172 
EOLIAN GEOLOGY 
Other conspicuous flat-topped mountains are situated in the Zuni 
basin, just over the continental divide in western New Mexico. 
The region is the largest, highest and dryest desert plain in this 
country. Structurally and topographically it constitutes an essen¬ 
tial section of the great Colorado dome, arching regularly from 
the Rio Grande to the Rio Colorado. Save in one place — the 
Zuni swell — its broad surface is unbroken by tectonic features. 
The region is preeminently one of plateau-plains standing at all 
heights above the general plains-surface. 
In the region under consideration extravasation of lavas has 
evidently gone on at frequent intervals from the very beginning 
of Tertic times almost, it may be said, to within the memory of 
men still living. The older trachytic and andesitic lava-sheets of 
the Mt. Taylor district now stand 1,000 feet above the country 
around and upon this mesa rests the old volcanic cone itself, higher 
and more impressive than Vesuvius. To the north of Mt. Taylor 
there are abundant evidences of still earlier volcanic 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 upon 
which they stood. Cabazon, a huge volcanic pipe, stands 1,200 
feet above its base and is a landmark for 100 miles about. 
Much younger and 500 feet below the Mt. 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 sur¬ 
face. Even more recent are the coulees from the Tintero, on the 
Mesa Redonda, west of Mt. Taylor, which finally enter the chan¬ 
nel of the Rio San Jose, at a level considerably below that of the 
great flow just mentioned. 
Four distinct and notable periods of volcanic extravasation, are 
then recorded between 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 read¬ 
ily established. 
On the continental divide the streams are their smallest. On a 
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 vigorously to 
