78 The Shoshone Island. 



That great revolutionary movement, the 

 elevation of the Cascade mountains, while 

 important in its relations to both islands, did 

 far more to give variety of surroundings to 

 the Shoshone than to the Siskiyou region. 

 To both regions the changes it caused make 

 the event itself the geological epoch of the 

 times. In our narrative it will furnish us with 

 a convenient division line between the records 

 of the Cretaceous and those of the Tertiary 

 rocks, for its was the closing work of Cre- 

 taceous times to separate, and the opening 

 work of the Tertiary to begin, the new record. 

 But another feature of the region we are de- 

 scribing owes its entire existence to the ele- 

 vation of this range. It is the enormous 

 masses of volcanic rock that exist along the 

 eastern slopes of the Cascades. Ordinarily 

 volcanoes throw out well defined streams of 

 lava, so that their masses are easily measured; 

 but this upthrust of Cascade lavas was most 

 of it in sheeted masses, bursting to the surface 

 like sheets of water through broken ice to 

 freeze over its rents. 



