728 DYNAMICAL GEOLOGY. 



a result of the pressure in action during an uplift. The slates are 

 transverse to the force. The pressure tends to turn all pebbles or 

 particles so as to place their natter side in this transverse position, 

 and any bubbles present become flattened out in the same way. 

 Again, in all such action, force taking place in oscillations would 

 tend to cause transverse lamination. 



The laminated structure of ice has been explained by Tyndall 

 on the same principle (p. 675). Tyndall has proved by experiment 

 that slaty cleavage may be produced by means of pressure in white 

 wax, clay, and similar substances, when they are left free to expand 

 in directions transverse to the pressure. 



When argillaceous and arenaceous layers alternate, the former 

 may receive the slaty structure and the latter not ; because the 

 arenaceous layers, if not too firmly solidified, can accommodate 

 themselves to the new condition by motion among their partially 

 adhering particles of sand ; if firmly consolidated, only joints will 

 be produced, though in some varieties they may be so numerous as 

 to occasion a coarsely laminated structure. 



2. Joints. — Joints are due to the same cause as slaty cleavage, 

 and may occur in slaty as well as other rocks. 



Two systems at right angles to one another often result from 

 one action, but that in the line of movement is much the least 

 distinct. 



Subjection to pressure from different directions, in different 

 periods, would produce different systems of joints, and, in slates, 

 sometimes a new direction of cleavage. 



3. EARTHQUAKES. 



1. General characteristics. — Earthquakes are vibrations of the 

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

 a sudden movement or shock of whatever kind, are conveyed in 

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

 is carried to the other. If the ear be placed near the ice in 

 winter, it will hear a crack made in it, although miles off. If the 

 earth's crust suffer an abrupt fracture somewhere in its depths 

 where tension has long been increasing and has finally forced a 

 relief, the vibration may move on through a hemisphere, and will 

 be almost regardless of the mountains on the surface. 



Earthquakes are of two kinds : — 



(1.) A simple vibratory movement, without any permanent dis- 

 placement of the rocks. 



(2.) A vibration accompanying an uplift. 



