§90 B. WILLIS A THEORY OF CONTINENTAL STRTIOTURE 



Heterogeneity may be a condition inherent to some extent in original 

 constitution, the term original being used to describe a constitution which 

 has characterized the continent since a time at least as remote as any of 

 which the rocks afford a record; and heterogeneity may also be an effect 

 of terrestrial forces acting, as by intrusion, to modify the original 

 structure. 



To discuss a possible original structure and to define some of the con- 

 ditions which have affected modifying forces are among the purposes of 

 this paper. In order that the argument may be followed more readily, 

 the hypothesis which has resulted from the investigation is here briefly 

 stated. 



Hypothesis ov continental Structure 



1. It is postulated that the continental mass which we call North 

 America is heterogeneous, and that the unlike parts of it are of notable 

 size, varying from a few hundred square miles in superficial area to sub- 

 continental dimensions. These parts are referred to as elements of the 

 continent, or continental elements. 



2. The distinction between the continental elements is based upon their 

 behavior during vertical movements, it being held to be demonstrable on 

 the evidence of erosion or sedimentation that certain masses have shown 

 a tendency to rise, whereas others have shown a like tendency to subside. 

 The boundaries of the elements are accordingly indicated where condi- 

 tions of general denudation pass into conditions of long-continued aggra- 

 dation, either subaerial or submarine. 



It follows that those continental elements which have tended to rise 

 are recognized by the unconformities or absence of sediments resulting 

 from conditions of erosion, whereas those elements that have tended to 

 sink are recognized by tbe accumulation of sediments upon them. 



According to their diverse tendencies with reference to relative vertical 

 movement, the two types of elements may be distinguished as positive or 

 negative. Those which have shown a decided tendency to rise are desig- 

 nated positive elements, and those wliich have tended to sink are termed 

 negative elements. 



3. It is observed that uplands of erosion and lowlands of aggradation 

 are commonly joined by a monoclinal flexure, which in some places in- 

 volves a normal fault. When the region is altogether under subaerial con- 

 ditions the flexure constitutes a warped surface, which is a mountain 

 slope. Such is the descent from the Blue ridge of Virginia to the Atlan- 

 tic coastal plain. Or, when the sunken region is so situated as to become 

 an arm of the sea, the flexure is the locus of a coastline and the depressed 



