GEOPHYSICS: a R BECKER 
85 
On the hypothesis that the origin of continents is due to the inferior 
diffusivity of certain areas of the earth's surface the condition of the 
ocean's bottom is very noteworthy. As is well known this floor is rel- 
atively featureless, consisting of vast plains, low ridges, and a few deeps, 
seven-eighths of its area lying at a depth of more than a kilometer below 
sea level. There are very few indications on the ocean floor of conti- 
nental topography, and yet if a continent were to be submerged to a depth 
of a hundred fathoms or more, that is below the reach of wave action, 
it is difficult to see how any process of base-leveling could reduce its 
accentuation. Neither on the hypothesis under discussion is it easy to 
see how a continent could be submerged though it is barely possible 
that a thin layer of rock of small diffusivity might be removed by erosion 
leaving exposed masses of diffusivity so high as to undergo rapid con- 
traction. Judging from the bathy metrical maps there are no important 
cases of this description, it would seem that, as the elder Dana so ably 
maintained, the oceanic areas have been persistent; and if so the sub- 
sidences which have occurred and recurred have been subordinate 
features of movements the net result of which in each case was uplift. 
This is in line with the results of Hayford, Helmert, and their associates. 
Since they have compelled us to concede that the earth is even now in a 
condition of approximate isostatic equilibrium, it seems impossible to 
believe that it has not been so in the past. Erosion has been in progress 
during every era from the Algonkian upward and there must have been 
a persistent and prevailing tendency to upheaval. Of a complete drown- 
ing of the continents, such as would occur during a prolonged era in which 
subsidence prevailed, there is now no trace. 
Two reasons have been suggested above for the high level at which the 
continents stand relatively to the ocean floor, viz., superior temperature 
and the existence of voids. The difference in level is 3.936 km. or 0.032 
of 122 km. If this difference were entirely due to excess of temperature, 
and if the linear expansion of average rock is 0.0008 per degree, the whole 
elevation of the continental columns would indicate a mean temperature 
difference of 40°C. If this elevation were due entirely to the existence 
of voids, these would amount to about half the maximum interstitial 
space found for me by Mr. Melcher in experiments on the crushing of 
sulphur in sealed brass tubes. It is evident that the two causes in com- 
bination might bring about elevations not only corresponding to the 
mean height of the continents but also to those of lofty mounta n ranges. 
Until erosion began the terrestrial mechanism must be regarded as a 
heat engine of the irreversible type. It could potentialize energy and do 
