Present Problems oi Geophysics. — Becker. 1 1 
mile becomes liquid, its rigidity 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 ex- 
pulsion. 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 influences tending to their expulsion 
would seem to be at a maximum immediately after the 
fusion of enough material to supply an eruptictn. 
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 sat- 
isfactory, so far as the explanation of fusion is concerned. 
Only those who liave studied the minute evidence of me- 
chanical action, in mountain ranges can appreciate the evi- 
dence they present of stupendous dissipation of energy. This 
has not indeed been enough to fuse rocks, but it is hard to 
conceive that it is always insufificient to furnish the latent 
heat of fusion to rocks already close to tlieir melting point 
under the prevailing- pressure. From this point of view, vul- 
canism is a feature of orogenic movement and it is to be 
looked for where relative motions are concentrated in zones 
so narrow that the local dissipation of energy is relatively 
intense. It is also possible that percolating waters, by re- 
ducing the melting points of rocks, sometimes bring about 
fusion without change of temperature. Such a hypothesis 
might fit the volcanoes of the Hawaiian islands where there 
is no known faulting in progress. 
The physics of magmatic solution is a great subject 
which is experimentally almost untouched, although a vast 
amount of geological speculation has been based upon as- 
sumed properties of magmas. It is only within a few 
months that even satisfactory melting-point determinations 
of those most important rock-forming minerals, the lime- 
soda feldspars, have been made. The feldspars are only one 
series of isomorphous mineral mixtures. Their study is fun- 
damental and must be followed by that of the remaining 
