CARBONIFEROUS UNDIVIDED. 369 



From special reports the following details are cited : 



The Carboniferous of northwestern Colorado south of the fortieth parallel is 

 described in the Anthracite-Crested Butte folio by Eldridge,^"^ who distinguished 

 the following, in ascending order : Leadville limestone, Weber formation, and Maroon 

 conglomerate. 



In the Leadville district, central Colorado, Emmons ^^^ distinguished the fol- 

 lowing subdivisions of the Carboniferous : 



Carboniferous, 3,700 to 4,200 feet: 



Upper Coal Measures: Blue and drab limestones and dolomites, with red sandstones and shales. 



Mud shales at top. 1,000 to 1,500 feet. 

 Weber grits: Coarse white sandstones, passing into conglomerates, and siliceous and highly- 

 micaceous shales, with occasional beds of black argillite and blue dolomitic limestone. 

 Weber shales: Calcareous and carbonaceous shales, with quartzite. 



Thickness of Weber grits and shales, 2,500 feet. 

 Blue [Leadville] limestone : Compact, heavy-bedded dark-blue dolomitic limestone. Siliceous 

 concretions at top, in form of black chert. 200 feet. 



The LeadvUle ("Blue") limestone is regarded by Girty as equivalent to the 

 upper part of the Ouray limestone of southwestern Colorado. The ''' Weber shales" 

 are treated by Emmons ^^^* as a subdivision (150 to 300 feet thick) of the "Weber 

 grits." The siliceous beds of the "Upper Coal Measures" he states ^^^^ are "not 

 distinguishable from those of the Weber grits at the base, but pass upward into red- 

 dish sandstones, which in their turn are sometimes difficult to distinguish from the 

 overlying red sandstones of the Trias." 



This sequence at Leadville is at the north end of the belt of Carboniferous rocks 

 which extends thence southward west of the Colorado Front Range into New 

 Mexico, to the vicinity of Santa Fe. Beyond that point after a brief interruption 

 the strata continue in the upper Rio Grande valley. 



In Colorado east of the Front Range the Carboniferous presents a less complete 

 section than that found west of the range at Leadville and thence southward. In 

 the Pikes Peak quadrangle occur the Millsap hmestone and the Fountain forma- 

 tion. The Fountain is not very definitely correlated with other Carboniferous 

 formations. The MUlsap is a thin-bedded dolomitic limestone, which rests uncon- 

 formably upon the Fremont limestone (late Ordovician) and which, having been 

 deeply eroded before the close of the Carboniferous, is but 30 feet thick in this 

 locality. It is regarded as approximately equivalent to the Leadville limestone of 

 north-central Colorado. The Fountain is made up of red sandstone and conglom- 

 erate and forms a part of the so-called "Red Beds," which were deposited uncon- 

 formably upon Ordovician and pre-Cambrian and composed in part of detritus 

 from the gneisses and granites. The local thickness is 1,000 feet. 



The age of the Fountain formation has been in doubt on account of the scarcity 

 of fossils. It is now regarded by Girty as wholly Pennsylvanian and by Henderson 

 as Mississippian below and Pennsylvanian above. Henderson *^^ states : 



At the northern boundary of Colorado we found a chert concretion zone containing Spirifer 

 cenironatus WincheU, Cransena subelliftica var. hqjrdingensis Girtj-, and Spiriferina sdidirostris 

 White, a fauna which is considered Mississippian and is found on the east side of the Front 

 Range at Canyon City and elsewhere. The first-named species and others of like age were 

 long ago reported from that region, but the record had little value because the position in the 

 formation was not given. We traced the chert zone for 8 or 9 miles southward, finding it 

 48011°— 12 24 



