436 



CHARLES R. KEYES 



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(Fig. 2). A similar arrangement represent- 

 ing the structure of the Estancia Plains is 

 shown in Fig. i. 



The important factor to be taken into 

 account in this region is the fact that the 

 local geologic structure is not nearly so 

 simple as it may at first glance appear. 

 Everywhere there is greater or less com- 

 plication. The great backslopes of the 

 tilted mountain blocks, instead of being 

 continuous stratum-planes, are found to be 

 faulted at frequent intervals; and the dips 

 of the rocks change within very short 

 distances. 



Rocky Mountain type of structure. — The 

 compressive type of mountain structure finds 

 expression in the Estancia Plains region only 

 in the extreme northeastern part, where the 

 Rocky Mountains end by plunging down- 

 ward beneath the plains surface. Only a 

 single southward-pitching arch is represented 

 within the limits of the district under con- 

 sideration. Farther to the eastward the 

 details of structure are more complete. 

 The cross-section to the plains of Las Vegas 

 beyond is well worth much more considera- 

 tion than can be given it here (Fig. 3). 



There is abundant evidence of marked 

 compressive action within the plains area 

 here described, the geologic date of which is 

 probably somewhat earlier than that which 

 the southern nose of the Rockies represents. 

 This period of compression was Early 

 Cretacic. While the evidences are very clear 

 within the Hmits of the Estancia region there 

 are more abundant exemphfications a short 

 distance outside of this area. On the 



