THE PRESIDENTIAL ADDRESS 5 
of this character; while, in between, there came times of broad 
expansions of continental land and regressions of the sea. ‘These 
changes were in his view on far too grand a scale to be compared 
with, or explained by, the trivial upheavals and depressions of land 
margins of the present day, which he showed could mostly be 
correlated with volcanoes or earthquakes, or with such incidents as 
the imposition or relief of ice-sheets on an elastic crust in connexion 
with glacial conditions. 
It became necessary for him to replace or supplement oscillations 
of the earth-crust by a world-wide periodic ebb and flow of the 
oceans, to and from the continents ; positive movements of trans- 
gression carrying the sea and its deposits over the lands, drowning 
them and their features under tens or hundreds of fathoms of water ; 
and negative movements or regressions when the oceans retreated 
to the deeps, leaving the continents bare or encrusted with recently 
formed sediments. 
Although the facts cried out for this generalisation Suess was at 
a loss to supply any mechanism competent to produce the wonderful 
rhythm. The problem was difficult because a liquid must maintain 
a horizontal, i.e. an equipotential, surface. It was manifestly im- 
possible to withdraw from the earth, and later to replace upon it, the 
vast quantity of water that would be required ; and, though a shifted 
water-level, or even a varied water-surface relative to the continents, 
might be caused by polar ice-caps, by redistribution of the continents 
carrying their local effects on gravitation, by variations in the rate of 
the earth’s rotation, or other far-reaching causes, none of these would 
supply an explanation that fitted all the facts. Regressions of the 
sea could be to some extent explained if Suess’s main postulate, 
that the great ocean basins had been slowly sinking throughout 
geological time, were granted. But this explanation only rendered 
more impotent the raising of ocean levels by deposits of sediment, 
and this was almost the only valid cause for transgressions that he 
had been able to suggest. 
Further, it is not possible to ignore the definite relationship that 
exists between the pulsation of the oceans and the raising of moun- 
tains by lateral or tangential stress. Periods of positive movement 
or advance of the seas were times of comparative tranquillity, when 
tangential pressure was in abeyance. Periods of negative movement 
and retreat were invariably marked by the operation of great stresses 
by which the earth’s face was ridged and wrinkled in the throes of 
mountain-birth. 
The theory that continuous cooling and shrinkage of the interior 
of the earth afforded an explanation of mountain ranges and other 
