Yol. 51.] THE MEAN RADIAL VARIATION OF THE GLOBE. 99 



13. On the Mean Radial Variation of the Globe. 

 By J. Logan Loblet, Esq., F.G.S. (Read January 23rd, 1895.) 



[Abstract.] 



The Author submits considerations (chiefly derived from the 

 characters of the earlier sediments) which lead him to suppose that 

 crust-folds have not been produced by continuous contraction of 

 the Earth, and that the planetary heat and mean radius of the 

 Earth have been practically invariable during the period which has 

 elapsed since Cambrian times. 



Discussion. 



Prof. Hughes pointed out that the criticisms and speculations of 

 the Author involved the assumption that the rocks of a contorted 

 series once occupied the area that would now be covered by them if 

 laid out evenly. But he thought that the examination of folded 

 rocks in small specimens or in mountain-masses showed clearly that 

 the larger surface was produced at the expense of their thickness, 

 and that in many cases they never extended over a larger area than 

 they do now. 



JJr. Du Riche Preller said that, to take an instance, the range 

 of the Alps represented an upward thrust of roughly 4 miles 

 in vertical depth, equal to about 0*1 per cent, of the radius of the 

 Earth. He asked the Author whether, in support of his theory, he 

 assumed that the upheaval of the Alps was simply compensated by 

 a corresponding sinking of 4 miles somewhere at the bottom of 

 the sea. The speaker contended that the Alps were the result of 

 lateral, horizontal pressure from north and south, which continued 

 in post-Cambrian and down to Tertiary times ; and, contrary to 

 the Author's theory, they were, like all mountain-ranges, due 

 to comparatively local folds produced by shrinkage of the earth's 

 crust. 



Prof. Hull admitted the Author's view that the temperature of 

 the outer crust had probably reached its normal state in Cambrian 

 times, but was unable to see how the phenomena of flexures and 

 foldings of strata, of faults, and of the fan-shaped structure of some 

 mountains could be accounted for, except on the supposition of 

 a somewhat rigid crust resting on a molten and continually con- 

 tracting interior magma. If, as all observations proved, the internal 

 heat was constantly being given off by conduction, or convection, 

 there must be simultaneous contraction of the interior magma. 

 This contraction would have the effect of bringing into play the 

 tangential forces of the outer crust which had arrived at a normal 

 condition of temperature, resulting in plications and similar 

 phenomena, lie thought that a diminution of temperature by one 

 degree of the interior magma would produce in a spheroid such as 



