234 



THE POPULAR SCIENCE MONTHLY 



Fig. 6. Tilted and Beveled Cretacic Sandstone, overlain by Cemented Hori- 

 zontal Breccia, near Los Cerrillos. New Mexico. 



manifest that notable leveling and lowering has gone on at a rapid rate. 

 Since the San Mateo and Datil plains-surfaces were flooded with lava 

 vast degradation has taken place. 



Extensive erosion is everywhere manifest but of peculiar type. There 

 is little of the sharp incision of the plains-surface such as normally char- 

 acterizes stream-action, especially in a high-lying region having slopes of 

 high gradients. Erosion is of the broad-basin type — wide, flat-bottomed, 

 even plains between abruptly upturned rims against resistant rock- 

 masses. As the lava-flows and coulees became more and more numer- 

 ous the separate basins became divided and smaller, but general lowering 

 of surface went on without interruption. There can be no question but 

 that the lava-capped mesas at varying heights represent former levels 

 of the general plains-surface. 



In the outpouring of the molten rock over the surface of the plains 

 the lava-streams naturally flowed down the lowest lines of the plains, 

 but as removal of the weaker beds on either side took place each flow 

 was soon left as an elevation. A particularly instructive instance is 

 shown near the Zuhi pueblo, where an old valley is exposed in a section 

 filled with basalt, the level of the latter in the mesa-face being several 

 hundred feet above the present floor of the plain. 



There is further strong evidence of the strictly eolic nature of the 

 landscape sculpturing. These summit plains of the continent are a 



