70 CHARLES R. KEYES 



instances mentioned by Spurr the younger strata would seem to be 

 entirely wanting. In many New Mexico ranges the great Carbonif- 

 erous limestones form the major portion, and they give evidence of 

 an older and very different record of events from that which includes 

 the uprising of the present mountain blocks. 



2. The structures of each mountain range must be determined 

 separately and upon the evidence which it alone presents. Even in 

 neighboring ranges one may disclose a history very much longer and 

 older than another. 



3. In the New Mexico area orogenic movement, while more or 

 less rhythmic in character, was doubtless continuous since Paleozoic 

 times. At least three periods of marked activity have been recog- 

 nized. 



4. Modern ranges, the "block" mountains rising out of the Basin 

 plains, in which the Mesozoic strata are eroded from the summits, 

 are likely to have the structures of the first period of orogenic move- 

 ment (early Cretaceous) most in evidence, and the later effects may 

 be less strongly emphasized. In the specific cases of New Mexico 

 this period was one of folding and overt hrusting. 



6. Modern mountains, around which there are late lava flows, 

 are likely to show the effects of the third (Pleistocene) movements. 

 These are chiefly normal faulting. 



