294 Prof. O. Lloyd Morgan—Elevation and Subsidence. 
zero: and that on relief of pressure such liquid expands into ice. 
Conversely Amagat has shown (Comtes Rendus, vol. 105, p. 165), 
that the solidifying point of carbon tetrachloride (a substance that 
contracts on solidification) may be raised from —19-5° C. to + 19-5°C. 
by increasing the pressure from 210 to 1160 atmospheres (approxi- 
mately). I would suggest that similar changes must occur as the 
fluid or viscous matter in the liquid substratum is subjected to 
increased or diminished pressure. 
Increased pressure would thus tend to squeeze the magma into the 
solid condition or to induce crystallisation therein. Diminished 
pressure would tend to allow the partially solidified or crystallized 
magma to expand into the fluid state. 
Take the case of an area undergoing continuous sedimentation. I 
would suggest that the increased load must tend to squeeze the magma 
in the underlayers into the solid condition. But in the solid con- 
dition the rock occupies less space. Contraction must take place and 
the contraction is manifested at the surface as subsidence. Further- 
more, without committing myself to the acceptance of the theory 
held by those who attribute subsidence to mere weight, I would 
suggest to the upholders of that theory that the added weight of the 
sediment above would entail on this hypothesis an added weight 
below—that is, if we suppose that the solidified rock adheres to the 
lower surface of the crust in this region. 
In a region undergoing denudation, on the other hand, the lighten- 
ing of the load would entail the melting of some of the solidified or 
crystallized magma. Such melting would be accompanied by expan- 
sion, manifesting itself at the surface by an uplift. 
By the expansion of the melting underlayers tensile stress in the 
overlying strata would be called into play, and this would throw 
these strata into a state of tensile strain, thus giving origin to normal 
faults (to account for the formation of which tensile stress must on 
any theory be called into play), to the gradual gaping of mineral 
veins, and to dykes into which the molten matter would be injected 
by the expansive force. 
Without denying as a factor that secular refrigeration on which 
Mr. Prestwich relies (Geology, vol. ii. p. 216), I would suggest 
that we have on this hypothesis an efficient primary cause of volcanic 
eruptions. In this way lava is pressed upwards towards the surface. 
The expansion of the contained water vapour does the rest of the 
business. 
It is clear that the process I have here suggested would be partially 
checked on the one hand by the assumed rise of the isogeotherms 
beneath the subsiding area, such rise, primarily due to sedimentation, 
being increased by the latent heat rendered sensible during solidifi- 
cation; and on the other hand by the converse depression of the 
isogeotherms and rendering latent of the heat of fusion beneath areas 
of denudation. 
In the area of subsidence lateral pressure would to some extent 
be brought to bear in aid of vertical pressure ; for it is evident that 
such subsidence is equivalent to the flattening of a portion of the 
