132 



Prof. J. Prestwich. 



to the explanation of volcanic action. He supposes a solid crust of about 

 25 miles thick resting on a fluid substratum of highly heated rocky 

 matter in a state of igneo- aqueous fusion, and shows that if a crack were 

 produced by any cause in the under surface of the crust it would become 

 filled with the water substance or vapour given off from the fluid 

 magma at a high tension. Whenever the rent, commencing below, 

 opens upwards, vapour at a high tension will escape, and after a certain 

 time will be followed by the magma itself, which will overflow at the 

 surface because the water- substance expanding, owing to the dimi- 

 nished pressure, will render the whole column of less weight than 

 an equal column of the crust. On this view he considers that any 

 disruption in the crust which is sufficient to permit the passage of 

 steam at an enormous pressure, would originate a volcano ; and " much 

 of the lava poured out might consist of the materials of the crust itself, 

 fused by the passage of the gases through it, and so vary in its compo- 

 sition at different vents, and even at the same vent at different times." 



I need not dwell on the other objections I feel to these hypotheses 

 because the special one before-named applies equally to this, — namely, 

 that, if they were true, all rocks formed under such conditions should 

 exhibit evidence of the presence or of the escape of vapour. All 

 volcanic matter should be more or less scoriaceous, whereas there are 

 many lavas which are little, and others not at all scoriaceous ; while 

 the great sheets of basaltic rocks which have welled out from fissures 

 at former geological periods, are likewise neither scoriaceous, except 

 very superficially if at all, nor are they accompanied as a rule by debris 

 indicating explosions and projections due to the presence of vapour 

 and gases. Why also should not all rocks of igneous origin, as well as 

 volcanic rocks proper, be scoriaceous, if such were the conditions of 

 the molten magma beneath the solid crust ? The general want of 

 hydration in volcanic rocks and their associated minerals'is likewise 

 incompatible with such conditions. 



It has been contended by some writers that large subterranean 

 cavities may exist at depths in the earth's crust, and that the vapour 

 of water under high pressure may be stored up in such underground 

 cavities. But the pressure of the strata is so great at depths, that, as 

 in deep coal pits, where no permanent cavities can be formed, owing 

 to the " creeping " and falling in of the strata, it would be impossible 

 for such cavities to exist in Sedimentary Strata, while in Igneous 

 rocks the initial plasticity of the rock and pressure would effect the 

 same object. Even if such cavities did exist, they could only be 

 maintained by the action of an elastic fluid, whose pressure would 

 exceed that of the superincumbent strata. Geology affords no evi- 

 dence of such underground reservoirs, or of any having existed in 

 former times. No great explosions of pent up steam show themselves 

 during the disturbances, shocks and rents accompanying earthquake 



