STRATIGRAPHIC TAXONOMY 605 



Stones Eiver are ascribable to continental creep and consequent subsidence 

 of the median areas of the continent. 



In the succeeding Blount (upper Chazyan) and Black River stages, 

 however, warping — and therefore shifting of seas — prevailed to an extra- 

 ordinary degree. Suboceanic spreading had again overcome seaward 

 creep of the land and caused great withdrawal of seas. When they came 

 in again the submergences were for a long time confined to the middle 

 parts of the Appalachian Valley tract and to other troughs nearer the 

 eastern and southern margins of the continent. This restriction is in- 

 ferred from the sharply limited distribution of the formations and faunas 

 of the Blount group. Moreover, as has been pointed out in other parts 

 of this work, the distribution of the several formations of this group 

 within the general area in which it is represented by deposits, is far 

 from uniform. Obviously, the decided variations are due to surface 

 warping. 



The extent and frequency of the warping which characterizes this 

 middle Ordovician period of oscillation is even more clearly expressed 

 by the geographic variability of the post-Lowville Black River deposits. 

 During the Blount stage warping seems to have affected only the now 

 more or less strongly folded submarginal areas. Nor did it transgress 

 these limits in the opening phase of the Black River, for the Lowville, 

 which represents the first age of the Mohawkian, spreads widely and 

 evenly inland and in a manner suggesting median subsidence due solely 

 to continental creep. Compared with the Lowville, the distribution of 

 the succeeding Black River deposits is strikingly irregular. Compared 

 even with each other, the areal patterns formed by the sediments of the 

 Watertown, Decorah, and Kimmswick ages are extremely different. In 

 fact, I know of but a single area, namely, in Hancock County, Tennessee, 

 in which anything like a complete sequence of Black River deposits is 

 to be found. 



Disregarding both local and continental tilting, which continued to 

 be a prominent factor of Ordovician diastrophic history to the close of 

 the Mohawkian epoch, relatively even and broad epeirogenic movements 

 became increasingly dominant during the Trenton ages and attained 

 the maximum for the concluding half of the Ordovician during the 

 Cincinnatian. 



In the Ordovician, then, we have an introductory stage characterized 

 by gradual submergence of the Mississippi Valley, continued emergence 

 of the Appalachian tract, and absence of warping in areas where the 

 record of such deformation might be preserved ; a second stage beginning 

 with submergence of Appalachian troughs and emergence of the Missis- 



