312 A. KEITH OUTLIXES OF APPALACHIAX STRUCTURE 



which characterized the earth movements at the end of the Paleozoic. 

 In Central America there seems as yet to be no mountain-building wliicli 

 can safely l)e connected with that of the Appalachians. 



There has l^een little description or discussion bv oeolo2;ists in general 

 of the width of the Appalachian system as a structural unit. It has 

 commonly been considered to include the gentle folds of the great Car- 

 boniferous synclinorium lying just west of the strongly deformed belt 

 and extending westward to the Cincinnati arch. Xo eastern limit has 

 been assigned except tacitly by the discussion of the Paleozoic folded 

 sediments but not of older rocks on the east. This has been due in the 

 past to the lack of information concerning the pre-Paleozoic rocks and 

 to the labor and difficulties involved in securing that information. The 

 author has spent many years in the examination of the Precambrian 

 rocks of the Appalachians with full realization of the important part 

 which they have played in Appalachian deformation, and has in all his 

 descriptions treated the easteiTi and older rocks as part of the Appa- 

 lachians. The Appalachian system, in his estimation, is limited on the 

 southeast by the Cretaceous cover of the Coastal Plain as far north as 

 Hudson Eiver, and in Xew England and Canada by the Atlantic Ocean. 

 The Appalachian folds are strong where they 2)ass beneath the sediments 

 of the Coastal Plain or the Atlantic, and there is no indication that the 

 orioinal eastern limit of the svstem is visible. 



A northwestern limit is very difficult to set, since the gentle folds pro- 

 duced at the end of the Paleozoic form a system integral with the Appa- 

 lachians which extends across the Mississippi Valley to the Cretaceous 

 rocks of the great Central Plains. Xowhere is it possible to draw a line 

 which will separate great groups of rocks of unlike structure or even of 

 unlike stratigraphy. The fold S3'stems of that time are shown in plate 

 4:. In Alabama and Tennessee at the south and in Xew York and Can- 

 ada at the north the intensity of the folding diminishes rapidly north- 

 westward at the margin of the Cuml^erland Plateau of the south and of 

 the Adirondack and Laurentian mountains of the north. In those areas 

 there would be the best opportunity for setting a western limit for the 

 Appalachians. Folds of Appalachian type appear west of this line, how- 

 ever, even where it is most pronounced, while in the central Appalachians 

 of Virginia and Pennsylvania strong Appalachian folds are found far to 

 the west of it, and there is a gradual transition from the strong folds of 

 the east to the gentle basins of the great Carboniferous synclinorium. 



The strongly folded belt in Tennessee, Georgia, and the Carolinas lias 

 an exposed width across the strike of 2 TO miles; its width in .Quel)ec, 

 Maine, and Xew Brunswick is practically the same, and across Xew- 



