1907.] AND MOUNTAIN FORMATION. 377 



and the lid then remains quiescent till the accumulating- pressure 

 again requires relief, when the shaking is renewed. Thus the 

 process is periodic, and the period depends on the rapidity with 

 which the steam is developed. In the case of earthquakes, as 

 already remarked, the steam is not free, but absorbed in the molten 

 rock, and when the agitation begins this gives a similar quivering 

 motion to the block of the earth's crust overlying it, and ceases only 

 when readjustment occurs, usually by the neighboring fault slipping 

 in some way to give more space to the swelling lava beneath. The 

 period is not so regular, as in the case of the tea-kettle, because the 

 resistance to be overcome is never the same in two successive earth- 

 quakes; nor is an equal amount of relief afforded even when the 

 underlying force is identical, because the crust moves as soon as 

 its resistance is overcome, and it ceases to move when the displace- 

 ment is sufficient to restore equilibrium. As the resistances to the 

 blocks of the earth's crust are constantly varying, owing to their 

 complicated mutual relations, and the steam also accumulates at 

 various rates and unequally under different parts, the intervals be- 

 tween successive shocks may be approximately, but are never ex- 

 actly, equal. Settlements after great shocks, and slight stresses, are 

 relieved by small movements, but eventually the tension becomes 

 great enough to demand a displacement that will provide more 

 space beneath the crust, and then the resulting fault movement 

 usually is conspicuous enough to show at the surface of the ground. 

 The crust is about fifteen or twenty miles thick, and molten lava 

 seldom reaches the surface, except when the crust is uplifted and an 

 outlet facilitated by the opening of cracks, as in mountain chains, 

 which may be along the shore, or in the depths of the sea, projecting 

 above the water as islands. Hence not all islands are volcanic, 

 and by no means all mountains become volcanoes; but such out- 

 breaks frequently happen in the neighborhood of deep seas, where 

 the expulsion from beneath the sea is most violent, and the crust 

 is abruptly and sharply upheaved, so as to afford a chance vent for 

 the imprisoned vapor. 



§ 6. The molten rock beneath the oceans probably experiences 

 an enforced creep towards all available avenues of escape, but the 

 movement usually is not rapid enough to prevent the bottom from 



