MOUNTAIN FORMATION 



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 neighbouring "fault"" slipping in some 

 way so as to give more space to the swelling lava beneath. 

 Of course, many of the cracks caused by this swelling are 

 never seen ; and the molten lava seldom reaches the sur- 

 face except when through volcano vents or cracks in 

 mountains that are near the sea-shore ; but such outbreaks 

 are probably more common in the deep sea. 



To see how effective the pressure arising from the 

 depths of the ocean may be in driving water into the 

 crust of the earth, we may observe that the tendency to 

 penetrate is everywhere proportional to the depth of the 

 sea. Now everybody knows that if a cistern be placed at 

 the top of a house and connected with a fountain in the 

 garden the fountain ought to throw a jet as high as the 

 cistern because water, as the saying goes, always rises to 

 its own level. As a matter of practice the water does not 

 rise so high because of the resistance of the air. But for 

 theoretical purposes we may consider the proportion true, 

 and we might similarly say that the pressure in a sea one 

 mile deep would thus throw a stream a mile high ; in a 

 sea two miles deep, two miles high; and so on. Now 

 some of the ocean depths exceed five miles, the greatest, 

 near Guam, being 5269 fathoms, almost exactly six 

 miles. Is it therefore any wonder that the deeps east of 



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