DIASTROPHISM AND THE FORMATIVE PROCESSES 325 



These evidences of pronounced folding movements in different 

 quarters of the globe at the close of the Ordovician are supported 

 by the extensive withdrawals of the sea from the line of maximum 

 transgression in the mid-Ordovician, to the shore lines implied by 

 the early Silurian beds in many other regions which were not 

 affected by the more violent disturbances. This was a time of 

 general disturbance; but it remains for future study to establish 

 just how nearly synchronous these widespread disturbances were. 



SILURIDES 



During the greater part of the Silurian period there was a rela- 

 tive freedom from diastrophic disturbances of the major sort. 

 But toward the close marked disturbances set in and may be said 

 to have terminated the epicontinental phase of the Silurian sedi- 

 mentation in various parts of the globe. North America was 

 affected chiefly by movements of the milder sort but certain por- 

 tions of Europe suffered from intense orogenic deformation. In 

 North America a comparatively rapid emergence of the continent 

 began with the Guelph and continued until the Silurian period 

 closed with the land nearly as extensive as it was at the beginning 

 of the period, when most of the North American continent stood 

 above water.^ While the very widespread unconformities indicate a 

 general epeirogenic movement at this time, or else a withdrawal of 

 the sea, or both, very little evidence of pronounced folding has as 

 yet been found on the mainland of this continent. Some slight 

 folding, however, has been recognized on the Atlantic border. 

 Campbell, in a summary of the periods of Appalachian folding in 

 Virginia, has described a deformative movement which, preceding 

 the deposition of the Walker black shale, has folded the limestones 

 and marks the division line between the Silurian and Devonian. 

 It is the principal period of deformation in the region before the 

 great Appalachian revolution which came at the close of the 

 Paleozoic.^ 



Siluride movements following the extensive Silurian sedimenta- 

 tion were especially prominent in the higher northern latitudes. 



' Charles Schuchert, " Paleogeography of North America," Bull. Geol. Soc. Am., 

 XX (1910), 491, 540. 



=> M. R. Campbell, Bull. Geol. Soc. Am., V (1894), 189-90. 



