THE VOLCANIC ROCKS OF ICELAND. 



89 



pyrites, and sometimes free from it. The recently mentioned 

 palagonitic substance of the tuff surrounding the solfatara of 

 Krisuvik consists of — 



Silica .... 

 Alumina . . . 

 Peroxide of iron 

 Lime . . . . 

 Magnesia . . 

 Potash . . . 

 Soda . . . . 

 Water . : . 

 Phosphoric acid 

 Residue . 



104. 



37-95 

 13-61 

 13*75 



6-48 

 7*13 

 0-42 

 1-72 

 12-68 

 0-43 

 7-25 



101-42 



} 



Oxygen found. 

 20-09 



10-49 



5-47 



Oxygen calculated. 

 20-60 



10-30 



5-15 



It may be seen that the composition corresponds almost com- 

 pletely with the formula proposed for palagonite, 3RO 2Si 0^4- 

 '2R^ O^ SiO^ + Aq, and consequently approximates very closely 

 to the composition of the pyroxenic rocks. A fumarole clay 

 formed from this tuff contained nearly 30 per cent, of very finely 

 developed crystals of iron pyrites ; and the argillaceous matter 

 separated from them consisted of — 



101-22 



The mixture of which the argillaceous mass consists corresponds 



with the formula RO 2Si03 + 2R203 SiO^ + Aq. Exactly the 



same decomposition is observed in the pyroxenic rocks. At the 



north-eastern foot of the Namarfjall, near Reykjahlidh, there are 



