HYPOTHESES BEARING ON CLIMATIC CHANGES 675 
ages, so much neglected, but also for the great continental eleva- 
tions and their superposed plateaus, and the deep oceanic depres- 
sions with their abyssmal anti-plateaus—— phenomena with which 
current hypotheses have struggled so unsatisfactorily. It is also 
necessary to find an explanation for the unequal distribution of 
densities which have been partially revealed by gravity observa- 
tions, but which are more broadly suggested by the unsym- 
metrical aggregation of the hydrosphere. The total shrinkage 
of the earth from first to last, under the hypothesis here pro- 
posed, would perhaps be sufficient to reduce its volume as much 
as one-half or even more, this, of course, depending on the 
original density. While the most of this contraction would 
antedate known geological history, the process can scarcely be 
supposed to have been complete in pre-Cambrian times, or even 
to be complete now. A part of the condensation must, there- 
fore, quite certainly have fallen within geological history, and a 
part must remain yet to be accomplished, for, in addition to the 
retardation of the process of condensation caused by the heat 
generated, by the rigidity of the outer rocks and by the rapid 
rotation of the sphere, the maximum condensation of the mass 
could only be attained by means of a general rearrangement of 
the heterogeneous material of the meteor-built globe through 
the agency of diffusion, segregation, re-combination, re-crystal- 
lization and other processes which aid in giving a maximum 
compactness to mixed material. This internal readjustment 
must necessarily have been a slow process if the globe has been 
solid throughout its entire history, and must doubtless be yet 
incomplete. This progressive rearrangement of internal material 
_adds a special agency of contraction to loss of heat, change of 
rotation and similar processes now recognized and which would 
act under this hypothesis essentially as under the current view. 
If we make the plausible assumption that a slow process of 
diffusion, differentiation, concentration and gravitative readjust- 
ment has been in progress throughout the whole history and is 
yet active, and that matter has crept up from the hot compressed 
center into the superficial parts where relief of pressure would 
