EARTHQUAKES. 741 



strain attending the bending, especially along the axes of folds ; and, 

 under this strain, those of an anticlinal axis should open upward, and 

 those of a synclinal downward, as is the usual fact. In some cases, 

 where the rocks were stiff, they have been broken into numberless 

 masses, which, under the pressure, have slid among one another, in a 

 very promiscuous way. Ordinary faults are nothing but the dropping 

 of the rock on one side of a fracture by gravity, or else a pushing of 

 it up or down sidewise, or in some cases horizontally, along a plane of 

 fracture, by the pressure which caused the breaking. 



The cause appealed to, moreover, is precisely that demanded, to ac- 

 count for the great systems of fractures in rocks, called joints, and the 

 lamination of slate transverse to the bedding (p. 89) ; for these depend 

 on the working of lateral pressure with extreme slowness through long 

 periods of time. The joints are parallel to some axis of upheaval. The 

 pressure which turns an argillaceous rock into a roofing slate, places 

 all flattened particles in positions tranverse to the force, and flattens 

 out all compressible grains and air-bubbles ; and thus lamination at 

 right angles to the pressure is a necessary result. Sand-beds under 

 the same circumstances may have all their bedding obliterated, through 

 the shaking they experience, and become jointed instead, as illustrated 

 on page 89. Slaty cleavage has been produced by Tyndall in wax, 

 as well as clay, by simple pressure, and the laminated structure of 

 glacial ice (p. 682) has been explained by him in the same way. 



2. Eakthquakes. 



1. General Characteristics. — Earthquakes are vibrations of the 

 earth's crust. The vibrations, begun at a line of fracture, or by a sud- 

 den movement or shock of whatever kind, are conveyed in the rockv 

 crust, just as the sound of a scratch at one end of a log is propagated 

 to the other. An abrupt fracture of the crust, along a line where the 

 force from lateral pressure has long been increasing, may send a vibra- 

 tion through a hemisphere, which will move on almost regardless of 

 the mountains on the surface. 



An earthquake is either (1) a simple vibratory movement, from a 

 slight yielding to a strain or pressure or other cause, without any per- 

 manent displacement of the rocks ; or (2) a vibration, consequent on a 

 permanent displacement or change of level. The latter is far the 

 most violent, as the simple impulse of vibration has an additional on- 

 ward progression, equivalent to the uplift or displacement. 



Besides these wave-movements in the rocks, there is also, in most 

 cases, the very rapid wave which gives sound to the ear. The sound- 

 wave may be felt before the translation- wave, and may travel farther. 

 At the shock of St. Vincent, in 1812, sounds like thunder were heard 



