THE VOLCANIC ROCKS OF ICELAND. 69 



3. Formation of Rocks by Pneumatolytic Metamorphism. 



The processes considered under this head refer to manifold 

 products which result from the action of volcanic gases and 

 vapours upon the rocks which have been already treated of. 

 They are of no less importance than the latter, and acquire for 

 geologists an especial interest from the fact, that the processes 

 to which they owe their origin admit of direct observation. In 

 order to comprehend the various reactions which take place 

 during these processes, it must be remembered that the mass of 

 most rocks consists of a mechanical mixture of solid rock with 

 water ; and that the action of the melted masses coming in con- 

 tact with these beds impregnated with water was twofold, and 

 took place at two distinctly different periods. It must have 

 commenced with a vaporization of the water ; and it was not until 

 after this had ceased that the hitherto prevailing temperature, 

 determined by the existing pressure, could increase and be raised 

 so high that the thorough plutonic reaction between the heating 

 and heated rocks commenced. All those apparent contradictions 

 which are presented by the pha^nomena of plutonic contact ad- 

 mit of easy explanation by the aid of this physical necessity. 

 For this purpose three possible cases must be kept in view. The 

 first is when a melted eruptive mass, possessing the lowest pos- 

 sible temperature and moving very slowly, comes in contact with 

 a rock which receives a very rapid supply of water from fissures 

 and spring strata. Here all the conditions are combined, which 

 would tend to prevent any trace of a direct igneous action upon 

 the substance of the adjoining rock. The first result of the con* 

 tact is the formation of a solid crust, frequently of a glassy, sco- 

 riaceous and basaltic character, such as may be observed in many 

 places where basalt has penetrated, and especially when in the 

 form of dykes. This solid crust may be compared to a badly 

 conducting furnace-wall, through which the equalization of tem- 

 perature in the adjoining rock, maintained constantly at the 

 boiling temperature of water, can only take place slowly. A 

 plutonic metamorphism or interfusion of the adjoining rock 

 would here be a physical impossibility. The second case, fre- 

 quent in Iceland, but more rare in our basalts, is that in which 

 the last-mentioned conditions ai'e either absent or comparatively 



