THE ORDOVICIAN PERIOD. 321 



seem not to be well differentiated. This is indicated by the sections 

 of Texas * and Indian Territory. 2 



General conditions in the eastern part of the continent. — It is worthy 

 of note that in mid-Ordovician time, limestone was forming over a 

 very wide area in the eastern part of North America, namely, from 

 New England on the east to Georgian Bay on the northwest, Indian 

 Territory and Texas on the southwest, and Alabama on the south. 

 Limestone appears to have been forming in the Great Basin region 

 of the west at the same time. At no previous epoch in geological 

 history, so far as now known, was there anything like so wide-spread 

 deposition of limestone within the limits of North America. This 

 is to be accounted for by the fact that the sea occupied a large part 

 of the continent, that such lands as existed were low, and that drain- 

 age from them brought little sediment to the sea. Meanwhile the 

 forms of life which secrete lime carbonate were depositing their shells 

 widely over the bottom of the interior epicontinental sea. 



It is perhaps equally worthy of note that in the latter part of this 

 period mud (now shale) was deposited over an almost equally exten- 

 sive area extending from New York southwest to Alabama and north- 

 west to Manitoba and beyond. 



The inauguration of the epoch of clastic sedimentation may have 

 been brought about by movements which increased the capacity of 

 the ocean basins, thereby drawing off the waters, attended by gentle 

 warpings which resulted, among other changes, in the uplift of an area 

 about Cincinnati 3 (the Cincinnati arch) and the closing of the southern 

 outlet of the bay which occupied the south end of the Appalachian 

 synclinorium. It appears to have been about the same time (Utica 

 epoch) that subsidence to the northeast reestablished connection 

 between the Atlantic and the interior sea by way of the St. Lawrence 

 valley. It seems not unreasonable that these geographic changes 

 should have been attended by an increase in clastic sedimentation. 

 The wide-spread late Ordovician shale formation may mean either 

 that the land areas were so far elevated as to allow the streams to 



1 Geol. Surv. of Texas, Second Ann. Rept., p. 565. 



2 Taff, Atoka (I. T.) folio, U. S. Geol. Surv. 



3 Ulrich and Schuchert, op. cit. p. 645. The time of the origin of the Cincinnati 

 arch has never been fixed with certainty. Hayes and Ulrich place its inception still 

 earlier in the Ordovician. See Columbia (Tenn.) folio, U. S. Geol. Surv. The region 

 was probably subject to disturbances at various times.. 



