EECENCY 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 ex- 

 ceedingly 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 mount- 

 ains 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 valleys — 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 leng'th a period of fault- 

 ing 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 accumula- 

 tion in this way will probably reach three or four hundred feet in somo 

 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 



