igoS] THE PHYSICS OF THE EARTH. 167 



when this collapses under gravity the first great wave comes 

 ashore. The elevation then subsides into a depression as at first, 

 and the currents again flow in and force up the level a second 

 time ; and with the second collapse another wave is sent ashore ; 

 and so the oscillation of the sea continues, sometimes for a day or 

 two before it finally quiets down. 



Xow these sea waves of the first class furnish an exceedingly 

 important criterion as to the nature of what is going on beneath the 

 earth's crust. The sinking of the sea bottom often happens in 

 the deep trench south of the Aleutian Islands, and repeated drops 

 of this kind have obviously produced the deep trough parallel to 

 these islands. For it is observed that the earthquake usually raises 

 one or more of the islands to the north, w'hen the sea bottom sinks 

 to the south. Xow the islands could not be upraised unless some- 

 thing was pushed under them, and the bed of the trough could not 

 sink down unless it was in some way undermined. Accordingly it 

 follows that molten rock is expelled from beneath the bed of the 

 trough to the south and pushed under the adjacent islands to the 

 north, which are thus uplifted. The bed of the sea often sinks during 

 the earthquake arising from this subterranean movement, and then 

 the water withdraws from the shore and afterwards returns as a 

 great seismic sea wave. It will be observed that the subcrustal move- 

 ment is from the sea towards the land, because steam accumulates 

 under the ocean, but scarcely at all under the land. 



Thus these seismic sea waves become very important criteria 

 for determining whether the sea bottom has sunk; and if it has 

 sunk we know that lava was expelled from under the sea and pushed 

 under the land. Seismic sea waves therefore may be regarded as 

 Z'cry delicate levels, for determining the movement of the sea bot- 

 tom ; and from the nature of this movement we can often decide 

 what the effect of the earthquake has been. ^loreover these waves 

 enable us to tell with certainty that the chief function of earth- 

 quakes is the elevation of the land along the coast by the expulsion 

 of lava from beneath the bed of the sea. It is not too much to say 

 that the true nature of earthquakes and their function in the uplift 

 of mountains and plateaus could not be certainly made out except for 

 the exceedingly important criterion furnished by seismic sea waves. 



