EXDUCH.] WIND ElVER RANGE DEVONIAN. 7.3 



about 250 feet tliick overlying tliem. Toward the north these latter in- 

 crease veiy much in their vertical dimensions. The work of Professor 

 Bradley and others has shown tliat a continuation of the same Silurian 

 area northward 'SAill fiu*nish us with ample material to make the sub- 

 <livision more complete. Within oiu" district, howe^'er, tlie group is so 

 restricted that this cannot properly be done. I close the Silurian occur- 

 rences of this region with the beds of limestone, dolomitic in part, which 

 have been described above. This furnishes for the Silurian strata on the 

 eastern base of Wind Eiver IMoimtains a maximum thickness of 550 feet. 

 Near JMiner's Delight this is reduced to about 2G0 feet, widening some- 

 what again farther southeast. 



DEVONIAN. 



Any positive determination of strata as Devonian meets with consider- 

 able dilhculty along the main chain of the Eocky Mountains. There is 

 scarcelj^ an applicable change in the lithological character of the rocks 

 composing them as compared to older or younger strata. Besides this, 

 we find it to be a widely perceptible rule that the faunal remains of De- 

 vonian and Lower Carboniferous strata show a remarkable affinity of 

 form. Basing om' deductions upon evidence of the latter only, we will 

 either generally arrive at no satisfactory residts whatever, or we will fre- 

 quently be misled. In reference to this point the late Professor Meek 

 says:* "They {tJiefosGils) belong, without exception, to genera that are 

 common both to the Carboniferous and Devonian, while a smaller por- 

 tion of the genera is also represented in the Silurian." 



Professor Comstock at first referred certain strata occurring above 

 what he considers the Niagara Group to the Devonian, but subsequently 

 retracted this view of the subject.t 



So far as my examinations extend, I can see no e\adence which would 

 justify the determination as "Devonian" of any of the strata exposed in 

 that section of country. Neither the character of the fossils nor the 

 arrangement of beds, as compared to other regions, speak for any such 

 identification. Eemembering the fact that the region we are speaking 

 of is located essentially along the terminal line of the older Palceozoic 

 depositions, we will readily understand why certain groups should either 

 be wanting entirely or shoidd appear in so subordinate a manner as to 

 make their specific recognition an imjiossibility. 



CARBONIFEKOUS. 



Among the Paloeozoic formations found within our district, the Car- 

 boniferous occupies the most prominent position. Although closely re- 

 sembling in certain of its characteristics the same formation as exhibited 

 farther south, we here find some interesting variations. In Colorado 

 three divisions can usually be recognized. Tliey are constant throughout 

 nearly the entire ptate, and are, individually, well defined. On the 

 western slope of the Wind Eiver Eange, and at other localities Avithin 

 our district, we are enabled to separate the entire series into the same 

 immber of groups. The terjn "Subcarboniferous" has been used for 

 the purpose of indicating certain beds below the horizons established by 

 typically Carboniferous fossils. I take occasion to apply it in the present 

 instance for classificatory reasons, but desire to state that the groui3 



* Kcp. U. S. Geol, Survey, 1872, p. 432. 



t Coiaparo Am. Jour. Sci., vol. vi, 1873, andEei). Rec. Nortliwestom Wyoming, 1873, 

 Wasliiiigtou, 1874, j). 112. 



