PREFACE 



TO THE SECOND EDITION 



The second edition is an extensively revised version of the first. Seven 

 new chapters have been added, one on the Precambrian orogenic belts 

 and six on the igneous provinces of the western cordillera. Igneous rocks 

 are accorded a more significant place here than in the first edition. South- 

 ern Mexico and Central America are treated in a separate chapter as is 

 also the Canadian Arctic. The colored maps of the summary in Chapter 3 

 have been extensively revised, and several new ones are included. 



Better index maps have been added throughout and an attempt has 

 been made to produce an understandable text independent of outside 

 sources of information. However, such maps as the geologic and tectonic 

 maps of the United States and Canada and the several state maps will 

 be indispensable for instructional purposes and should be available to 

 the student or professional geologist reading the book. 



The second edition marks a time of major transition in structural geol- 

 ogy. In the past geologists have seen evidence in nearly every mountain 

 system of crustal compression, but now a number of authorities postulate 

 earth expansion, differential uplift, and crustal tension. The folds and 

 thrust sheets are being interpreted as gravity slide phenomena from re- 

 gions of marked uplift. Vertical movements along with distention and 

 wrenching are considered to be the primary aspects of crustal deforma- 

 tion — not horizontal compression. 



The writer sees much in favor of the hypothesis of primary vertical 

 movements and has perhaps accorded it greater attention than some will 

 like. However, he has also attempted to present the geology of the several 

 provinces as the authorities have depicted them. Certain sections of the 

 book, therefore, reflect the orthodox concepts of compression, win 

 other parts will seem to emphasize primary vertical movements with sec- 

 ondary folding and thrusting. It will take another ten years to resoK e the 

 irregularities and to warrant the preparation of a more definitive third 

 edition. 



A. J. Eardli v 



June, 1962 



xv 



