128 Adventures in Scenery 



period volcanic outbursts occurred and great lava flows spread 

 over large areas. Throughout the long time of the Tertiary 

 and Quaternary periods erosion was actively going on. A large 

 part of the Tertiary lava flows and other rocks were worn away 

 until now only remnants of once continuous formations are 

 left. Disturbance of the rocks by faulting completed the 

 work of deformation and resulted in the present relief. 



Geological conditions have resulted in the accumulation of 

 mineral deposits. These constitute the greatest resource of the 

 region, and have been the incentive for the early exploration 

 and much of the later industrial development. Of metallic 

 ores those of gold, silver, copper, and iron have been princi- 

 pally mined, but lead, zinc, quicksilver, and many rarer metals 

 also have been found. Non-metallic minerals, as salt, potash, 

 niter, borax, and gypsum occur in many places, some in com- 

 mercially important quantities. 



Much literature relating to the minerals and geologic fea- 

 tures of the region is available. (See Appendix.) 



