412 B. WILLIS A THEORY OF CONTINENTAL STRUCTURE 



for. which were, however, more or less effective in producing vertical dis- 

 placements so long as the mechanical relations of the masses were not 

 materially affected by tangential pressure. 



(b) The criteria of unconformity and sedimentation are applied to an 

 analysis of North America to distinguish the lighter elements which have 

 often been land areas from the heavier elements which have more often 

 been submerged. The elements thus recognized are enumerated and de- 

 scribed as positive (lighter) and negative (denser) elements. 



(c) The effects of tangential pressure are considered in a large way, 

 and it is recognized that North America has been narrowed by compres- 

 sion from the Atlantic and Paciiic sides. The pressures are attributed 

 to deep-seated suh oceanic spread,, and their effects in modifying the rela- 

 tions of elevation or depression dwe originally to isostatic adjustment are 

 discussed. 



(d) Displacements of lighter and denser masses are regarded as one 

 cause of variations of temperature and pressure to which we may ascribe 

 the formation of igneous masses, and zones on which such masses border 

 are discussed as favorable places for the development of intrusive and 

 extrusive bodies. The courses taken by igneous bodies rising toward the 

 surface are in some degree directed by the thrust planes set up in conse- 

 quence of tangential movements, and the distribution of bathyliths and 

 volcanics is accordingly modified, as in the Cordillera. 



The resultant conclusion is that the dominant geologic phenomena of 

 the history of North America are in accord with the view that the conti- 

 nent consists of heterogeneous elements, which primarily exhibited more 

 or less simple isostatic relations, but which in consequence of tangential 

 pressure and igneous intrusion are no longer in close isostatic adjustment 

 among themselves, although the composite continental plateau as a whole 

 apparently is in equilibrium with the adjacent ocean beds. 



