328 A. KEITH OUTLINES OF APPALACHIAN STRUCTURE 



CROSS-FOLD SYSTEM 



On those folds which distinctly parallel the sharp Appalachian folds 

 and show plainly the same salients and recesses, there are high places 

 and low places. For instance, the low place between Cincinnati anfl 

 I^ashville domes is the direct continuation eastward of the Illinois coal 

 basin and joins a very deep portion of the Appalachian coal basin. 

 Similarly the sag in northern Ohio between the Cincinnati dome and the 

 Ordovician uplift at the west end of Lake Ontario lies directly between 

 the Appalachian coal basin in its deepest part in West Virginia and the 

 Michigan coal basin; thence northwestward it passes through the Cam- 

 brian depression of Lake Superior. Where this axis crosses the strong 

 folds of the Appalachians it coincides with the Virginia recess and with 

 a major depression of the Appalachian axis. 



The cross-anticline of the Adirondacks is very plain ^s far east as 

 Connecticut River ; westward it forms a barrier between the Saint Law- 

 rence Valley and the Great Lakes and passes northwestward into the 

 area of uplift immediately north of the Great Lakes. Parallel to this a 

 similar major syncline crosses the Appalachian folds of northern Ver- 

 mont and Xew Hampshire and forms the upper Saint Lawrence Valley 

 between Montreal and Ottawa. Northwestward it extends up the valley 

 of Ottawa River, marked by numerous inliers of Paleozoic rocks in the 

 Precambrian, and passes into the great basin of Hudson Bay. A part 

 of this system of great cross-folds is the uplift connecting the Ozark 

 dome of Missouri with the ISTashville dome of Tennessee. The influence 

 of this cross-fold is readily seen in the general altitude of the close 

 Appalachian folds in northwest Georgia. West of the culmination of 

 the Ozark dome the arch appears to divide, the principal portion running 

 nearly west through Missouri and into eastern Kansas. 



The most southerly of the cross-synclines is much concealed by the 

 young deposits of the Mississippi embayment. It causes the great deep- 

 ening and expansion of Pennsylvanian rocks in central Alabama near 

 the Cretaceous cover and is exhibited in the great Pennsylvanian syn- 

 clinorium of Arkansas River in Arkansas and Oklahoma. Parallel to 

 and south of this a belt of major uplift in Arkansas and Oklahoma con- 

 nects the Ouachita, Arbuckle, and the Wichita Mountains. South of 

 til is belt other folds, probably of the same system, are exhibited in Texas 

 and Louisiana. Detailed information about these is scanty, however, 

 and is derived mainly from drill records. 



