,507.] AND MOUNTAIN FORMATION. 415 



thinks cannot take place in the first twenty miles of the earth's crust, 

 because the pressure is not great enough to prevent secular leakage 

 under the constant pressure of the oceans, which in many places is 

 sufficiently powerful to throw a column of water five miles high. 



No doubt this view will be generally adopted by physicists and 

 geologists. One may indeed see that it is true by remembering that 

 at a depth of twenty miles the pressure is only about 8,600 atmos- 

 pheres, or a little over eight times what it is in the deepest oceans. 

 Such pressure might indeed tighten the crystalline elements of 

 granite, but it could not obliterate the crystals ; and hence capillary 

 forces would be intensified rather than diminished by this tighten- 

 ing up of such a coarse-grained structure. 



We cannot experiment on rock subjected to such pressure as 

 exists at a depth of twenty miles, but we can observe lava expelled 

 from volcanoes, some of which was once compressed at this depth ; 

 and we can examine the granite at the base of vertical walls where 

 faults have moved under earthquake forces for many thousands of 

 feet. In neither case does the structure of the rock indicate that 

 great modification would occur at so small a depth as twenty miles. 



But the most conclusive answer which nature gives to this inquiry 

 is furnished by the vast vertical walls of granite so often lifted 

 thousands of feet by fault movements in such places as the Andes 

 of Peru, Chile and Patagonia, where the leakage of the ocean has 

 found relief by the expulsion of lava under the land along the 

 margins of the sea. The meaning of the mountain formation along 

 such a sea coast admits of no possible doubt. Accordingly we are 

 enabled to interpret the movements beneath the crust, and to infer 

 the nature of the underlying rock, even where it is wholly inacces- 

 sible to observation. 



Blue Ridge on Loutre, 



Montgomery City, Missouri, 

 Jan. 14, 1908. 



