Rev. 0. Fisher — Formation of Mountains. 63 



Still further, if instead of 7000° Far. we assume 4000° to be the 

 temperature for melting rock, which seems to be justified by Mr. 

 Mallet's experiments, then the value of 2 & e, or the thickness of the 

 coating referred to, would be less than 150 feet. In the latter case 

 the time since solidification commenced would be about thirty-three 

 millions of years. 



If we compare the values thus found upon two different supposi- 

 tions respecting the temperature of melting rock (one of them being 

 extravagantly large) with the value for the same measurement as 

 determined by estimating the actually existing inequalities of the 

 earth's surface, we cannot but be struck with the immense dis- 

 crepancy between them, the latter being from 13 to 80 times as 

 large as the former. I am consequently led to doubt the necessity 

 for accepting Sir W. Thomson's restrictions upon the manner in 

 which the earth has come into its present state, especially since it 

 seems now generally admitted that Bischoff's results concerning the 

 contraction of melted rock cannot be relied upon. This was pointed 

 out in 1868 by Mr. David Forbes, and quite recently by Mr. Mallet, 

 who has determined the contraction in passing from a molten to a 

 solid state to be scarcely 6 per cent., instead of 25 per cent., as stated 

 by Bischoff. Probably, therefore, when we take into account the 

 intermediate condition of viscosity, we need not assume the breaking 

 up and sinking of a crust formed over a molten globe. This view 

 is supported by what Mr. Scrope tells us about a lava stream re- 

 maining liquid, and even more or less in motion in its central and 

 lower portion for years.^ Indeed, Sir W. Thomson is careful not to 

 exclude as impossible ''the case of a liquid globe gradually solidifying 

 from without inwards, in consequence of heat conducted through the 

 solid crust to a cold external medium." 



If this has been what has happened, there may have been a much 

 larger nucleus inclosed within the crust in early times than we have 

 at present, and thus the corrugations formed would have been larger. 

 And a great portion of that nucleus consisting of superheated rocks 

 in a state of igneo-aqueous fusion, much of the water may have 

 escaped in steam during the frequent volcanic outbursts of pristine 

 ages, so that a large portion, at any rate, of the oceans now 

 .above the crust may have been originally confined beneath it ; and 

 thus a much greater amount of contraction may have taken place 

 than mere cooling would account for. 



It is obvious that this reasoning will apply equally well to the case 

 of a solid globe originally covered with a sufficiently deep layer of 

 molten rock, which is the condition supposed by Sir W. Thomson 

 to be the most probable, a view strongly supported by Dr. Sterry 

 Hunt,- and more in consonance with the rigidity considered requisite 

 to obviate the production of internal tides. But at the same time it 

 is to be remarked, that a highly fluid original condition of the 

 interior may have lasted long after mountains commenced to be 

 formed, and yet its condition need not continue such at the present time. 



1 Volcanoes, 2nd ed. p. 84. 



2 American Journal of Science, vol. v. p. 26 i. 



