_C. &. Dutton on the Contractional Hypothesis. 121 
compounds of higher average density. But it is not clear how 
such changes could take place at depths greater than those 
assigned as the limits of sensible cooling, and such an assump- 
tion must appear gratuitous until suppo: y evidence. The 
want of such evidence compels us to confine possible changes 
of density (so far as strict reasoning is concerned) to horizons 
not lower than two or three hundred miles. Although no esti- 
mate can be made of the contraction of this portion, it is prob- 
ably safe to say that its volume cannot have diminished so 
much as one-tenth; and if we were to assign thirty miles as the 
diminution of the earth’s mean radius since the first formation 
residue must have occurred before the beginning of the Ter- 
tary ; and yet the whole of this contraction would not be suffi- 
client to account for the disturbances which have occurred since 
the close of the Cretaceous. In all mountain regions the dis- 
Wherever found, their excessively disturbed condition must 
utterly prohibit the belief that it is the result of secular con- — 
traction of the interior. Bearing in mind that a shrinkage of 
one-fifth of linear dimensions implies an increase of 95 per cent 
im mean density, and that according to this hypothesis such in- 
crease is zero at the surface, it puzzles the imagination to con- 
ceive what must have been the condition of the eart 
while the Laurentian sediments were accumulating, if we are 
to assume that their present distortion is due merely to secular 
contraction. - g 
he determination of plications to particular localities pre- 
sents difficulties in the way of the contractional hypothesis 
which have been underrated. It has been assumed that if a 
