10 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



of all the orogenic cycles that have affected each part of the country, etc." 

 Collet (1927) uses the word phase as a tectonic unit of the Middle 

 Oligocene orogenic paroxysms of the Alps, viz., the St. Bernard phase, the 

 Dent Blanc phase, the Monte Rosa phase, the phase of Adriatic sub- 

 sidence, and the phase Insurbrienne. This usage emphasizes the mass and 

 spatial aspect because all the nappes mentioned evolved within a short 

 time — a succession of events is not implied. On the other hand, van 

 Waterschoot van der Gracht ( 1931 ) uses the term more in a time aspect 

 in describing the structural relations in the Mid-Continent area, for he 

 designates the successive episodes of disturbance as the early Wichita 

 phase, the late Wichita phase (early Pennsylvanian ) , and the Arbuckle 

 phase (late Pennsylvanian). 



Others terms such as epoch, stage, and impulse, have been used but to 

 a lesser extent than phase. 



CLASSIFICATION USED FOR CRUSTAL DISTURBANCES 



Revolution 



If revolutions are chapters of diastrophism in earth history, it is clear 

 that they have both time and spatial aspects. To say they terminate the 

 great eras of time reflects the state of advancement of the science 45 

 year ago. Most of the time divisions were originally set apart by uncon- 

 formities, and early became more or less fixed by the fossil content of 

 formations between the unconformities at the type localities. Since then, 

 evidence of many new and important disturbances has been discovered 

 within the periods and eras thus set apart. Crustal deformation has come 

 to be known not as a repetition of pulsations that occurred precisely at 

 the close of periods and eras, but as developmental sequences of deforma- 

 tional events which frequently occurred over protracted periods of time 

 with shifting scenes of activity. 



A revolution will be considered to encompass the deformational events 

 of the hinterland, the geosyncline, and the foreland, and to include both 

 orogenic and epeirogenic processes. Setting time limits is an arbitrary pro- 

 cedure, and in doing so one must be mindful of usage which will help 

 determine the best limits of the revolution in question. 



System 



The major structural divisions of revolutions will be called systems. A 

 system is thus primarily a spatial division and is determined by a unity 

 of the structural features in it, such as the folds and thrusts of a geo- 

 syncline in contrast to the basins, shelves, and arches of the foreland, or 

 by isolation of a somewhat similar structural assemblage from another 

 by younger overlapping deposits, such as separate the Ouachita Moun- 

 tains from the Marathon Mountains. 



As far as noted, systems have been named after the outstanding range 

 or geographic feature in the division. This precedent will be followed 

 structurally where possible, but some exceptions seem necessary. For 

 instance, in organizing the structures of the central stable region of the 

 United States the area proved so large that no one geographic name 

 seemed suitable for the greatest arch, so its outstanding structural char- 

 acter was used, namely, the Transcontinental Arch. 



Phase 



Each system has its developmental history, and the structural events 

 of this history will be called phases. Although the types and extent of the 

 structures developed will be considered part of the phase, emphasis is 

 laid on the time aspect. It may be necessary to consider as phases two 

 contemporaneously evolving parts of a system, but in organizing structural 

 elements of the continent I have not run into this difficulty. 



In the Alps, the phases have been given geographic names, and the 

 practice was followed in this country by Van der Gracht (1931), who 

 discussed the Wichita and Arbuckle phases of the Wichita system. Since 

 in this book the emphasis will be placed on time, I have concluded that 

 time names will be most meaningful. For instance, if it is written, the 

 early Pennsylvanian phase of the Ancestral Rockies system, the student 

 cannot miss the intended meaning; but if the Wichita phase of the An- 

 cestral Rockies appears, the student may be confused if he has not read 

 the chapter on the Wichita system. 



Time names can be inappropriate only where the time of the disturb- 

 ance is not accurately known and future research shows the designation 





