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.

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.

The Charleston earthquake.  On the evening of August 31, 1886, the city of Charleston, S.C., was shaken by one of the greatest earthquakes which has occurred in the United States.  A slight tremor which rattled the windows was followed a few seconds later by a roar, as of subterranean thunder, as the main shock passed beneath the city.  Houses swayed to and fro, and their heaving floors overturned furniture and threw persons off their feet as, dizzy and nauseated, they rushed to the doors for safety.  In sixty seconds a number of houses were completely wrecked, fourteen thousand chimneys were toppled over, and in all the city scarcely a building was left without serious injury.  In the vicinity of Charleston railways were twisted and trains derailed.  Fissures opened in the loose superficial deposits, and in places spouted water mingled with sand from shallow underlying aquifers.

The point of origin, or focus, of the earthquake was inferred from subsequent investigations to be a rent in the rocks about twelve miles beneath the surface.  From the center of greatest disturbance, which lay above the focus, a few miles northwest of the city, the surface shock traveled outward in every direction, with decreasing effects, at the rate of nearly two hundred miles per minute.  It was felt from Boston to Cuba, and from eastern Iowa to the Bermudas, over a circular area whose diameter was a thousand miles.