96 BUN8EN ON THE FORMATION OF 



its constituents by extraction, and that the calcareous spar and 

 pyrites present in it have not been introduced from outwards, 

 but were formed by simple re-ariangement of the constituents 

 upon the spot. For if the radicals contained in the calcareous 

 spar and the pyrites are restored to the rock as lime and prot- 

 oxide of iron, we obtain the composition of a tracheo-pyroxenic 

 rock perfectly corresponding with theory ; — 



115. 



Found. Calculated. 



Silica 56-48 56-48 



Alumina and protoxide of iron . 26*62 25*65 



Lime 9*78 8*91 



Magnesia 4*33 4*99 



Potash 0*34 1*38 



Soda 2*15 2*59 



100*00 100*00 



This constitution corresponds to a mixture of 1 part trachyte 

 and 2*521 pyroxenic substance. The penetrated rock has there- 

 fore here also mixed with the penetrating mass. These exam- 

 ples, to which I could add a great number of others, may suffice 

 to explain the fact, that the gases penetrating the rocks toge- 

 ther with water and aqueous vapour, are capable of metamor- 

 phosing their substance on the spot without any removal of the 

 products of decomposition. 



It only remains for me at the close of this general view, to 

 mention the phaenomena which present themselves where the 

 pneumatolytic and zeolitic metamorphoses take place together. 

 The result of this state of things is the production of argilla- 

 ceous amygdaloid conglomerates, extremely rich in zeolites, and 

 which alternate in irregular order as thick beds or masses with 

 the pyroxenic eruptive rocks. They may be regarded as pyro- 

 caustic products of an immense subterranean fumarole action, 

 in which sulphuretted hydrogen or sulphurous acid were less 

 active than aqueous vapour, either alone or mixed with carbonic 

 acid. The already-mentioned districts round Silfrastadir are 

 especially remarkable in this respect. This small spot is situated 

 in the valley of the Heradsvotn, which opens into that of the 

 SkagaQordhr, and is enclosed by rocky precipices, consisting of 



