Growth of Continents. 
505 
ial as compared with the sea bed, solidification would go on 
more rapidly at the latter place than at the former, until finally 
the thickness of the solidified material below the sea beds might 
approximate to that below the continental areas. But as rocks 
solidify, and especially as they crystallize, they contract a very 
considerable amount. 1 Barus 2 has shown this combined contrac¬ 
tion for diabase, a rock of average composition, to be about 12 
per cent. Furthermore, it has been supposed that the regions 
below the sea have a higher conductivity than the continents and 
their downward extensions. If this be so, the former regions 
would ever continue to coo] more rapidly than the latter, and would 
contract more. 3 Both of these contractions would result in 
concentration of the rocks of the sea bed and in bringing them 
nearer to the center of the earth than the continental masses. 
Hence gravity would be more effective on the mass below the sea 
than elsewhere, and differential stresses would result. The quanti¬ 
tative value of the increased effectiveness of gravity on the mass 
of the sea beds should be estimated upon various numerical 
suppositions, but the amount of contraction in cooling, solidifi¬ 
cation, and crystallization is so great that it can hardly be 
doubted that its effectiveness would be considerably increased. 
As a result of the unequal contraction the continental ship 
would be no longer in isostatic equilibrium. Differential vert¬ 
ical movement would be set up, and material below the sea 
areas would tend to flow toward the land areas, and thus tend 
to elevate the continents, but the average of the movements 
would be downward. Whether the continents absolutely or 
relatively rise under these stresses would depend upon the aver¬ 
age amount of contraction of the earth. It is probable that 
this contraction would more than counteract the tendency to 
uplift, and therefore that relative elevation, and not absolute 
1 Manual of geology, by J. D. Dana: 4th ed., 1895, pp. 264-265. 
*The contraction of molten rock, by C. Barus: Am. Journ. Sci., 3rd 
ser., Vol. 42, 1891, pp. 498-499. 
8 Davison expresses the idea that early in the history of the earth below the 
level of no lateral stress “ crust stretching by lateral tension must have 
taken place chiefly beneath the ocean basins, deepening them and intensi¬ 
fying their character.” (Phil. Trans. Royal Soc., Vol. 178, 1887, p. 241.) 
