park, on 

 Fan Creek and the north branch of Gardiner River, that have yielded 

 an abundance of Ostrea anomioides, a species that occurs in the Colorado 

 formation at several localities in Montana. 



Montana formation. — The Fort Pierre and Fox Hills divisions of the Meek 

 and Hayden section are frequently combined under the name Montana 

 formation for reasons similar to those that caused the union of the Fort 

 Benton and Niobrara. In the western part of the Rocky Mountain Cre- 

 taceous area it is often difficult to draw a sharp line between even these two 

 broader divisions. The lower part of the section is distinctively Colorado 

 and the upper part distinctively Montana, but there is frequently a doubtful 

 zone in which the faunas are more or less blended. This is especially true 

 in northern Utah, at Coalville, and in western Wyoming, where both the 



'Ann. Rept. U. S. Geol. Snrv. Terr, for 1872, p. 475. 



