THE PRESIDENTIAL ADDRESS 9 
and extrusion of molten rock. In the second place the continental 
masses, now truly floating in a substratum which has become fluid 
and less dense than before, will sink deeper into it, suffering dis- 
placement along the rift cracks or other planes of dislocation. As 
a result the ocean waters, unchanged in volume, must encroach 
on the edges of the continents, and spread farther and farther 
over their surfaces. 
Thus we have the mechanism which Suess vainly sought, causing 
positive movements of the oceans, their waters spreading over wide 
stretches of what was formerly continental land, and laying down 
as sediment upon it the marine stratified rocks which are our chief 
witness of the rhythmic advances of the sea. 
This condition, however, cannot be permanent, for by convection 
of the fluid basic substratum, supplemented by the influence of tides 
within it, and the slow westward tidal drag of the continental masses 
towards and over what had been ocean floor, there will now be 
dissipation of its heat, mainly into the ocean waters, at a rate much 
faster than it has been or could be accumulated. Resolidifica- 
tion ensues, and again there are two main consequences. First, 
the stratum embedding their roots having now become more 
dense, the continental masses rise, and as they do so the ocean 
waters retreat from their margins and epicontinental seas, leaving 
bare as new land, made of the recently deposited sediments, the 
areas previously drowned. Secondly, the expanded crust, left 
insufficiently supported by the withdrawal of shrunken substratum, 
will suffer from severe tangential stress, and, on yielding, will 
wrinkle like the skin of a withering apple. The wrinkles will be 
mountain ranges, formed along lines of weakness such as those at 
continental margins ; and they will be piled up and elevated to suffer 
from the intense erosion due to water action upon their exposed and 
upraised rocks. 
In this, again, we have a mechanism which supplies what was 
needed by Suess, and one, moreover, which secures the required 
relationship between continental and mountain movement, between 
the broader extensions of continental land and the growth of 
mountains with their volcanoes and earthquakes and the other 
concomitants of lateral thrust. 
Thus a Thermal Cycle may run its full course from the solid 
substratum, through a period of liquefaction accompanied by crustal 
tension, back to solidification and an era of lateral stress : and the 
stage is set for a new cycle. 
Professor Arthur Holmes, in checking Joly’s calculations, has 
concluded that the length of the cycles in a basic rock substratum 
should occupy from 25 to 40 million years, a period much too short 
B2 
