THE CRUST OF THE EARTH. 



of hypotheses for the explanation of natural phenomena, no cause 

 can be admitted, except such as can he shown to have real 

 existence, and which, being admitted, shall appear to be sufficient 

 to produce the phenomena which it is put forward to explain. In 

 accordance with this rule, geologists are therefore required to show 







Fig 45. 



that the elevated internal temperature of the globe, those 

 upheavings and disruptions of land, those sedimentary deposits 

 from water, those ejections of fluid and pasty matter from clefts 

 and caverns, those abrasions of the solid crust by the corroding 

 action of water, and its modifications by the action of the atmos- 

 phere, are severally real agencies still in visible operation, and 

 producing effects similar in kind though different in degree 

 from those ascribed to them in geology, and also that even the 

 difference in degree, which must be admitted to be often very 

 considerable, admits of satisfactory explanation. 



Before, therefore, proceeding further in the exposition of the 

 phenomena disclosed by the labours of geologists, we shall here 

 pause for the purpose of showing the reality of the various phy- 

 sical causes to which geologists have ascribed the phenomena. 

 We shall see that like phenomena have been, and still are, 

 developed on the surface of the earth ; and that the reasons why 

 these agencies were more energetic at remote epochs than at 

 present are sufficiently obvious. 



108. It has been already very fully explained in our Tract on 

 4 'Terrestrial Heat," that in descending deeper and deeper through 

 the crust of the globe, the temperature continually rises ; so that 

 at a depth which forms a very insignificant fraction of the semi- 

 diameter of the globe, the materials which constitute it must 

 have a temperature altogether incompatible with their continuance 

 in the solid state, a temperature, in short, which is above the 

 point of fusion of the most refractory of these materials. 



109. The superficial heat of the earth may be considered, 

 therefore, to result from the combined effects of solar and internal 

 heat. In the present condition of the globe, the effects of the 



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