THE TERTIARY PERIOD 



437 



FIG. 454. Bad Land topography in South Dakota. (Darton, U.S. Geol. 



Sura.) 



slowly worn down, but their cores and remnants of the old 

 lava flows may still be recognized. 



Most of the ore deposits which have given the western states 

 their renown as mining districts are connected with the volcanic 

 intrusives of Tertiary times. The gold, the discovery of which 

 caused the rush of immigrants to California in 1849 and succeeding 

 years, was found partly in gravels in the valleys of Tertiary rivers. 

 The famous gold mines of Cripple Creek, Colorado, and the copper 

 mines of Butte, Montana, and parts of Utah, all get their ores 

 from veins adjacent to bodies of porphyry and other igneous rocks 

 which were forced into the older formations in the Tertiary period. 

 In this respect there is a contrast between the western and eastern 

 mountains of the United States. 



The climate of the mountain region could not have been 

 as arid as it is to-day, for the luxuriant vegetation which 

 flourished there in the Tertiary period shows that the rain- 

 fall was plentiful. In some of the driest parts of Utah and 



