56 p. B. KING 



cene, etched out the mountains and canyons; mountain barriers 

 prevented free circulation of moisture-laden winds from the Pacific 

 and heightened the climatic contrasts. Since mid-Tertiary time, 

 regional relief, local relief, and climatic contrasts have been greater 

 in the Cordillera than at any earlier period. 



Throughout geologic time, the Cordilleran system has been 

 bordered on the west by the deep Pacific Ocean basin, floored by 

 crustal material different from that of the continent. It is unlikely 

 that any additional lands ever existed offshore that have since 

 foundered to oceanic depths. More likely, continental area has been 

 added at the expense of ocean basin by various accretionary proc- 

 esses. On the other hand, land connections persisted intermittently 

 along the strike of the Cordilleran system, between North America, 

 Asia, and South America, as the coastal areas of all three are part of 

 a circum- Pacific belt of mountain structures whose origin, like the 

 North American Cordillera, extends far back into the geologic past. 



Acknowledgments 



This paper was compiled largely from existing publications, but its 

 context has been shaped to a considerable degree by the climate of thought 

 and opinion of the writer's many geological friends and colleagues, in 

 California and elsewhere in the west. Their observations and ideas, too 

 numerous to recall and mention separately, have given the writer many 

 insights into the interpretation of the printed record. 



In particular, one or more of the versions of the paper have been 

 read by C. R. Longwell and G. D. Robinson of the U. S. Geological Sur- 

 vey, J. H. Mackin of the University of Washington, and A. O. Woodford 

 of Pomona College. Also, C. A. Repenning of the U. S. Geological Survey 

 has furnished helpful information on the history of the Colorado River 

 from his unpublished work. Their generous help and their thoughtful 

 criticisms are gratefully acknowledged, and have been carefully considered 

 in preparing final revisions. 



Nevertheless, decisions as to the context of the paper are those of 

 the writer, and he assumes all responsibility for the views herein ex- 

 pressed, whether right or wrong. As one critic remarked, no two geologists 

 would summarize this complex and diverse subject in the same manner, 

 either in the subjects selected for coverage or in the interpretations 

 which are made of them. The perfect story of the evolution of the surface 

 features of western North America is yet to be written. The present paper 

 is an interim report which it is hoped will stimulate further thought, 

 especially in that fascinating no-man's land which lies between the 

 geological and the biological sciences. 



