August 1, 1892.] 



KNOWLEDGE 



147 



directly over tlie " centre of impulse," and that the waves 

 run roighlv in circles or in ellipses from such a central 

 spot. But the shapes as mapped out are often very 

 irregular. This will be due to the nature of the surface of 

 the ground and of the rocks below. It appears that in 

 mountainous countries, like Switzerland and the Pyrenees, 

 the great undulations are propagated in the direction of the 

 valleys. In striking against the tilted strata at the bases 

 of mountain masses they behave like waves of a river 

 which dash against a bank, breaking up and changing their 

 courses, and running along at the foot of the heights in the 

 same direction as the stream of the valley. Earthquakes, 

 though -^-ioleut in their etfects, are fortunately of very short 

 duration. The great Calabrian earthquake lasted barely 

 ten seconds ! 



We must now pass on to our second question, and 

 consider the crl't'cts of earthquakes. First, they make many 

 noises, variously described as resembling explosions of 

 mines, distant artillery, peals of thunder, roar of cataracts, 

 &c. Sometimes the shocks are felt before they are heard. 

 Take the famous Lisbon earthquake of 175.5. Towns in 

 Portugal were overthrown, and places even in Morocco 

 suffered considerable damage. The undulations extended 

 over one-twelfth of the earth's surface ! Thousands of 

 persons were killed in Lisbon, and the sea was greatly 

 disturbed. In England, lakes and pools oscillated to and 

 fro like water in a basin suddenly tilted. Even Iceland 

 was affected. The sea rose in a great wave round the 

 coasts of Britain, and ten hours after, the sea round the 

 West Indies was greatly disturbed ! Shocks occurred for 

 some months afterwards. 



Sometimes, as at .Jamaica in 1G02, the sea-wave does 

 more damage than the land-wave. At Port Royal, 2500 

 ho'.ises were covered with water to a depth of 33 feet. It 

 is interesting to note the behaviour of animals before an 

 earthquake. They seem to be able to detect slight tremors 

 of the ground which we ourselves do not notice, and which 

 precede earthquakes. Eats and mice leave their holes. 

 The ground is frequently rent asunder, and sometimes 

 permanent changes of level take place. 



But it is not of the effects of seismic disturbances that 

 we wish now to speak ; they are well kuown, and have 

 been repeatedly described. Their nature and origin, though 

 less clearly understood, afford more interesting matter for 

 a brief paper such as the present. Let us therefore 

 pass on at once to consider our third question : how 

 earthquakes are caused. It has been shown in Switzer- 

 land that they 'are more frequent at night than during the 

 daytime ; and during winter than during summer. Facts 

 of this nature seem to indicate that the contraction of rock 

 masses, due to a lowering of temperature, such as absence 

 of sunlight would involve, is intimately connected with 

 whatever causes are at work in the earth's crust to produce 

 earthquakes. Such contraction might produce dislocations 

 in the rocks, and these would set up vibrations. Again, 

 by burying charges of gunpowder and guncotton, and 

 exploding them, Prof. Milne has succeeded in producing, 

 on a small scale, phenomena closely resembling seismic 

 disturbances. Experiments of this nature lead to the 

 conclusion that some sudden blow, or impact, is the most 

 frequent cause of earthquakes. But we must be careful 

 not to assume that only one cause exists, and that aU 

 earthquakes are due to the same cause. Evidently this is 

 not the case. During volcanic eruptions, and also previous 

 to an eruption, the ground trembles, and rumblings are 

 heard, as of earthquakes. In Switzerland, avalanches of 

 snow in slipping down a mountain side cause slight earth- 

 tremors. The occasional falling down of great masses of 

 rock produces similar effects. 



The distribution of earthquakes helps to throw light on 

 this difficult and, as yet, rather obscure subject. Thus 

 they are found to be more frequent in mountainous regions 

 than in flat, low countries. They have a connection also 

 with volcanic regions, but rather an indirect than a direct 

 one ; for it is clear that earthquakes in general are not 

 due to volcanoes or volcanic phenomena. Some geologists 

 wrongly considered that all earthquakes were to be 

 regarded as incomplete or unsuccessful attempts to estab- 

 lish a volcano. In other words, they are not caused by 

 the struggles and efforts at escape made by superheated 

 steam retained at high pressure l3elow the surface of the 

 earth. Steam undoubtedly is the power chiefly concerned 

 in the production of volcanoes and of volcanic phenomena. 

 But although earthquakes are concomitants of volcanic 

 action, they are not to be attributed generally to the same 

 causes. Volcanoes are associated with great mountain 

 chains, because it is only along these lines of weakness in 

 the earth's crust — where the strata has been contorted, 

 crumpled, folded, and cracked, over and over again, on a 

 stupendous scale — that the masses of heated rock below 

 the surface, charged as they are with superheated steam 

 at enormous pressure, can hnd relief and come up to the 

 surface. 



By burying telephones and microphones in the earth, it 

 has been found that slight noises and tremors which would 

 otherwise never be noticed — unless by animals — can be 

 detected. Transient shiverings of the earth's crust are thus 

 found to be very frequent. Even in Britain we have a soil 

 subject to storms of microscopic earthquakes which, in 

 other countries, would be the forerunners of actual earth- 

 quakes. Wherever these little earthquakes occur the 

 earth sends forth a medley of confused sounds — crackings 

 and snappings — probably caused by the rocks creeping 

 toward relief from the strains which urge them to change 

 their position. Thus we begin to reahze that the world is 

 quivering Uke a mass of jelly ! It is hardly too much to 

 say that this method of observation has enabled us in part 

 to perceive the constant working of the great telluric 

 machinery which continually builds our lands ! Between 

 these tiny movements and those which cause ordinary 

 earthquakes there is only a difference of degree. They 

 are essentially of the same nature. By means of delicate 

 spirit levels, the bubbles of which move ver\' easily, certain 

 other movements, called " earth pulsations," have been 

 detected by Prof. Milne. All these phenomena must be 

 taken into account if we wish to find a satisfactory ex- 

 planation. Mr. Mallet submitted for consideration the 

 following possible causes : — (1) The sudden formation of 

 steam by water coming in contact with highly-heated rock; 



(2) The escape of steam at a high pressure through fissures 

 in the rocks and its condensation on reaching the sea ; 



(3) Volcanic explosions ; (4) (Ireat fractures and disloca- 

 tions in the earth's crust, suddenly produced by pressure 

 or contraction, in any direction. The first three of these 

 suggestions are not sufficient to account for earthquakes 

 which occur outside volcanic regions ; the last one seems 

 to supply what is wanted, namely, an explanation which 

 connects earthquake phenomena with those movements of 

 the crust of the earth which (as shown in a piredous 

 article)'' raise our continents, elevate our mountain chains, 

 and afford means of escape for highly-heated rocky matter 

 and associated steam from those deeply buried regions 

 where the internal and external portions of the earth react 

 upon each other. In mountain building and the folding 

 of strata we may look for the main cause of earthquakes. 

 It is titanic work, and must necessarily involve innumerable 



* Knowledge for June, 1891. 



