Page:Popular Science Monthly Volume 81.djvu/238

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are the denuded necks and dikes. These lava-sheets form the foundation of many a notable mesa.

Outside the limits of the lava-fields the massive and more indurated beds which are included between the thick sections of weaker strata take the place of the lava-flows in the formation of the mesas.

In the region under consideration extravasation of lavas has evidently gone on at frequent intervals from the very beginning of Tertiary times almost, it may be said, to within the memory of men still

PSM V81 D238 Outliers of the great acoma mesa.png

Fig. 4. Outliers of the Great Acoma Mesa. The plateau-plain is 500 ft. above the general plains surface.

living. The older trachytic and andesitic lava-sheet of the San Mateo, or 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 this peak, which rises 13,000 feet above sea-level, 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 (Fig. 8). Cabazon, a huge volcanic pipe, stands 1,200 feet above its base and is a landmark for eighty miles about.

Much younger and 500 feet below the San Mateo plain is Acoma mesa, 30 miles long and 15 miles wide, capped by basalt. At its foot, another 500 feet down, is a great basaltic flow, 50 miles long by 20 miles broad, covering the present plains-surface. Even more recent are