Page:Popular Science Monthly Volume 81.djvu/239

Rh the coulèes from the Tintero, on the Mesa Redonda, west of the San Mateo, that finally enter the channel of the Rio San José at a level considerably below that of the great flow already mentioned.

Four distinct and notable periods of volcanic extravasation are thus recorded, between the first and last flows of which more than 1,000 feet of rock were removed from the entire region about. There are in the district many other lava-flows at other elevations; but between the four especially mentioned definite time-relations are readily established.

At the present time particular interest attaches to the mesas and their origin. Normal water-corrasion manifestly did not accomplish

the strange sculpturing of the country. In these relief features we seem to be introduced to an erosive force as potent as water but which we are just beginning fully to appreciate. Mesas appear to furnish the most direct and convincing testimony we have of the tremendous power of the wind in affecting general erosion under conditions of aridity.

That water could not possibly produce such effects is shown in a number of ways. On the continental divide the streams are their smallest. On a vast plain so situated drainage features are necessarily insignificant. Rainfall 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