342 SEE— THE CAUSE OF EARTHQUAKES. [October 19, 



VI. Confirmation of the Foregoing Theory of Mountain 



Building by Geodesy. 



§ 27. General considerations on the attraction of mountains. 



If the theory above outHned be admissible it will follow that 

 all mountains are filled with light material like porous lava, which 

 was originally injected as hot lava under great steam pressure, full 

 of bubbles, and is thus similar to dense pumice. When the lava 

 solidified, the bubbles dried up, and left a honeycombed structure 

 of great strength, but comparatively small weight. The deep in- 

 terior of all large mountains should be filled with material of this 

 kind, which gives strength, but has low density, and the effect of its 

 injection is to reduce the average density of the mountain below 

 that of an equal mass of similar material taken from a plain. 



It is remarkable that although some long tunnels through moun- 

 tain chains have been bored by human effort, none of them has gone 

 more than a mile or so deep, and thus all are too shallow to give us 

 any experimental knowledge of the materials in the depths of the 

 mountains. No large mountain has been eroded to great depth, 

 and hence our only knowledge of the interior of mountains is de- 

 rived from volcanic action, which blows out the inner portions of 

 some of them. The mountains which happen to break out as vol- 

 canoes, either because the subterranean steam pressure is suddenly 

 applied and abnormally great, or because they are weak in some 

 point of their construction, are obviously not different from ordi- 

 nary chains and peaks, until after they break forth. 



Eruptions are always accompanied by violent earthquakes, and 

 a mountain does not take fire and form ashes and cinders by burn- 

 ing, but blows out the volcanic materials already stored up in vast 

 quantities. Thus we naturally infer that all mountains are essen- 

 tially alike, but we are never able to see the inner contents except 

 of the few of them which have become volcanoes. And if we are 

 able to explore the interior of all mountains, it is only by studying 

 the materials expelled from volcanoes by the explosive power of 

 steam. It is doubtful if even the oldest mountains show erosion a 

 mile deep, and hence we cannot penetrate the earth's crust to any 

 depth except by an analysis of the materials which come out of the 

 mountains after they are blown open in eruption. It is scarcely 



