1869.] HUNT — ON VOLCANOES AND EARTHQUAKES. 393 



matter, that we are to find an explanation of all the phenomena 

 of volcanoes and earthquakes, of elevation and subsidence, and of 

 the movements which result in the formation of mountain chains, 

 as ingeniously set forth by Mr. Shaler. The slow contraction of 

 the gradually cooling globe, a most important agency in the latter 

 phenomena, is evidently not excluded by this hypothesis. It may 

 be added that a similar structure of the globe, viz., a solid nucleus 

 and a solid crust separated from each other by a liquid stratum? 

 was long ago suggested by Halley in order to explain the phe- 

 enomena of terrestrial magnetism, Scrope has completed this 

 hypothesis by the suggestion that variations in tension or pressure 

 may cause portions of matter beneath the surface to pass from solid 

 to liquid, or from a liquid to a solid state, and in this way help us 

 to explain the local and the temporary nature of volcanic activity. 



This theory of Hopkins and Scrope, apparently so complete in 

 itself, is an approximation to the one which I adopt, though 

 differing from it in some most important particulars. While 

 admitting with them the existence of a solid nucleus and a solid 

 crust, with an interposed stratum of semi-liquid matter, I consider 

 this last to be, not a portion of the yet unsolidified igneous matter, 

 but a layer of material which was once solid, but is now rendered 

 liquid by the intervention of water under the influence of heat 

 and pressure. When, in the process of refrigeration, the globe 

 had reached the point imagined by Hopkins, where a solid crust 

 was formed over the shallow molten layer which covered the solid 

 nucleus, the farther cooling and contraction of this crust would 

 result in irregular movements, breaking it up, and causing the 

 extravasation of the yet liquid portious confined beneath. When 

 at length the reduction of temperature permitted the precipitation 

 of water from the dense primeval atmosphere, the whole cooling 

 and disintegrating mass of broken-up crust and poured-out 

 igneous rock would become exposed to the action of air and 

 water. In this way the solid nucleus of igneous rock became 

 surrounded with a deep layer of disintegrated and water-impreg- 

 nated material, the ruins of its former envelope, and the chaotic 

 mass from which, under the influence of heat from below and of 

 air and water from above, the world of geologic and of human 

 history was to be evolved. 



As we descend in the sedimentary crust of the earth, we 

 observe a regular increase of temperature, due, as is supposed, to 

 the slow upward passage of the central heat. In the present 



Vol. IV. A No. 4. 



