106 NEWER PLIOCENE PERIOD. [Ch. IX, 



sive force of volcanic heat is sufficiently great to overcome the 

 resistance of columns of lava, several miles or leagues in height, 

 forcing them up from great depths, and causing the fluid 

 matter to flow out upon the surface. To imagine, therefore, 

 that this same power, which is so frequently exerted in dif- 

 ferent parts of the globe, should occasionally propel a column 

 of lava to a considerable height, yet be unable to force it 

 through the superincumbent rocks, is quite natural. 



Whenever the superimposed masses happen to be of a yield- 

 ing and elastic nature, they will bend, and instead of break- 

 ing, so as to afford an escape to the melted matter through a 

 fissure, they will allow it to accumulate in large quantities 

 beneath the surface, sometimes in amorphous masses, and 

 sometimes in horizontal sheets. So long as such sheets of 

 matter retain their fluidity, and communicate with the column 

 of lava which is still urged upwards, they must exert an 

 enormous hydrostatic pressure on the overlying mass, tend- 

 ing to elevate it, and an equal force on the subjacent beds 

 pressing them down, and probably rendering them more com- 

 pact. If we consider how great is the volume of lava that 

 sometimes flows out on the surface from volcanic vents, we 

 must expect that it will produce great changes of level so often 

 as its escape is impeded. 



Let us only reflect on the magnitude of Iceland, an island 

 two hundred and sixty miles long by two hundred in breadth, 

 and which rises, at some points, to the height of six thousand 

 feet above the level of the sea. Nearly the entire mass is repre- 

 sented to be of volcanic origin ; but even if we suppose some 

 parts to consist of aqueous deposits, still that portion may be 

 more than compensated by the great volume of lava which 

 must have been poured out upon the bottom of the surround- 

 ing sea during the growth of the entire island ; for we know 

 that submarine eruptions have been considerable near the coast 

 during the historical era. Now if the whole of this lava had 

 been prevented from reaching the surface, by the weight and 

 tenacity of certain overlying rocks, it might have given rise to 



