PRESIDENT’S ADDRESS—SECTION C. 83 
two miles, and the level of greatest cooling 54 miles below the 
surface. The radial contraction would have been six miles, and 
the mean height of the surface elevations formed by compression 
would be 64 feet. But if the temperature of consolidation was 
4000° F., the level of no strain would be three-quarters of a mile, 
and the level of greatest cooling 31 miles below the surface. 
The radial contraction would have been two miles, and the mean 
height of the surface elevations only eight inches. 
These calculations assume that the surface maintains a con- 
stant temperature ; but as this temperature depends upon the 
sun, and as astronomers assure us that the sun is cooling, the 
surface of the earth must be cooling also; so that the level of no 
strain must be less than the calculated distance—that is less 
than two miles, probably less than one mile below the surface. 
Four other objections can now be brought against the con- 
traction theory. (1) It cannot explain the fact that rocks have 
been depressed far below the level of no strain, and brought up 
again. (2) It cannot explain the contortion of a series of beds far 
thicker than the whole shell of compression. (3) The contortions 
do not resemble those produced by a lateral thrust, which is 
greatest on the surface and diminishes downwards. (4) The granitic 
and gneissic cores of mountain ranges could not have been forced 
up by so superficial a cause. Professor Claypole notices these 
objections, and, curiously enough, supposes that the depth of the 
level of no strain has been miscalculated. But this is not 
possible. Two distinct lines of reasoning lead to the same result; 
and, indeed, the contraction theory had been virtually slain by 
Captain Dutton and Mr. Fisher before the existence of the level 
of no strain was discovered. Professor Claypole argues that as 
the centra of earthquakes are sometimes twelve miles or more 
deep, therefore the level of no strain must be more than twelve 
miles deep. This would be true, provided these earthquakes were 
necessarily due to compression caused by contraction. But 
tensile strains, which are relieved suddenly, are much more likely 
to produce earthquakes than compressive strains, and consequently 
the level of no strain lies probably above the earthquake region. 
Mr. Davison has suggested a test by which the contraction 
theory may be tried, although it is one difficult to apply. He 
says that the depth of the level of no strain varies as the square 
root of the time since consolidation, while compression varies 
nearly inversely as the square root of the time, so that folding by 
compression ought to have been much more rapid during the 
early stages of the earth’s history than during the latter, and 
the amount of rock folded in any given time ought to decrease 
nearly in proportion as the square root of the time increases. 
Satisfactory evidence to test this deduction is not, perhaps, 
available at present, but the fact that there are extensive regions 
of the earth’s surface which have never been folded since the 
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