80 PRESIDENT’S ADDRESS——-SECTION C. 
geological reasons for thinking that a ¢/7zx fluid substratum exists, 
although, if pressure be a very important agent in solidification, 
the earth may have solidified both from the centre and from the 
surface. Still, as we have to descend into the earth for nearly 
half its radius before we arrive at the density which iron has at 
the surface, we can with difficulty believe that the outer half is 
solid, unless it be formed of materials less dense than iron, which 
is very improbable. The hypothesis was originated to meet the 
argument founded on the amount of the precession of the 
equinoxes, and, as this objection to a fluid interior has been 
withdrawn, the hypothesis of a ¢Azz fluid substratum will probably 
be abandoned, for it affords but little help towards explaining the 
supposed absence of bodily tides. 
CAUSES OF THE OSCILLATIONS. 
We are now prepared to examine the principal theories that 
have been proposed to explain the movements of the surface. It 
would not be necessary, even if it were possible, for me to 
discuss them all. So long ago as 1834 Mr. Grenough, in his 
presidential address to the Geological Society of London, said, 
“the assigned causes of elevation are exceedingly various. One 
author raises the bottom of the sea by earthquakes ; another by 
subterraneous fire ; another by aqueous vapour; another by the 
contact of water with the metallic bases of the earths and alkalis. 
Heim ascribes it to gas, Playfair to expansive forces acting 
from beneath, Necker de Saussure connects it with magnetism, 
Wrede with a slow continuous change in the position of the 
earth’s axis. Leslie figured to himself a stratum of concentrated 
atmospheric air under the ocean, to be applied, I suppose, to 
the same purpose” ; and since then others have been added to 
the list, such as extravasation of water-substance and changes 
in the velocity of the earth’s rotation. 
Contraction Theory.—TVhe theory that has gained the greatest 
celebrity is the one which attributes the movements of the 
surface to the tangential pressures set up by a cooling and 
contracting globe: a theory which, originating with Descartes 
and Sir I. Newton, was revived in 1816 and 1827 by Cordier, 
followed up by Elie de Beaumont and Constant Prevost in 1829, 
and has since been advocated by Sedgwick, De la Beche, and 
numerous other distinguished geologists. It supposes that the 
earth consists of a shrinking nucleus surrounded by a solid 
crust which, no longer contracting, is gradually left unsupported, 
and periodically adjusts itself to the shrinking nucleus by folding 
along bands which form mountain ranges. 
This theory, so simple and so dramatic, was widely adopted 
both in Europe and in America, but several objections were 
brought forward in 1874 by Captain Dutton in the American 
