LAVA FLOWS OF THE REGION 717 



flows aud coulees became more and more numerous the separate basins 

 became diWded 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. 



The lavas manifestly 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 (plate 43, figure 1). A particularly instructive in- 

 stance is shown near the Zuni pueblo, where an old valley is shown in 

 section filled with basalt, being exposed in a mesa face several hundred 

 feet above the present floor of the plain. Still more interesting is the 

 coulee from the Sierra Lucero, at the northernmost extremity of the 

 Mimbres range and 40 miles west of Magdalena. In this district the San 

 Augustine plains are some 60 miles long and from 15 to 30 miles wide. 

 A high rim completely surrounds them, giving them interior drainage. 

 The substructure is mainly soft Cretaceous sandstones. From a mid- 

 point on the south rim, not so very long ago, Lucero sent out a narrow 

 coulee to the northward a distance of 15 miles, until it reached the oppo- 

 site wall of the plains in the Sierra del Datil. Since that time the gen- 

 eral surface of the plains has been lowered 50 to 100 feet or more. A 

 small playa now occupies a part of the western division of these plains. 



Most of the basin plains of tliis region have rock-floors (plate 43, fig- 

 ure 2), It would be impossible for these basins to be filled with the 

 debris from the rims as has been ascribed to the Great Basin region. 



To the north of the lava-capped mesas, mesa country still prevails, but 

 the table tops are indurated layers, such as are seen in Toyalane and El 

 Moro. When gently tilted, typical cuesta topography appears. 



EoLic Character of the regional erosive Activities 



That this high, dry, almost waterless waste on the continental divide 

 should owe its landscape features chiefly to deflation seems to need little 

 argument in this place. In the case of a district with alternating hard 

 and soft strata, the Toyalane type of mesa might not always offer con- 

 clusive evidence in support of this contention. Against the lava flows of 

 diverse ages as of the Lucero type no such objections can be raised. 



These summit plains of the continent are a region of continual high 

 wind and constant sand-storm. Nowhere else in the arid region of the 

 Southwest is wind-scour in active operation so advantageously viewed. 

 Nowhere else in this country are deflative effects and desert-leveling so 

 well displayed. Nowhere else in all the world is general lowering of a 



