GEOLOGY OF THE WEST 



COAST REGION OF THE 



UNITED STATES 



BY C. F. TOLMAN, JR. 



Associate Professor of Economic 

 Geology, Stanford University 



NATURE has left voluminous records of the 

 geological history of the west coast of the 

 United States in the numerous and thick 

 sedimentary formations of the region. The records 

 are fragmentary and incomplete for the pre-Cam- 

 brian eras, those antedating the appearance of life 

 on the earth or at least containing no evidence of 

 the teeming population that existed in the oceans 

 from the Cambrian period on. Beginning with the 

 Cambrian and extending down to the present time, 

 each period is represented by marine deposits 

 which register the geography, climate, and some- 

 thing of the denizens, of the times in which they 

 were formed. The story of vulcanism and earth 

 movements is read not only in the character and 

 distribution of the igneous rocks of this region, but 

 also in the metamorphism and deformation of the 

 sedimentary rocks. This second record, imprinted 

 by metamorphism, often erases, partially or com- 

 pletely, the earlier records of the sea. The history 

 of the relatively recent periods, of the geologic 

 yesterday, is written in more detail than elsewhere 

 in the world. It treats of marine, terrestrial, and 

 glacial conditions, of the base leveling of mountain 

 ranges followed by vulcanism, earth movements 

 and the re-birth of mountain systems. 



In the attempt to select and describe a few of 

 the salient features of this long and complicated 

 history, it has been difficult to make the treatment 

 accurate, or to give alternate interpretations which 

 may be as worthy of consideration as those pre- 

 sented here. The limited space allotted to this 

 article has rendered it inadvisable to cite the con- 

 tributions of each of the geologists who have added 

 to our knowledge of west coast geology. The brief 

 bibliography includes a few important papers bear- 

 ing on the subjects emphasized in this summary. 

 In the accompanying geological map the formations 

 have been grouped according to the divisions 

 adopted in the text of this article. (See Pis. XXXIX 

 and XL.) 



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