Origin of Continents. 
479 
Furthermore, it is believed, as explained in another place, 1 
that some estimates of the amount of shortening of the outer 
part of the earth have been too great. 
As a result of the contraction of the interior of the earth and its 
change of form, due to the causes given, and doubtless others, the 
outer part becomes too large to fit the inner part. Pulled down by 
gravity, enormous lateral stresses are set up, which could not 
be resisted by the rock, though it were 30 times as strong 
as the finest steel, or from 600 to 1,000 times as strong as 
granite. 2 Consequently the outer part adjusts itself to the les¬ 
sened nucleus by various earth movements. 
However, the above changes, so far as they occurred before 
the formation of the outer solid crust of the earth, can have 
produced no effects which would be permanently retained. Af¬ 
ter such a crust was formed they would all result in epiorogenic 
and orogenic crustal movements. 
Origin of Continents. — Dana 3 and Gilbert, 4 assuming that the 
earth in the remote past was in a liquid condition, have sug¬ 
gested that the continental plateaus originated at places where 
the molten lava first began to solidify. These would probably 
be places of low heat conductivity. As the rock changed 
to the solid condition it contracted 5 and sank below the 
liquid surface, which in turn solidified and sank. Much of 
this material which passed below the surface might have 
been re-melted, but by the long-continued process at last a 
solid mass was formed, which extended from the surface far 
below. This solid mass being built up of successive portions of 
the outer part of the liquid world would be lighter than the re¬ 
mainder, and therefore would project above it. Even if solidifi- 
1 Estimates of crustal shortening, and causes for the same, by C. R. Van 
Hise: Journ. of Geol., Vol. VI, Jan.-Feb., 1898. 
2 Some mechanical conditions of the earth’s mass, by R. S. Woodward: 
Phil. Soc. Washington, Vol. XI, 1888-91, p. 532. 
3 Manual of geology, by J. D. Dana: 2nd ed., 1874, p. 738. 
4 Continental problems, by G. K. Gilbert: Bull. Geol. Soc. Am., Vol. 4, 
1893, pp. 183-190. 
5 Report on earthquakes and volcanic action, by Wm. Hopkins: Brit. 
Assoc. Rep., 1847, p. 46. Treatise on natural philosophy, by Thompson 
and Tait: new ed., Part II, 1890, p. 483. Popular lectures and addresses, 
by Sir Wm. Thompson: Vol. II, 1894; p. 306. 
