Crosby.] 502 " [March 7, 



and Carboniferous here are not separated by a great thickness of 

 similarly conformable Ordovician, Silurian and Devonian strata, 

 but the Carboniferous appears to lie directly upon the Potsdam. 

 In other words, the intervening epochs are a complete stratigraphic 

 blank, a lost interval of vast duration. 



This conformable contact of the Potsdam and Carboniferous, 

 which would be sufficiently remarkable if limited to the Black Hills, 

 is known to exist over almost the entire Rocky Mountain region. 

 A few of the numerous areas of Paleozoic strata scattered over 

 this vast territory exhibit, between the Potsdam sandstone and the 

 Carboniferous limestones, traces or meagre developments of the 

 Ordovician, Silurian and Devonian formations. But it is difficult 

 or impossible to define the exact horizons of these beds ; and con- 

 tinued investigation only establishes more clearly the general fact 

 that few or none of the many distinct periods and epochs between 

 the Potsdam and Carboniferous in the Appalachian region can be 

 recognized in the Rocky Mountains. Farther west, however, in 

 western Utah and Nevada, or toward the western shore of the Pal- 

 eozoic ocean, the Ordovician, Silurian and Devonian appear to be, 

 in thickness and distinctness, somewhat comparable with the same 

 formations in the east. 



Although several slight unconformities have been observed 

 among the Paleozoic beds in the Grand Canon and Nevada sections, 

 indicating corresponding oscillations of the floor of this part of the 

 Paleozoic ocean, the perfect and widespread conformity of the Pots- 

 dam and Carboniferous absolutely forbids the supposition that the 

 western part of the United States was dry land during any consider- 

 able part of the intervening epochs of Paleozoic time. The surface 

 of the Potsdam is not eroded, and its strata do not exhibit the 

 disturbance which would almost necessarily have attended the el- 

 evation of such a vast expanse of the sea-floor. 



The lowest Paleozoic strata of North America rest everywhere 

 unconformably upon an eroded Archaean foundation ; and hence few 

 facts in American geology are more firmly established than that the 

 part of this continent subsequently covered by the Paleozoic sea, 

 stretching from the Adirondacks and the Blue Ridge to the Sierra 

 Nevada or beyond, had been dry land for a long period anterior to 

 the general interchange of continental and oceanic conditions which 

 spread the sea over its surface. The Blue Ridge, Adirondacks, and, 

 perhaps, some of the New England Archaean ranges, the Highlands 



