DEC. 4, 1923 PROCEEDINGS: WASHINGTON ACADEMY OF SCIENCES 449 
The hypothesis of large displacements of the earth’s axis of rotation in the 
body of the earth (or of the poles on the surface) forms part of Wegener’s 
scheme, although it is not a necessary corollary of the assumption of 
continental migration. In general, large displacements of the pole, apart 
from oscillations of short period, imply extensive rearrangements of mass 
within or upon the earth, rearrangements too large for geologists to accept. 
On Wegener’s scheme of continental blocks of “sial’’ floating in ‘‘sima’”’ even 
the shifting of a large continental block over many degrees of great circle 
would involve such a small effective rearrangement of mass that the resultant 
displacement of the pole would be unimportant. Wegener cites two articles 
as tending to show that large displacements of the pole might take place 
without correspondingly large rearrangements of mass; one citation is based 
on a misapplication of the words quoted; the other article cited involves 
fundamental fallacies in mechanics. 
If the earth were a rigid spheroid, an increase in its speed of rotation or in 
the tide-producing action of a satellite would tend to move a particle on its 
surface toward the equator. This idea has been invoked as a_ possible 
explanation of the equatorward drift of the continents. But the continents 
are not particles but masses of matter forming part of a continuous crust and 
therefore impeded in their motion by their surroundings; furthermore, the 
earth is not rigid but is presumably plastic under the action of long-continued 
forces. Therefore, the result of such an increase in the speed of rotation or 
in tidal action would be merely an increase in the flattening, to which the 
earth would adjust itself by plastic flow or by ruptures here and there, the 
continents remaining in the same general relative position with respect to 
their surroundings. 
There is, however, a small residual equatorward force that acts on an 
object floating on the earth’s surface. It is due to the change in direction 
of gravity with elevation and is approximately proportional to the distance 
between the center of gravity and the center of buoyancy of the floating body. 
This force is invoked by Wegener as an explanation of the equatorward 
movement of a continental block of “sial’’ floating in “‘sima.”’ The force is 
so small (about 1/1,000,000 of gravity) that it seems inadequate to overcome 
the resistance of the “‘sima.’”’ In rebuttal to this objection it is urged that a 
very small force acting through geologic ages might produce considerable 
effects, since, in the yielding of a viscous liquid, time is the all-important 
element rather the magnitude of the force. This argument assumes that 
so-called solids like the “‘sima”’ are really extremely viscous liquids. There 
is, however, a real distinction between soft solids and viscous liquids, as was 
pointed out long ago by Clerk Maxwell, and as has been more recently 
verified by Bingham and Durham. It seems far more probable that the 
-“sima’’ is a solid with a yield-point well above the stresses due to this ex- 
tremely minute equatorward force than that it is a viscous liquid; if the 
“sima”’ is a true solid, the force:in question would be ineffective in producing 
equatorward displacements of the continents. 
The fact that the higher portions of the earth’s crust are lighter than the 
deeper-lying portions and the hypothesis of isostasy based on this fact both 
suggest the conception of floating continental blocks, of which Wegener has 
made such free use. But this whole hypothesis of a floating crust is rather a 
convenient simile than an adequate statement of all the facts, and must not be 
pressed too far. On the hypothesis of isostasy the stresses in the crust are 
not hydrostatic (that is, such as occur in flotation) until the depth of com- 
