"CERATOPS beds" OF WYOMING AND MONTANA 265 



of the Bull Mountains. If these resemblances could be accepted as 

 identity in each case the smaller thicknesses in the Bull Mountains 

 could be explained as due cither to slower deposition, owing to greater 

 distance from the source of materials, or to partial removal by erosion 

 during one or more periods. From eastern Montana westward to 

 the Crazy Mountains there appears to be a progressive thickening 

 of the beds lying between the marine Cretaceous and what has hither- 

 to been generally called the typical Fort Union, and there is a similar 

 thickening southward from Hell Creek to Converse County, Wyom- 

 ing, but in all this region the lowest bed containing the Triceratops 

 fauna is never very far above the same general horizon of the 

 marine Cretaceous, and the evidence for an important period of 

 extensive erosion at its base is very slight — not greater than the evi- 

 dence for such erosion at the base of the typical, or "upper," Fort 

 Union. 



Area east of the Bighorn Mountains near Sheridan, Wyoming. — The 

 formations of this area have been named and described by N. H. 

 Darton." The coal-bearing rocks have also been studied by J. A. 

 Taff.^^ The highest recognized marine Cretaceous is the Parkman 

 sandstone with a fauna related on the one hand to that of the Claggett 

 and on the other to the Fox Hills fauna. Evidence was obtained by 

 C. A. Fisher and myself in neighboring areas on the north that there 

 are marine shales above the Parkman sandstone, but in this area the 

 beds are not all well exposed and the details of the stratigraphy — 

 especially the contacts between some of the formations — have not 

 been worked out. 



Above the Parkman sandstone is the Piney formation, consisting 

 of dark shales with coaly seams alternating with beds of massive 

 light-colored sandstone. In .the Dayton quadrangle according to 

 Darton its thickness is 2000 to 3000 feet and it is believed to be entirely 

 of fresh-water origin. At a locality one-half mile north of Parkman, 

 Wyoming, a sandstone in the Piney about 1000 feet above the Park- 

 man sandstone has yielded some fragments of dinosaur bones together 

 with imperfectly preserved species of Unio and other fresh-water shells 



'^ Bald Mountain-Dayton and Cloud Peak- Fort McKinney folios (Nos. 

 141 and 142), Geol. Atlas U. S., 1906. Geology of the Bighorn Mountains, 

 Prof. Paper, U. S. Geol. Survey, No. 51, 1906. 



^* Bull. U. S. Geol. Survey, No. 341, pp. 123-150, 1909. 



