NIAGARA PERIOD. 245 



the rising of the bottom. In the Oneida epoch, which opened the 

 Upper Silurian, we have evidence only of an exposed coast-line in 

 the face of the waves, — the former barrier being more sunken, and 

 the bottom of the Interior Continental sea risen quite above the 

 level of the waters, so as to make dry land where before was a pro- 

 fusion of marine life. In the Medina epoch the bold sea-coast 

 becomes a flat coast-region, with sand-flats and marshes, and to the 

 northward the waters spread west of New York State. In the Clin- 

 ton epoch the coast-region is similar to that in the Medina, but the 

 waters cover largely the Mississippi basin, and the interior sea once 

 more begins to appear. In the Niagara epoch the Interior Conti- 

 nental basin is again a continental sea full of marine life, and through 

 a large part of it limestone reefs are in progress. 



If the above is a correct view of the geographical changes, it is 

 seen that they consisted in a grand though small oscillation over 

 the continent, — a rising and sinking again of the interior ; and in 

 its course the interior seas became dry land at the close of the 

 Lower Silurian. If so, there is no need of any other explanation 

 of the extinction of life that then happened, or of the exception 

 to the thoroughness of the extinction in the Eastern border basin 

 at Anticosti, — if actually an exception (p. 231). 



In the course of these oscillations, from the beginning of the 

 Trenton to the close of the Niagara period, 12,G60 feet of rock were 

 deposited along the Appalachians. From this datum the slowness 

 of the oscillation may be estimated. The whole amount of change 

 of level over the Interior Continental basin may not have exceeded 

 1C00 feet. 



But the Appalachian deposits show a vast amount of subsidence 

 which was in slow progress as the accumulations went on. Without 

 the subsidence, great breadth of deposits might have been formed, 

 but not great thickness. 



With regard to the continent beyond the Mississippi we have small basis for a 

 conclusion. About the Black Hills and Laramie Range Dr. Hayden found the 

 Carboniferous strata resting on those of the Potsdam period, and, therefore, an 

 absence of all the formations of the Lower Silurian above the Potsdam, of all 

 the Upper Silurian, and of all the Devonian. Similar facts are reported from 

 Arkansas. About the El Paso Mountains in New Mexico, between the rivers 

 Pecos and Grande (near lat. 32°), Dr. G. G. Shumard found a limestone of the 

 Trenton or Hudson periods, containing the fossils Orthis testndinaria, 0. occi- 

 dentalis H., Rhynchonella copax Conrad, and others; but to this succeeded the 

 Carboniferous. It would seem, therefore, that a part of the region beyond the 

 Mississippi was in no condition for the formation of limestones or sandstones 

 between the Lower Silurian and the Carboniferous ; and the most probable sup- 

 position is that the land was not under water, although it was afterwards ex- 



