| OCR Text |
Show RECENCY OF FAULTS. 39 series of beds, displayed in all parts of the district, composed of the waste of volcanic rocks. The rocks which furnished these sands and marls are nowhere discernible. Either they have been buried beneath the later lava-floods or have been wholly removed by erosion. Deep in the recesses of some of the plateaus, at a very few places where the grander gorges have eaten their way into them, the oldest observed Tertiary eruptives, the pro-pylites, are revealed. Of these earliest' propylitic eruptions we know exceedingly little historically. They are covered with great floods of andesite and trachyte. There is evidence that these eruptions had their periods of activity alternating with long periods of repose. These periods represent an immense amount of devastation wrought upon the older volcanic mountains by the elements, for their debris is found in the form of huge beds of conglomerate stratified in a manner which leaves no doubt in my mind that the process of accumulation was the exact counterpart of that which is now building similar beds in the valleysâ€"a purely alluvial process. The earlier andesitic mountains were almost utterly destroyed by this process. Then came another period of activity, followed by another period of denudation. We have older and younger conglomerates. The older contain the andesitic and some trachytic fragments; the younger contain trachytic, doleritic, and even basaltic fragments. But both conglomerates represent an enormous period of denudation, for the aggregate thickness of the beds will frequently exceed 2,000 feet, covering very large areas. At length a period of faulting set in. These conglomerate beds were sheared or flexed, and now form the walls and summits of the great plateaus for many scores of miles in alternation with the remnants of the old volcanic sheets. Again the process of degradation set to work tearing down these tables, the streams rolling the fragments down into the valleys and building up along the foot of each wall a row of very low alluvial slopes, often beautifully stratified, and the exact counterparts of the conglomeritic strata which are now seen edgewise in the plateau-walls. Since the uplifting began the amount of accumulation in this way will probably reach three or four hundred feet in some places, though it is not probable that the average will exceed 200 feet. But this modern accumulation has been made under peculiarly advantageous circumstances. The process will become slower and more difficult as the |
