684 AV. J. MILLER PRE-CAMBRL\X FOLDING IX NORTH AMERICA 



cases such distinctions have not as yet been very satisfactorily made out, 

 as, for example, in the Appalachian-Piedmont region and western Xew 

 Eno'land, where Paleozoic deformation has very laraelv overwhelmed and 

 obscured whatever pre-Cambrian folding there may have been. Struc- 

 ture sections in various United States Geological Survey folios illustrat- 

 ing the region from Xew Jersey to Georgia show l^oth pre-Cambrian and 

 Paleozoic rocks to have been folded in similar manner apparently by the 

 same force or forces. 



Mr. Arthur Keith has, however, recently told the writer that there is 

 evidence for at least some pre-Cambrian deformation. In the Xantahala, 

 Xorth Carolina, folio Keith says : 



"The various deformations ( Precainbriaii and Paleozoic) have greatly 

 changed the aspect of the rocks (Cambrian and pre-Cambrian) — so much so, 

 in fact, that the original nature of some of the oldest formations can be at 

 present only surmised." 



Also : 



"Structures in the Arehean uplift in the southeastern part of the «)uad- 

 rangle do not differ radically from those in the sediments." 



But in regard to the pre-Cambrian Carolina and Koan gneisses, he 

 says that they have "passed through two deformations, one producing 

 the foliation and a second folding the foliation planes.'' Since these 

 gneisses were originally probably either granites or injection gneisses, 

 is it necessary to assume that their foliation was produced by severe com- 

 pression? Could not this structure be either a primary foliation or a 

 magmatic injection structure produced under conditions of only very 

 moderate lateral pressure, according to tbe principles set forth in the 

 writer's paper^ already referred to? 



Professor Bascom in a recent letter says, in regard to southeastern 

 Pennsylvania, that the 



"pre-Cambrian rocks were folded (in pre-Cambrian time), but I am not pre- 

 pared to say 'notably' ; close folding can not be proven : there is genera] diver- 

 gence of strike in some places over long distances ; folding about igneous in- 

 trusions of pre-Cambrian age can be positively differentiated from Paleozoic 

 folding, of course relatively local in character. ... Of course, the very 

 intense Paleozoic folding now dominates and obscures all previous folding." 



From these statements it seems reasonable to believe that such pre- 

 Cambrian deformation as can be made out. may have been largely or 

 wholly caused bv the intrusives without severe compression of the region. 



«W. .1. :SIiller: .Jour. .Geol.. vol. 24. 1916, pp. 600-612. 



