OSCILLATORY CHARACTER OF CONTINENTAL SEAS 339 



long time with apparently no very important oscillations until the sea 

 withdrawal preceding the last stage of the period. The latter suggests 

 merely restriction of the Ozarkian sea, accessible deposits of this age, so 

 far as known, being found chiefly or only in the Mississippi Valley. 



The Canadian seas exhibit a more striking example of the proposition. 

 In New York the earliest deposits of this period are found only on the 

 west and south flanks of Adirondackia, while all later Canadian forma- 

 tions in the State are confined to the Champlain trough and its southern 

 continuation. Both are well represented in the northern and middle 

 divisions of the Appalachian Valley, but the upper is absent south of the 

 Staunton axis. In the Mississippi Valley the period seems to be repre- 

 sented by its early to middle stages only, and the same is true of central 

 Texas. But the later as well as the middle stages are well developed in 

 western Texas, Oklahoma and rather generally in the Great Basin of the 

 far west. Thus, it will be observed that while the early Canadian sub- 

 mergence simulated the geographic pattern prevailing during the middle 

 stage of the Ozarkian, subsequent Canadian patterns differed widely. 



Comparison of the Ordovician seas indicates a similar sequence of 

 movements. The proposition holds, also, though not uniformly, for the 

 succeeding Eopaleozoic periods. In the closing stage of the Ordovician 

 decided restriction of seas occurred. The extraordinarily wide submer- 

 gences of the interior areas of the continent which prevailed in the late 

 middle Ordovician stages were almost entirely replaced by emergent 

 phases in the Cincinnatian. Deposits of this age are practically confined 

 in North America to the southeast of a line connecting the mouth of 

 Ohio River and the north shore of the Gulf of Saint Lawrence, and the 

 last of the truly marine Cincinnatian is known only from the Cincinnati 

 dome. 



The Eichmond, to whicli age tlie typical Medina of the Appalachian 

 region is referred, is regarded as beginning the Silurian. The oldest of 

 the Richmond faunas — the Arnheim — invaded from the Gulf of Mexico 

 and like the last of the Cincinnatian (McMillan) is found only on the 

 flanks of tlie Cincinnati dome and on the west side of the Nashville dome. 

 But before the close of the Richmondian the seas of this time had spread 

 over the interior areas of the continent to an extent and in a manner 

 closely simulating the great, though somewhat composite, early Trenton 

 submergence. As is well known, the distribution of the middle and late 

 Silurian deposits is much less extensive and, especially in the eastern 

 half of the continent, very different. 



The cause of the relative greatness and distinctness of the introductory 

 submergences of most periods is twofold. In the first place, during the 



