ld 
PRESIDENTS ADDRESS—SECTION C. 79 
the earth consisted of an immense mass of incandescent liquid, 
covered by a shell only a few miles thick, this shell would have 
no stability, and catastrophes of some sort would be common. 
We are told that if there were a surging mass of molten lava 
everywhere, not far beneath our feet, earthquakes and volcanic 
eruptions would be far more frequent than they are. But why 
should molten lava surge beneath a crust of rock twenty or 
thirty miles thick? The difficulty is rather to discover causes 
sufficiently powerful to explain the observed movements ; for the 
largest bodily tide would be under two feet, and could not influence 
much the position even of molten lava in a volcano like Stromboli, 
for the friction on the sides of the pipe would reduce the move- 
ment almost to nothing. So far as I can judge from published 
opinion, the tendency of geological thought during the last twelve 
years has been in the direction of the idea that the interior of 
the earth is fluid. Deductive reasonings against that idea have 
fallen one by one, while extended observation has more and more 
confirmed the geological argument in favour of a motile interior, 
Tt must be remembered that the one argument for solidity— 
that of the tides—is an exceedingly complicated one, while the 
arguments for fluidity are simple. If it should ultimately turn 
out that the bodily tides are quite insignificant, or even absent, 
it would not necessarily follow that the earth is solid. It would 
be far more likely that the whole of the conditions of the tidal 
problem had not been taken into consideration, than that 
depressions of five or six miles in depth could take place on the 
surface of a rigid solid body. It seems to be certain that the 
present inequalities of the surface are far greater than can be 
accounted for by the contraction through cooling of a solid globe ; 
and if the interior was not solid when the first crust was formed, 
it cannot be solid now. 
Notwithstanding the opinions held by astronomers like Laplace, 
Sir J. Herschel, Sir G. Airy, M. Delaunay, and Professor 
Newcomb, some geologists have been so much impressed with 
the arguments advanced in favour of a solid earth that they 
have thought it necessary to frame some hypothesis which would 
reconcile physical arguinents with geological facts. Mr. Hopkins’ 
hypothesis of the existence of subterranean lakes of molten rock 
is one of these, which was at one time held to be probable by 
many geologists, but is now universally abandoned. Another is 
Sir W. Thomson’s suggestion that the earth may be a cold sphere, 
around which a stratum of meteoric matter has accumulated, 
heated to the temperature of fusion by collision with the earth. 
Another hypothesis of the same character is the existence of a 
thin fluid substratum between a solid nucleus and a solid crust. 
This idea was originated by Mr. Poulett Scrope, and has been 
advocated by Professor Shaler, Professor Le Conte, M. Roche, 
and the Rev. O. Fisher. There are no special physical or 
