84 PRESIDENT’S ADDRESS—SECTION C. 
commencement of the Paleozoic era seems hardly consistent with 
it, and I think that most geologists would allow that since the 
close of the Jurassic period rock-folding has been quite as active 
as during any former period of equal length. But however this 
may be, the contraction theory is evidently inadequate to 
explain the formation of geosynclinals and synclinoria, and it 
cannot therefore be the true explanation of mountain ranges, 
while it has always been thought incapable of explaining con- 
tinental elevations. Indeed, its effects must be so insignificant 
that they may be dismissed from our consideration ; for, except 
in the deep ocean beds, they must be quite obliterated by 
denudation and deposition. 
Gradation Theory.—Another way in which the equilibrium of 
the earth’s crust is disturbed is the removal of rock by wind or 
running water, and its deposition in another place. The theory 
which finds in this the explanation of surface movements may 
be called, if we adopt Mr. W. M‘Gee’s terminology, the Gradation 
Theory. The transterence of matter acts in two ways; first, by 
altering the load on two portions of the earth’s crust, and 
secondly, by changing the positions of the isogeothermal surfaces 
in the earth, or, in other words, by altering the temperature of 
portions of the interior. Although the Gradation Theory includes 
the combined effects of both these reactions, it will be better to con- 
sider each separately, and I will take first the alteration in load. 
This idea was first broached by Sir J. Herschel in a letter to 
Sir C. Lyell, writtenin 1836, but he did not support it by any 
geological evidence. He says, supposing the earth’s crust to float 
on a sea of lava the effect of transference of pressure brought 
about in this way would be an extremely minute flexure of the 
strata ; but, supposing the layer next below the crust to be partly 
solid and partly fluid, composed of a mixture of solid rock, liquid 
lava, and other masses in various degrees of viscidity and 
mobility, great inequalities might subsist in the distribution of 
pressure, and the consequence might be local disruptions of the 
crust where weakest, and escape to the surface of lava. At 
a later date, in his Physical Geography (1861), he speaks more 
strongly in favour of the theory, and says that any amount of 
pressure and relief which the geologist can possibly require to 
work out his problems are available. The theory was supported 
by Professor James Hall in 1859, and has been widely accepted 
in America, as well as by several French geologists. In England 
it was advocated by Dr. C. Ricketts in 1871, and lately the Rev. 
O. Fisher and several others have written in its favour. In 1845 
Sir C. Lyell added the idea that the depression of a convex 
surface, like that of the earth, would produce foldings and 
crumblings. 
The principal evidence in favour of subsidence being caused by 
deposition is the fact that, at the mouths of large rivers, the 
