the Earth's Contraction from Cooling, 277 



transformed into heat being its source, in conformity with the 

 views ably set forth by Mallet ? 



(2) Is the ejecting force in eruptions a result of the same 

 lateral pressure ? 



1. Source of igneous fusion*. — The suggestion of Professor 

 Hopkins with regard to the earth's interior, which has been sus- 

 tained on a preceding pagef, offers for the source of igneous 

 ejections the old one of viscous rock underneath the earth's 

 crust ; but viscous rock at first as a layer, and later as remnants 

 of the same layer, left after the long cooling. This author sug- 

 gests that these isolated or detached portions may be the source 

 of modern volcanic eruptions; and this view is favoured by 

 Scrope, the eminent vulcanologist J. 



The theory does not require that the isolated fire-lakes should 

 exist now at the depth of the original viscous layer ; for in the 



* I say nothing here on the nature or the degree of igneous fusion in 

 plastic rock, as this is foreign to the subject under discussion. 



t Professor Hopkins's argument from the amount of precession and 

 nutation, in the Transactions of the Royal Society, 1839, 1840, and 1842, 

 led him to the conclusion that "the thickness of the solid shell could not 

 be less than about one fourth or one fifth of the radius of its external sur- 

 face." In his paper in the Report of the British Association for 1847 he 

 repeats his conclusion, and then considers the possible steps in the process 

 of refrigeration. Among the different conditions discussed, he supposes as 

 one (the one he favours) the temperature of fusion to be increased much 

 by the pressure — at a rate much more rapid than " 1° F. for an increase of 

 pressure equivalent to about five atmospheres," and adds that then " we 

 should probably have the condition under which solidification would com- 

 mence at the centre. In this case, after incrustation had begun at the 

 surface, the earth would consist of a solid central nucleus and a solid shell 

 with fluid matter between them, as already explained, till the solidification 

 should be complete." He then remarks, with regard to the present state 

 of our globe, that " under the condition here assumed, with the observed 

 rate of increase of terrestrial temperature in descending, there could be no 

 reasonable doubt of the earth's entire solidity." He next treats more in 

 detail of the process of solidification, and says, having in view the several 

 conditions discussed, that "the incrustation may have constituted the very 

 first step in the solidification, or it may have taken place after the forma- 

 tion of a solid nucleus surrounded by a superficial fluid envelope, according 

 as the solidification began at the surface or centre ; but in either case, as 

 I have already intimated, the superficial crust when first formed must ne- 

 cessarily have reposed on a fluid mass beneath." He next explains his 

 views as to the formation of the crust, and the final obliteration of the vis- 

 cous layer; and states, with reference to volcanoes : — " By a continuation 

 of this process it is obvious that the superficial crust of our globe must at 

 length arrive at what I conceive, for reasons above assigned [in a paragraph 

 on p. 33, speaking of the isolated sources of volcanoes], to be its present 

 condition, — that of a solid mass containing numerous cavities filled with 

 fluid incandescent matter, and either entirely insulated or perhaps com- 

 municating in some cases by obstructed channels." These cavities are 

 made the sources of the material of volcanic eruptions. 



X Volcanoes, 2nd edit. 8vo, pp. 264, 2(>8. 



