376 SEE— THE NEW THEORY OF EARTHQUAKES [November 15. 



tain range elevated above the water. A case such as that offered 

 by the Aleutian Islands proves this beyond all doubt; yet in most 

 cases the process of formation has been slow, and the present state 

 of the sea bottom does not show exactly how the development came 

 about. From the study of those islands near which depressions 

 exist we may in time gather much light on the details of island 

 development, and in this way add to our knowledge of the geology 

 of the sea. While the illustrations already given show the general 

 nature of the process, it is clear that the sea bottom is still too little 

 explored to enable us to treat of all cases intelligently, and we must 

 therefore wait for more light as the surveys of the oceans are 

 gradually extended. It is obvious that exact surveys of the 

 sea bottom are needed not only for the laying of cables, but also 

 for the study of the earth's geological history; and the time may 

 come when exact ocean surveys may be as important for our knowl- 

 edge of the physics of the earth as geological or geodetic surveys 

 of the land. 



§ 5. On the Nature of the Molten Rock Which Moves Beneath 

 the Earth's Crust. — In the paper on the Cause of Earthquakes we 

 have pointed out the leaky character of the earth's crust, and shown 

 how steam will naturally form in the hot rocks beneath till the lava 

 becomes so fully saturated by this vapor that it swells and requires 

 more space. The steam is not held to be free, but absorbed in the 

 hot rock, causing it finally to expand till it becomes irresistible; 

 and when the elastic tension of this steam-saturated lava becomes 

 great enough the crust begins to shake and the paroxysm continues 

 till it moves along the nearest fault line. When the underlying 

 molten rock has thus obtained more space, the agitation ceases till 

 the tension again becomes too powerful for the crust to withstand, 

 when another readjustment will take place. Thus the process is 

 repeated at successive somewhat irregular intervals of time. The 

 shaking is due to the enforced movement of the molten rock be- 

 neath the crust, and the paroxysm thus developed sometimes shakes 

 down cities and devastates whole countries. 



A familiar illustration of this process is seen in the lid of a tea- 

 kettle when the steam pressure accumulates till it sets the lid quiver- 

 ing ; as the steam escapes at the sides the agitation slowly dies down, 



