Fig. 18. The United States
According to G. K. Gilbert, the San Francisco Mountain group is formed of a variety of lava known as trachyte, that is of comparatively recent ejection, possibly of a geological age called the Tertiary. The lava forming the mountains escaped through a number of crater cones, some of which can still be seen in the neighborhood. Some of these craters are now in almost as perfect a condition as the day they were formed. Indeed, to one looking at them from a neighboring elevation, they appear so fresh, and so little affected by the climate, that one might almost believe that the lava had just flowed out of the craters, and has not yet hardened. Nevertheless, geologists are sure they have been formed long before man appeared on the earth. In one of these craters a lake of fresh water has collected.
Another extinct volcano of the United States is Mt. Taylor in New Mexico, nearly east of the San Francisco Mountains. This mountain rises from the surface of a high table-land, or, as it is called in this part of the world, a mesa. The surface of the plateau is covered with a thick lava stream from which Mt. Taylor rises to a height of 11,390 feet above the level of the ocean. This mesa, or table-land, is forty-seven miles in length from northwest to southeast, and about twenty-three miles in breadth. Its general elevation is about 8,200 feet. The plateau rises about 2,000 feet above the surface of the level land that surrounds it. All these 2,000 feet have been removed by erosion. The table-land from which Mt. Taylor rises has not been eroded by the action of the rain, rivers, and other weathering agencies like the surface of the country surrounding it, because of a covering of lava that has been spread over its surface to a depth of about 300 feet.
Mt. Taylor is formed almost entirely of lava that has escaped through a single opening and has built up a high cone around it. The volcano is now quite extinct, so that the original form of the mountain has been greatly changed by erosion.
You will remember, when we were discussing the general subject of volcanoes, in the beginning of this book, that we spoke of volcanic mountains being bottled up after an eruption, by the hardening of the lava which remained in the crater and the tube that connects the crater with the place from which the lava had been derived. We then spoke of this hardened mass being known as a volcanic plug, or stopper, explaining how the volcano could never again erupt through its old crater unless it could develop sufficient force to blow out or remove this stopper.
Now besides the crater at the top of Mt. Taylor there were several others in the eroded region surrounding the mesa, or high table-land, from which Mt. Taylor rises. When, therefore, the erosion which removed the 2,000 feet of rocks on all portions of the old mesa that were not protected by the coating of lava, these old mountain plugs were too hard to be worn away or eroded, and were, therefore, left projecting into the air like vast pyramids.
If you should ever visit Mt. Taylor and should go to the eastern border of this mesa, and look over the eroded plain, you would see in the lowlands a part of the places from which the 2,000 feet of matter have been slowly eroded. Dutton describes the beautiful panorama that is to be seen as follows:
"The edge of the mesa suddenly descends by a succession of ledges and slopes, nearly 2,000 feet into the rugged and highly diversified valley-plain below. The country beneath is a medley of low cliffs and bluffs, showing the browns and pale yellows of the Cretaceous sandstones and shales. Out of this confused patchwork of bright colors rise several objects of remarkable aspect. They are apparently inaccessible eyries of black rock, and at a rough guess, by comparison with the known altitudes of surrounding objects, their heights above the mean level of the adjoining plain may range from 800 to 1,500 feet. The blackness of their shade may be exaggerated by contrast with the brilliant colors of the rocks and soil out of which they rise, but their forms are even more striking."