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the plateau-plains stand at many different heights above the general 

 plains-surface. To this particular great genetic significance is attached. 

 Several notable peculiarities distinguish the Colorado dome. Around 

 its southern slope there is, as Gilbert observes, one of the great lava- 

 tracts of the world, second in magnitude in our country only to the 

 great northwestern lava-field, and fifteen times as large as the classical 

 district of extinct volcanoes in central France. Sweeping in a broad 

 crescent, 250 miles long, with the lofty cone of San Mateo on one horn 

 and the towering San Francisco volcano on the other, the main body 

 of lava-flows superposed in countless numbers, covers an area half the 

 size of New York state. Beyond the borders of the crescent are num- 

 berless cinder-cones, coulees, and minor lava-sheets which spread out 

 over the soft sedimentaries constituting the chief substructure of the 

 plains in this part of the country. Farther west, in Arizona, the hard 

 carboniferous limestone is the principal surface-rock, the shales once 

 overlying it having been recently stripped off. Other volcanic evidences 



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Fig. 3. Tooth of Time, New Acoma Pueblo, New Mexico. Last vestige of a once 



extensive plateau-plain. 



