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, suflBciently 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 ; ft'eeze 
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. VI. 3 a 
