Shaler.] 12 [June 6, 



"While the contour of the continental fol'ds, as exhibited both in 

 land surface and sea floors, evinces the gradual operation of the gen- 

 eral contraction of the earth on a crust of great thickness, we have 

 in mountain chains another effect of contraction, which can not, from 

 the evidence, be properly referred to the shrinicing of the whole 

 mass. It is evident that if tlie continental folds are compensative 

 wrinkles formed in the adaptation of a crust to a diminished nucleus, 

 the mountain chains can not be of the same nature ; it is not to 

 be believed that a crust would bend from the action of the same force 

 into the broad, low curves of the continents, and into the sharp 

 defined and narrow fractures of a mountain range. 

 i Accepting, as established, the fact that mountain chains are the 

 result of lateral pressure, and indirectly of contraction from loss of 

 heat, and denying that they are the result of the accommodation of the 

 crust to the nucleus, it is at once manifest that we must seek their 

 origin in the changes going on within the crust itself, and in no way 

 connected with the regions below. And within that crust we can find 

 forces operating to produce contraction quite sufficient to account for 

 all the facts. 



According to the computations of Thompson,* we 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° Fah. for each foot-f 



40,000 " " 1 " " 



160,000 " « I " « 



4,000,000 " " ^\ " « 



100,000,000 « « -JL " « 



* Thompson (Wm.) on the secular cooling of the Earth. Trans. Eoy. Soc, Edin- 

 burgh, xxm, Sec. 1. 



t The effect of these changes in temperature may be estimated from the follow- 

 ing 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 experi- 

 ments 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, 2d Edit., Appendix, p. 221. 



