CROSS-STRUCTURES 327 



United States Geological Survey^ and the northeastern swerve has re- 

 cently been described by Wentworth as the Cumberland thmst block. 

 Northwest of this thrust block the main axis of the Carboniferous syn- 

 cline is pushed northwestward out of the usual trend. Along the north- 

 east margin of this thrust block various structures swerve out of the 

 usual trend into nearly east-west courses, although the swerving is not 

 as prominent at this end of the block as it is at the southwest end. The 

 facts summed up show that this excess northwestward motion w^as shared 

 by all of the rocks from the Pennsylvanian down into the Archean. 



A similar local cross-warp is seen in northern Vermont, north of the 

 latitude of Eutland, where all of the structures run decidedly west of 

 north. The Appalachian Valley is here at its highest and narrowest 

 point, so that the youngest known beds to be involved are Middle Ordo- 

 vician. These beds, the Cambrian strata, and the gneisses and schists 

 of the Precambrian are alike turned aside from their normal courses. 

 This cross-warping runs a little north of west and is part of the general 

 Vermont salient. 



In addition to the cross-warps and cross-folds of the Appalachians 

 there are cross-zones along which the pitch of most structures is in- 

 creased. These are of similar nature with the cross-folds, but the pitch 

 is continuous in the same direction instead of being in two directions as 

 in the folds. A notable example of this feature crosses the Appalachians 

 in New York along the Mohawk River and causes the rise of the Pre- 

 cambrian Adirondack mass from the Paleozoic beds of the Catskills and 

 Allegheny Plateau. The same zone of cross-dip causes a sharp deepening 

 of the Appalachian troughs east of Albany and in northwestern Massa- 

 chusetts; it also brings down the Devonian beds at Bernardston, Massa- 

 chusetts, on the Connecticut, and in northeastern Massachusetts it 

 deepens several synclines of Carboniferous phyllite. In the areas between 

 these synclines beds of Ordovician, Cambrian, and Precambrian age are 

 affected. 



A similar zone of cross-dips is seen in northern Alabama, extending 

 nearly east and west across the State. These dips, which with reference 

 to the general system constitute a pitch of the folds, are readily visible 

 to the eye in many places. The pitch causes a westward extension and 

 general deepening of the Pennsylvanian coal measures in Alabama, and 

 it is exhibited in the pitch of the entire Paleozoic folded series southward 

 beneath the Cambrian along the Eome fault in Alabama and adjoining 

 northwest Georgia. 



XXII — Bi LL. Gkol. Soc. Am., Vol. 34, 1922 



