194 CA.RBONIFKROUS FORMATIONS AND FAUNAS OF COLORADO. 



KiH'lioii beeoiiu's iuoyv :uk1 more s;iiidy. hut i-lici't fray-niciils wci-c I'ouiul aliiKjst up 

 to the fossiliferous limestone of tlio Ilerniosu. 



This formation is especial Iv ehuracterized by its de(>p-red, IViahle. sandy strata, 

 whieli are variably calcareous and often shal}'. They are seldom ver\' distinctly 

 bedded, and disintegrate so rapidly on weathering that good exposures are rare. In 

 the lower part of the formation dark chert nodules abound, often making a large 

 part of Hat lentils or discontinuous layers. Many of the cherts carry a Lower 

 Cai'boniferous invertebrate fauna, like that at the top of the Ouray limestone. In 

 a single locality in the Needle Mountains quadrangle some thin limestones contain a 

 fauna of Upper Carboniferous age, which is elsewhere discussed. 



The Molas formation is distinguished on lithologic grounds because it records 

 a preceding interval of erosion, by which formations of Mississippian age were in 

 large measure destroyed, and iu a subordinate degree because of certain faunal 

 peeiiliai'ities which further collections may or may not affect. 



Above the Rico formation come the Juratnas formations, which are the Dolores 

 Red Beds, 1,600 feet in thickness, the La Plata sandstones, 250 feet or more, and the 

 McElmo shales and sandstones, having a total thickness of nearlj^ 900 feet. 



Comparing this .series with that in the Elk Mountains, where, in the Crested 

 Butte quadrangle, the geographically nearest detailed section occurs, we do not 

 find, the same patent equivalence, either in the rock groups themselves or in the 

 formation names employed for them, which was a feature of the central Colorado 

 sections. However, the Hermosa formation bears comparison in many ways with 

 the lower portion of the Maroon conglomerate, though it evidently consists of finer 

 materials and contains a larger proportion of shales and limestone. The lower 

 member of the Maroon measures 2,000 feet, and the Hermosa 1,800 feet, with a 

 maximum of 2,000 feet. The Weber formation of the Crested Butte has no dis- 

 tinctly marked eqt^ivalence in the San Juan, unless it be the Molas formation, which 

 is somewhat different lithologicallj'. There is, however, a certain resemblance 

 between the fauna of the Weber and the rather distinctive faunas at the base of the 

 Hermosa formation, which would indicate that the Weber formation had a correlate 

 in the San Juan region, if not a very distinct one lithologically. 



The lithologic and color changes from the lower to the upper division of the 

 Maroon formation have a counterpart in the change from the Hermosa formation to 

 the maroon-colored Rico formation. While the Maroon of the typical area has a 

 maximum thickness of 2,500 feet, this character is qualified by the fact that the 

 thickness in the southern part of the Crested Butte area is very much decreased. 

 The Dolores formation has apparently no lithologic equivalence in the Crested Butte 

 region, but it greatly resembles the bright-red Triassic 'sandstone which overlies the 

 Maroon at Lenado Canyon, near Aspen, and the Wyoming .sandstone, which overlies 



