148 CAR1K)NIFER0US FORMATIONS AND FAUNAS OF COLORADO. 



w Imt silicoous. Like tlio Cainhrian, these beds aw said to tiiieiveii toward tlie south 

 and to laiige I'roiii 250 to 400 feet, it sccins to nie pr()l>al)le that Spurr's Silurian 

 corresponds to only the middle division of the. throe which Eldrido'c makes in the 

 Yule limestone, for the upper division of Eldrido-(>'s section Spurr cori'clates with 

 the I'arting- quartzite. The lower division may not exist at Aspen or elsewhere, 

 and is in fact very variable at Crested Butte, being even wantinu' in places. It is 

 more like Sj)urr's Cambrian than his Silurian, and there seems a possibility that the 

 line between Cambrian and Ordovician may have been variously taken in diflerent 

 ureas. The occurrence of a glauconitic horizon, to which Spurr calls attention, in 

 the upper part of the Sawatch in the Crested Butte and Aspen districts, and in 

 Peale's Calciferous group on Eagle River, seems to warrant a provisional correlation. 

 Above the glauconite at Aspen occur sandy dolomites of uni-ecorded thickness, while 

 above the glauconitic sandstone in Peale's section are found about 17.5 feet of sand- 

 stone and quartzite that may represent the quartzite series at Crested Butte which 

 Eldridge, apparently upon fossil evidence, separated from the Cambrian and placed 

 in the Ordovician. The lower member of the original Yule limestone, therefore, 

 probabh' represents the transitional beds elsewhere grouped with the Sawatch 

 quartzite. 



In Peale's Eagle River section the quartzite and sandstone of his Calciferous 

 group are succeeded bj' 219 feet of light colored magnesian limestone. This is the 

 Yule limestone. Spurr found this formation to be .thinning toward the north. Its 

 thickness in the northern portion of his area was 250 feet, and its reduction to 219 

 feet in Peale's section is quite in line with this observation. 



Tilden describes the entire section below the Leadville limestone at Red Clitl' 

 as consisting wholly of quartzite and sandstone, with a total thickness of but 250 

 feet. He denominates a few feet at the top of this series as "Silurian quartzite," by 

 which may be meant the Parting formation, and while it seems more probable that 

 this, like the beds below, is Cambrian, thei'e is evidently nothiugin the sub-Leadville 

 series which invites comparison with the Yule limestone. 



In the Tenmile district the Yule limestone is represented by a series of light 

 drab-colored limestones, which are often magnesian and always more or less siliceous, 

 sometimes passing into calcareous sandstone. Their thickness averages 120 to 160 

 feet, and the scantj^ paleontologic e\idence determines their age to be probablj^ 

 Ordovician. 



In the Leadville district this formation was originally called the White limestone, 

 and was said to consist in the main of light drab dolomite, with from 10 per cent 

 upward of silica. It is characterized by concretions of white, semitransparent 

 chalcedony or chert. The average thickness is 120 to 160 feet. 



Farther south in the Mosquito Range our information is less satisfactory. 

 Stevenson found the Silurian, under which title are included all the sedimentarj^ for- 



