26 



level of the overwhelming waters. There is a very general im- 

 pression that volcanic action was on a grander scale in past 

 geological ages than at present, but there is good reason to believe 

 that " volcanic energy " is a " constant quantity " and that though 

 it changes its scene of action, it does not alter or vary in amount 

 (judd). Earthquakes are so rare in England, and so comparatively 

 slight, that few of us perhaps quite realise how constant and 

 stupendous are these forces, even at the present moment. It has 

 been calculated (^Huxley) that there are certainly not less than 

 three shocks of earthquake a week, and could we obtain records 

 daily from the whole surface of the globe, it would be found that 

 valcanic action was positively unceasing in one place or another. 

 Even in England during the relatively short period of historical 

 times, there have been recorded 500 shocks of earthquake, and, 

 taking the records of the whole inhabited world, no less than 70,000, 

 by means of which many millions of lives have been lost. Earth- 

 quakes and volcanoes are generally spoken of as different manifest- 

 ations of the same explosive force, but I think there are good 

 grounds for believing that, taking into account the cavernous or 

 cellular condition of parts of the earth's crust, caused by the solvent 

 action of water, and the enormous cavities created by the evolution 

 of carbonic acid gas, there must occasionally be tremendous con- 

 vulsions caused by subsidences of the roofs of these vast caverns, 

 accompanied by terrific reports from the fratture of rocky beds and 

 strata, and that these causes alone would account for many of the 

 vibrations and minor earthquakes and earth-tremblings, apart from 

 volcanic action. Of course I do not for a moment mean to assert 

 that these subsidences would be sufficient to cause the more stupen- 

 dous earthquakes felt for great distances, for these are undoubtedly 

 of volcanic origin. Ti.ese gigantic throes, the earth waves and 

 upheavals, are the result of pent-up forces striving to find an outlet. 

 All experience teaches that earthquakes and volcanoes stand in 

 inverse ratio one to the other. 1 mean that the maximum of one 

 corresponds to the minimum of the other, so that earthquakes are 

 always more violent and numerous immediately before the eruption 

 of the nearest volcanic outlet, an outlet, by the way, often situated 

 hundreds of miles distant. An earthquake may be appropriately 

 described as an earth wave — a billow of solid earth moving, just 

 like a wave of the sea, often with a velocity of from 20 to 28 miles 

 a minute (Humbolt) and with an undulatory motion such as you 

 may see when a heavy carpet is shaken at one end. It has two 

 distinct movements, one vertical, i.e., up and down, the other 

 horizontal ; and it is this second linear movement which is so fatal 

 to buildings, for, of course, it operates at right angles to them, 

 tending to rend or overthrow them, as shown on this diagram. 



