292 Prof. C. Lloyd Morgan—Elevation and Subsidence. 
uplift manifests itself in the mountain-building at the close of sedi- 
mentation. It is difficult, however, to understand why the effects 
should be so long deferred. 
As a corollary from the rise of the isogeotherms beneath areas 
undergoing sedimentation, we have the depression of the isogeotherms 
beneath areas undergoing denudation. Here, therefore, cooling 
should produce contraction, and there should be subsidence. ‘This is 
not in accordance with observation. 
(3.) The intimate connection between subsidence and sedimenta- 
tion on the one hand and elevation and denudation on the other hand 
has often been insisted on. This connection may be regarded in two 
ways. Many geologists maintain that denudation is consequent upon 
upheaval, and that continued deposition is consequent upon (or is 
conditioned by) continued subsidence. ‘To others, however (and I 
count myself among the number), this mode of looking at the facts 
is not wholly satisfactory. They regard the uplift as in some way 
the direct result of the lightening of the load by denudation, and the 
subsidence as the direct result of the added load by sedimentation. 
(a) Those who hold the latter view would seem generally to attri- 
bute the elevation and subsidence to the mere weight of the material 
added to or removed from a flexible crust resting upon a fluid or 
viscous substratum. 
(b) Mr. O. Fisher has suggested (Proc. Camb. Phil. Soc. vol. vi. 
pt. 1) that water gas may be dissolved in molten rock just as carbonic 
anhydride may be dissolved in water. Applying Henry’s law of 
the absorption of gases, he considers that on the pressure being 
relieved from that required for saturation, vesicles of gas may 
separate from the solvent and may coalesce and rise through the 
magma expanding as they reach regions of diminished pressure. 
From this expansion an uplift of the overlying area would result. 
(c) Prof. Joseph Le Conte has suggested (Nature, vol. xxix. p. 213) 
that the principle of flotation comes into play. He assumes that the 
crust of continental areas is more conductive and therefore cools and 
thickens more rapidly than that of oceanic areas. Thus inequalities 
of thickness of the crust would result, and by flotation these in- 
equalities, produced in this way on the under side next the liquid 
substratum, would be reproduced on the upper side next the 
atmosphere. 
Whether such flotation would take place depends upon whether 
the solid produced by cooling is heavier or lighter than the liquid 
from which it is derived and on which it rests. Now, although 
there is a want of agreement as to the amount of contraction which 
rocks undergo on solidification and crystallization, it is well nigh 
universally admitted that solidification does involve such contraction 
and increase of density. The thickened area would therefore tend 
to subside rather than to be upheaved. 
(d) It is the object of this paper to suggest other ways in which 
the loading and unloading of the earth’s crust may indirectly bring 
about subsidence and elevation. I must, however, first ask and seek 
to answer one or two preliminary questions. 
