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IN THE FAROE ISLANDS. 223 



^ surfaces of submarine lavas uneven and rough, as in the ex- 

 ^ amples before the Society. 



In considering this subject, it ought to be observed, that 

 steam is not like permanently elastic fluids, which, when they 

 escape, overcome any pressure of water, and rise quickly to the 

 surface. In the case of water above a greatly heated surface, 

 there is a constant production, as well as a constant condensa- 

 tion of steam ; and the water itself may, to a considerable ex- 

 tent, become greatly heated. But steam produced under the 

 lava, by its flowing over a wet, and, if tuffa be present, over a 

 spongy surface, would act in a very different manner, as I have 

 shown in my account of the JMineralogy of Iceland. It will 

 have a tendency upwards, and will act upon the hot mass, ren- 

 dering it more or less vesicular, according to its degree of flui- 

 dity *. 



Thus 



* " When the lava is very hot, and consequently very liquid, the steam will 

 have less difficulty in penetrating it than when it is viscid. We may con- 

 ceive cases in which the lava burst forth in such a high state of liquidity, as 

 to permit the whole of the moisture to pass through it in the form of steam ; 

 in such a state of viscidity, as to admit of its escaping very slowly, so that the 

 lava may become solid, and, by confining the steam, more or less vesicu- 

 r lar -|- ; and, lastly, so tough, that the exertions of the elastic vapour shall be 



t* confined entirely to the lower surface of the lava. In the first case, a com- 

 pact mass of stone would be formed, having no appearance of the action of 

 heat ; in the second, on account of the pressure of the superincumbent wa- 

 I ter being sufficient to prevent the escape of carbonic acid and other volatile 

 ingredients, a vesicular and araygdaloidal mass would be produced ; and from 

 the last would result a mass entirely compact, excepting at the under surface." 

 Travels in Iceland, chap. tx. 



To this passage of the text the following note is subjoined : 

 " In such a case, it is possible that the steam, when condensed, would, in 

 some instances, remain confined in the stone in the form of water ; and thus 



the 



