516 GEOLOGY 



extensive emergence of land in the interior at the close of the Ordovi- 

 cian period. Throughout much of the western part of the continent 

 also, the land may have emerged at about this time, for the Silurian 

 system is wanting, or has not been recognized, in many regions 

 where the Ordovician is present. 



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

 able was in the Taconic Mountains, where both the Cambrian and 

 Ordovician systems were thick. Both were folded and lifted above 

 the sea beneath which they had accumulated. The eroded rem- 

 nants of the folds often show a complicated structure. The date 

 of the folding is known, because Silurian formations overlie the 

 Upper Ordovician unconformably about the borders of this moun- 

 tain region. It is not to be inferred that all the mountain-making 

 movements which have affected western New England occurred 

 at this time. There had been folding earlier, in pre-Cambrian 

 times, and there were later movements, as will be noted. 



Between folding and the more gentle movements already noted 

 there are all gradations. The " Cincinnati arch" is an example. 

 This arch is a very low anticline with a general north-south course, 

 extending through Cincinnati. The beginning of this arch may 

 have been as early as mid-Ordovician. 1 Another similar arch 2 may 

 have come into existence at about the same time in Arkansas and 

 Oklahoma, corresponding in position with the mountain system 

 commonly known as the Ouachita Uplift, of which it was perhaps 

 the beginning. The strata of this region were notably folded at a 

 much later time. In some other places, as for example in New 

 Brunswick and Nova Scotia, there is unconformity between the 

 Ordovician strata and those which overlie them, indicating an 

 emergence after the deposition of the Ordovician formations. 



The crustal movements referred to above have been mentioned 

 as occurring at the close of the Ordovician. It would perhaps be 

 more accurate to say that their beginning marks the beginning of 



1 Hayes and Ulrich, Columbia (Term.) 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. Kept., Dept. of Geol. and Nat. Hist., 

 Resources of Indiana, 1899. 



2 Branner, Am. Jour. Sci., Vol. IV, 1897, p. 357. This very suggestive 

 article has bearings on many questions besides the Ouachita Uplift. 



