STRUCTURAL TERMINOLOGY 



11 



wrong. A geographic name avoids this difficulty, it is true, but for the 

 most part stratigraphy has advanced to the point, in the United States at 

 least, that the times of the deformation are fairly accurately known and 

 not likely to be changed much in the future. The advantage to the student 

 weighs so heavily against the possible chance of error that time names for 

 the phases will be used. 



Most of the chapters deal with systems and their phases. Such organiza- 

 tion seems adequate to explain the structural evolution of the continent. 

 Originally it was planned to organize the book according to revolutions, 

 but setting limits led to many difficulties, and the idea was abandoned. 

 As a result, the concept of revolution is not emphasized. 



Orogeny 



With the decision reached to divide the great deformational belts into 

 mountain systems, and to treat the several episodes of deformation of each 



system as phases, the proper usage of the term orogeny teemed d- 

 each phase is an orogeny. Thus we speak of the "I. .it.- ( retaceoui and 

 Early Tertiary Rocky Mountain systems," and for one ol th< 

 the Central Rockies, we note its episodes of deformation, namely, the 

 Montana phase, the Paleocene phase, and the Eocene phase. These pb 

 are commonly the orogenies, which respectively would be the earl) 

 Laramide orogeny, middle Laramide orogeny, and late 1 •aramide OXOgi 

 See table of contents for the various orogenies recognized in North 

 America. 



An orogeny should be given a geographic name, like a formation, and 

 if the time of deformation is found to be earlier or later than previously 

 recognized on the basis of later research, then the name remains the same, 

 but a somewhat different age is assigned it. 



An orogeny should not be limited to a phase of folding and thrusting, 

 but should include all forms of diastrophism, according to Billings ( 1960). 



