ON THE OEIGIN OF MOUNTAIN EANGES. 159 



heat of the earth would not produce contraction ratlior than 

 expansion ; at least if it was sufficiently great to introduce 

 motamorphism or any chemical change." (Oeol. Mag., vol. 

 X. p. 261). This is the view to which I believe most physical 

 geologists will incline, and that the more since it seems to 

 accord with the gradual sinking which accompanies con- 

 tinuous depression. I cannot but feel surprise that Mr. 

 Reade should pass over this aspect of the question in almost 

 complete silence. 



And if the expansion docs take place, with the rise of the 

 isogootherms, as is assumed by Mr. Reade, how is it that 

 the vast masses of preparatory sediment (40,000 feet or 

 more in some cases) continue to be formed in a sinJdng 

 area ? Why is the effect of expansion, wliich one would 

 expect to take place pari passu with sedimentation, delayed 

 for long geological ages ? Docs Mr. Reado answer that the 

 rise of the isogootherms is a slow and gradual process ? So 

 is sedimentation. A further question suggests itself. Why, 

 when expansion does begin to take effect, is mountain up- 

 heaval so rapid ? I do not mean rapid as historians count 

 time, but rapid as geologists count time. Mr. Reade again 

 and again reminds us that mountain building is a slow pro- 

 cess; and so it is in one sense. But whereas the preparatory 

 sedimentation for tho Alps occu]iied all the Mesozoic and 

 Eocene time, tho first uplicaval of that range can bo dated 

 distinctly as Oligocene. Compared with the ages of previous 

 sedimentation, tho upheaval of the Alps may be said to be 

 sudden. 



Let us now look at Mr. Roade's figures. He tells us that 

 if we assume that in one area of the globe the temperature 

 plane of 3,000° Fah. rises from a depth of thirty-five miles 

 to a dei)th of twenty-five miles, tlie mean increase of tem- 

 perature for tho whoio overlying twenty-five miles of rock 



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