1887.J Distribution of Strain in the Earth 9 8 Crust. 



327 



conclusions are given for their qualitative rather than their quantita- 

 tive value. They depend also on the assumption that the earth's 

 surface is smooth and spherical. 



The following laws are also shown to be approximately true : — ■ 



1. The depth of the surface at which folding by lateral pressure 

 vanishes, and the depth of the surface at which stretching by lateral 

 tension is greatest, both increase as the square root of the time that 

 has elapsed since the consolidation of the globe. 



2. Folding by lateral pressure was effected most rnpidly in the early 

 epochs of the world's history as a solid globe, and, since then, the 

 total amount of rock folded in any given time decreases nearly in 

 proportion as the square root of the time increases. 



3. A law, similar to No. 2, for stretching by lateral tension. 



II. The Rev. 0. Fisher s Argument on the Insufficiency of the Contraction 



TJieory. 



The argument is described (see ' Phil. Mag.' for Feb., 1887). It 

 is shown to be inconclusive on the following grounds : — 



1. It assumes that the cooling of the earth to its present condition 

 was instantaneous. 



2. If instantaneous cooling were possible, there would, it is shown, 

 be no folding at all, but only stretching by lateral tension. 



3. It assumes that the earth's surface was initially smooth and 

 spherical, whereas Professor B. Peirce and Professor Gr. H. Darwin 

 have both shown that vast continental wrinkles would be formed on 

 the surface of a once viscous earth by the diminishing velocity of 

 rotation resulting from tidal friction. 



III. The Effects of Crust-stretching and Folding on the Evolution of the 



Earth's Surface-features. 



1. Owing to the pressure of the continental masses, crust- stretching 

 by lateral tension takes place principally beneath the ocean-basins, 

 therefore deepening them and contributing to their permanence. This 

 effect must have been greatest in early geological periods, when the 

 surface of greatest stretching was close to the surface of the earth. 



2. In another part of the paper it is shown that the amount, of 

 crust- stretching is considerably greater than the amount of crust- 

 folding, due directly to secular cooling. Folding beneath the ocean- 

 bed will therefore do little but diminish its rate of subsidence. The 

 effects of folding in changing the forms of the earth's surface features 

 must be most apparent in continental areas, especially along those 

 coasts where the slope towards the ocean-depths is most rapid (i.e., 

 in the districts where earthquake and volcanic action are known to be 

 most prevalent). In the coast regions, also, the products of conti- 



