ISO PEOFESSOE EPWAED HULL. LLD. ; F.E.S.. OX 



thickness, ultimately resting on a semi-fluid interior of lava, 

 rock in a molten condition due to heat, such as is erupted from 

 volcanoes in action. That this is the condition of the interior 

 immediately under the crust may be inferred from experiments 

 earned out over large portions of the land areas, which go to 

 show that the temperature increases with the depth at an 

 average rate of about 1 = Fahr. for every 60 feet of depth. 

 These experiments only reach to about 3,500 feet from the 

 " invariable stratum,*' only a short distance downwards.* There 

 is no indication that the temperature tends to decrease as the 

 depth increases, and the molten condition of liquid lava 

 extruded from great depths by volcanic action indicates the 

 contrary. We may feel confident that the mass of matter 

 enveloping the centre of the globe nearly retains the tempera- 

 ture of its original condition owing to the exceedingly slow 

 radiation of heat from the surface. 



With a continuance of this increasing heat a depth must be 

 eventually reached at which there will be a temperature equal 

 to that of rock fusion at the surface : though the pressure at 

 those depths may affect the conditions of fluidity tending to the 

 solidity of the crust. Taking the rate of increase at l c Fahr. 

 for every 60 feet below the invariable stratum, the following 

 will be the heat at various depths : — x 



Depth. Temperature. 

 50 feet ... 50° Fahr. 



1.000 

 4,000 

 7.S26 

 34.702 

 150,000 



63° „ 

 116° „ 



212 = r . Boiling point of water. 

 773° „ Critical point of water. 

 3 ; 174~ Approximate fusion point 

 of rocks. 



the last case, about 28 miles, being the minimum thickness of 

 the crust. 



Assuming this to be the composition of the globe, it is clear 

 that it can give way to lateral pressure exerted at opposite 



* The invariable stratum is the depth of the annual mean temperature 

 of the locality — a few feet from the surface. 



t Prestwieh, Geology, Chemical, Physical and Stratigraphical, toL ii, 

 p. 537. The question of a solid mobile crust resting on a viscous interior 

 of molten matter due to heat, is ably dealt with, and is one I myself 

 hold strongly in opposition to the theory of a solid interior. 



