PRESIDENTS ADDRESS—SECTION C. 95 
diminish towards the centre, where it will vanish. It is hardly 
possible that the crust is sufficiently strong or sufliciently elastic 
to resist all outward movement ; far more likely it yields to the 
pressure, and in that case it seems probable that, owing to 
unequal yielding of the crust, slow currents might be set up in 
the fluid layers immediately underlying it. The daily oscillations 
of the pendulum observed by Professor Milne in Japan, and by 
M. Plantamour on Lake Geneva, may, in fact, be due to tidal 
pulsations.* 
The short period semi-diurnal tides must cause confused 
movements, if any, but the longer period lunar fortnightly and 
19-yearly tides, as well as the solar semi-annual and annual 
tides, would tend to produce steadier currents. However, the 
effects of these bodily tides must be very small. 
If internal currents really exist it seems probable that eleva- 
tion and subsidence of the surface would follow on the principle 
stated by Professor Phillips. If movements take place in an 
internal fluid which increases in density downwards, the mean 
density along any one radius would be variable, and, as the mass 
along each radius would remain nearly constant, the length of 
the radii would vary or would try to vary. Now, as the tempe- 
rature of fusion, as well as the conductivity, is different in 
different rocks the solid crust cannot have a uniform thickness ; 
and as the strength or coherence of a rock is independent of both 
those properties, the surface of the fluid interior must be opposed 
by unequal resistances. This being so, suppose a slow current of 
less dense and more superficial material to set from A to B, and 
a deeper return current of denser matter, but of smaller volume, 
to set from B to A, then there will be a tendency to elevate the 
crust in the neighbourhood of B and to depress it in the neigh- 
bourhood of A. If we further suppose that at some place in the 
neighbourhood of B the crust was weaker than elsewhere, then 
that part would be more elevated, and the current of superficial 
molten matter would set to it. The position of this weak place 
might have been determined by previous sedimentation having 
raised the melting point in the old crust, while the newer 
sediments were not so consolidated as the old ones, as suggested 
by Dr. Sterry Hunt and Professor Dana. Or, as suggested by 
Mr. Fisher, contraction of the lower sedimentary rocks by 
metamorphism might have formed fissures into which the sub- 
jacent molten rock could force its way. If the crust was 
sufficiently strong to bear the strain, regional elevation would 
take place, or a mountain range of the Uinta type might be 
formed. But if the crust gave way the molten matter would be 
forced into the fracture, would crush and contort the rocks on 
each side, and a mountain range on the alpine type would be 
* Since this address was delivered I have learnt that Mr. H. C. Russell has observed earth 
pulsations in Sydney. (Vide Roy. Soc. N.S.W., vol. XIX., 1885, p. 51). 
