140 CARBONIE'EROUS FORMATIONS AND FAUNAS OF COLORADO. 



The Upper Carboniferous formations, however, ai'c much more important in the 

 niiittor (if thickness; their outcrop covers extensive areas, and their distribution con- 

 forms (iidy partially to that of the earlier Paleozoics. The largest body of Paleozoic 

 outcrop in Colorado is that in the central portion of the State, and as the Paleozoic 

 section is rathei' surprisingly uniform over this area the most advantageous manner 

 of approaching the subject will be to consider the sediments and faunas of this region 

 first, and to compare with them those of surrounding districts. 



CAMBRIAN. 



The Cambrian of central Colorado has been named the Sawatch quartzite. In 

 the Crested Butte quadrangle this formation, which is extremely variable in thickness, 

 permits of being divided into two portions. The lower division, which is from 50 to 

 200 feet thick, is a white quartzite, and has a persistent conglomerate of pure white 

 quartz at the base. The upper division, with a maximum thickness of 150 feet, is a 

 red, ferruginous, and sooaewhat calcareous sandstone. In it a few fossils of the 

 Potsdam type have been found. This division is in places entirely wanting, as at the 

 head of Taylor Creek. 



The Cambrian about Aspen bears the same name and is essentially identical in 

 its physical characters with that of the Crested Butte quadrangle. It is thicker to 

 the south, where it measures 350 or 400 feet, but gradual!}' diminishes northward, 

 where it is 200 feet or less. Spurr does not recognize two divisions, as does Eldridge, 

 but desci'ibes it as consisting at the base of a thin bed of conglomerate, which soon 

 passes upward into fine white quartzite. The upper third is not so compact and pure 

 as the rest, and consists of dolomitio quartzite, glauconitic grit, and sandj' dolomite. 

 The glauconitic grit has a peculiar reddish color, and others of the upper beds are 

 stained with iron. 



In his Eagle River section Peale described 604 feet of siliceous rocks which I 

 think must represent the Sawatch quartzite. At the base is 400 feet of white quartz- 

 ite which he identifies as Primordial, the remainder of the section being referred to 

 the Calciferous. This consists of 10 feet of glauconitic sandstone at the base, with 

 quartzite and sandstone above. Both Eldridge and Spurr have described the Sawatch 

 as being glauconitic, and Spurr regards this as a characteristic feature. Peale does 

 not mention the reddish sandstones to which Eldridge and Spurr make reference, 

 but if the glauconitic sandstone of his section can appi;oximately be correlated with 

 the similar bed in the Sawatch quartzite at Aspen, the horizon appears to be repre- 

 sented, even if that lithologic character is lost. 



At Red Cliff, according to Tilden, the entire sedimentarA- series below the 

 Leadville limestone has a thickness of onl}^ 250 feet, and consists throughout of 

 quai-tzite and sandstone. A few feet at the top of this series are referred to the 



