THE ORDOVICIAN PERIOD. 335 



metamorphosed, and the eroded remnants of the folds often show 

 a complicated structure (Fig. 153). The orogenic disturbance may 

 have extended south of New England, but its limit in this direction 

 is undetermined. 



About the borders of the mountain region, the Upper Ordovician 

 shale is overlain unconformably by rocks of Silurian age, showing that 

 the period of upturning and metamorphism followed the deposition of 

 the youngest of the Ordovician formations, and preceded the depo- 

 sition of the oldest formation which overlies the metamorphosed rocks 

 unconformably. Thus the date of the upturning is, within limits, 

 determined. The Silurian strata which overlie the Upper Ordovician 

 do not belong to the first epoch of the period, so that in this region, 

 there is an interval unrepresented by sedimentary formations. 



The formations involved in the Taconic moimtains were formerly 

 known as the Taconic system. Their metamorphic character and 

 complex structure long prevented their proper correlation. Within 

 recent years it has been shown that the series includes the systems 

 mentioned above. 1 It is not to be inferred that all the mountain- 

 making movements which have affected western New England occurred 

 at this time. Later movements, involving younger strata, affected 

 this region, as will be noted in succeeding pages. 



Between folding and the more gentle movements there are all degrees 

 of gradation, and there were areas where only incipient folding or 

 bowing of the strata took place. The " Cincinnati arch " and the 

 " Nashville (or central Tennessee) dome " farther south have already 

 been referred to. The deformation here was not great, but it appears 

 to have been prolonged, and to have begun perhaps as early as the 

 mid-Ordovician period. From this time on, the region seems to have 

 been near sea-level, being now submerged and now land. 2 



Recently attention has been directed to another similar arch which 

 may have come into existence at about the same time. This is in 

 Arkansas, 3 Indian Territory, and Oklahoma, corresponding in position 

 with the mountain system commonly known as the Ouachita uplift, 



1 Walcott, Am. Jour. Sci., Vol. 35, 1888. This paper gives bibliography (p. 30). 



2 Hayes and Ulrich, Columbia (Tenn.) folio, U. S. Geol. Surv.; also Foerste, Geol. 

 Soc. of Am., Vol. XI, p. 604, and Vol. XIII, p. 531, Science, New Ser. Vol. X, p. 488, and 

 24th Ann. Rept., Dept. of Geol. and Nat. Hist. Resources of Indiana, 1899. 



3 Branner, Am. Jour. Sci., Vol. IV, 1897, p. 357. This very suggestive article has 

 bearings on many questions besides the Ouachita uplift. 



