Page:South African Geology - Schwarz - 1912.djvu/95

 terrifying effect of this has led to too much concentration on this particular kind. The destructiveness of an earthquake depends on the amplitude and period of the wave. The amplitude is the actual throw of the ground at the particular place: a throw of 1 in. will destroy a town if the period be rapid. The period is the time required for a point in the ground to rise and fall back to the same place. Periods of two or three seconds are disastrous with even small throws, whereas periods of half a minute to a minute are not serious. The shake spreads from the origin in concentric shells, and the destruction of houses in the epicentral and adjoining areas is marked by cracks parallel to the surface of the sphere at that place; that is to say, going north from the epicentre the cracks will incline to the north at greater and greater angles as the distance increases, and going south, east, or west they will incline south, east, or west in the same way. By constructing lines at right angles to the cracks of the buildings, therefore, they will be found to converge to a point which is the origin or focus.

Secular Upheaval and Subsidence.—When in nine million years the whole of the mass of South Africa standing above the sea has been removed and transported to the sea adjacent, the crust of the earth will have been lightened by many millions of millions of tons, and the sea floor weighted by the same amount, less the weight of the substances that have gone into solution. Now we saw that when the force of gravity was lessened on one side of the earth by the sun and moon pulling together on the other side, the water of the ocean bunched itself into a tidal bulge. The sun and the moon have only a slight effect on the rocks of the earth, but the earth