VOLCANIC PHENOMENA. 559 



Earthquakes. — Earthquakes are waves propagated in the earth's 

 crust from certain centres of disturbance in or beneath it. In most 

 cases the underground pressure appears to act on a centre, the force 

 dying away in ripples in continually widening circles, or extending 

 further in some directions according to the nature of the rocks 

 affected. 



The majority of earthquakes, according to Prof. Milne, are due 

 to explosive efforts at volcanic foci ; and the greater number of 

 these explosions take place beneath the sea, and are probably due 

 to the admission of water through fissures to the heated rocks 

 beneath. Those which make themselves known to us are no doubt, 

 and fortunately so, of a comparatively mild form ; but others are 

 often sufficiently violent to rupture the earth's surface. At the 

 same time the earth's horizontal movement is usually only the 

 fraction of a millimetre (oV in.), seldom exceeding 3 or 4 

 millimetres, while the vertical movement is rarely more than 

 one millimetre. 



While earthquakes accompany and often precede volcanic erup- 

 tions, so they sometimes cease to occur when the outburst takes 

 place ; but it is remarkable that they are felt in our own country, 

 and in other lands far away from active volcanoes. Hence, if due 

 to similar causes, the subterranean forces must in these instances 

 exhaust themselves in producing the earthquakes, and they may be 

 regarded as " an uncompleted effort to establish a volcano." ^ 

 Some earthquakes, however, may be simply due to shrinkage and 

 fracture of the earth's crust, and of such disturbances we have 

 abundant evidence in the joints, faults, and contortions seen in our 

 rocks in many places. The oxidation of metallic substances in the 

 interior of the earth is also admitted to be a likely source of local 

 disturbance. 



On April 22, 1884, an earthquake occurred, the most serious that has happened 

 in this country for four centuries ; its intensity was about one-twentieth that of the 

 Lisbon earthquake of 1755- Originating in tlie neighbourhood of Colchester, 

 this subterranean disturbance made itself felt over an area estimated at about 

 50,000 square miles. The shock was over in ten or twenty seconds, according 

 to various accounts ; but the results were of a very destructive character in 

 Colchester and the immediate neighbourhood, the main line of damage being 

 between Wivenhoe and Peldon. It was felt in various directions with decreasing 

 force. At Ipswich, and throughout Suffolk, it was perceived, and the shock 

 extended as far as Yarmouth, Norwich, Lynn, Cambridge, Ely, and Boston ; to 

 Northampton, Leicester, and Wolverhampton ; and even as far as Stroud, Bristol, 

 and Street in Somersetshire. It was felt near Reading, in and around London, at 

 Ryde, Portsmouth, near Brighton, Hastings, Dover, at Rochester, and Wesgate- 

 on-Sea. 



There is no evidence to connect this earthquake directly with volcanic phenorriena, 

 and it may rather be attributed to one or more subterranean rents produced by 

 shrinkage, which led to no material shifting of the rocks, and which, owing to the 

 tenacious nature of the subsoil at Colchester, chiefly London Clay, did not manifest 

 itself in any conspicuous manner at the surface. No perceptible change of surface- 



^ J. Milne, Earthquakes, 1886, pp. 76, 275, 295 ; R. Mallet, The Earthquake 

 Catalogue of the British Association, 1852-1858, Reprinted 185S ; Prestwich, 

 G. Mag. 1870, p. 541 ; C. Walford, Journ. Statistical Soc. xli. 482. 



