On the Formation of Mountain Chains. 515 



According to the computations of Thomson,^ wo may assume that 

 at the close of ten thousand years after solidification of the surface of 

 the earth had taken place, the rate of increase in temperature would 

 be 2° Fahrenheit for each foot of descent, and with the lapse of time 

 the rate of increase in going towards the centre would be less and 

 less rapid in about the proportion indicated in the table below. 



10,000 years after freezing of surface, 2° Fahr. for each foot.^ 



40,000 „ „ 1 



160,000 „ „ \ 



4,000,000 



>j >> To " *' 



100,000,000 „ „ Vo 



If this calculation is correct, (and that it is in a general way cor- 

 rect, does not seem to admit of much doubt, provided we accept the 

 hypothesis of original igneous fluidity,) it follows that the gradual 

 cooling of the deeper portions of the crust must result in the forma- 

 tion of a strong lateral pressure at every point near the surface. The 

 truth of this proposition is readily seen, when we consider that while 

 the original surface, which in ten thousand years after the hardening 

 of the crust had been reduced to the temperature of the atmosphere, 

 retained the same temperature in the ages which followed, the por- 

 tions of the crust beneath were constantly parting with their heat, 

 and approaching nearer to the thermal condition of the surface. 

 There would be no shrinkage of the surface layer from the loss of 

 heat, while from this cause the contraction of the deeper portions 

 would be considerable. This would give precisely the conditions 

 requisite to produce a rending and upfolding of the superficial strata 

 of the outer shell. Immediately after the formation of a crust, the 

 progressive diminution of the interior heat would begin to produce a 

 tension on the surface, which would augment as the ceaseless flow of 

 heat went on, until either a rupture of the contracting beds, or the 

 folding together of the superficial layers, relieved the strain. Both 

 these methods of accomplishing the movement of contraction, have 

 been most probably operative at difierent times and places in the 

 earth's history. Furthermore, as the upper portions of the crust, or 

 region of slight contraction, is of much less thickness than the region 

 which, by its considerable contraction, produced the tension, we 



' Thomson (Wm.) on the Secular CooHng of the Earth. Trans. Eoy. Soc. Edin- 

 burgh, xxxiii. sec. 1. 



2 The effect of these changes in temperature may be estimated from the following 

 table of the expansion of various substances under the influence of heat : 



For each degree of Fahrenheit, 



Granite expands about 000004825 



Marble „ „ 000005662 



Sandstone „ „ 000009532 



A stratum of granite five hundred miles in diameter would contract, on passing 

 from a temperature of 3,000 degrees Fahrenheit to the average temperature of the 

 earth, about seven and a half miles ; in the case of a sandstone area of the same 

 diameter, the contraction would amount to about fifteen miles. 



The computations on which these estimates are founded were based on experiments 

 made by Mr. H. C. Bartlett, of the United States Engineers, and published in the 

 Amer. Jour. Science, vol. xxii. p. 136. See also for other data on this point Ninth 

 Bridgewater Treatise, C. Babbage, 2nd edit. Appendix, p. 221. 



