THE ABSENCE OF THE DEVONIAN. 259 



of faunas characteristic, in other regions, of upper Cambrian, of some 

 horizons of the Silurian, of lower Carboniferous, and of the Coal Measures. 

 From time to time individual forms, apparently indicative of a Devonian 

 age, have been found ; but in every case a more exhaustive examina- 

 tion of the locality has shown their association to be overwhelmingly Car- 

 boniferous or Silurian. The Devonian, therefore, seems to be wanting in 

 the Rocky Mountain regiou, as it has thus far been found to be in New Mexico, 

 Texas, Arkansas, and the Black Hills. To account for its absence in the 

 latter region, Mr. W. O. Crosby * has advanced the ingenious theory that, 

 in the cycle of deposition succeeding the Cambrian, the ocean had in De- 

 vonian time reached the abyssal depth at which, according to Murray, sedi- 

 mentation is no longer possible. While I must admit that evidence of shal- 

 low-water deposition is less conclusive in this interval than in those which 

 succeeded, and that portions of the Colorado islands were then submerged 

 which were not subjected to sedimentation during the succeeding intervals, I am 

 unable to accept this explanation for the Rocky Mountain region, and am 

 more inclined to attribute the absence of Devonian to a partial recession of the 

 ocean. The direct evidence of such recession is, it must be confessed, as yet 

 very slight, being limited to an unconformity by erosion between Silurian 

 and Carboniferous, observed in a single locality only,| and to the existence 

 of a thin and not always persistent sandstone between Silurian and Carbon- 

 iferous limestones. 



This supposition corresponds better with the course of events on the east- 

 ern continent as recently traced out by Prof. J. D. Dana.j The break 

 which he shows to exist at the close of the Lower Silurian does not corre- 

 spond exactly in geological succession with the gap which appears to exist 

 in the Rocky Mountain region ; but the exact position of this gap in the 

 geological column is not yet determined. It is quite possible, moreover, that 

 the elevation of laud may not have been strictly contemporaneous in both 

 continents, and that the succeeding subsidence which allowed the reoccupa- 

 tion of the region by oceau waters may have proceeded more rapidly in the 

 one than in the other. 



Early Paleozoic Land. 



The laud areas that existed during this time, or rather the degrees to which 

 the present elevated regions were submerged so as to admit of sedimentation, 

 were somewhat as follows : 



Colorado Island. — At the north the Laramie hills extension of the Colo- 

 rado range was submerged beyond the state line, and the shore-line extended 



* Proe. Bos. Soe. Nat. Hist., vol. XXIII, March, 1S88. 

 t Monographs U. S. Geol. Survey, No. XII, 1886, p. 50. 

 X Bull. Geol. Soc. Am., vol. I, 1889, p. 36. 



