482 
Van Else—Earth Movements. 
the slower contraction of the continental regions than of the 
oceanic regions because of the lower conductivity of the former, 
and with the effects of changing rotation. As has been seen, as a 
result of decreased speed of rotation there is decreased oblateness, 
and increased density caused by the increased effectiveness of 
gravity. Both of these effects require very important adjust¬ 
ments of the outer part of the earth. The actual work of the 
tides may give a direction to these adjustments in the manner 
explained by Darwin, and thus the forces of changing rotation 
concur with the immediate stresses of tidal movements in form¬ 
ing differences in elevation between the oceanic basins and the 
continental plateaus, the two being in approximate equilibrium. 
Davison 1 further suggests, in support of Darwin’s theory, that 
“Soon after the formation of these wrinkles, that is, in 
the initial period of the Earth’s history as a solid, or nearly 
solid, globe, the unstrained shell must have been very close 
to the surface of the Earth, and the surface of greatest stretch¬ 
ing also so near to it that stretching by lateral tension must 
have affected the form of the surface features. But, owing to 
the pressure of the continental wrinkles, the amount of stretch¬ 
ing under them must have been very much less than under 
the great oceanic areas. Thenceforward, therefore, crust¬ 
stretching by lateral tension must have taken place chiefly be¬ 
neath the ocean basins, deepening them and intensifying their 
character. And, in leading to the continual subsidence of the 
ocean-bed, it is evidently a physical cause of the general per¬ 
manence of oceanic areas: a cause, it is true, continually 
receding from the surface, and diminishing in intensity with 
the increase of time, but probably even now not quite inef¬ 
fective. ” 
The above suggestions as to the origin of the continental 
masses are not all exclusive of one another, but on the contrary 
several of them to a large extent supplement one another. How¬ 
ever, the explanation must be considered as provisional and 
1 On the distribution of strain in the earth’s crust, resulting from secu¬ 
lar cooling; with special reference to the growth of continents and the 
formation of mountain chains, by Charles Davison: Phil. Trans. Roy. 
Soc., Vol. 178, Part A, 1887, p. 241. 
