THE VOLCANIC ROCKS OF ICELAND. 61 



basalt is mixed with a large excess of fused hydrate of potash, 

 and the basic alkaline silicate formed removed by means of 

 water. The substance obtained after washing and elutriation is 

 hydrated, and pulverulent when dry ; it gelatinizes even with 

 the weakest acids, is readily decomposed by carbonic acid and 

 sulphuretted hydrogen, and has the following composition, cor- 

 responding with that of the purest Icelandic palagonite : — 



The residue after elutriation consists of a mixture of silicates, 

 whose mean composition differs from that of palagonite only by 

 about a one-fifth smaller per-centage of silica, which remains in 

 the alkaline water used for washing the substance, from which 

 distinctly-formed zeolitic crystals are sometimes separated having 

 the composition SCaO 2Si03-f Aq. I shall again refer to these 

 further on. 



This conversion of pyroxenic rocks into palagonite is accom- 

 panied by a very remarkable phaenomenon. A considerable 

 quantity of pure hydrogen is disengaged, resulting from the 

 oxidation of protosilicates of iron to persilicates taking place at 

 the cost of the equivalent of water contained in the hydrate of 

 potash. The consequence of this is, that there is no trace of 

 protoxide of iron in the palagonites, and the protoxide of iron in 

 the pyroxenic rocks always appears as peroxide in the palago- 

 nitic tuffs. The protoxide and peroxide of manganese behave 

 in a manner similar to the protoxide of iron, either free or com- 

 bined with silica, passing, with disengagement of hydrogen, into 

 manganate of potash, which reaction may be partly the cause of 

 the frequent incrustations and dendritic deposits which are met 

 with in the palagonitic rocks. The occurrence of metallic cop- 

 per, evidently reduced from chloride of copper, in the palagonite 



