THE CENTRAL REGION 259 



fectly shown because largely concealed beneath the coastal plain on the 

 south and beneath the sea on the north — exhibits an eastward overturn- 

 ing, as if toward its eastern border. 



The central Eegion 



The central region in ]N"ew England is not one of engulfment of the 

 newer and highest strata in the folded system, but, on the contrary, a 

 tract in which the Precambrian is about as high as elsewhere. South of 

 New York City, in the area of crystalline rocks of the piedmont region 

 lying between the typical Appalachian structures along the western side 

 of this mountain system and the place beneath the coastal plain in which 

 one would expect to find the extension of the Carboniferous folds of east- 

 ern Massachusetts and JSTova Scotia, the local structures indicate that the- 

 central axial part of the range brought ancient rocks to high levels. Fur- 

 thermore, the general plan of the cross-section of the foldings shows, 

 along with some shifting of the sites of geanticlines and neighboring- 

 geos}Ticlines, a central tract of uplift and erosion of older rocks bordered 

 on one or both sides by geos}Ticlinal troughs of deposition whose final 

 structural expression exhibits outthrust toward the outer borders of the 

 compressed region. This fan-shaped structure is, however, not without 

 much diversity where igneous intrusions have appeared during or just 

 after the episodes of folding. For this reason large tracts of Precambrian 

 rocks make the axial areas of the Acadian range, and thick sections of 

 Paleozoic strata, including Carboniferous formations, sometimes witlr 

 coal, now lie here and there on or near the margins of the complex moun- 

 tain structure. The greater Appalachian system in eastern ISTorth Amer- 

 ica has gained the reputation of being wholly overthrust westward because^ 

 its western border, exhibiting that structure, is everywhere exposed to 

 examination, and because its eastern border is largely concealed or has 

 not been considered in the description of its broad physiognomy. So far 

 as we are permitted to observe the eastern border of the Acadian area, 

 strata varying in age from the Carboniferous downwards to the Precam- 

 brian are at the present surface above sealevel, and eruptives of Carbon- 

 iferous or immediately post-Carboniferous age play a larger role in the 

 surface exposures than is the case west of the Connecticut Valley. One 

 must conclude, therefore, that volcanic phenomena prevailed on the east- 

 ern or southern flanks of the mountain system in Pennsylvanian times, 

 in contrast with the paucity or absence of eruptives of this date on the 

 western overthrust belt of the same system. A like one-sidedness in the 

 distribution of igneous intrusions appears to characterize the less well 



