716 C. H. KEYES TOYALAXE AND LUCERO 



beds which are included between the thick sections of weaker strata take 

 the place of the lava flows in the formation of the plateau plains. 



Lava Flows of the Eegion 



In the region under consideration extravasation of lavas hits gone on 

 at frequent intervals from the very beginning of Tertiary times almost, 

 it may be said, to within the memory of men still living. The older 

 trachytic and andesitic lava-sheets of the Mount Taylor district now 

 stand 1,000 feet above the country around, and on this mesa rests the old 

 volcanic cone itself, higher and more impressive than Vesuvius. To the 

 north of Mount Taylor there are abundant evidences of still earlier vol- 

 canic activities, as shown in the forest of volcanic necks of that area, 

 from which is swept almost every vestige of their cones and the plains on 

 which they stood. Cabazon, a huge volcanic pipe, stands 1,200 feet above 

 its base and is a landmark for 80 miles about. 



Much younger, and 500 feet below the Mount Taylor plain, is Acoma 

 mesa, 30 miles long and 15 miles wide, capped by basalt. At its foot, 

 another 500 feet lower down, is a great basaltic flow, 50 miles long by 20 

 miles broad, covering the present plains surface. Even more recent are 

 the coulees from the Tintero, on the Mesa Eedonda, west of Mount Tay- 

 lor, which finally enter the channel of the Eio San Jose at a point con- 

 siderably below the level of the great flow just mentioned. 



There are, then, four distinct periods of volcanic extravasation, be- 

 tween the first and last of which more than 1,000 feet of strata were 

 removed from the entire region about. There are in the district many 

 other lava-sheets at other heights ; but between the four especially noted 

 definite time relations are readily established. 



On the continental divide the streams are their smallest. On a vast 

 plain so situated drainage features are necessarily insignificant. Rain- 

 fall is the scantiest. These three conditions combined with arid climate 

 give water action small opportunity to vigorously erode. On every hand 

 the country clearly shows it. It is equally manifest that notable leveling 

 and lowering has gone on at a rapid rate. Since the Mount Taylor and 

 Datil plains levels were flooded with lava, general removal of surface rock 

 has taken place to the extent of more than 1,000 feet, as already stated. 



The extensive erosion is of a peculiar type. There is little of the sharp 

 incision of the plains surface such as normally characterizes 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 be- 

 tween abruptly upturned rims against resistant rock-masses. As the lava 



