16 COLORADO FORMATION AND ITS INVERTEBRATE FAUNA, [bull. 106. 



believed to be a part of the Laramie. 1 It has recently been shown to 

 occupy a much lower horizon, probably about that of Dakota. 



As explorations were extended throughout the West it was found 

 that, while the divisions of the above sections can be recognized over a 

 large area, there are regions in which they can be applied only with 

 the greatest difficulty, if at all. They had been separated in the first 

 place mainly on account of lithologic differences, though the lines thus 

 drawn happened to coincide with decided faunal breaks in one or per- 

 haps two instances. Naturally, the strati graphic units thus defined 

 are more or less restricted in their geographic distribution, and are 

 sometimes represented in other regions by deposits of an entirely dif- 

 ferent character. For example, in large areas west of the Continental 

 Divide the limestone and marls of the Niobrara are not found, they 

 being variously represented at different localities by shales and sand- 

 stones. Under such circumstances it is very difficult to discriminate 

 between the dark shales of the Fort Benton and those of the Fort 

 Pierre, unless characteristic fossils are found. This difficulty was 

 encountered by the geologists of the United States Geological Explo- 

 ration of the Fortieth parallel, and as a solution of it Mr. Clarence 

 King, the chief of that survey, proposed the new term Colorado Group 

 "for the great clay group" of the Cretaceous, making it include the 

 equivalents of the Fort Benton, Niobrara, and Fort Pierre groups of 

 Meek and Hayden. 



The name " Colorado Group" was first published in 1875 on one of 

 the atlas sheets of the Fortieth Parallel Survey, and it was more fully 

 defined by Messrs. King, Hague, and Emmons in vols. I and n of the 

 final reports of the same survey. The name has since been used with 

 the same meaning by several other geologists. 2 



But the lithologic classification of the Cretaceous formations adopted 

 by the Fortieth Parallel Survey is often as difficult to apply as the older 

 one which it was intended to replace, even in the region for which it 

 was first proposed; for on approaching the western shore line of the 

 Cretaceous sea along the eastern base of the Wasatch mountains, the 

 Colorado formation ceases to be a " great clay group n bounded by sand- 

 stones above and below. On the contrary, there are several alterna- 

 tions of similar heavy beds of sandstone with dark shales throughout 

 the entire series, and no good lithologic or structural reasons can be 

 given fojr separating them into groups. The section at Coalville, Utah, 

 to be given further on^is a good example of this. Becent paleontologic 

 studies have shown that on the maps of the Fortieth Parallel Survey the 

 upper limit of the Colorado formation at that place is really drawn not 

 higher than the top of the Niobrara. It may be added that in that 



l Vide White, C. A.: On the Bear River Formation. Am. Jour. Sci., vol. xliii, 1892, pp. 91-97; and 

 Stanton, T. W. : Stratigraphic Position of the Bear River Formation. Ibid., pp. 98-115. 



•For the nomenclature and grouping adopted by various authors who have written on the Cretaceous 

 of this region see White, C. A. : A Review of the Cretaceous Formations of North America, Bull. 

 No. 82, U. S, Qeo\. Sur., from which most of the historical statements here given were obtained. 



