82 
GEOPHYSICS: G. F. BECKER 
very considerable intumescence or uplift would result and an additional 
diminution in the diffusivity, because joints interfere with the conduction 
of heat. This diminution would further increase the depth to which 
sensible differences in temperature on the same horizontal plane would 
extend. Besides systematic rupture it is evident that the lateral pres- 
sures on the square column would or might bring about deformations 
and the crumpling of layers originally plane. 
It would seem then that the initial difference in diffusivity need not 
be great eventually to ensure a considerable upHf t within the hypothetical 
square since so soon as it sufhced to estabHsh a temperature-difference 
of a few degrees the process of upHft would be increased by the effects 
of rupture. 
Supposing no water to exist upon the earth, the square column under 
discussion might attain a notable elevation. The average land surface 
now stands about 4 km. or two and a half miles above the average sea 
bottom and if the material from which the salt of the ocean has been 
derived represents eroded continents of present area, the average land 
surface might have stood some 2.5 km. higher than it now does. Whether 
so lofty a mass could sustain its own weight need not be discussed here, 
the present object being merely to bring out a particular feature of the 
whole problem. 
The mere configuration of the present continents standing at a mean 
elevation of nearly 13,000 feet above sea bottom manifestly represents 
a vast amount of energy of position, or potential energy, and to this must 
be added that of the total mass which has been eroded from the conti- 
nents, something Hke half as much as now remains. (Thus, according 
to the computations made by F. W. Clarke in his Data of Geochemistry. 
the sodium of the ocean represents a spherical shell 2050 feet in thickness. 
If this is supposed piled onto continents of an area one-fourth that of the 
globe, its thickness would be 8200 feet, or somewhat more than one half 
of 13,000 feet.) 
The internal temperature of the continental mass likewise calls for 
consideration. At the present day the mean depth of the ocean is 
about 3496 m., the means elevation of the land is about 440 m. 
and the mean thermometric gradient 1° in 38 m. or possibly as 
high as 1° in 32 m. The temperature at the bottom of the sea is not 
far from zero, while at the same level beneath the continents it is over 
100°. Thus the mean temperature of the actual continents down to 
the level of sea bottom is more than 50°C. above the temperature of the 
sea-bottom itself, while for a long distance below this level the sub- 
continental masses must be hotter than the sub-oceanic layers. (Even 
