THE ORDOVICIAN PERIOD. 333 



the sinking of the ocean bottoms and the draining off of the epicon- 

 tinental waters to a large extent, attended by some slight deformation 

 of the general surface of the continents and some notable flexure of 

 special tracts. The withdrawal of the waters appears to have con- 

 verted extensive stretches of Ordovician epicontinental sea-bottom 

 into land. The land area of the Lake Superior region (Fig. 129) was 

 greatly enlarged. From it land probably extended, via Missouri, 1 

 as far south as Texas, and perhaps beyond. It may have extended 

 as far west as the Rocky mountains, and east to the Cincinnati arch. 

 If this be true, a great expanse of land, the surface of which consisted 

 of Ordovician strata, came into existence at this time in the very heart 

 of the continent. The elevation of this land must have been slight, 

 for it seems to have suffered little erosion before most of it was again 

 submerged and covered by sediments of lesser age. It is indeed the 

 wide-spread absence of the lower part of the Silurian system (the 

 Oswegan series, p. 370), rather than pronounced stratigraphic un- 

 conformity between the Ordovician and the Silurian of the interior, 

 which indicates the extensive emergence of land referred to. There 

 is, however, an unconformity between the Ordovician and Silurian 

 systems, in some places, as in Iowa, 2 Arkansas, 3 and Tennessee, 4 and 

 the Devonian or later beds sometimes rest on the Ordovician, with no 

 Silurian between. Throughout much of the western part of the con- 

 tinent also, the land seems to have emerged at about this time, foi 

 the Silurian system is wanting in many regions where the Ordovician 

 is present. 5 



The folding movements were less wide-spread. The most consider- 

 able was in western New England and eastern New York, in the region 

 of the Taconic mountains. Here both the Cambrian and Ordovician 

 systems were thick, and at this time they were folded and lifted above 

 the seas beneath which they had accumulated. Under the influence 

 of the intense dynamic action involved in the folding, the rocks were 



1 The pre-Cambrian area of Missouri is often referred to as an island at this time. 

 It has been urged, however, that the Cambrian, Ordovician , Devonian , and Carbonifer- 

 ous beds once overlay it. See Keyes, Science, Vol. VII, p. 388, 1898, and Bull. Geol. 

 Soc. Am., Vol. XIII, pp. 268-292. 



2 See Silurian, pp. 371-378. 



3 Williams, Am. Jour. Sci., Vol. 48, 1894, p. 325. 



4 Pp. 371-378. 



5 See sections of Appendix, Vol. III. 



