PRESIDENTS ADDRESS—SECTION C. 81 
Journal of Science, and, independently, by myself in the Geological 
Magazine. It was pointed out that horizontal thrusts through 
more than a hundred miles of rock were impossible ; that the 
theory gave no explanation of tension in rocks, everything 
being done by compression, while normal faults proved that 
rocks nearly everywhere had undergone tension; that long 
mountain chains with parallel foldings, such as actually exist, 
could not be the result of a collapsing spherical shell, for that 
would give rise to a network of small hills. It was also pointed 
out that the theory failed altogether to explain continental 
elevations, as well as the numerous oscillations in level that had 
occurred in many parts of the world ; and, finally, that as the 
cooling could not have penetrated more than two or three 
hundred miles below the surface, the main body of the earth 
was as hot as ever, so that there was no shrinking nucleus for 
the crust to adjust itself to. Then, in 1881, the Rev. O. Fisher 
calculated that, supposing the earth’s crust to have solidified 
at a temperature of 7000° F., the elevations caused by subsequent 
contraction would average eight or nine hundred feet; while 
if the temperature of solidification was 4000° F., then the average 
height of the elevations would be less than two hundred feet. 
But the average height of the actual inequalities of the earth is 
certainly not less than 9500 feet, so that they cannot be explained 
as the result of contraction. Prevost, however, recognised that 
the oceanic depressions could not be due to tangential thrust, 
although he gave no clear explanation of them. Professor 
J. D. Dana, in 1847, first put forward the idea that they were 
due to unequal radial contraction during cooling, and in this he 
was supported by Archdeacon Pratt and Mr. Robert Mallet. 
There are, no doubt, geological reasons for thinking that the land 
area has been increasing, and that the ocean bed has been 
getting deeper since Paleozoic times; but in 1881 Mr. Fisher 
calculated that the mean radial contraction could not have been 
more than two miles, so that a differential contraction of three 
miles, which is the average depth of the ocean, was not probable. 
It was, however, reserved for Mr. Mellard Reade to give the 
contraction theory its death blow. In his “ Origin of Mountain 
Ranges,’ published in 1886, he pointed out that only a very 
small depth of the crust was subject to compression, and that in 
this thin layer the compression must be greatest at the surface 
and diminish downwards untila level of no strain was reached. 
below which the crust must be in a state of tension. 
The reason for this is easy to see. If we suppose the earth 
to commence cooling by radiation from a melted condition, it is 
evident that the cooling will be most rapid at the surface, for 
the greatest differences in temperature are there. This will 
continue until the surface approaches the temperature due to 
the radiation of heat from the sun, when the shell of greatest 
F 
