486 Mr. T. M. Reade on the Origin of Mountain-Ranges 



fact seems to me never to have been fully realized by those 

 theorists who favour either of these views. 



The Nucleus of the Earth possesses a high Temperature. 



There is such a general consensus of opinion that the earth 

 at a depth of from 25 to 30 miles below the surface is at a 

 temperature equal to that of molten rock at the surface that 

 it is unnecessary for me to go over the arguments in favour 

 of this widely prevalent view. If we assume that it is so, a 

 very little calculation will show that matter at the depth of 

 say 30 miles is subject to an enormous pressure, to which we 

 can find no parallel by experimental methods at the surface. 

 30 miles = 158,400 feet ; so that if we estimate that a column 

 of the crust of the earth one inch square has a mean weight 

 per foot of 1*5 pounds, the pressure at the depth of 30 miles 

 will be in round numbers not less than 100 tons per square 

 inch, or 14,400 tons per square foot. It has been proved by 

 the experiments of the late Mr. Hopkins that there is in 

 certain solids a relation between the melting-point and the 

 pressure ; so that, if the rock at the depth of 30 miles is at a 

 temperature sufficient to melt it under ordinary pressures at 

 the surface, the additional pressure of 100 tons per square 

 inch may solidify it by raising its melting-point, or at least 

 render it plastic. If the pressure increase more rapidly than 

 the temperature as the earth is penetrated, what may be only 

 semi-solid at 30 miles may become rigid at greater depths. 



These points, from their nature, are incapable of direct 

 demonstration, but possess a high degree of probability. 



Shell of Greatest Mobility. 

 Although not accepting the hypothesis that there is a fluid 

 zone under the Earth's crust, it would follow from the pre- 

 ceding considerations that the shell occupying the space be- 

 tween the solid rigid crust and the compressed rigid nucleus 

 would respond to changes of pressure or temperature more 

 readily than either the crust or the nucleus. 



Facts of Physical Geology. 



All great Mountain- Ranges are composed of great thicknesses of 

 Sedimentary and other Deposits. 

 That all great mountain-ranges are composed of great thick- 

 nesses of sedimentary and volcanic deposits and igneous intru- 

 sions is a fact admitting of demonstration. It is true of the Alps, 

 the Andes, the Himalayas*, the Rocky Mountains or North- 



* Mr. C. S, Middlemiss, in his extended criticisms on the ' Origin of 

 Mountain-Ranges ' (Memoirs of the Geological Survey of India, vol. xxiv. 

 part 2 ; Physical Geology of the Sub-Himalaya of Garhwal and Kumaun), 



