‘99 PRESIDENTS ADDRESS—SECTION C. 
than the unmetamorphosed rock and the water did before. If 
the water can penetrate to the minerals there is room for the 
minerals to expand, and there will be no important increase 
in bulk of the rock. Hydrothermal metamorphism has often 
occurred in large masses of rock which show no signs of having 
been under great stress, and, consequently, could not have exerted 
great pressure on the surrounding rocks during the process. 
Changes in density giving rise to changes in total bulk must be 
due to changes in temperature, which may be brought about 
either by mechanical or by chemical means. 
The hypothesis of the mechanical origin of the heat has been 
advocated by G. L. Vose, Professor Wurtz, and R. Mallet. The 
idea is founded on the supposition that the contraction of the 
earth by radiation furnishes the necessary energy, and it falls 
with the contraction theory. No other mechanical theory 
attempts to explain the origin of the movements, and there is 
only one chemical theory, viz., the oxidation of a metallic nucleus 
by the infiltration downwards of surface water. 
This theory was originated by Sir Humphrey Davy in 1808, to 
account for volcanoes, and, although he abandoned it in 1828, it 
was ably supported by Dr. C. Daubeny. Sir H. De la Beche, in 
1834, was also inclined to think that it might account for 
oscillations of the surface, the subsequent radiation of the heat 
causing depression. ‘For while,” he says, “intense heat was 
developed by the combination of the oxygen of one charge of 
water with the metallic base, no more water could approach the 
lower body from above until the heat was sufficiently radiated 
or conducted away, and therefore there would be no gradual and 
continued expansion unchecked by contraction.” At the present 
day this theory is almost universally abandoned, although it still 
seems to be looked upon with a favourable eye by Professor 
Judd. But we have no reason to suppose that unoxidised sodium 
or potassium ever formed a portion of the earth since it had a 
solid crust, and iron, which probably exists in the interior, would 
not furnish the necessary heat by the decomposition of water. 
Also, the whole of the present ocean would not oxidise a layer 
more than two miles deep ; so that if oxidation has been the cause 
of the movements, an enormous amount of water must have been 
decomposed. But, as dry land was probably in existence in the 
Archean era, and has certainly existed continuously since the 
Silurian period, we cannot admit the disappearance of such a large 
body of water. 
Mr. Mellard Reade says: “It is not improbable that large 
masses of the heated globe, far below our thirty-mile zone, 
undergo slow changes which produce fluctuations of temperature 
even in this super-heated zone.” Also, that ‘“‘ Chemical re-action 
can hardly yet have ceased, considering the multifarious materials 
of which the globe is composed, and chemical reaction may mean 
