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



stricter! is sufficiently large to show great variations in the nature and 

 thickness of the sediments and considerable modifications of the faunas. 

 With the 150 species of fossils all from one comparatively limited hori- 

 zon, now for tho first time brought together, it will be much less diffi- 

 cult to extend the correlation of beds belonging to the same horizon in 

 this country, and to make comparisons with Cretaceous faunas as de- 

 veloped elsewhere. 



The variations above mentioned can be best shown by describing a 

 few typical sections in different parts of the area. The sections and 

 their fauna! lists will be given in greater detail in some cases where 

 important new correlations are made. 



GEOLOGIC DESCRIPTION. 

 THE EASTERN BORDER. 



On the eastern border of the area in which the Colorado formation 

 occurs the country is comparatively level; the Cretaceous beds are 

 nearly horizontal and are frequently covered with drift, loess or other 

 recent deposits, so that there are not many good exposures showing 

 the character and the thickness of the different divisions. Conse- 

 quently, but little detailed work has been done here and very few local 

 sections have been published. It is known that in Iowa and Mhine 

 sota, east of the main Cretaceous area, there are many detached rem- 

 nants that have escaped the general erosion and are evidence of the 

 eastward extension of the Cretaceous sea. Only a few of these isolated 

 localities have yielded characteristic fossils, but since they are in an 

 undisturbed region and not very far distant from the typical Meek and 

 Hayden section, the beds can usually be identified with reasonable cer- 

 tainty by their stratigraphic and lithologic relations. 



In the valley of Sauk river, Minnesota, there are exposures of Fort 

 Benton shales that have been described by Mr. J.'H. Kloos. 1 He states 

 that in that region the Cretaceous rests on the older ciystalline rocks, 

 and only the Fort Benton division seems to be represented. One sec- 

 tion obtained from a well showed 73 feet of dark blue clay and shales 

 with two thin seams of lignite. The few fossils that were found were 

 submitted to Prof. F. B. Meek, and proved to be characteristic Benton 

 forms. He identified Inoceramus problematicus (=1. labiatw) and Am- 

 monites percarinatus ( = Prionotropis woolgari) among them. 



Prof. N. H. Winchell 2 states that similar beds extend farther south, 

 along the Minnesota river and the Cottonwood, and at other localities 

 there are exposures of sandstone that are referred to the Dakota, while 

 at New [Jim there is an outcrop of limestone that is believed to repre- 

 sent the Niobrara. 



1 Kloos, J. H. Cretaceous Basin in the Sank Valley. Am. Jour. Sci., 3d ser., vol. Ill, 1872, pp. 17-26. 

 s Wiiichell, N. H. The Cretaceous in Minnesota, Minn. Acad. Nat. Sci. Bull. vol. 1, 1878, pp. 349-390. 



