MACKIE — BRITISH EARTHQUAKES. 



409 



Shropshire, Leicestershire, Cheshire, and generally over the same 

 area as the one a few weeks since. Our list is, however, if 

 not quite perfect, sufficiently complete to show how numerous, 

 even in England, earthquake-shocks are, and how generally not dan- 

 gerous nor violent in their effects. To establish this point was not 

 our purpose in this article, but rather to raise discussion on an im- 

 portant question. All earthquakes are more or less of the nature of 

 what are properly termed " shocks/' and more commonly they are the 

 effects of something sudden, more like the snap of a bar of metal or 

 stone under tensile strain than an explosion of gunpowder or gases. 

 Perhaps very generally they are the " shocks " of the rupture of 

 masses of dense strata, or the sudden slippings of one great rock-for- 

 mation over another. We ourselves pay little regard to the idea of 

 a central fused mass, and the possibility of tidal waves in such an 

 igneous ocean, and desire rather to seek for the cause with an un- 

 biassed mind, and as proceeding from natural causes such as we are 

 acquainted with. It seems to me that in the crystallization of rock- 

 masses we have a power of the most enormous character, working, it 

 is true, by atoms ; and so does molecular force, and what more tre- 

 mendous, more irresistible ? Enclose water in a casket of iron ; freeze 

 it, and the casket bursts like a fragile china-cup. If in the crys- 

 tallization of vast rocks, under the pressure of superincumbent 

 strata, an expansion of their volume takes place, the superincum- 

 bent beds will be subjected to tensile strain, at first moderate, sub- 

 sequently intense, and then a fracture or snap, or giving way. 

 If the rock-masses contract in crystallizing, fracture from tensile 

 strain is equally possible, A bar of iron, broken by tensile strain, 

 snaps with the report of a cannon ; so the snapping of the 

 solid rock-masses below may produce sounds like thunder or the 

 rumbling of waggons. The snapping of great masses of sub- 

 terranean rock might give rise to great chasms, to uplifts of the 

 edges, and large tracts of strata, while the vibration of the snap would 

 ring through the earth for miles upon miles ; its occurrence would be 

 sudden, unpredicted, immediately dependent, perhaps, upon a sudden 

 change of temperature, upon long-continued rain, or drought, — in- 

 deed, upon any unusual condition. Certainly, too, it is not a little 

 singular that earthquakes are most frequent in the vicinity of the 

 crystalline rocks of mighty mountains ; in Chile along the line of 

 the Andes, along the mountainous tracts of Wales, and the same in 

 Scotland, along the Pyrenees, and most frequently of all along the 

 vol. ti. 3 a 



