S50 C. Davison — Least Age of the Stratified Rochs. 



the following laws are very nearly ti'ue, and that they do not depend 

 on the initial temperature of the earth (except in so far as the rate 

 of conductivity of rock may vary with the temperature) : — 



(a.) Folding by lateral pressure takes place only to a limited 

 depth from the earth's surface, below this depth giving place to 

 stretching by lateral tension ; and the depth at which folding 

 changes to stretching varies directly as the square root of the time 

 since consolidation. Also, the rate at which the unstrained surface 

 recedes from the surface of the earth varies inversely as the square 

 root of the same time. 



(&.) The total amount of rock folded in any given interval also 

 varies inversely as the square root of the time since consolidation.^ 



On both accounts, then, it would seem probable, though not, it 

 should be remembered, actually proved, that the mean rate of sub- 

 sidence, due to folding by lateral pressure, should vary according to 

 the same law. 



Assuming this to be the case, let the total amount of subsidence 



in t years be x feet, so that -r measures the rate of subsidence at 

 the time t. Also, let a feet be the average thickness of the stratified 

 rocks deposited throughout geological time, and T years the time 

 required for its deposition. 



Then, — =: -7= , where X is a constant ; and x = ^iXy/t, no 

 dt yt 



constant of integration being required, for x and t vanish together. 



But, when t =. T, we have x =: a, and — = --— , taking 

 the mean rate of subsidence at its highest value ; 



so that 



If we knew the average thickness of the sediment deposited 

 throughout the whole of geological time, the value of T obtained 

 from the last equation would give a lower limit to the length of 

 time required for the formation of the stratified rocks ; but unfor- 

 tunately, the necessary data are not to be had. The maximum 

 thickness of the sedimentary rocks has, however, been estimated by 

 Dr. S. Haughton and Prof E. Hull, at 177,200 feet ; and I cannot 

 think we shall be far wrong if we take this as the average thickness 



1 " On the Distribution of Strain in the Earth's Crust, resulting from Secular 

 Cooling ; with special reference to the Growth of Continents and the Formation of 

 Mountain Chains," read May 5, 1887. 



