i9o8.] THE PHYSICS OF THE EARTH. 161 



base, etc., are by no means so penetrable as the sedimentary rocks, 

 and hence water has more difficulty in passing through them. And 

 as the layers of this material composing the earth's crust ate 

 about fifteen miles deep, it has been generally held that water 

 would have difficulty in making its way down into the heated 

 layers just beneath the crust.^ Indeed it has been practically as- 

 sumed that the ocean bottoms are water-tight, in spite of the 

 great fluid pressure constantly exerted by the mere depths of 

 the water over a large part of the bottom of the sea. This fluid 

 pressure in many places is gregit enough to throw a column of water 

 to the free surface, over five miles high; and it operates not only 

 from day to day, year to year, but also from century to century, 

 age to age. If granite is at all penetrable by water, is it therefore 

 any wonder that a gradual secular leakage should go on, and at 

 length, by a kind of slow perspiration of the stone, give rise to 

 sufficient accumulation of steam beneath the crust to produce a 

 swelling of the saturated mass, and require a readjustment of the 

 overlying rocks? 



Now it happens that by nature all the granitic rocks are crystal- 

 line, and thus somewhat coarse-grained in structure ; so that they 

 absorb water from the ground and moisture from the air. The 

 crystalline structure permits penetrability to a greater degree than 

 would fine-grained and very hard rocks such as agate ; but no rock 

 has such fine pores as the metals, and especially vitreous bodies like 

 glass, to which agate is an approximation. And as all the metals 

 are proved by experiment to be leaky under great fluid pressure, and 

 glass is shown to obey the same law, it obviously follows that all 

 rocks are leaky under great fluid pressure. Consequently under the 

 incessant pressure of the oceans water must make its way into the 

 heated layer just beneath the earth's crust- 

 Heretofore the possibility of earthquakes cracking the ocean 

 bottom has been generally recognized, but it has been held that 



^ This statement is perhaps too positive, for Sir William Ramsay, the 

 celebrated British Chemist and Physicist, writes me that he has long be- 

 lieved that the ocean bottom leaks and that the formation of minerals 

 takes place chiefly in the bed of the sea. Undoubtedly this view will come 

 to be generally accepted. Similar views seem to be held by Lord Rayleigh, 

 Sir Wm. Huggins, Arrhenius and many other eminent physicists. 



