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Show U-IN-KA-RET MOUNTAINS. 201 there were valleys along their courses. Other streams had their sources far away to the south, and came down into the Colorado, and it is probable that they also ran through valleys. Then these displacements began; they were not formed suddenly; the rocks were not flung down during some great convulsion, but settled slowly, so that this change in the contour of the surface had no effect on the course of the streams. Thus the downfall of the beds was not faster than the wearing away of the channels, for the displacements by faults and folds has not determined nor modified the direction of the principal streams. As the rocks fell, molten lava was thrust up, not suddenly, nor all at once, but from time to time-now here, now there- pouring out a sheet of molten rock in one eruption, and again in another, and this commenced away back in that time before the shales and sandstones seen in the Vermilion Cliffs had been carried away from the benches and plateaus to the south. Doubtless these first floods of lava found their ways into valleys-valleys in that elder time-and covered great beds of these sandstones and shales. When the lavas cooled,'the rocks which they formed were much harder than the sandstones by which they were underlaid, and the beds which formed the surface of the country elsewhere; and as the degradation of this region by rains and rivers continued, the surrounding country was carried away, and the sandstones and shales, protected by the harder beds of basalt, remained; and now mountains stand in such places, doubtless marking the sites of ancient valleys. So the uncovered sandstones wasted away, and the lava-capped beds remained, leaving at first low tables, covered with sheets of basalt. Still, from time to time, new beds of lava were poured out-not over the old beds, usually, but on their borders, increasing their protected area; and, as the surrounding sandstones were still farther carried away, still, pari passu with erosion, came floods of lava, and thus the mountains which remain have a strangely complex constitution. We may call them eruptive mountains, for, had no eruption occurred, no mountains would have been left; all of the sandstones would have been carried away. But yet the great mass of the material of which the mountains are made is not eruptive matter; the mountains are great beds of sandstone and shale, covered with blankets of basalt, and, in a general way, the older beds of lava have the higher position on the mountains. ; 20 COL |
