EARTH’S CRUST RESULTING FROM SECULAR COOLING. 
241 
that many geologists are of opinion that the great continents have always been more 
or less in their present positions. 
(22) Now, 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 stretching 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 stretching 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 beneath 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 permanence 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 ineffective. 
(23) Again, the amount of crust-stretching by lateral tension being greatly in 
excess of the amount of crust-folding by lateral pressure due to secular cooling, it 
follows that folding beneath the ocean-bed will do little but diminish its rate of sub¬ 
sidence. The effects of folding in changing the form of the Earth’s surface features will 
therefore be most apparent in the continental areas, especially in those regions where 
the change of vertical pressure above the folded layers diminishes most rapidly, i.e., 
near the coast-lines where the slope towards the ocean depths is greatest. It is 
perhaps worthy of remark that these are the districts where earthquake and volcanic 
action are now most prevalent.! 
In the coast regions, moreover, the products of continental denudation are chiefly 
deposited, and the rock-folding due simply to secular cooling produces in vast masses 
of sediment still more crushing and folding. | The direction of the folds will be 
perpendicular to the average slope of the surface above them, i.e., they will generally 
be parallel to the coast-line. Hence the continents will grow by the formation of 
mountain chains along their borders. 
(24) In a given time, the amount of rock-folding resulting from secular cooling w T as 
greatest in the first epochs of the Earth’s history, and diminished as the time increased. 
It does not necessarily follow that the early mountain ranges were the loftiest and most 
massive, but probably they were ; and very possibly also the displacement, by crushing 
and folding, of two neighbouring portions of rock was greatest in early times. But, 
* ‘ Phil. Trans.’, 1879, pp. 589, 590. 
t Professor J. Milne, “Note on the Geographical Distribution of Volcanoes,” ‘ Geol. Mag.,’ vol. 7, 
1880, pp. 166-170. 
f Professor J. D. Dana thus summarises the history of the Alleghany and other mountain-chains :— 
“ First, a slow subsidence or geosynclinal .... and, accompanying it, the deposition of sediments to a 
thickness equal to the depth of the subsidence; finally, as a result of the subsidence, and as the climax 
in the effects of the pressure producing it, an epoch of plication, crushing, &c., between the sides of the 
trough.” ‘ Phil. Mag.,’ vol. 46, 1873, p. 49. 
MDCCCLXXXVII.—A. 2 I 
