EAELIER TEETIAEY (EOCENE AND OLIGOCENE). 769 



and others believe is of Cretaceous age and older than the Fort Union, representing 

 in fact the upper part of the Laramie. i 



Tertiary strata occur in the Laramie Basin and in the valleys of the adjacent 

 slopes on the east and northeast. They have been mapped by Darton,^*^ who 

 assigns the deposits on Little Medicine Bow Creek, at the north end of the basin, 

 to the Chadron formation, the lower formation of the White River group (Ohgocene) . 

 The age of the other occurrences has not been determined. 



The elevated basin of Middle Park, Colorado, contains strata which are cor- 

 related by Cross with the Denver formation. They were first described by Mar- 

 vine,^" who distinguished a "doleritic breccia" and a "lignitic formation," the latter 

 5,500 feet thick. Cross ^^^ reviewed Marvine's observations in detail in the field 

 and reported on the stratigraphy and correlation of the formations. The strata 

 are unconformable to the marine Cretaceous, and their correlation with the Denver 

 formation is determined on their stratigraphic condition, hthologic similarity, and 

 closely related floras. Cross ^^^ says: 



The "doleritic breccia" of Marvine is a series of dark tuffs, conglomerates, and breccia 

 beds, made up of a large series of andesitic fragments, of types identical with those in the Den- 

 ver formation. These beds are coarser in texture and are more variable than the Denver strata 

 but resemble them very much in many details. The sharp line drawn by Marvine between 

 the "breccia" and his "Lignitic" series does not appear justifiable. While Marvine does not 

 i-efer to volcanic materials in the upper series, there is in fact a gradation between the lower, 

 dark, ahnost purely andesitic strata and the lighter-colored beds above, in which granitic 

 debris usually predominates, although micaceous and homblendic andesites are abundant for 

 more than 2,000 feet upward in the series, as far as the writer's observations go. 



Plant remains are the only fossils as yet known from the Middle Park strata. These were 

 found by the Hayden Survey party in the "Lignitic" series only, but they occur also in the 

 dark tuff layers of the lower beds. A number of the species described by Lesquereux as from 

 Middle Park are now known to have come from the Eocene lake bed at Florissant, Colo. The 

 entire known fossil flora of the Middle Park series has been studied by Mr. Knowlton. * * * 

 Twenty-five satisfactory species are known from these strata, and by far the strongest alliance 

 is with the flora of the Denver formations. 



Along the Grand River near Hot Sulphur Springs the stratigraphic relations of the Middle 

 Park beds are clearly shown. They here rest upon the upturned and eroded section of the 

 Mesozoic series, from the Jura to the Fox HiUs, and overlap the former to the granite. 



According to Shaw,^^® the following Tertiary formations occur in the Glenrock 

 coal field in east-central Wyoming: 



Red and light-gray conglomerate and sandstone. 



Wbite clay, more or less calcareous and commonly sandy (^\^lite River formation). 



Fort Union (?) formation (shales, sandstones, and coal). 



K 13-14. GREAT PLAINS OF SOUTH DAKOTA, WYOMING, COLORADO, AND NEBRASKA. 



, Eocene strata are lacking in part of the northern Great Plains, the lower Oligo- 

 cene being the lowest Tertiary. In considerable areas it rests on the eroded marine 

 Cretaceous, though there are outlying remnants on later formations, and it consti- 

 tutes the White River group of Hayden, which includes the Chadron and Brule 

 formations of Darton. Darton ^^^^ says : 



Early in the study of Great Plains geology the Tertiary formations were divided into the 

 White River group below and the Loup Fork above. This distinction is a clearly recognizable 



48011°— 12 49 



