DISCOVERY 



315 



agreed that the focus of the shock is deep-seated, at a 

 position in the Earth's interior where the conditions 

 are unknown. It used to be fashionable to speak of 

 the Earth's crust as about forty miles thick, and to 

 believe that shocks occurred within that solid covering. 

 Now, from observations of the variation in the time 

 occupied by the primary waves in arriving at the 

 antipodes, there is evidence to suggest that the seat 

 of an earthquake may be as deep as 400 miles. The 

 constituents of a stai" millions of miles away, by the 

 aid of the spectrograph, are infinitely easier to in- 

 vestigate than the conditions a few miles beneath our 

 feet. 



As an instance of the diversity of opinion, two 

 theories stand out in bold relief ; 



(a) The Earth is a cooling mass in which its outer 

 shell has already become tolerabty cool and solid. 

 The interior continues to cool and contract, leaving 

 the overcoat too large for the body. The overcoat 

 wrinkles and creases like the skin of a drying apple. 

 When the wrinkling reaches the elastic limit, the rocks 

 snap, producing an earthquake. 



{b) The Earth is a hot bodv whose outer shell is 

 cooling and has become solid, and in its continued 

 contraction becomes too small and too tight for the 

 plastic interior, with the result that it occasionally 

 cracks along lines of weakness. 



Connection with Volcanoes 



It is, perhaps, only natural that one ilies to the 

 volcano in the hope of fathoming the secrets of the 

 interior, and it is a common impression that volcanoes 

 are closely bound up in the earthquake phenomenon. 

 This hypothesis appears to break down on closer 

 examination. 



When a volcano is in eruption, it almost invariably 

 follows that earthquakes will be felt in the vicinity, 

 but these shocks rareh' provide sufficient energy to 

 operate seismographs bevond a radius of a few hun- 

 dred miles ; suggesting that they are only minor 

 adjustments after lava has been ejected, and are of a 

 comparatively local character. On the other hand, 

 the majority of the greater earthquakes emanate from 

 regions where no volcanoes exist. This has been put 

 forward as evidence that volcanoes are the safety- 

 valves which prevent earthquakes. Not aU the vol- 

 canoes in Japan, of which there are many, saved 

 Tokio and Yokohama from destruction. 



Another theory suggests tidal forces, where the 

 Earth is supposed to be liquid within, and fitted with 

 a thin, cold skin. The Sun and Moon as they pass 

 set up a tide in the liquid interior similar to the tide 

 in the sea, and so on occasions disrupt the crust and 

 produce an earthquake. In this case it should follow 



that at the time of new moon there would be a pre- 

 ponderance of shocks. The examination of 15,000 

 earthquakes showed a preponderance amounting to 

 4 per cent . at this period ; on continuing the examina- 

 tion and using 140,000 earthquakes, the law of average 

 asserted itself and wiped out even this small excess. 



The Latest Theory 



The suggestion which has gained most favour, and 

 seems to fit in with manv observations, is the theorj' 

 of Isostacy, which contends that all visible land is 

 composed of the lighter parts of the Earth's crust and 

 is floating upon the components of greater density. 



Every mountain has its stream ; every stream is 

 gradually but continually conveying fine sand in 

 suspension and soluble salts in solution to the sea. 

 In the millions of years that have passed it is surprising 

 that all land has not been washed into the sea, for 

 there is ample room. If all the visible land were laid 

 upon the bed of the oceans, there would be water to 

 a depth of 1,200 ft. over all. 



There certainly must be some process of restora- 

 tion. The suggestion is that the Earth's outer shell is 

 composed of great areas of m.aterial of different 

 densities, and that the lighter areas float upon and in 

 the heavier ones, in precisely the same way as icebergs 

 float in the sea. If the difference in w-eight is smaU, 

 the amount showing above the surface is also small 

 compared with the amount immersed. In the case of 

 the iceberg there is nine times as much below as 

 above ; and whenever the sun melts a layer from the 

 top, the whole floats upward to re-establish the 

 equipoise. 



So with the mountain; as the tops are worn off 

 they gradually rise above the plain, and of necessity 

 require to break away round their lower edges where 

 they meet the lower- lying ground in which they float. 



It is this periodical act of severance which consti- 

 tutes the earthquake. 



The evidence to support this hypothesis is fairly 

 comprehensive, and its chief points are — 



1. The materials of which mountains are composed 

 are generally lighter than that of the surrounding 

 plains. 



2. Evidence of sea- beaches on mountani-sides. 



3. Marine deposits found at high altitudes. 



4. That nearly all the great earthquakes occur either 

 at the foot of mountain ranges, or at the bottom of 

 steep declivities in the ocean bed. 



But it is unwise to dogmatise on the cause of earth- 

 quakes ; one can merely indicate the lines of modern 

 thought. Probably there is a modicum of truth in 

 them all. 



