8 THE PRESIDENTIAL ADDRESS 
long felt, and that Suess had so brilliantly diagnosed. As Darwin 
found the missing word for Lyell, so Joly in his theory of Thermal 
Cycles has indicated the direction of search for a mechanism to 
actuate the rhythm of Suess. 
In Joly’s conception the running down of the earth’s energy, 
though a continuous process, was, through the intervention of radio- 
activity, converted into a series of cycles, during each of which 
relative movements of sea and land must occur ; downward move- 
ments of the continents, associated with positive encroachments of 
the sea ; upward movements, with retreat of the sea, the formation 
of wide land masses, and the ridging of strata to form mountain 
ranges. ‘Thus he forged a link that could unite the continental or 
epeirogenic movement with orogenic or mountain movement. 
The visible parts of mountains and continents, as well as their 
lower and hidden portions, or ‘ roots,’ are made of comparatively 
light rocks. In order to stand up as they do their roots must be 
embedded in denser matter, in which they ‘ float ’ like ice-bergs in 
water. A far larger mass must exist below than is visible above, 
and the bigger the upstanding part the bigger the submerged root. 
Over the larger area of the ocean floor, on the other hand, the 
thickness of material of low density must be very slight, and the 
denser layer must come close to the surface. 
The study of earthquakes, to which the Seismology Committee 
of the British Association has made outstanding contributions, has 
yielded, from the times taken in transmission of vibrations through the 
earth, the best information as to the nature and state of the interior. 
It has proved that the dense layer is solid at the present time. It is 
probably no coincidence that the earth is also but just recovering 
from what is possibly the greatest period of mountain-building, if 
not the greatest negative movement of ocean retreat, that it has 
ever experienced. 
But solidity cannot be the permanent condition of the sub- 
stratum. Heat is generated in it by its own radioactivity, but, 
according to the terms of the hypothesis, cannot escape, in con- 
sequence of the higher temperature generated in the continental 
rocks which cover it. It is therefore retained in the substratum 
and stored as latent heat of liquefaction, so that, within a period 
which has been calculated approximately in millions of years, com- 
plete melting of the sub-crust must ensue. 
The resulting expansion of the liquefied stratum will have at 
least two effects of great importance to us. In the first place the 
unexpanded superficial layers will be too small to fit the swelling 
interior. They will, therefore, suffer tension, greater on the ocean 
floor than on land, and cracking and rifting will occur, with intrusion 
