PRESENT PROBLEMS OF GEOPHYSICS 513 



The causes and conditions of earthquakes afford a separate topic of 

 great interest. That some of them are of volcanic origin is evident; 

 others appear to be due to paroxysmal faulting, yet there is very 

 possibly a common underlying cause. 



On no subject are opinions more divergent than concerning the 

 origin and mechanism of volcanoes. To the ancients they were the 

 mouths of the river Phlegethon. To those who adhere to the Cartesian 

 doctrine they are communications with the liquid interior of the earth. 

 Most geologists think of them as connected with hypogeal reservoirs 

 of melted matter subsisting for indefinitely long periods of time. 

 Finally, it is conceivable that the lava may be extruded as soon as 

 the melted mass has accumulated in sufficient quantity, somewhat as 

 water may break through an obstructing dam after its depth reaches 

 a certain value. The continual movements of the rocks show that they 

 must be to some extent in a state of elastic strain, so that a given cubic 

 mile of rock resists surrounding pressure in virtue both of its rigidity 

 and of its compressibility. If that cubic mile becomes liquid, its rigid- 

 ity is gone and the change of shape of surrounding masses may aid in 

 its expulsion. Of course imprisoned gases, especially the " juvenile 

 waters " of Professor Suess, may also play a very important part in 

 expulsion. But the more I have studied the matter, the less probable 

 it seems to me that considerable bodies of melted lava can remain 

 quiet for long periods of time in the depths of the earth. The influ- 

 ences tending to their expulsion would seem to be at a maximum 

 immediately after the fusion of enough material to supply an erup- 

 tion. 



Relief of pressure is often invoked to explain fusion of lava, but it is 

 not a wholly satisfactory cause. If a deep crack were to form, the 

 rock at the bottom might melt indeed, but, as the crack filled, the 

 pressure and the solidity of the source would be restored. To me, 

 Mallet's hypothesis is more satisfactory, so far as the explanation of 

 fusion is concerned. Only those who have studied the minute evi- 

 dence of mechanical action in mountain ranges can appreciate the 

 evidence they present of stupendous dissipation of energy. This has 

 not indeed been enough to fuse the rocks, but it is hard to conceive 

 that it is always insufficient to furnish the latent heat of fusion to 

 rocks already close to their melting-point under the prevailing press- 

 ure. From this point of view, volcanism is a feature of orogenic 

 movement, and it is to be looked for where relative motions are con- 

 centrated in zones so narrow that the local dissipation of energy is 

 relatively intense. It is also possible that percolating waters, by 

 reducing the melting-points of rocks, sometimes bring about fusion 

 without change of temperature. Such an hypothesis might fit the 

 volcanoes of the Hawaiian Islands, where there is no known faulting 

 in progress. 



