478 HISTORICAL GEOLOGY 



peatedly (i) to the continent of Appalachia, a broad upland, sometimes 

 high and sometimes low, and (2) to the Appalachian trough west of 

 it in which much of its waste was poured. Two points have been em- 

 phasized : (1) that the trough or geosyncline sank as it was weighted 

 with sediments, and (2) that, as Appalachia was worn down by the 

 streams, a compensating rise took place. With the exception of com- 

 paratively short periods of emergence, sediments were accumulating 

 in the Appalachian trough from the beginning of the Cambrian until 

 the Permian, during which time more than 25,000 feet of sediments 

 were laid down. One of the most important upward movements of 

 the trough occurred near the close of the Ordovician, apparently at 

 about the time the Taconic deformation (p. 422) was taking place. 

 Others occurred in the Silurian and between the Mississippian and 

 Pennsylvanian periods. With these and other minor exceptions 

 the great Appalachian trough was the site of deposition during the 

 long periods of the Paleozoic, and a thickness of more than five miles 

 of sediment accumulated. 



Towards the close of the Carboniferous the most striking event in 

 the geological history of eastern North America was consummated. 

 At this time the sediments of the Appalachian trough yielded to the 

 strain that had long been accumulating and folded into a great moun- 

 tain system (Fig. 351, p. 361), the axes of the folds extending in a 

 northeast-southwest direction, one range reaching from Nova Scotia 

 to Rhode Island, another from New York to Alabama, and a third in 

 Arkansas forming the Ouachita Mountains. 



The probable cause of the yielding of this particular portion of the 

 crust to lateral pressure was the fact that the geosyncline was a zone 

 of weakness " just as the bend in a crooked stick determines the point 

 at which it will break when pressure is applied at the ends." The 

 rocks in all portions of the trough were not equally deformed : those 

 in Pennsylvania and West Virginia have been, for the most part, com- 

 pressed into gentle folds, while those in the southern Appalachians 

 in Tennessee and elsewhere were broken by so many thrust faults 

 that the reconstruction of the region is often difficult. The intensity 

 of the folding diminished from east to west. In eastern Pennsylvania, 

 for example, the folds are more compressed and faults are more 

 common than in the central part of the state, while in the western 

 portion the rocks were little disturbed and are almost horizontal. 

 The greater deformation on the eastern side of the trough is also seen 

 in the character of the coal in eastern and western Pennsylvania. 



