Note that the crystalline rocks a have been crumpled and crushed. Comparing their structure with that of folded mountains, what do you infer as to their relief after their deformation? To which surface were they first worn down, _mm´_ or _nm_? Describe and account for the surface _mm´_. How does it differ from the surface of the crystalline rocks seen in the Torridonian Mountains (Fig. 206), and why? This surface _mm´_ is one of the oldest land surfaces of which any vestige remains. It is a bit of fossil geography buried from view since the earliest geological ages and recently brought to light by the erosion of the canyon. [Illustration: Fig. 208. View of the North Wall of the Grand Canyon of the Colorado River, Arizona, showing the Unconformities illustrated in Figure 207] How did the surface _mm´_ come to receive its cover of sandstones _b_? From the thickness and coarseness of these sediments draw inferences as to the land mass from which they were derived. Was it rising or subsiding? high or low? Were its streams slow or swift? Was the amount of erosion small or great? Note the strong dip of these sandstones _b_. Was the surface _mm´_ tilted as now when the sandstones were deposited upon it? When was it tilted? Draw a diagram showing the attitude of the rocks after this tilting occurred, and their height relative to sea level. The surface _nn´_ is remarkably even, although diversified by some low hills which rise into the bedded rocks of _c_, and it may be traced for long distances up and down the canyon. Were the layers of _b_ and the surface _mm´_ always thus cut short by _nn´_ as now? What has made the surface _nn´_ so even? How does it come to cross the hard crystalline rocks a and the weaker sandstones _b_ at the same impartial level? How did the sediments of _c_ come to be laid upon it? Give now the entire history recorded in the section, and in addition that involved in the production of the platform _P_, shown in Figure 130, and that of the cutting of the canyon. How does the time involved in the cutting of the canyon compare with that required for the production of the surfaces _mm´_, _nn´_, and _P_? CHAPTER X EARTHQUAKES Any sudden movement of the rocks of the crust, as when they tear apart when a fissure is formed or extended, or slip from time to time along a growing fault, produces a jar called an earthquake, which spreads in all directions from the place of disturbance.