366 INDEX TO THE STRATIGEAPHY OF NORTH AMERICA. 



water in which it was deposited. Here at Eureka the material is exceptionally coarse, with 

 abundant evidence of shallow-water deposition and the existence of a land surface not very- 

 far removed at the time the beds were laid down. 



Beds of the Upper Coal measures are found conformably overlying the Weber conglom- 

 erate. * * * The thickness attained by the rocks of this epoch is nowhere exposed in the 

 district, the overlying beds having either suffered removal by the denudation or else been con- 

 cealed beneath flows of igneous rocks. * * * The beds are estimated at 500 feet. In the 

 northern and central portions of the State of Nevada the Upper Coal measure limestones attain 

 a development of nearly 2,000 feet. * * * 



In the field the Upper Coal measures may be distinguished readily from the Lower Coal 

 measures by their lighter color and the greater prevalence of fine-grained beds. These colors 

 are hght bluish gray and drab, the latter possessing a conchoidal fracture and compact texture. 

 These compact limestones frequently present forms of erosion quite different from the coarse- 

 grained and granular limestones of the Lower Coal measures. Throughout the horizon the 

 limestones are interstratified with belts of grit and sihceous pebbles, held together by a calca- 

 reous cement, in wMch are intercalated thin beds of purer limestone. One or two prominent 

 beds are apparently made up of quartz pebbles and fragments of an older limestone, carrying 

 such fossils as Fusilina cylindrica and Produdus semireticulatus, as if indicating that they had 

 been derived from the underlying Carboniferous rocks. The fossils, however, which are aU 

 Coal measure species, might be derived quite as well from the Upper as from the Lower beds. 

 A chemical examination failed to detect any beds of dolomite in the limestones, the highest 

 amount of magnesium carbonate obtained being 1.33 per cent. This is not without interest, 

 as it is the only limestone horizon in the Paleozoic series at Eureka free from dolomitic strata. 



J 12. SOUTHERN UTAH AND NORTHERN ARIZONA. 



The eastern margin of the Great Basin in southwestern Utah is mapped accord- 

 ing to information supplied by F. B. Weeks." The data for the eastern portion of 

 the State, comprising the Green River and Grand River sections and the San Rafael 

 Swell, are derived from McGee's map °" and appear to represent observations by 

 Powell and Dutton that are not otherwise recorded. 



The later Paleozoic section of northern Arizona comprises the Carboniferous 

 formations known as the Redwall limestone (Mississippian and lower Pennsyl- 

 vanian), the Aubrey group (upper Pennsylvanian), and the Moencopie formation 

 (Permian?). The Redwall and Aubrey were first described by Gilbert,^'"' who 

 gives the following generalizations regarding the stratigraphy, beginning at the top 

 with the "Aubrey" limestone (renamed Kaibab limestone by Darton^*''* in 1910, 

 Aubrey being retained for the group), which in some areas is overlain by Triassic 

 strata and in other areas by the Moencopie formation ^*''^ (Permian?): 



The Aubrey limestone has a thickness of 820 feet on Kanab Creek, and this is about its 

 maximum. * * * Lithologically the limestone is characterized by a great abundance of 

 chert, which, toward the top, sometimes constitutes half the mass. Near the middle it is, in 

 some places, interrupted by a belt of shale, with gypsum. * * * 



The Aubrey sandstone series has a thickness, in the Aubrey Cliffs and along the Grand 

 Canyon, of about 1,000 feet. In every exposure a portion of this body is massive and cross- 

 bedded and another portion soft and gypsiferous, but the order of these parts is not constant. 

 In the Aubrey Cliff the compact rock is at the top and, together with the Aubrey limestone, 

 holds a sheer bluff, at the foot of which the softer portion spreads a broad slope. Along the 



- Personal communication. 



