If)!) CAKHONIFEEOUS FORMATIONS AND FAUNAS oF COLORADO. 



of the (ii'iyiiiiil Yul(> with the Harding sandstone and Fremont limestone. If future 

 iiifniination proves tlio correlation thus suggested, it is evident that the Manitou 

 foi'uiation nuist bo missing from the section of central Colorado, while the Parting 

 quartzito nmst be missing on the eastern side of the Front Range. 



No lithologit' o(iui\'alent of the Yule limestone and no strata of Ordovician age 

 are known in tlic San Juan region. Cross and Spencer make a three-fold division of 

 the beds which the}' refer to the Devonian, qualifying the reference by the opinion 

 that the twni lower members, a quartzite and a shale, may be pre-Devonian. The 

 quartzite I have already suggested as a possible equivalent of the Sawatch quartzite. 

 The shaly member may belong in the Ordovician, but shows a different phase of 

 deposition from that which produced the calcareous division of the Yule limestone. 

 It more probably I'epresents the upper portion of the t3'pical Yule, which Spurr 

 correlates with the Parting quartzite and refers to the Devonian. 



SILURIAN. 



There is but little evidence of the existence of Upper Silurian strata in Colorado. 

 Emmons states (Leadville monograph, p. 61): "Casts of a Ii/ii/?iehonella, hetween 

 H. neglecta and R. incUanensis of the Niagara epoch, were found in the prosjaect 

 shaft in California Gulch, not far below the White Limestone quarr}-, in such a 

 position that they must have been derived from the beds of this horizon at least 50 

 feet above the base of the formation." The evidence of this one species is unim- 

 portant, nor is the deduction drawn from it by Mr. Emmons that any portion of the 

 Yule limestone is of Upper Silurian age; and in view of the extreme rarity of Upper 

 Silurian strata in the west, considering besides that none are known elsewhere in 

 Colorado, and that the Yule limestone has in many places furnished an Ordovician 

 fauna, it is only fair to reason that Upper Silurian time is not, so far as known, 

 represented in the I'ocks of the State, and that the evidence for its existence near 

 Leadville is due to the imperfect condition of the fossils or to ill-considered 

 identifications. 



DEVONIAN (?). 



Above the Yule limestone over much of central Colorado occurs a second 

 quartzite horizon, similar to the Cambrian at the base of the section. This forma- 

 tion was first discriminated hj Emmons, at Leadville, where he called it the Parting" 

 quartzite, and it has since been recognized at a number of localities. At Leadville 

 this formation is a white quartzite, of rather variable thickness but remarkable per- 

 sistence, and it was regarded as constituting the upper limit of the Silurian. In 

 thickness it varies from 40 to 70 feet. On the east fork of the Arkansas evidence of 

 nouconformit}^ by erosion was observed at its upper boundarj"-, " which renders it 

 possible that the Upper Silurian and Devonian formations may be entirely wanting 



