FORCE ENGAGED IN MOUNTAIN-MAKING. 815 



Mr. G- H. Darwin, by using an equation for the cooling of "a solid extending to 

 infinity in all directions, on the supposition that at an initial epoch the temperature has 

 had two different constant values on the two sides of a certain infinite plane." deduced 

 by Sir W. Thomson in his paper on the Secular Cooling of the Earth, arrives at the 

 conclusion (see Nature for February 6, and American Journal of Science, April, 1879) 

 that the seat of the maximum rate of cooling moves inward as the time increases; and 

 that the equation, x 2 = SOOt, expresses the thickness {x) of the crust in feet in terms 

 of the time (t) of cooling in years; so that "if the time which has elapsed from the in- 

 itial state be 200,000,000 years, x =400,000 feet, or a little less than eighty miles." 

 He adds: "Sir W. Thomson shows in his paper on the Secular Cooling of the Earth, 

 that the solution of his ideal problem will be very nearly correct for the case of the 

 earth, which is supposed to be a hot sphere cooling by radiation. It follows, therefore, 

 from the numerical result which is given above, that the seat of the maximum ratio of 

 cooling must probably be something like one hundred miles below the earth's surface. 

 It does not of course necessarily follow that the seat of the maximum rate of contrac- 

 tion of volume should be identical with that of the maximum rate of cooling; yet it 

 seems probable that it would not be very far removed from it. The Rev. O. Fisher 

 very justly remarks that the more rapid contraction of the internal than the external 

 strata would cause a wrinkling of the surface, although he does not admit that this can 

 be the sole cause of geological distortion. Does it not seem possible that Mr. Fisher 

 may have underestimated the contractibility of rock in cooling, and that this is the sole 

 cause of geological contortion ? " 



2. Modifying Conditions. — Were the conditions of the globe 

 strictly equable in all respects there could have been no continents. 

 The circumstances which modified the action in progress include the 

 following : — 



(1.) The .Earth's flotation: Producing (a) a bulging over the equa- 

 torial regions of the surface-layer, and, to a less degree, of all the con- 

 centric beds of increasing density beneath it; (5) through the earth's 

 relation to the sun, a decreasing surface temperature from the equator 

 to the poles, becoming pronounced as solidification went forward ; and 

 (c), through relations to the sun and moon, tidal waves moving west- 

 ward, as long as the degree of liquidity was such as to admit of it. 



But the above-mentioned modifying circumstances, although dif- 

 ferentiating in their action, could not alone have led to the making of 

 continents. There must have existed, also, for this result — 



(2.) Differences between great areas in the Constitution of the 

 Earth's Material. — The fact that the continental and oceanic areas 

 were defined in the first cooling of the globe, signifies that in the cool- 

 ing, or the radiation of heat into space, there were areas of greatest 

 and least contraction. 



This difference in cooling and in the resulting level of the surface 

 must have been owing to some difference of quality or condition in the 

 material. One quality has been brought to light by pendulum exper- 

 iments in India, which, if general for the globe, as has been supposed, 

 is of great significance in this connection, namely : that, without ex- 

 ception, gravity is greater at the coast stations where observations 

 were made than at the continental stations, and greater at the island 



