by Sedimentary Loading and Recurrent Expansion. 493 



their bulk. From a great number of experiments made by 

 me on sandstone, slate, limestone, and marble, I have estimated 

 the coefficient of expansion of average rock at 2*75 feet lineal 

 per mile for every rise of 100° Fahr. ; but there is every 

 reason to believe that the coefficient of expansion rises at 

 higher temperatures than those at which my experiments 

 were conducted. It has been urged by some of my critics 

 that I have not allowed for the compression of the sediments 

 filling the earth-trough into denser masses, but have credited 

 all the expansion to mountain-building *. It has been over- 

 looked that I have already explained that the weight of the 

 mass alone will, by compressive extension, consolidate the 

 beds below by reducing their thickness. Also the denser 

 sedimentary rocks are often denser only by infiltration. This 

 is particularly the case with sandstones, where the conversion 

 into the final stage of quartzite is by the deposit of secondary 

 silica in the interspaces of the grains, not by condensation. 



Clays contract on heating ; but, according to my views, 

 the contraction of such beds in an earth-trough will be vertical 

 only, by reason of the superimposed weight. A stage of 

 metamorphism is at least arrived at, as we see in clay-slate, 

 when the materials of that rock, originally clay, become meta- 

 morphosed so as to behave like other rocks, and expand with 

 a rise of temperature. 



Even if these criticisms possess much force, they do not 

 apply to the rocky crust of the Earth already consolidated 

 forming the earth-trough in which the sediments are laid 

 down. There will be little or no loss by condensation in them, 

 only straining or change of form. It is obvious that deep- 

 seated rocks must be so compressed by simple gravitation, 

 that lateral pressure will have little effect in further con- 

 densing them. 



Recurrent Expansion Cumulative in its Effects. 



If a given area of the crust of the Earth is raised in tempe- 

 rature, when the limit of elasticity is reached the surplus 

 material must be disposed of by a change of form : it will rise 

 in the line of least resistance. 



Assume that the surplus due to the cubical expansion of a 

 horizontal sheet is thrown into a ridge, and that then a fall of 

 temperature takes place to the same extent. The material 

 ridged up can never be drawn back again ; it becomes a per- 



* Hutton, Presidental Address, Section C, Melbourne Meeting of the 

 Australasian Association for the Advancement of Science, p. 89. 



