Sir J. Herschel on the Theory of Volcanic Phcenomena. 213 



Assuming a high central temperature, which many geologists ad- 

 mit, and with which all are familiar; the author agrees with Mr. 

 Lyell's observations, that the ordinary repose of the surface argues 

 a wonderful inertness in the interior, where in fact he conceives that 

 everything is motionless; debarred therefore from the invasion of a 

 circulating current or casual injection of intensely hot liquid matter 

 from below, he conceives that the phaenomena may be explained as 

 follows : 



Granting an equilibrium of temperature and pressure within the 

 globe, the isothermal strata near the centre will be spherical ; but 

 where they approach the surface they will by degrees con form them- 

 selves to the configuration of the solid portion, that is, to the bottom of 

 the sea and the surface of continents. If we suppose therefore a state 

 of equilibrium, and that under the concave bottom of any great 

 ocean the lines of equal temperature be parallel to its concavity ; 

 when this comes to be filled up by the deposition of matter brought 

 down by rivers, etc., the formerly concave bottom may become hori- 

 zontal or even convex, and the equilibrium of temperature will im- 

 mediately be disturbed ; because the form of a stratum of tempera- 

 ture depends essentially on the bounding surface of the solid above 

 it, that form being one of the arbitrary functions which enter into 

 its partial differential equation. The temperature, therefore, will im- 

 mediately begin to migrate from below upwards, and the isothermal 

 strata will gradually change their forms from the concave to the hori- 

 zontal or convex form. The former bottom of the ocean will then 

 (after the lapse of ages, and when a fresh state of equilibrium is at- 

 tained) acquire a temperature corresponding to its then actual 

 depth ; while a point as deep below it, as itself is below the surface, 

 will have acquired a much higher temperature, and may become ac- 

 tually melted, and this without any bodily transfer of matter in a 

 liquid state from below. But if the temperature of this supposed 

 deep stratum be already at the melting point, then will this rise to 

 the former bottom of the ocean and the strata become melted, 

 water included, with which, from the circumstances of the case, 

 they must be saturated. 



If the process of deposition go on, until by accumulation of press- 

 ure on the bottom or sloping sides, some support gives way, — a piece 

 of the solid crust breaks down and is plunged into the liquid below, 

 and a crack takes place, extending upwards. Into this the liquid 

 will rise by simple hydrostatic pressure. But as it gains height it 

 is less pressed ; and if it attain such a height that the ignited water 

 can become steam, the joint specific gravity of the column is sud- 

 denly diminished and up comes a jet of mixed steam and lava ; till 

 so much has escaped that the deposited matter takes a fresh bearing, 

 when the evacuation ceases and the crack becomes sealed up. 



By taking this viewof the process of heating from below, we have 

 a strictly theoretical explanation of the effects of heat on newly de- 

 posited strata ; and this, simply because the fact of new strata ha- 

 vmgbeen deposited, theconditions of the equilibrium of temperature 

 become altered, and they draw the heat to them, or rather retain it 



