the Earth in Relation to Mountain-Building. 213 



shell — of compression greatest at the external surface, gradually 

 diminishing downwards until it is nil at the under surface, 

 and an inner shell of much greater volume, in which the beds 

 are all in a state of tension by contraction, the contraction 

 being greatest at the zone of greatest cooling, and nil on the 

 inner and outer surfaces of the shell. Hence it follows that 

 at the zone of contact of these imaginary shells the rock will 

 be neither subject to tension nor compression. 



Below these shells exists an uncooled nucleus, geologically 

 speaking, of hitherto nearly constant volume. 



In a recent paper* Mr. C. Davison arrives at practically 

 the same results, although he makes no reference to my work, 

 which was published in 1886. If Mr. Davison reached his 

 results without knowing of mine it strengthens my case 

 considerably, and I am glad to have confirmation from so able 

 a mathematician. 



But while we are practically agreed as to the strains set up 

 in the crust of a cooling earth, we entirely differ as to tbeir 

 geological consequences. Without committing myself to Mr. 

 Davison's numerical results, I will for present purposes accept his 

 calculation that the shell of compression is now five miles deep. 



I confess I cannot follow Mr. Davison in considering that 

 " owing to the pressure of the continental masses, crust- 

 stretching by lateral tension takes place principally beneath the 

 ocean-basins." Assuming that the isogeotherms follow ap- 

 proximately the surface-contours of the land, whether beneath 

 or above the level of the oceans, the greater pressure will be over 

 the ocean-basins, as the weight of ocean-water will be addi- 

 tional to that of an equal depth of crust. It is, however, quite 

 as probable that the cooled crust is thicker under the oceans, 

 and such is assumed by many physicists. It also appears to me 

 to be a physical misconception to suppose that a portion of 

 the earth's crust would stretch like a cooling bar of iron held 

 firmly in a vice at either end, or that the pressure of the con- 

 tinental masses would take the place of the vices by holding 

 down the contracting body. I have tried to form a mental 

 picture of the relation Mr. Davison wishes to show existing 

 between coast-lines, earthquakes, sedimentation, and mountain- 

 building, but must confess my inability to do so with the aid 

 only of the abstract. 



The statement that " the rate of mountain-making dimi- 

 nishes with the increase of time " is opposed to all geological 

 fact. The greatest mountains are the youngest ; and Mur- 



* " 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." Received by the Royal Society, 

 April 7, 1887, and just published in abstract. 



