

MetamorjpMsm. 405 



The condition of the earth's interior can never "be known by 

 direct investigation. The deepest mine is not nearly three 

 thousand feet below the surface, not a fifth part of the amount 

 of the inequalities either of mountain or ocean. Within that 

 depth there are no doubt indications of a gradual increase of 

 temperature, but it is quite unsafe to assume that this change 

 continues or that it is connected with any uniform increase 

 towards the centre, as it may be purely superficial in every 

 important sense of the word. Volcanoes are phenomena which 

 no doubt indicate that in certain districts and under certain 

 conditions there is a disturbing force at work beneath the 

 earth's surface, and that a high temperature exists at the 

 moderate depths at which volcanic action seems always to 

 originate. 



But all this is a very small scintilla of evidence on which to 

 base so important a conclusion as that the interior of the earth 

 is in a state of igneous fluidity. The skin we can examine is 

 a film so thin in proportion to the diameter of the earth that it 

 does not correspond to more than the coat of varnish on a 

 three-foot globe.* It cannot possibly justify any deductions as 

 to the state of the interior, since even the phenomena of vol- 

 canic action are local. The only real evidence on the subject 

 is derived from investigations purely physical. Thus by an 

 elaborate calculation, the details of which will be found in the 

 Philosophical Transactions for 1857, Mr. Hopkins concluded 

 that the increase of temperature in deep mines cannot be due 

 to any such cause as central heat. Long before this the same 

 mathematician had shown that the earth could not have a fluid 

 nucleus approaching within several .hundred miles of the sur- 

 face, and very lately one of our most distinguished physicists, 

 Professor W. Thomson, of Glasgow, has shown the probability 

 that unless the earth were more rigid than steel to a depth of 

 at least two thousand miles, it could not maintain its figure 

 against the tide-producing forces of the sun and moon so as to 

 allow the phenomena of the ocean tides and of precession and 

 nutation to be as they are.f 



Putting aside then for the present the assumption that the 

 earth has once been in a state of fluidity from heat, that it has 

 cooled down gradually until a thin film has formed on the sur- 

 face, and that this film has since been cracked and the fissures 

 filled from the molten sea of fire below, let us endeavour to 

 seek for some explanation of known phenomena without this 

 hypothesis, and, if possible, without any preconceived notion or 

 theory whatever. Let us consider the state of the earth's 

 outer film, or crust, the changes to which it is subjected, and 



* Three thousand feet corresponds to l-200th of an inch on a three-foot globe. 

 f Proceedings of Royal Society for April 14, 1862. 



VOL. IV. — NO. VI. E E 



